An approach to the diagnosis of spindle cell lesions of the breast

Histopathology 2016, 68, 33 44. DOI: 10.1111/his.12865 REVIEW An approach to the diagnosis of spindle cell lesions of the breast Emad A Rakha, Mohammed A Aleskandarany, Andrew H S Lee & Ian O Ellis Department of Histopathology, Nottingham University Hospitals NHS Trust and the University of Nottingham, Nottingham City Hospital, Nottingham, UK Rakha E A, Aleskandarany M A, Lee A H S & Ellis I O (2016) Histopathology 68, 33 44. DOI: 10.1111/his.12865 An approach to the diagnosis of spindle cell lesions of the breast Although most breast spindle cell lesions (BSCLs) are rare, they constitute a wide spectrum of diseases, ranging from reactive processes to aggressive malignant tumours. Despite their varied histogenesis and behaviour, some lesions show an overlap of morphological features, making accurate diagnosis a challenging task, particularly in needle core biopsies. Clinical history and immunohistochemistry can help in making a correct diagnosis in morphologically challenging cases. To make an accurate diagnosis, it is important to maintain a wide differential diagnosis and be familiar with the diverse morphological appearances of these different entities. BSCLs can generally be classified into bland-looking and malignantlooking categories. In the former, the commonest diagnosis is scarring. However, it is important to distinguish low-grade spindle cell metaplastic breast carcinoma from other benign entities, as the management is clearly different. In the malignant category, it is important to differentiate metaplastic carcinoma from other malignant primary and metastatic malignant spindle cell tumours of the breast, such as malignant phyllodes tumour, angiosarcoma, and melanoma. This review focuses on the classification and histological and molecular diagnosis of various BSCLs, with an emphasis on the diagnostic approach, including in core biopsies. Keywords: breast, core biopsy, diagnosis, immunohistochemistry, spindle cell lesions, update Introduction Although most breast spindle cell lesions (BSCLs) are uncommon, they constitute a wide spectrum of diseases with varied histogenesis. They range from reactive processes to aggressive malignant tumours. An overlap in the histological appearances of some lesions may lead to misinterpretation or misdiagnosis of benign and malignant entities, particularly in the limited material of the needle core biopsy (NCB). Ancillary techniques such as immunohistochemistry (IHC) are helpful in arriving at a correct diagnosis in Address for correspondence: I Ellis, Histopathology Department, Nottingham University Hospital NHS Trust, City Hospital Campus, Hucknall Road, NG5 1PB Nottingham, UK. e-mail: ian.ellis@ nottingham.ac.uk morphologically challenging cases. To ensure accurate diagnosis, it is important to maintain a wide differential diagnosis and be familiar with the diverse morphological appearances of the different entities. Although the stromal component of phyllodes tumours can be part of the differential diagnosis of BSCL, phyllodes tumours constitute a distinct group of breast fibroepithelial lesions, 1 and will not be considered further. Although the most important challenge when assessing a BSCL on NCB is to determine whether the lesion represents a metaplastic breast carcinoma (MBC), subclassification of different lesions is often associated with important management implications. This review focuses on the classification, histological and molecular diagnosis and differential diagnosis of various BSCLs, with an emphasis on the most common entities. 2015 John Wiley & Sons Ltd.

