The World Health Organization defines PEComas as mesenchymal

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1 ORIGINAL ARTICLE Perivascular Epithelioid Cell Neoplasms of Soft Tissue and Gynecologic Origin A Clinicopathologic Study of 26 Cases and Review of the Literature Andrew L. Folpe, MD,* Thomas Mentzel, MD, Hans-Anton Lehr, MD, PhD, Cyril Fisher, MD, Bonnie L. Balzer, MD, PhD,* and Sharon W. Weiss, MD* Abstract: PEComas, occasionally associated with the tuberous sclerosis complex, are defined by the presence of perivascular epithelioid cells that coexpress muscle and melanocytic markers. This family of tumors includes angiomyolipoma (AML), clear cell sugar tumor of the lung (CCST), lymphangioleiomyomatosis (LAM), and very rare tumors in other locations. Because non-aml/non-lam PEComas are extremely rare and their natural history and prognostic features undefined, we present our experience with 26 PEComas of soft tissue and the gynecologic tract, the largest series to date. We also performed a detailed review of the literature, with special attention to features predictive of clinical behavior. All PEComas exclusive of AML and LAM were retrieved from our consultation files. Immunohistochemistry for pan-cytokeratin (CK), S-100 protein, smooth muscle actins (SMA), desmin, vimentin, HMB45, Melan-A, microphthalmia transcription factor (MiTF), TFE3, CD117, and CD34 was performed. Clinical follow-up information was obtained. Fisher s exact test was performed. The median patient age was 46 years (range, years); there was a marked female predominance (22 females, 4 males). Sites of involvement included the omentum or mesentery (6 cases), uterus (4 cases), pelvic soft tissues (3 cases), abdominal wall (2 cases), uterine cervix (2 cases), and vagina, retroperitoneum, thigh, falciform ligament, scalp, broad ligament, forearm, shoulder, and neck (1 case each). The tumors ranged from 1.6 to 29 cm in size (median, 7.8 cm). Tumors were epithelioid (N = 9), spindled (N = 7), or mixed (N = 10). Multinucleated giant cells were present in 18 cases. High nuclear grade was noted in 10 cases, high cellularity in 7 cases, necrosis in 8 cases, and vascular invasion in 3 cases. Mitotic activity was 0 to 50 mitotic figures (MF)/50 high power fields (HPF) (median, 0 MF/50 HPF) with atypical MF in 6 cases. IHC results were: SMA (20/25), desmin (8/22), HMB45 (22/24), Melan-A (13/18), MITF (9/18), S-100 protein (8/24), CK (3/23), vimentin (12/14), TFE3 (5/17), c-kit (1/20), and CD34 (0/7). Clinical follow-up (24 of 26 patients, 92%; median, 30 months; range, months) showed 3 local recurrences and 5 distant metastases. At last available clinical follow-up, 2 patients From the *Department of Pathology and Laboratory Medicine, Emory University, Atlanta, GA; Dermatopathologische Gemeinschaftspraxis, Friedrichshafen, Germany; Institut Universitaire de Pathologie, Centre Hospitalier Universitaire Vaudois, Lausanne, Switzerland; and Department of Pathology, Royal Marsden NHS Trust, London, UK. Reprints: Andrew L. Folpe, MD, Department of Pathology, H-175, Emory University Hospital, 1364 Clifton Road NE, Atlanta, GA ( afolpe@emory.edu). Copyright Ó 2005 by Lippincott Williams & Wilkins (8%) were dead of disease, 4 patients (17%) were alive with metastatic or unresectable local disease, and 18 patients (75%) were alive with no evidence of disease. No patient in our series had a history of tuberous sclerosis complex. Recurrence and/or metastasis was strongly associated tumor size. median size (8 cm), mitotic activity greater than 1/50 HPF, and necrosis. We conclude that PEComas of soft tissue and gynecologic origin may be classified as benign, of uncertain malignant potential, or malignant. Small PEComas without any worrisome histologic features are most likely benign. PEComas with nuclear pleomorphism alone ( symplastic ) and large PEComas without other worrisome features have uncertain malignant potential. PEComas with two or more worrisome histologic features should be considered malignant. Occasional PEComas express unusual markers, such as S-100 protein, desmin, and rarely CK. The role of TFE3 in PEComas should be further studied. Key Words: perivascular epithelioid cell, immunohistochemistry, sarcoma (Am J Surg Pathol 2005;29: ) The World Health Organization defines PEComas as mesenchymal tumors composed of histologically and immunohistochemically distinctive perivascular epithelioid cells. 16 The concept of a family of neoplasms derived from these distinctive cells was first advanced by Bonetti et al in a letter published in the American Journal of Surgical Pathology in 1992, in which they noted the presence in both angiomyolipoma (AML) and clear cell sugar tumor of the lung (CCST) of an unusual cell type. immunoreactive with melanocytic markers, and exhibit(ing) an epithelioid appearance, a clear-acidophilic cytoplasm, and a perivascular distribution. 6 The PEComa family of tumors has subsequently growntoincludeaml,ccst,lymphangioleiomyomatosis (LAM), and a number of unusual visceral, intraabdominal, soft tissue and bone tumors, which have been described under a variety of names, including clear cell myomelanocytic tumor of the falciform ligament/ligamentum teres, abdominopelvic sarcoma of perivascular epithelioid cells, and primary extrapulmonary sugar tumor, among others. 16 Although there is a strong association between the tuberous sclerosis complex (TSC), AML, and LAM, this association is much less clear for the rarer PEComas. Altogether, fewer than 65 non-aml, non-lam, non-ccst PEComas have been reported. 1,4,5,9,10,13 15,18,19,21,23 27,29,31,35,36,38 40,42,44,45,48,49,52,54, Am J Surg Pathol Volume 29, Number 12, December 2005

