Bizarre parosteal osteochondromatous proliferation

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1 * * Bizarre Parosteal Osteochondromatous Proliferation A Case Report with Literature Review Chi-Fu Kao Yang-Chih Lin Yu-Hung Wu Be-Fong Chen* We report the case of a 12-year-old female with a slowly erythematous enlarging nodule on her right big toe. Its characteristic radiographs and the histopathologic findings exhibited classic Bizarre parosteal osteochondromatous proliferation (BPOP) features. We review the relevant literature and discuss the clinical manifestations, pathology and differential diagnosis of this lesion.(dermatol Sinica 22 : , 2003) Key words: Bizarre parosteal osteochondromatous proliferation, BPOP, Nora's lesion, Blue bone, Spindle cell stroma, Atypia chondrocyte X ( 22 : , 2004) INTRODUCTION Bizarre parosteal osteochondromatous proliferation (BPOP) is a rare condition that most commonly affects the small bones of the feet and hands. 1, 2 Although benign, it behaves aggressively with rapid growth and a high rate of recurrence after local excision. 3, 4 It may be misdiagnosed as another benign or malignant 2, 3, 5 tumor. We report a case of BPOP and review the relevant literature. CASE REPORT A 12-year-old girl presented with a slowly enlarging, painless, firm nodule on her right big toe. It had been present for about one month. She denied any trauma to the affected site, and she was in good general health. On physical exam, there was a 1.5-cm erythematous periun- From the Departments of Dermatology and Pathology,* Mackay Memorial Hospital Accepted for publication: June 03, 2004 Reprint requests: Yang-Chih Lin, Department of Dermatology, Mackay Memorial Hospital, No. 92, Sec. 2, Chung-Shan N Rd., 10449, Taipei, Taiwan TEL: ext FAX:

2 Fig. 1 A 1.5 cm mildly tender erythematous nodule on the periungual area of the right big toe that had been slowly enlarging for about one month. Fig. 2 X-ray demonstrated a well-circumscribed, bony mass contiguous to the cortical surface of the distal phalanx but without continuity with the cortex or medullary canal. gual nodule on the right big toe. The initial impression was an ingrown nail with abundant granulation tissue (Fig. 1). However, the lesion failed to improve despite topical cauterization with AgNO 3, five days of oral antibiotics and non-steroidal anti-inflammatory drugs therapy. And the wound bled easily when changing dressings. Therefore, CO 2 laser vaporization was undertaken. During the procedure, the lesion was found more difficult to remove than granulation tissue. Only upper part of the tumor can be removed and sent for pathological exanimation. The patient then underwent the study of Roentgenology, and the radiograph demonstrated a well-circumscribed, bony mass with broad base directly attaching the cortical surface of the affected bone. It showed neither lack of continuity with the bony cortex nor medullary involvement (Fig. 2). Microscopic examination showed that the lesion was composed of lobulated proliferative cartilage with an irregular cartilagebone interface and spindle cells in the stroma (Fig. 3). The cartilage that was likely to form a cap was poorly organized with mild atypia but no hyperchromatism. There was calcification as well as enchondral ossification in the deeper parts. The osteoid tissue stained with a bluish tinctorial hue, and spindle-shaped fibroblasts were arranged between the osteoid tissue without a specific pattern or cytologic atypia. Given the characteristic radiology and pathology findings, the diagnosis of BPOP was made. From image study, the lesion was not totally removed by CO 2 laser. Therefore, a wide excision of the lesion was arranged and performed by orthopedist. The lesion did not recur over four months of follow-up. DISCUSSION BPOP, also called "Nora's lesion", was first described by Nora et al. in It is a benign lesion that usually involves the small tubular bones of the hands and feet, such as the proximal and middle phalanges, metacarpal, and metatarsal bones. 1, 3, 4 There have also been occasional reports of BPOP arising from long bones, 2, 4, 5 the maxilla, and, rarely, the skull. It commonly affects young adults in their 20s and 30s, without sexual predisposition. 3, 4 It typically presents as a mildly tender or sometimes painless mass that grows over a period of weeks to months. 1, 3 Underlying trauma has been 4, 6-8 mentioned in a number of reports, but the true pathogenesis is unknown. Yuen et al. 8 suggested that BPOP, florid reactive periostitis, and subungual exostosis might all represent different phases of a proliferative periosteal reactive process. They hypothesized that the actual lesion that resulted would depend on whether the periosteum was breached, the stage of the Dermatol Sinica, December