34 E A Rakha et al. Classification of BSCLs All spindle cell lesions of soft tissues can occur in the breast. BSCL can be exclusively composed of spindle cells (pure), or be composed of spindle cells mixed with other non-spindle cell components (mixed). BSCL can be composed purely or predominantly of a mesenchymal element (monophasic), or be composed of mixed mesenchymal and epithelial components (biphasic). Pure spindle cell MBC is a monophasic lesion but is of epithelial origin. Mesenchymal tissue includes components similar to those present in normal breast tissues, such as fat, fibroblasts, and myofibroblasts (homologous), or tissues not normally present in the breast, such as skeletal muscle, cartilage, or bone (heterologous). Homologous or heterologous mesenchymal elements in BSCL may be benign or malignant. Adipose tissue within a BSCL may be a component of the lesion, as in spindle cell lipoma and myofibroblastoma, or represent entrapped fat cells resulting from neoplastic infiltration by the lesion, and this latter type is mainly seen at the periphery of the lesion. Similarly, epithelial components can represent entrapped normal mammary epithelial tissues, such as in fibromatosis, part of the spindle cell proliferative process, such as in MBC, or a distinct component of the lesion, such in fibroepithelial lesions. A benign epithelial component can be seen in both benign and malignant BSCL; however, a malignant epithelial component denotes a malignant lesion. Some BSCLs, such as myofibroblastoma, are typically devoid of mammary glandular elements. 2 Mammary parenchymal cells show a high degree of phenotypic plasticity, which is seen in both metaplastic and neoplastic lesions. 3 8 Mammary epithelial cells may show a wide range of metaplastic changes, including apocrine, clear cell, squamous, secretory and mucinous metaplasia, whereas neoplastic mammary epithelial cells may undergo genetic reprogramming, resulting in a change of morphology to spindle mesenchymal-like, squamous, and small cell, in addition to other types of transdifferentiation, such as salivary gland, secretory and clear cell differentiation. Proliferating myoepithelial cells show different morphological appearances, including spindle cells. 9 Proliferating intralobular and interlobular mammary stromal cells may result in the formation of spindle cell lesions. BSCL can also arise from non-mammary specific tissue, such as skin, smooth muscle, deep fascia, blood vessels, nerves, and other soft tissue elements. 10,11 The diagnostic approach in assessing BSCL should include the following points: 1. Cytonuclear atypia: high-grade cytonuclear atypia indicates a malignant process with a differential diagnosis of MBC, sarcomas, melanomas, and metastatic spindle cell tumours. In contrast, absent or low-grade atypia may represent a reactive, benign or low-grade malignant process. Frequent mitotic figures, including atypical forms, are a feature of high-grade MBC and other malignant spindle cell tumours. Although some reactive BSCLs, such as nodular fasciitis, may show frequent mitotic figures, these are typically of normal forms and not associated with cytonuclear atypia. 2. Component cells: pure spindle cells, or mixed with other cells, including adipose tissue, muscle cells, inflammatory cells, or parenchymal elements, including epithelial cells and myoepithelial cells, ducts, and/or lobules. 3. Growth pattern: fascicular, storiform, diffuse, or whirling. 4. Margins: infiltrating or well defined. 5. Cellularity: low, moderate, or high. 6. IHC: this is very helpful in challenging cases. Biomarkers indicating epithelial differentiation are particularly important. IHC can be used for the diagnosis of specific lesions or to rule out other entities (see below for details). 7. Other features: the presence of thick bundles of hyalinized collagen between the spindle cells is an important feature of myofibroblastoma. Necrosis is usually seen in high-grade malignant lesions. The presence of in-situ carcinoma or conventional mammary-type invasive carcinoma in the surrounding breast tissue favours the diagnosis of MBC. Some MBCs are associated with papillomas or radial scars. 12 Patient age, lesion size, location relation to skin and deep structures and rate of growth are additional helpful features. Tables 1 and 2 show the most common BSCLs and their salient features. In brief, BSCLs can be classified into bland-looking breast spindle cell lesions and malignant-looking breast spindle cell lesions. BLAND-LOOKING BSCLS These include reactive and neoplastic mesenchymal and epithelial/myoepithelial lesions that can be classified as: 1. Lesions of fibroblastic myofibroblastic origin, including: fibromatosis, 13 nodular fasciitis, 14 16 myofibroblastoma, 17 solitary fibrous tumour, 18,19 pseudoangiomatous stromal hyperplasia