2 Am J Surg Pathol Volume 29, Number 12, December 2005 PEComas of Soft Tissue and Gynecologic Origin To better understand the pathologic spectrum and immunophenotype and clinical behavior of these rare tumors, we studied 26 PEComas of soft tissue and gynecologic origin. We also performed a detailed literature review of all previously reported soft tissue, bone, and gynecologic PEComas, with particular attention to features that might be predictive of clinical behavior. METHODS Thirty-two cases of PEComa were retrieved from our consultation archives. One of these cases had been previously reported as a clear cell myomelanocytic tumor of the thigh. 19 These cases had been previously coded as perivascular epithelioid cell neoplasm or clear cell myomelanocytic tumor. Upon re-review, we excluded 6 cases that either showed focal adipocytic differentiation or lacked expression of either gp100 protein (HMB45) or Melan-A. Almost all included cases showed typical morphologic features of epithelioid AML 12 epithelioid cells with clear to lightly eosinophilic cytoplasm arranged around and comprising the walls of abnormal, thick-walled blood vessels) or clear cell myomelanocytic tumor 18 (fascicles and nests of spindled cells with clear to lightly eosinophilic cytoplasm arrayed around an elaborate capillary-sized vascular network, reminiscent of renal cell carcinoma, as well as occasion thick-walled vessels similar to those seen in epithelioid angiomyolipoma). These cases all expressed HMB45, Melan-A, or both markers. Coexpression of melanocytic and myoid markers was required only for cases that deviated significantly from the above morphologic descriptions. All cases were studied with respect to patient age and sex, tumor site and size, growth pattern (circumscribed or infiltrative), cellularity and nuclear grade (low to moderate versus high), mitotic figures/50 high power fields (HPF), atypical mitotic figures, coagulative tumor cell necrosis, 3 vascular invasion, epithelioid or spindled morphology, and the presence of multinucleated tumor giant cells. Cases with high nuclear grade showed nuclear pleomorphism, irregular nuclear contours, and prominent nucleoli, whereas cases with low to moderate nuclear grade showed regular nuclear contours, only moderate nuclear variability in size, and only occasional small nucleoli. Cases with high cellularity showed high nuclear to cytoplasmic ratios, nuclear crowding, and nuclear overlap, whereas cases with low to moderate cellularity lacked these features. Immunohistochemical studies for pan-cytokeratin (AE1/AE3, 1:50, Dako Corp, Carpinteria, CA), desmin (D33, prediluted, Ventana Medical Systems, Tucson, AZ), S-100 protein (polyclonal, 1:800, Dako), a-smooth muscle actin (1A4, 1:160, Dako), gp100 protein (HMB45, 1:40, Dako), Melan A (A103, 1;40, Dako), microphthalmia transcription factor (34CA5, 1;10, Novocastra, Newcastle upon Tyne, UK), vimentin (V9, 1:80, Dako), TFE3 (polyclonal, 1:160, Santa Cruz Biotechnology, Santa Cruz, CA), CD117 (polyclonal, 1:200: Dako), and CD34 (QBEND10, 1:320, Dako) were performed. Sections were deparaffinized and rehydrated. Epitope retrieval was performed in citrate buffer (ph 6) using an electric pressure cooker, for 5 minutes at 120 C with cooling for 10 minutes before immunostaining. Epitope retrieval was not used for S-100 protein. All tissues were then exposed to 3% hydrogen peroxide for 5 minutes, primary antibody for 25 minutes, biotinylated secondary linking antibody for 25 minutes, streptavidin enzyme complex for 25 minutes, diaminobenzidine as chromogen for 5 minutes, and hematoxylin as counterstain for 1 minute. These incubations were performed at room temperature; between incubations, sections were washed with Tris-buffered saline buffer. Cases were scored as negative, 1+ (5% 25% positive cells), 2+ (26% 50% positive cells), and 3+ (.51% positive cells). Appropriate negative controls were used. RESULTS Clinical Findings The clinicopathologic features of the PEComas included in this study are summarized in Table 1. The 26 tumors occurred in 22 women and 4 men, with a median patient age of 46 years (range, years). Sites of involvement included the omentum or mesentery (6 cases), uterus (4 cases), pelvic soft tissues (3 cases), abdominal wall (2 cases), uterine cervix (2 cases), and vagina, retroperitoneum, thigh, falciform ligament, scalp, broad ligament, forearm, shoulder, and neck (1 case each). Only the scalp tumor occurred in a suprafascial location. The tumors ranged from 1.6 to 24 cm (median, 7.8 cm). Follow-up information was available for 24 of 26 patients (92%) with a mean duration of 30 months, a median duration of 30 months, and a range of 10 to 84 months. Local recurrences and distant metastases were noted in 3 cases (13%) and 5 cases (21%), respectively; metastatic sites included the liver (3 events), lung (2 events), and bone (1 event). Two patients suffered metastases to more than one site. Adjuvant radiotherapy and/or chemotherapy were given to 3 and 5 patients, respectively. At the time of the last available clinical follow-up, 2 patients (8%) were dead of disease, 4 patients (17%) were alive with metastatic or unresectable local disease, and 18 patients (75%) were alive with no evidence of disease. No patient had a history of tuberous sclerosis complex. Histopathologic Findings Figure 1 illustrates the morphologic findings. The tumors grew in both circumscribed (14 cases) and infiltrative (12 cases) patterns, with negative initial surgical margins in 18 cases, positive margins in 4 cases, and an uncertain margin status in 4 cases. All cases were composed of clear to lightly eosinophilic cells, which grew in a fascicular, nested, and occasionally sheet-like pattern, and often were arranged in a radial fashion around blood vessels. A prominent intrinsic vasculature was a feature of all cases and ranged from delicately arborizing capillaries, reminiscent of those seen in renal cell carcinoma, to thicker, often hyalinized arterioles and small arteries. The tumors most often showed an admixture of epithelioid and spindled cells (10 cases, 38%) but were judged to show.95% spindled morphology, identical to so-called clear cell myomelanocytic tumor, 18 in 7 cases (27%) and.95% epithelioid morphology, identical to so-called clear cell sugar tumor 22 or epithelioid angiomyolipoma, in 9 cases (35%). Overall cellularity was low to moderate in most cases (19 cases, 73%), with a minority (7 cases, 27%) having high cellularity. Similarly, 16 cases (62%) had low to at most q 2005 Lippincott Williams & Wilkins 1559