3 process at which the lesion evolved, and local anatomic factors. 8 Its aggressive behavior, tendency to recur, and radiologic and pathologic characteristics lead to confusion with lesions such as osteochondroma and osteosarcoma, chondrosarcoma, fibrosarcoma, subungual exostosis, florid reactive periostitis, and myositis ossificans. 1, 2, 3, 4 Clinically, the symptoms of BPOP were related to the local tumefaction 1, however, the pictures of BPOP which affected distal tubular bones and resembled ingrown nail were seldom reported before. 2 In general speaking, the consistency of BPOP is slightly harder than granulation tissue. Thus, careful history taking (recently nail clipping or tight fitting shoes), physical examination, and most importantly, radiological study could help in differentiation these two lesions. Grossly, it resembles an osteochondroma, that is, a lesion with a nodular surface covered by glistening cartilage and a distinctive blue hue of the bone within. 3, 4 Microscopically, there is cartilage with calcification and ossification, cancellous bone, and a spindle cell stroma. 2-4, 7 The lobulated, hypercellular cartilage and fibrous tissue usually form a cap, occasionally with large chondrocytes peripherally. 3, 6 Binucleated chondrocytes may be seen and the chondrocytes are not arranged in columns as they are in osteochondroma. 1, 3, 6 Instead, the cartilage is disorganized and irregular, with patchy areas of ossification. The calcified cartilage-bone interface 1, 3, 6 may form trabeculae, which stains deep blue on hematoxylin and eosin stain, and was therefore called "blue bone". 2 These trabeculae are eventually covered and replaced by osseous tissue with a rim of osteoblasts. 2, 4 Proliferation of spindle fibroblasts in the intertrabecular spaces 3, 4, 7 of disorganized immature bone is also found. Typically, the chondrocytes show mild atypia and the spindle cells also have moderate mitotic activity. 3 It is this appearance that led to the designation "bizarre". These bizarre findings often lead to misdiagnosis so that BPOP needs to be distinguished from chondrosarcoma, fibrosarcoma, low grade parosteal osteosarcoma and conventional osteosarcoma pathologically. 2-4 (A) (B) (C) (D) Fig. 3 (A) Pathology examination showed that the lesion was composed of lobulated proliferative cartilage with an irregular cartilage-bone interface and spindle cells in the stroma. The cartilage formed a cap. (H & E, 20x) (B), (C) High power views showed that the cartilage was poorly organized with mild atypia but no hyperchromatism. (H & E. B, 100x; C, 400x) (D) There was calcification as well as enchondral ossification. The osteoid tissue stained with a bluish tinctorial hue, and spindle-shaped fibroblasts were arranged between the osteoid tissue without cytologic atypia. (H & E, 100x) 314 Dermatol Sinica, December 2004