Spindle cell lesions of the breast 35 Table 1. Key features of the most common bland-looking spindle cell lesions of the breast Entity Key features Malignant epithelial Fibromatosis-like MBC Low-grade spindle cell proliferation with pale eosinophilic cytoplasm and slender nuclei with tapered edges and finely distributed chromatin but with focal plump fusiform and polygonal tumour cells, with more rounded nuclei arranged in epithelioid clumps mainly seen centrally in the tumour Infiltrative with entrapped normal breast structures Variable cellularity and collagenization Scattered inflammatory infiltrate composed of lymphocytes and plasma cells with occasional lymphoid follicles at the edges of the tumour DCIS rarely (10 15%) present IHC: positive for CKs and/or p63. CK-positive cells are seen in almost all cases; they usually appear as cords or sheets of polygonal cells, but rarely as isolated positive cells. Often positive for SMA, particularly in CK-negative cells. Typically negative for CD34, hormone receptors, and HER2 Mesenchymal fibroblastic/myofibroblastic proliferation Fibromatosis Myofibroblastoma Solitary fibrous tumour Nodular fasciitis Scar Pseudoangiomatous stromal hyperplasia Locally infiltrative non-metastatic lesion frequently arises from deep fascia Usually solitary, non-tender, ill-defined mass May be spiculate on mammography, mimicking carcinoma. US and MRI are more sensitive for its detection Long fascicles; variable collagen deposition and cellularity, diffusely infiltrative with entrapped fat at the periphery. The lesion nuclei are characteristically spaced Lymphocytes often seen at the edges IHC: nuclear staining of b-catenin and cytoplasmic staining of SMA. Negative for CD34, CKs, p63, desmin, and S100 Benign, solitary, and slowly growing, often as a circumscribed tumour Variable morphology but typically fascicular growth of spindle cells with bands of collagen fibres, devoid of mammary ducts and lobules, amianthoid fibres, often contain variable adipocytic component, may be cellular. No regularly spaced nuclei IHC: positive for CD34, desmin, SMA, ER, PR, CD99, bcl-2, and CD10 Solitary circumscribed firm tumour similar to the more common soft tissue solitary fibrous tumour and, to some degree, to breast myofibroblastoma Patternless pattern and haemanigopericytoma-like vasculature. Ovoid spindle cells with inconspicuous nucleoli. May show bundles of hyalinized collagen and perivascular inflammatory infiltrate IHC: Strong nuclear positivity for STAT6, and negative for desmin Rapidly growing; may be tender or painful, self-limiting mass-forming composed of clonal cellular proliferation Well-circumscribed; tissue culture-like fibroblasts with plump vesicular nuclei; myxoid stroma; extravasated RBCs. Mitosis may be frequent, but no abnormal forms IHC: positive for actin History of trauma/procedure and presence of biopsy site-associated changes such as haemosiderin deposition, fat necrosis, foamy macrophages, and foreign body giant cells Distinction of postoperative scar from residual fibromatosis may be extremely difficult, but a fascicular growth pattern and entrapment of breast parenchyma are not common features seen in a scar Reactive spindle cell nodule is likely to represent an exuberant reparative process (i.e. young scar), but may reach a large size and be associated with breast fibrosclerotic lesions IHC: positive for SMA, but negative for b-catenin, desmin, and CD34 Usually comprises a component of other breast lesions, but may present as a pure circumscribed nodular mass. Dominant feature of gynaecomastia in males Collagenous stroma separated by slit-like, interanastomosing pseudovascular spaces lined by spindle cells resembling endothelial cells IHC: positive for CD34 and PR, variable positivity for actin and desmin, and negative for other endothelial markers

36 E A Rakha et al. Table 1. (Continued) Entity Inflammatory myofibroblastic tumour Key features Painless circumscribed firm mass Significant inflammatory component dominated by plasma cells. Fascicles of spindle cells with eosinophilic cytoplasm and ovoid or tapering nuclei IHC: positive for SMA and ALK; may show CK positivity Fibroepithelial lesions Fibroadenoma For details of the diagnostic features, see the articles on phyllodes tumour in this issue of Histopathology Benign phyllodes Nerve sheath tumours Schwannoma Neurofibroma Painless lumps, mainly in the skin May show a biphasic pattern with compact hypercellular Antoni A areas and myxoid hypocellular Antoni B areas, and the characteristic nuclear palisading. Wavy nuclei and ill-defined cytoplasm. Amianthoid fibres or collagenous spherules IHC: positive for S100, with strong diffuse nuclear and cytoplasmic staining in Schwann cells Non-encapsulated, all elements of peripheral nerves including axons (silver-positive). Less of a fascicular pattern than fibromatosis Unlike schwannoma, no Verocay bodies, no nuclear palisading, and no hyalinized thickening of vessel walls IHC: positive for S100, which highlights Schwann cells with no staining of the intervening stroma and perineural cells. CD34-positive (focal), factor 13a-positive (focal) Smooth muscle tumour Leiomyoma Slowly growing mass, often in the subareolar area. May be tender or painful. Deep lesions need to be distinguished from other breast lesions with prominent smooth muscle differentiation Interlacing fascicles of spindle cells with cigar-shaped nuclei and eosinophilic cytoplasm. Atypia and mitotic figures favour the diagnosis of leiomyosarcoma IHC: positive for SMA, desmin, and caldesmon Adipose tissue Spindle cell lipoma A rare variant of breast lipoma characterized by a mixture of collagen-forming uniform spindle cells, mature fat cells, and varying degrees of myxoid change May contain entrapped breast parenchymal elements IHC: positive for CD34, and negative for SMA, desmin, and S100 CK, Cytokeratin; DCIS, Ductal carcinoma in situ; ER, Oestrogen receptor; IHC, Immunohistochemistry; MBC, Metaplastic breast carcinoma; MRI, Magnetic resonance imaging; PR, Progesterone receptor; RBC, Red blood cell; SMA, Smooth muscle actin; US, Ultrasound. Most of the lesions apart from nodular fasciitis show rare or no mitosis. Atypia and necrosis are not features of this category. All entities, apart from MBC and some cases of inflammatory myofibroblastic tumour and leiomyoma, typically lack CK expression. (PASH), 20 scar/reactive spindle cell nodules, 21 and inflammatory myofibroblastic tumour. 