3 Folpe et al Am J Surg Pathol Volume 29, Number 12, December 2005 TABLE 1. Clinicopathologic Features and Clinical Follow-Up No. Submitting Diagnosis Age/Sex Location Size (cm) Nuclear Grade Cellularity 1 GIST 67/F Mesentery 13 High Moderate 2 PEComa 97/F Mesentery 4 Intermediate Moderate 3 Fibrohistiocytic tumor 59/M Omentum 6.5 Intermediate Moderate 4 Mesenchymal neoplasm 15/F Falciform ligament 4.5 Intermediate Moderate 5 PEComa 40/M Omentum 22 Intermediate Moderate 6 Sarcoma 80/F Mesentery 9.5 High High 7 Melanoma 46/F Mesentery 12 Intermediate Moderate 8 Clear cell sarcoma 57/F Vagina Unknown Intermediate Moderate 9 PEComa 28/F Cervix 3 Intermediate Moderate 10 Rhabdoid tumor 48/F Cervix 2 High Moderate 11 PEComa 16/F Broad ligament 4 Intermediate Moderate 12 Leiomyosarcoma, melanoma 59/F Uterus 14.5 High High 13 Malignant PEComa 56/F Uterus 9 High High 14 Malignant PEComa 36/F Uterus Large High High 15 Combined smooth muscle/endometrial stromal tumor 42/F Uterus 7 Intermediate High 16 Liposarcoma 80/M Scalp 2 High Low 17 None 43/F Thigh 3.5 Intermediate Moderate 18 Pleomorphic sarcoma 71/M Forearm 9 High High 19 Rhabdomyosarcoma 77/F Neck 2.6 High Moderate 20 Clear cell sarcoma 49/F Shoulder Large High High 21 PEComa 22/F Abdominal wall 8.5 Intermediate Moderate 22 PEComa 24/F Abdominal wall 10.5 Intermediate Moderate 23 Epithelioid leiomyoma 19/F Pelvic soft tissue 2.1 Intermediate Moderate 24 PEComa 18/F Pelvic soft tissue 6 Intermediate Moderate 25 Leiomyosarcoma 72/F Pelvic soft tissue 24 Intermediate Moderate 26 Low grade sarcoma 46/F Perinephric soft tissue 11 Low Moderate No. Necrosis Mitotic Figures /50 HPF Atypical MF 1 No 0 No No Epithelioid; MNGC and SC present 2 No 0 No No Epithelioid; MNGC and SC present 3 No 0 No No Mixed; MNGC and SC present Vascular Space Invasion Morphology Clinical Follow-Up No additional surgery or adjuvant therapy; ANED at 84 months No additional surgery or adjuvant therapy; ANED at 38 months No additional surgery or adjuvant therapy; ANED at 41 months 4 No 0 No No Spindled Re-excised; ANED at 35 months 5 No 4 No No Spindled Extensive intra-abdominal recurrence/local metastases at 24 months; lost to further follow-up 6 Yes.50 Yes Yes Mixed No additional surgery or adjuvant therapy; ANED at 19 months 7 Yes 5 No Yes Epithelioid Multiple cycles of adjuvant chemotherapy; extensive intra-abdominal recurrences and liver metastases at 22 months; DOD at 27 months 8 No 0 No No Mixed; MNGC present No additional surgery or adjuvant therapy; ANED at 54 months 9 No 0 No No Epithelioid; MNGC present Hysterectomy and lymph node dissection; ANED at 36 months 10 No 0 No 0 Epithelioid; MNGC and Adjuvant radiotherapy; ANED at 21 months SC present 11 No 0 No No Spindled; MNGC present Re-excised; ANED at 18 months 12 Yes 10 No Yes Epithelioid; MNGC and SC present Adjuvant chemotherapy; liver and lung metastases at 30 months; alive with metastases at 30 months 13 No.50 Yes No Epithelioid; MNGC present Adjuvant radio- and chemotherapy; lung and bone metastases at 11 months; alive with metastases at 11 months 1560 q 2005 Lippincott Williams & Wilkins

4 Am J Surg Pathol Volume 29, Number 12, December 2005 PEComas of Soft Tissue and Gynecologic Origin TABLE 1. (continued) Clinicopathologic Features and Clinical Follow-Up No. Necrosis Mitotic Figures /50 HPF Atypical MF Vascular Space Invasion Morphology Clinical Follow-Up 14 Yes.50 Yes No Mixed; MNGC present Hysterectomy and adjuvant chemotherapy; lung metastases at 12 months and liver metastases at 36 months; DOD at 39 months 15 No 0 No No Epithelioid; MNGC and SC present No additional surgery or adjuvant therapy; ANED at 11 months 16 No 10 Yes No Epithelioid; MNGC and Lost to follow-up SC present 17 No 0 No No Mixed; MNGC present Re-excised; ANED at 35 months 18 Yes.50 Yes No Mixed; MNGC present Re-excision and adjuvant radiotherapy; ANED at 10 months 19 No 1 No No Mixed; MNGC and SC present Re-excision and adjuvant radiotherapy; ANED at 6 months 20 Yes.50 Yes No Mixed; MNGC present Neo-adjuvant chemotherapy (adriamycin/iphosphamide); complete resection with residual viable PEComa; ANED at 11 months 21 No 0 No No Spindled; MNGC present No additional surgery or adjuvant therapy; ANED at 15 months 22 No 1 No No Spindled; MNGC present Too recent for follow-up 23 Yes 1 No No Mixed; MNGC present Lost to follow-up 24 No 0 No No Spindled No additional surgery or adjuvant therapy; ANED at 31 months 25 Yes 4 No No Mixed; MNGC and SC present No additional surgery or adjuvant therapy; alive with local recurrence at 15 months 26 No 0 No No Spindled No additional surgery or adjuvant therapy; ANED at 42 months intermediate nuclear grade, with 10 cases (38%) showing high nuclear grade. Multinucleated tumor giant cells were identified in 18 cases (69%) and giant cells with a central zone of eosinophilia surrounded by a peripheral cleared zone, reminiscent of the spider cells seen in adult rhabdomyomas, were present in 9 cases (35%). Six cases (23%) showed high nuclear grade and/or tumor giant cells in the absence of any other atypical feature (eg, high cellularity, mitotic activity, necrosis, vascular invasion). Mitotic activity ranged from 0 to 50 mitoses/50 HPF (median, 0/50 HPF), with atypical mitotic figures present in only 6 cases (23%). Coagulative tumor cell necrosis and angiolymphatic invasion were seen in 8 cases (31%) and 3 cases (12%), respectively. Immunohistochemical Findings The immunohistochemical results are summarized in Table 2. Figures 2 to 4 illustrate these immunohistochemical findings. All cases expressed at least 1 melanocytic marker; HMB45 was the most sensitive reagent (92% positive), followed by Melan-A (72% positive) and MiTF (50%). Smooth muscle actin expression was seen in 80% of cases, and in general was more often positive in epithelioid cells than in spindled cells. Coexpression of HMB45 and/or Melan-A with smooth muscle actin was seen in 20 of 24 cases (83%). S-100 protein was present in 33% of cases (nuclear and cytoplasmic staining), all of which were also positive for a muscle marker (smooth muscle actin, 7 cases; caldesmon, 1 case). Desmin expression was seen 8 cases, none of which showed morphologic features suggestive of a typical smooth muscle tumor, and only 1 of which occurred in the uterus.tfe3 expression was present in 5 cases, all of which showed either negative (3 cases) or 1+ (2 cases) MiTF expression. CD117 was 2+ positive in a single case; no case expressed CD34. Anomalous cytokeratin immunoreactivity was focally present in 3 cases. Literature Review Table 3 summarizes the clinicopathologic and immunohistochemical features of 61 PEComas previously reported in the English language literature. These cases occurred in 55 women and 6 men, with a median age of 38 years (range, 3 87 years). The tumors ranged from 0.5 to 29 cm (median, 5.3 cm) and occurred in a wide variety of gynecologic (N = 21), somatic soft tissue/skin (N = 28), visceral (N = 10), and bone (N = 2) locations. By report, all 61 cases (100%) were HMB45-positive, 15 of 35 cases (41%) were Melan-A positive, 33 of 56 cases (59%) were smooth or pan-muscle actin positive, 6 of 53 cases (11%) were S-100 protein positive, 16 of 52 cases (31%) were desmin positive, 0 of 9 cases (0%) were cytokeratin positive, and 2 of 6 cases (33%) were CD117 positive. Only 5 patients were stated to have the tuberous sclerosis complex, 33 patients were specifically stated not to have tuberous sclerosis complex, and no information was provided for 23 patients. Follow-up information was available for 45 of 61 previously reported cases (74%). Local recurrences and metastases were noted in 3 (7%) and 9 (20%) of patients, q 2005 Lippincott Williams & Wilkins 1561