4 Radiographically, BPOP is seen as a calcified, well-defined mass directly contiguous to the cortex of bone. It sometimes possesses a pedicle as a base. 6 There is no actual continuity with the cortex or medullary canal, which, in addition to the absence of cortical flaring, is helpful in distinguishing it from osteochondroma and other malignant tumors. 3, 4, 6 CT and MRI also help to define the margins of the lesion and determine if there is cortical or and medullary involvement. 3 BPOP may be particularly difficult to distinguish from osteochondroma, as both lesions have a similar bony matrix covered by a cartilage cap. However, the cartilage cap of the latter is uniform and better organized, and the chondrocytes are usually arranged in parallel rows. 2-4 Also, in contrast to BPOP, osteochondroma is continuous with the medullary cavity that is usually helpful in making the diagnosis. 2, 4 Subungual exostosis, a variant of osteochondroma, is characterized by a triad of pain, nail 9, 10 deformity, and specific radiographic features. It is differentiated by its characteristic anatomical sites and the lack of the classic chondroid materials seen in BPOP. 4 Florid reactive periostitis arises from the bony surface, and it shares histopathologic characteristics similar to those of BPOP. 2, 4 There is a spindle cell component as well as osteoid with an osteoblastic rim. However, florid reactive periostitis is less likely to have bizarre cartilaginous proliferation. 2, 4 Myositis ossificans (heterotopic ossification), regarded by some authors as an immature form of BPOP, is usually separate from the bone surface with periosteal reaction, and it lacks a cartilaginous cap. 2, 4 Parosteal osteosarcoma rarely occurs in the hands and feet, usually arising instead from a long bone. 2, 4 It may have a cartilage cap, and the stroma shows cytologic atypia. 4 In our case, the characteristic radiograph helps us rule out other conditions, especially osteochondroma and myositis ossificans. The histopathologic findings exhibited classic BPOP features that differed from the other conditions mentioned above. The treatment of choice for BPOP is surgical excision 2, 4, although up to 50% of lesions may recur between two months and two years after surgery. 1-4 Metastasis and death related to BPOP have never been reported. One case of fibrosarcoma coexisting with BPOP was reported in It was hypothesized that fibrosarcoma could develop from the mesenchymal cells involved in the reactive BPOP process. 11 In conclusion, BPOP is a benign tumor commonly affecting small bones of hands and feet and, behaving aggressively with rapid growth and high recurrence rate after local excision. An awareness of this benign tumor and its characteristic histopathologic and radiographic findings permit differentiation from other benign and malignant conditions, especially osteochondroma and osteosarcoma. A correct diagnosis may help avert overly aggressive treatment. ACKNOWLEDGEMENT We appreciate Dr. Mary Jeanne Buttrey's help in revising the manuscript. REFERENCES 1.Nora FE, Dahlin DC, Beabout JW: Bizarre parosteal osteochondromatous proliferations of the hands and feet. Am J Surg Pathol 7: , Abramovici L, Steiner GC: Bizarre parosteal osteochondromatous proliferation (Nora's lesion): a retrospective study of 12 cases, 2 arising in long bones. Hum Pathol 33: , Soon JL, Chang HC, Sim CS, et al.: A case of Bizarre Parosteal Osteochondromatous Proliferation of the hand. Singapore Med J 44: 27-30, Meneses MF, Unni KK, Swee RG: Bizarre parosteal osteochondromatous proliferation of bone (Nora's lesion). Am J Surg Pathol 17: , Cooper PN, Malcolm AJ: A bizarre parosteal osteochondromatous proliferation of the radius. Histopathology 22: 78-80, Horiguchi H, Sakane M, Matsui M, et al.: Bizarre parosteal osteochondromatous proliferation Dermatol Sinica, December

5 (Nora's lesion) of the foot. Pathol Int 51: , Smith NC, Ellis AM, McCarthy S, et al.: Bizarre parosteal osteochondromatous proliferation: a review of seven cases. Aust N Z J Surg 66: , Yuen M, Friedman L, Orr W, et al.: Proliferative periosteal processes of phalanges: a unitary hypothesis. Skeletal Radiol 21: , Wang TC, Wu YH, Su HY: Subungual exostosis. J Dermatol 26: 72-74, Haneke E, Baran R, Bureau H : Tumours of the nail apparatus and adjacent tissue. In Baran R, Dawber RPR, et al., eds. Disease of the nail and their management. London: Blackwell Scientific Publications, , Choi JH, Gu MJ, Kim MJ, et al.: Fibrosarcoma in bizarre parosteal osteochondromatous proliferation. Skeletal Radiol 30: 44-47, Dermatol Sinica, December 2004