5 Folpe et al Am J Surg Pathol Volume 29, Number 12, December 2005 FIGURE 1. Morphologic variation in benign-appearing PEComas. Morphologically typical, predominantly spindled PEComa, showing an elaborate network of small capillaries (A) and growth in short fascicles and small nests (B). The cells of both spindled and epithelioid PEComas typically show clear to lightly eosinophilic cytoplasm, normochromatic ovoid nuclei, and small nucleoli (C). Occasional PEComas display prominent perivascular hyalinization (D). A nested growth pattern is also commonly seen in epithelioid PEComas, as in this uterine tumor (E). Note also the prominent nuclear pleomorphism in the absence of other worrisome histologic features, such as necrosis or mitotic activity (inset). Other unusual histologic features seen in otherwise benign-appearing PEComas include multinucleation (F) and spider cell -like giant cells (G). respectively. At the time of the final clinical follow-up, 35 patients (78%) were alive without evidence of disease, 5 patients (11%) were alive with recurrent or metastatic disease, and 4 patients (9%) were dead of disease. TABLE 2. Immunohistochemical Results Total (%) Smooth muscle actin /25 (80) HMB /24 (92) Melan A /18 (72) MiTF /18 (50) Desmin /22 (36) S /24 (33) Pan-CK /23 (13) Vimentin /14 (86) TFE /17 (29) CD /20 (5) CD /7 (0) Statistical Analysis of Cases in Present Series Only Table 4 summarizes the statistical analysis for the present cohort and the combined analysis of the present cohort and the previously reported cases (see below). Because retroperitoneal and intraabdominal sarcomas commonly result in adverse patient outcome due to local recurrences, rather than distant metastases, the development of either an aggressive local recurrence or a distant metastasis ( aggressive behavior ) was used as the endpoint for statistical analysis. A significant association was noted between tumor size.8cm (median size), mitotic activity greater than 1/50 HPF, and necrosis with aggressive behavior. Patient age.46 years (median age), infiltrative growth pattern, gynecologic primary site, high cellularity, high nuclear grade, atypical mitotic figures, vascular invasion, and the growth pattern of the tumor (spindled or epithelioid) were not associated with aggressive behavior. The six tumors that displayed aggressive behavior were all large, had intermediate to high 1562 q 2005 Lippincott Williams & Wilkins

6 Am J Surg Pathol Volume 29, Number 12, December 2005 PEComas of Soft Tissue and Gynecologic Origin FIGURE 2. Morphologically typical PEComa (A), showing strong expression of HMB45 (B), smooth muscle actin (C), and S-100 protein (D). This degree of S-100 protein expression may be seen in PEComas and does not obligate a diagnosis of a true melanocytic tumor. cellularity, showed brisk mitotic activity, and contained necrosis in 4 cases. Figure 5 illustrates selected histologically and clinically malignant PEComas. Combined Statistical Analysis of Cases in Present Series and Previously Reported Cases Sufficient histopathologic information was present in the 45 previously reported cases with clinical follow-up to allow a repeated statistical analysis, combining the results from these cases with those from the present series. Table 4 summarizes this statistical analysis. A significant correlation with subsequent aggressive behavior was seen for the presence of tumor size.5 cm (combined median size), infiltrative growth pattern, high nuclear grade, necrosis, and mitotic activity.1/50 HPF. DISCUSSION The concept of a family of related tumors defined by the presence of distinctive perivascular epithelioid cells has evolved over the last 100 years, slowly at first, and with increasing momentum in recent years. Grawitz described renal AML in 1900, and the association between TSC and AML was recognized as early as LAM and CCST were formally described by the early 1970s, 20,32 and associations between LAM and TSC, and AML and LAM were noted in and 1974, 37 respectively. Hepatic AMLs were first reported in 1976 q 2005 Lippincott Williams & Wilkins 1563

7 Folpe et al Am J Surg Pathol Volume 29, Number 12, December 2005 FIGURE 3. Histologically malignant epithelioid PEComa, showing high nuclear grade, high cellularity, and mitotic activity (A). Although this tumor did show the classic myomelanocytic immunophenotype, with coexpression of HMB45 (B) and smooth muscle actin (C), it also showed strong desmin expression (D). Desmin expression may be seen in more than 25% of PEComas. by Ishak. 30 Using immunohistochemistry and electron microscopy, HMB45 positivity and pre-melanosomes were identified in AML and CCST in 1991, 22,41,53 and in LAM in Completing this picture, CCST was reported in association with LAM in a TSC patient in 1997, 17 and a CCST/LAM hybrid tumor reported shortly thereafter. 28 Thus, it is generally accepted that AML, LAM, and CCST are related to one another morphologically, immunohistochemically, and genetically, through their association with TSC. Although in a sense CCST of lung represents the first reported nonrenal, visceral PEComa, the 1996 report by Zamboni of a pancreatic tumor morphologically identical to CCST 55 effectively ushered in the modern era of PEComa consciousness among pathologists. Perhaps equally important was the expansion of the morphologic spectrum of renal and hepatic AML to include carcinoma-like purely epithelioid variants by Eble et al 12 and Tsui et al, 50 respectively, and our own description of purely spindled PEComas, so-called clear cell myomelanocytic tumor. 18 As detailed in Table 2, PEComas of both epithelioid and spindled-type have subsequently been reported in a wide variety of intraabdominal, soft tissue, and bone sites, including the falciform ligament/ligamentum teres, uterus, mesentery, heart, thigh, and skull base, among others. Combining our present results with those of previous studies, a much more complete picture emerges of soft tissue and gynecologic PEComas. As in the present series, these tumors occur most often in middle aged patients (median overall age, 38 years) and show a striking female predominance, 1564 q 2005 Lippincott Williams & Wilkins

8 Am J Surg Pathol Volume 29, Number 12, December 2005 PEComas of Soft Tissue and Gynecologic Origin FIGURE 4. Malignant-appearing PE- Coma with necrosis (A), showing strong nuclear expression of TFE3 protein (B). TFE3 was usually expressed by PEComas that showed only focal or absent MiTF expression, suggesting a potential for this related member of the MiTF family of transcription factors in the expression of melanocytic proteins. with an overall female-to-male ratio of nearly 7:1. Even when tumors occurring in gender-specific locations (eg, uterus, prostate) are excluded, the ratio of women to men remains 4:1. This is in agreement with the approximately 4:1 female-tomale ratio seen in both TSC-associated and sporadic AML. 11 Conceivably, this striking female predominance could be due to stimulation of the tumors by hormone receptors, although the small number of PEComas previously examined for estrogen and progesterone receptor expression have been negative. Tumors of the gynecologic tract account for just over 40% of cases, with the falciform ligament/ligamentum teres, omentum/mesentery, and pelvic/retroperitoneal soft tissues also being common sites of involvement. The tumors typically achieve a large size by the time of diagnosis, likely reflecting the difficulty of clinically recognizing tumors in these favored locations, with an overall median size of about 5 cm. Interestingly, only a very small minority (9%) of soft tissue and gynecologic PEComas are TSC-associated. Owing to their rarity, it has not as yet been possible to fully define criteria for malignancy in PEComas. The relevant chapter in the 2002 WHO Soft Tissue and Bone book states that. PEComas displaying any combination of infiltrative growth, marked hypercellularity, nuclear enlargement and hyperchromasia, high mitotic activity, atypical mitotic figures, and coagulative necrosis should be regarded as malignant, although no guidance was given as to the application of these criteria in a given case. As noted in Results, by combining our cases with those previously reported, we observed a significant association between tumor size.5 cm, infiltrative growth pattern, high nuclear grade, necrosis, and mitotic activity.1/50 HPF and subsequent aggressive clinical behavior. These results, we believe, allow the development of a provisional classification of PEComas (Table 5). Given the rarity of PEComas, it has until now been unclear whether any of these tumors can be declared to be benign, or whether all PEComas carry some risk of aggressive behavior. Our results indicate that a subset of PEComas can be provisionally labeled as either benign, or perhaps having a very low risk for aggressive behavior. Specifically, this includes cases that lack any worrisome feature (eg, large size, high cellularity, necrosis,.1 MF/50 HPF). To date, 22 histologically banal cases with adequate clinical follow-up have been reported; none has behaved in an aggressive fashion. It has been recognized for some time that AMLs may show striking nuclear atypia in the absence of other worrisome histologic features ( atypical AML ). 8 Our data suggest that similar degenerative or symplastic nuclear atypia, including multinucleated tumor giant cells, may occur in PEComas, without being associated with aggressive clinical behavior. In the present series, 6 cases displayed nuclear pleomorphism and/or multinucleated tumor giant cells alone; all showed nonaggressive behavior. Similarly, Fadare et al noted multinucleated giant cells with degenerative atypia in 1 case with nonaggressive outcome. 14 The larger number of such cases in the present series may represent a certain referral bias but is more likely the result of our specific attention to these histologic features. Multinucleated tumor giant cells, and in particular spider cell -like cells, are in fact sufficiently common in PEComas as to represent a useful diagnostic clue. It is important to recognize, however, that multinucleated and spider cell -like giant cells are not by themselves indicative of a pseudomalignant PEComa, as they may be seen in cases with other worrisome features, which ultimately behave aggressively. Although PEComas with nuclear atypia/multinucleated giant cells alone are probably benign, it is probably best to classify them as having uncertain malignant potential, until further cases become available for extended analysis. It is more difficult to precisely classify PEComas showing only a single worrisome feature other than nuclear pleomorphism/giant cells, in part owing to the rarity of such cases. For example, aggressive behavior has been noted in 2 of q 2005 Lippincott Williams & Wilkins 1565

9 Folpe et al Am J Surg Pathol Volume 29, Number 12, December 2005 TABLE 3. Reported Cases of Soft Tissue and Gynecologic PEComas in the English Language Literature (59 Cases) Site/Size (cm) Histopathology Immunohistochemistry Outcome Comments Reference Age/Sex Zamboni /F Pancreas/2 Circumscribed; epithelioid; nuclear grade and no necrosis or mitotic figures Pea /F Uterus/2.0 Circumscribed; epithelioid; low intermediate no necrosis or mitotic figures Al-Saleem /F Inguinal/10 Infiltrative; epithelioid; high nuclear grade and presence of necrosis not mentioned; atypical mitotic figures present Ruco /F Uterus/5 Infiltrative; epithelioid; high nuclear grade and necrosis present, 11MF/10HPF Michal /F Uterus/2.0 Circumscribed; epithelioid; 48/F Uterus/7.0 Circumscribed; epithelioid; 46/F Uterus/1.5 Circumscribed; epithelioid; 46/F Uterus/2.5 Circumscribed; epithelioid; Tanaka /F Ligamentum teres/9 Folpe /M Ligamentum teres-falciform ligament/20 11/F Ligamentum teres-falciform ligament/29 21/F Ligamentum teres-falciform ligament/8.5 Circumscribed; epithelioid; Infiltrative; spindled; Infiltrative; spindled; Infiltrative; spindled; necrosis and mitotic HMB45 and SMApositive; focally S100 protein-positive HMB45 and SMApositive; negative for desmin and S100 protein HMB45, MSA, desmin and S100 proteinpositive HMB45 and SMApositive; focally desmin-positive; negative for S100 protein HMB45, SMA and desmin-positive; protein HMB45, SMA and desmin-positive; protein HMB45, SMA and desmin-positive; protein HMB45, SMA and desmin-positive; protein HMB45, Melan-A and SMA-positive; S100 protein and desmin negative HMB45-positive; S100 protein-negative HMB45-positive; S100 protein-negative HMB45-positive; S100 protein-negative ANED at 3 months ANED at 2 years ANED at 1 year Not given ANED at 4 years ANED at 4 years ANED at 1 year ANED at 1 year ANED at 22 years Radiographically suspected lung metastasis at 3 months; dead of other causes at 12 months ANED at 5 years ANED at 2 years History of TSC 1566 q 2005 Lippincott Williams & Wilkins

10 Am J Surg Pathol Volume 29, Number 12, December 2005 PEComas of Soft Tissue and Gynecologic Origin TABLE 3. (continued) Reported Cases of Soft Tissue and Gynecologic PEComas in the English Language Literature (59 Cases) Reference Age/Sex Site/Size (cm) Histopathology Immunohistochemistry Outcome Comments 10/F Ligamentum teres-falciform ligament/5 Infiltrative; spindled; 6/F Ligamentum teres-falciform ligament/5 3/F Ligamentum teres-falciform ligament/5.5 Infiltrative; spindled; Infiltrative; spindled; 15/F Omentum/8 Infiltrative; spindled; Tazelaar /F Rectum/3 Circumscribed; epithelioid; low to intermediate nuclear grade and no necrosis or mitotic figures 20/F Perineum/2 Circumscribed; epithelioid; low to intermediate no necrosis or mitotic figures 29/M Left atrium Infiltrative; epithelioid; high nuclear grade and necrosis present; 45MF/10HPF 40/F Rectum/small Circumscribed; epithelioid; low to intermediate nuclear grade and no necrosis or mitotic figures Bonetti /F Terminal ileum and cecum/9 Epithelioid; intermediate to high nuclear grade; necrosis present; mitotic activity rare, lymphatic space invasion present 19/F Uterus/5.5 Epithelioid; intermediate to high nuclear grade; necrosis present; mitotic activity rare, lymphatic space invasion present HMB45-positive; S100 protein-negative HMB45, Melan-A and SMA-positive; S100 protein and desmin-negative HMB45, Melan-A and SMA-positive; S100 protein and desmin-negative HMB45, Melan-A and SMA-positive; S100 protein and desmin-negative HMB45-positive; protein, SMA, desmin HMB45-positive; protein, SMA, desmin HMB45-positive; protein, SMA, desmin HMB45-positive; protein, SMA, desmin HMB45 and Melan-A-positive; SMA, desmin, S100 protein-negative HMB45-positive; Melan-A, SMA, desmin, S100 protein-negative Not given ANED at 2 years ANED at 10 months ANED at 6 months ANED at 14 months ANED at 4 years Died immediately after surgical resection; no tumor identified at autopsy Hepatic metastases; DOD at 28 months Aggressive local recurrence at 1 month; lung and bone metastases at 11 months; alive with advanced metastatic disease at 18 months (continued on next page) q 2005 Lippincott Williams & Wilkins 1567

11 Folpe et al Am J Surg Pathol Volume 29, Number 12, December 2005 TABLE 3. (continued) Reported Cases of Soft Tissue and Gynecologic PEComas in the English Language Literature (59 Cases) Reference Age/Sex 40/F Pelvic soft tissue/2.5 Site/Size (cm) Histopathology Immunohistochemistry Outcome Comments Epithelioid; intermediate to high nuclear grade; necrosis present; mitotic activity rare, lymphatic space invasion present 41/F Uterus/6 Epithelioid; intermediate to high nuclear grade; necrosis present; mitotic activity rare, lymphatic space invasion present Folpe /F Thigh/3.5 Circumscribed; spindled; Vang /F Uterus/12 Infiltrative; epithelioid; 54/F Uterus/0.6 Infiltrative; epithelioid; 56/F Uterus/1.0 Infiltrative; epithelioid; 75/F Uterus/5.0 Infiltrative; mixed HMB45-positive; Melan-A, SMA, desmin, S100 protein-negative 47/F Uterus/4.5 Infiltrative; mixed intermediate nuclear grade; SMA and desminpositive; focally HMB45- positive; negative for S100 protein and Melan-A 49/F Uterus/4.0 Infiltrative; mixed 55/F Uterus/4.5 Infiltrative; mixed infarct-type necrosis present; mitotic 58/F Uterus/1.5 Circumscribed; mixed Insabato /M Tibia/2 Circumscribed; epithelioid; intermediate nuclear grade and low cellularity ANED at 6 months HMB45 and Melan-Apositive; SMA, desmin, present at presentation Ovarian mass S100 protein-negative (metastasis vs synchronous disease); ANED at 6 months HMB45 and SMApositive; focally ANED at 35 months* MiTF-positive; S100 protein, Melan-A and desmin-negative HMB45-positiive; focally Lost to follow-up positive for SMA and Melan-A; S100 protein and desmin-negative Focally HMB455 and Recent SMA-positive; negative for S100 protein, Melan-A, desmin HMB45-positive; Recent negative for SMA, Melan-A, S100 protein, desmin HMB45-positive; ANED at 2.6 years negative for SMA, Melan-A, S100 protein, desmin SMA and desmin-positive; focally HMB45- positive; negative for S100 protein and Melan-A SMA-positive; focally HMB45 and desmin-positive; protein, Melan-A Recent ANED 4.5 years ANED 2 months SMA-positive; focally Lost to follow-up HMB45-positive; negative for S100 protein, Melan-A, desmin HMB45-positive; negative ANED at 12 months for SMA, S100 protein, desmin, Melan-A History of TSC Included in present series (Case 18) History of TSC; also had LAM involving lymph node LAM in pelvic lymph node 1568 q 2005 Lippincott Williams & Wilkins

12 Am J Surg Pathol Volume 29, Number 12, December 2005 PEComas of Soft Tissue and Gynecologic Origin TABLE 3. (continued) Reported Cases of Soft Tissue and Gynecologic PEComas in the English Language Literature (59 Cases) Reference Age/Sex Site/Size (cm) Histopathology Immunohistochemistry Outcome Comments Govender /F Breast/7 Circumscribed; epithelioid; necrosis present; mitotic Dimmler /F Uterus/4 Infiltrative; epithelioid; no necrosis, low mitotic activity; lymphatic space invasion present Park /F Uterus/8 Infiltrative; epithelioid; intermediate nuclear grade; necrosis present; 1MF/50HPF Pan /F Bladder/4 Circumscribed; mixed necrosis present; mitotic Diment /F Thigh/10 Mixed high necrosis present; 8MF/10HPF Greene /F Uterus and pelvic side wall/13 Infiltrative; epithelioid; intermediate to high necrosis present; 8MF/10HPF Pan /M Prostate/8.5 Infiltrative; epithelioid; necrosis present; low mitotic rate Yanai /F Jejunum/7.5 Circumscribed; epithelioid; intermediate nuclear grade and necrosis present Fukunaga /F Broad ligament/5 Fink /F Broad ligament/17 Fadare /F Uterine cervix/2.2 Infiltrative; mixed 5 MF /50HPF Highly cellular, partially necrotic, pleomorphic sarcoma with spindled and epithelioid features Infiltrative; mixed occasional multinucleated cells with degenerative atypia,,1mf/50hpf HMB45, Melan-A ANED at 9 months and PR-positive; S100 protein, SMA, ER and desmin negative HMB45-and SMA positive; desminnegative Lung metastases at 7 years HMB45 and CD117- ANED at 18 months positive; SMA, desmin, S100 protein-negative HMB45 and SMApositive; negative for S100 protein, Melan-A, desmin, ER/PR HMB45 and SMApositive; negative for S100 protein and desmin HMB45, Melan-A, SMA and S100 protein-positive; desmin, ER/PR-negative HMB45-positive; negative for SMA and S100 protein Positive for HMB45; focally S100 proteinpositive; negative for SMA, desmin, and Melan-A HMB45 and caldesmon-positive; negative for SMA, desmin, MSA, Melan-A, S100 protein, CD117 HMB45 and Melan-A positive; focally SMA, desmin and S100 protein positive HMB45 and Melan-A positive; focally SMA, desmin and S100 protein positive ANED at 6 years Not given Recurred in pelvis and mesentery at 2 years; DOD several months later Received adjuvant chemotherapy; lung metastases at 3 years; DOD at 4 years 2 pelvic wall recurrences/local metastases at 13 months; ovarian metastasis at 25 months ANED at 8 months Received adjuvant RT; ANED at 15 months ANED at 35 months History of TSC; HMB45- positive PEC aggregates also present in myometrium, small bowel and ovarian hilum (continued on next page) q 2005 Lippincott Williams & Wilkins 1569

13 Folpe et al Am J Surg Pathol Volume 29, Number 12, December 2005 TABLE 3. (continued) Reported Cases of Soft Tissue and Gynecologic PEComas in the English Language Literature (59 Cases) Reference Age/Sex Site/Size (cm) Histopathology Immunohistochemistry Outcome Comments Gao /F Uterus/4 Circumscribed; epithelioid; intermediate nuclear grade and low no necrosis or mitotic activity Birkhaeuser /F Cecum/3.5 Circumscribed; mixed focally high nuclear grade; no necrosis or mitotic activity Sadeghi /M Common bile duct/2.4 Circumscribed; epithelioid; no necrosis; 1MF/20HPF Lehman /F Skull base/5 Infiltrative; epithelioid; intermediate nuclear grade and necrosis absent;.30mf/10hpf Harris /M Soft tissue near knee/5.5 Circumscribed; mixed high nuclear grade and multinucleated cells; necrosis absent; 6MF/10HPF Genevay /F Cecum/3.5 Circumscribed; mixed low nuclear grade and multinucleated cells present; no necrosis or mitotic activity 35/F Pararectal/not given Evert /F Rectovaginal space/8 Mentzel /F Skin, lower leg/0.5 42/F Skin, lower leg/0.6 Circumscribed; epithelioid; nuclear grade; necrosis present; up to 1 mitosis per 10 HPF Infiltrative; necrosis present; mixed spindled and epithelioid; epithelioid areas had high nuclear grade and multinucleation;.200mf/50hpf Infiltrative; epithelioid; no necrosis, no mitoses Infiltrative; epithelioid; no necrosis, no mitoses HMB45, Melan-A, SMA, desmin-positive; protein and CD117 HMB45, SMA and desmin-positive; protein and CD117 HMB45-positive; negative for Melan-A, SMA, S100 protein HMB45 and SMApositive; negative for S100 protein and desmin HMB45 and SMApositive; negative for S100 protein and desmin SMA-positive, focally desmin, Melan-A and HMB45-positive; negative for S100 protein, cytokeratin, CD117 HMB45 and Melan-A positive, focally SMA positive; negative for desmin, S100 protein, cytokeratin HMB45, Melan-A, SMA, CD117-positive; focally desmin-positive; negative for S100 protein HMB45 and MiTF positive, focally calponin positive; protein, desmin, Melan A, actin, cytokeratin MiTF positive; focally HMB45, Melan A and calponin positive; negative for actin, desmin, S100 protein, cytokeratin Not given ANED at 3 months Not given Paraspinal and lung metastases at 6 weeks; DOD at 3 months Inguinal lymph node and lung metastases at 13 months; AWD at 40 months Not given Not given Pulmonary metastases present at presentation ANED at 25 months ANED at 30 months 1570 q 2005 Lippincott Williams & Wilkins

14 Am J Surg Pathol Volume 29, Number 12, December 2005 PEComas of Soft Tissue and Gynecologic Origin TABLE 3. (continued) Reported Cases of Soft Tissue and Gynecologic PEComas in the English Language Literature (59 Cases) Reference Age/Sex 41/F Skin, lower leg/0.8 62/F Skin, popliteal fossa/1.2 30/F Skin, lower leg/3.0 66/F Skin, forearm/0.6 48/F Skin, lower leg/0.8 cm Site/Size (cm) Histopathology Immunohistochemistry Outcome Comments Infiltrative; epithelioid; no necrosis, no mitoses HMB45 and MITF ANED at 108 months positive, focally desmin positive; negative for actin, Melan A, S100 protein, cytokeratin, calponin Infiltrative; epithelioid; mostly occasional pleomorphic cells; no necrosis, no mitoses HMB45 and MITF positive; negative for actin, desmin, Melan A, S100 protein, cytokeratin, calponin Recent Infiltrative; epithelioid; no necrosis, no mitoses Infiltrative; epithelioid; no necrosis, no mitoses Infiltrative; epithelioid; no necrosis, 2 3 MF/10 HPF MiTF positive, ANED at 12 months focally HMB45, actin positive; negative for desmin, S100 protein, Melan A, cytokeratin HMB45 and MITF ANED at 10 months positive; negative for actin, desmin, Melan A, S100 protein, cytokeratin MiTF positive; ANED at 7 months focally positive for HMB45, negative for S100 protein, Melan A, actin, desmin, cytokeratin the 17 (12%) cases that were.5 cm at diagnosis but lacked any other atypical feature. Such cases, we believe, could be labeled as having uncertain malignant potential, with the hope that this label will ensure long-term clinical follow-up. Regrettably, there are insufficient data on cases with mitotic activity alone: 1 of the 2 reported case arguably fulfilling these criteria metastasizing to the lungs 7 years after initial diagnosis, 10 with the other being disease free at last follow-up. 35 Other histologic features, such as infiltrative growth, necrosis, or vascular invasion, always appear to co-vary with high nuclear grade, etc. Conversely, of the 17 cases with aggressive behavior in the present series and the literature, only 2 had less than 1 worrisome feature. 10,18 Indeed, this may somewhat understate the issue, as the presumed pulmonary metastasis from a falciform ligament tumor occurring in a 29-year-old man, which we previously reported, was not confirmed histologically. 10,18 Inclusive of both the cases in the present series and previously reported series, aggressive behavior has been seen in 12 of 17 (71%) of PEComas having 2 or more atypical features. Thus, it would seem that such cases may be confidently classified as malignant or at high risk of aggressive behavior, recognizing that clinically aggressive disease may not be seen in all histologically malignant neoplasms. Our immunophenotypic results are generally in accord with previously published results, although we have found these tumors to have a somewhat broader immunophenotype than has traditionally been thought. In agreement with the literature, we have found HMB45 to be the most sensitive melanocytic marker of PEComas, followed closely by Melan A. In contrast to our previous findings in AML, 56 MiTF was the least sensitive melanocytic marker of PEComas. MiTF has not been previously evaluated in soft tissue and gynecologic PEComas. Interestingly, expression of the closely related transcription factor TFE3 was seen in 5 cases, all of which showed focal or no MITF expression. This suggests a potential role for TFE3 in the development of the myomelanocytic phenotype in MiTF-negative cases, akin to the postulated role of another member of the MiTF/TFE3 gene family, TFE-B, in HMB45-positive, t(6;11) renal cell carcinomas carrying TFE-B gene fusions. 2 Further study of TFE3 in PEComas will of course be necessary to validate this hypothesis. Although it has been suggested that PEComas do not express S-100 protein, thereby allowing their distinction from melanoma and clear cell sarcoma; this is not the case. We observed S-100 protein expression in 33% of cases, a somewhat higher number than has been reported in the literature. Importantly, all S-100 protein-positive cases expressed smooth muscle actin, a finding not generally seen in true melanocytic lesions. Coexpression of melanocytic markers and smooth muscle actin, thought to be the hallmark of PEComas, was seen in over 80% of cases, also in agreement with previously reported cases. It is important to note that 20% of the present cases and a significant number of previously reported case are actin-negative, a finding that does not rule out the diagnosis of PEComa. Although it has been suggested that PEComas q 2005 Lippincott Williams & Wilkins 1571

15 Folpe et al Am J Surg Pathol Volume 29, Number 12, December 2005 TABLE 4. Statistical Analysis (Fisher s Exact Test) Feature Present Cases: With Recurrences or Metastases Present Cases: Without Recurrences or Metastases P Present Cases + Previously Reported Cases: With Recurrences or Metastases Present Cases + Previously Reported Cases: Without Recurrences or Metastases Size. median 6/6 7/ /18 19/ Mitoses.1/50 HPF 6/6 3/ /17 8/ Necrosis 4/6 3/ /18 8/ Patient age.46 years 3/6 8/ (NS) 9/17 22/ (NS) Infiltrative growth 5/6 6/ (NS) 12/15 26/ High cellularity 3/6 4/ (NS) 9/18 9/ High nuclear grade 3/6 5/ (NS) 9/18 9/ Atypical mitoses 2/6 3/ (NS) Not evaluable Not evaluable Not applicable Vascular invasion 2/6 1/ (NS) Not evaluable Not evaluable Not applicable.95% spindled 1/6 5/ (NS) 2/18 11/ (NS).95% epithelioid 3/6 5/ (NS) 11/18 25/ (NS) P should not express desmin, distinguishing them from true smooth muscle tumors, desmin was expressed in just over 30% of cases both in the present series and in the literature. None of the desmin-positive cases in the present series showed any morphologic features suggestive of ordinary smooth muscle neoplasms, and only 1 occurred in the uterus. With the recent therapeutic success in the treatment of gastrointestinal stromal tumors with imatinib mesylate, there has been considerable interest in the identification of other tumors expressing the CD117 tyrosine kinase receptor. CD117 expression has been previously reported in AMLs 34 and in occasional soft tissue and gynecologic PEComas 13,40 ; we observed CD117 expression in only 1 case. Given the low frequency of CD117 in PEComas, it is doubtful that there is a role for imatinib mesylate in the treatment of malignant examples. Finally, we did note anomalous cytokeratin expression in 3 cases, a previously unreported event. The differential diagnosis of soft tissue and gynecologic PEComas is fairly broad and predominantly guided by the morphology (epithelioid vs. spindled) and location of the tumor. Given their uniform expression of melanocytic markers, it is not surprising that PEComas are frequently confused with both conventional melanoma and clear cell sarcoma. A number of the morphologic features of PEComas, including the admixture of spindled and epithelioid forms, the occasionally prominent nucleoli, and the presence of multinucleated cells, are also seen in melanoma and clear cell sarcoma. In most cases, melanoma/clear cell sarcoma can be distinguished from PEComa by virtue of their strong expression of S-100 protein and smooth muscle actin nonimmunoreactivity. As noted above, all of the S-100 protein-positive PEComas in the present series also showed actin expression. Although PEComas may be actin-negative, one should obviously be quite wary of this diagnosis in an HMB45-positive, S-100 TABLE 5. Proposed Classification of PEComas Benign Uncertain malignant potential Malignant Criteria No worrisome features (,5 cm, non-infiltrative, non-high nuclear grade and cellularity, mitotic rate #1/50HPF, no necrosis, no vascular invasion) 1) Nuclear pleomorphism/multinucleated giant cells only or 2) Size.5 cm only Two or more worrisome features (.5 cm, infiltrative, high nuclear grade and cellularity, mitotic rate $1/50HPF, necrosis, vascular invasion) Percentage Fulfilling Criteria With Aggressive Behavior 0 of 22 (0%) 1) 0 of 6 (0%) 2) 2 of 17 (12%) 12 of 17 (71%) Comment 1) Symplastic PEComaprobably benign, but few reported cases 2) Large tumors should be extensively sampled to exclude areas with other worrisome features 1572 q 2005 Lippincott Williams & Wilkins

16 Am J Surg Pathol Volume 29, Number 12, December 2005 PEComas of Soft Tissue and Gynecologic Origin FIGURE 5. Histologic features of clinically malignant PEComas. Epithelioid PEComa with intermediate nuclear grade, but with extensive necrosis (A) and vascular invasion (B). This case (case no. 7) recurred extensively within the abdomen and resulted in the death of the patient. This PEComa (case no. 14) showed marked nuclear pleomorphism and frequent mitotic figures (C) and produced lung metastases in 12 months (D). Note the clear to lightly eosinophilic cytoplasm and the perivascular distribution of the cells seen even in the lung metastasis (inset). The primary tumor in this case (case no. 5) was principally spindled and lacked worrisome features (E). However, it recurred in less than 2 years and contained histologically malignant-appearing spindled areas, as well as areas identical to the primary tumor (F). Higher power examination of the recurrent tumor showed notably higher nuclear grade and cellularity, as well as mitotic activity (inset). protein-positive, actin-negative tumor. Rare spindled melanomas may show limited actin expression. 33 Although neither PEComas nor melanomas have a known specific genetic finding, cytogenetic and/or molecular genetic study can be extremely valuable in the diagnosis of clear cell sarcoma, which carries a specific t(12; 22)(q13; q13)(ews-atf1) gene fusion. 43 Gastrointestinal stromal tumors (GIST) may also enter the differential diagnosis of PEComas, as both tumors frequently occur within the abdomen, and both may show a mixture of epithelioid and spindled morphology. Morphologically, PEComas differ from GISTs in that they typically show clear to lightly eosinophilic cytoplasm, lack fibrillar-appearing cytoplasm, and typically show a very well-developed capillary vasculature (renal cell carcinoma-like). Multinucleated giant cells may be seen in both tumors. Immunohistochemical demonstration of melanocytic marker expression is probably the most reliable way to distinguish PEComa and GIST. Obviously, the potential exists to misdiagnose CD117-positive PEComas as GIST, if one is not aware of this potential pitfall. CD34, negative in PEComas, and positive in many GISTs, may also be useful in this differential diagnosis. Epithelioid PEComas may be confused with carcinomas such as clear cell carcinoma, particularly when they involve visceral locations, or locations such as the cervix and vagina. Although rare PEComas may show focal anomalous cytokeratin expression, they do not show the diffuse cytokeratin expression seen in clear cell carcinomas. Here again, demonstration of melanocytic marker expression is very valuable in distinguishing PEComa from carcinoma. Lastly, PEComas may be confused with true smooth muscle neoplasms with both spindled and epithelioid q 2005 Lippincott Williams & Wilkins 1573