22 2. Lesions of nerve sheath origin, including neurofibroma and schwannoma. 3. Stromal component of phyllodes tumours. 4. Lesions of smooth muscle origin: leiomyoma and low-grade leiomyosarcoma, which may arise from subareolar smooth muscle. 23,24 Those maintain their smooth muscle differentiation, which facilitates their recognition. 5. Other lesions that may show spindle cell morphology include mammary hamartoma and lesions of epithelial/myoepithelial origin, including low-grade fibromatosis-like spindle cell MBC, benign myoepithelial cell proliferation, which can be seen in adenomyoepithelioma, and cellular spindle forms of sclerosing adenosis. The lowgrade fibromatosis-like spindle cell MBC 25,26 is by far the most important differential diagnosis of bland-looking BSCL. This is a rare breast tumour that represents a bland variant of the very heterogeneous group of MBCs, and may be associated with a papillary lesion or radial scars. Although it is associated with an excellent prognosis as compared with other high-grade MBCs, 25 local recurrence and occasional metastasis may

Spindle cell lesions of the breast 37 Table 2. Key features of the most common malignant-looking spindle cell lesions of the breast Entity Key features Mammary epithelial tumours High-grade spindle cell MBC* Mostly circumscribed and/or cystic Classic sarcomatoid spindle cell breast carcinomas show variable combinations of malignant mesenchymal and epithelial tissue The carcinomatous component is frequently poorly differentiated adenocarcinomatous, but may be squamous Variable degree of cellularity, architecture (fascicular, storiform but, rarely, herringbone pattern), frequent mitotic figures, moderate to marked pleomorphism, and occasional areas of necrosis and inflammatory infiltrates Associated DCIS is not uncommon Entrapped benign breast parenchyma may be seen IHC: positive for CKs. Positive for p63 in squamous areas. Typically negative for CD34, hormone receptor, and HER2. Variable degree of positivity for myoepithelial markers Mammary myoepithelial tumours Malignant adenomyoepithelioma and myoepithelial carcinomas Malignant spindle cell component of malignant adenomyoepithelioma, it often shows epithelial with or without myoepithelial differentiation and should be considered as MBC arising in adenomyoepithelioma Pure myoepithelial carcinoma is extremely rare, and should show evidence of a residual benign myoepithelial component and predominant myoepithelial immunophenotypic differentiation (myoepithelial carcinoma is currently classified as metaplastic carcinoma) IHC: positive for markers of myoepithelial differentiation, including SMA, myosin, p63, and S100, plus ultrastructural evidence of pinocytotic vesicles, myofibrils, patchy basal lamina, and tonofilaments Primary breast sarcoma Malignant phyllodes Angiosarcoma Leiomyosarcoma For details of the diagnostic features, see the articles on phyllodes tumour in this issue of Histopathology Can be primary from breast parenchyma or the dermis, sporadically or developing after radiotherapy for previous carcinoma Typically presents with unusual skin changes or, occasionally, as nodules or a mass Poorly defined margins, variable size, invading fat and glandular elements, anastomosing vascular channels with foci of papillary and solid spindle and epithelioid cell proliferation with mitotic figures, occasional necrosis and haemorrhage Tumours with a vasoformative pattern need to be differentiated from acantholytic squamous cell carcinoma, whereas those with a solid pattern need to be differentiated from poorly differentiated carcinoma, melanoma, or spindle cell sarcoma IHC: positivity for CD31, CD34, factor VIII, ERG and D2-40 is useful in confirming the diagnosis Very rare; usually occurs in the subareolar area Similar to leiomyosarcoma elsewhere, and high-grade lesions need to be differentiated from metastatic tumours IHC: positive for desmin and SMA. May be a metastasis from elsewhere Metastatic malignant spindle cell tumours Melanoma Melanoma is one of the most common neoplasms to metastasize to the breast parenchyma It tends to show any growth pattern, including a spindle cell sarcomatous, angiosarcomatous, rhabdoid or malignant biphasic pattern. Clinical history and IHC are very useful for its diagnosis IHC: positive for S100 in most cases, with or without positivity for melan-a, HMB-45, and vimentin

38 E A Rakha et al. Table 2. (Continued) Entity Metastatic sarcomatoid carcinoma and sarcomas Key features Usually present as a circumscribed mass as part of the metastatic process Clinical history, comparison with the primary tumour and IHC are most useful for their diagnosis Examples include metastatic sarcomatoid renal cell carcinoma, rhabdomyosarcomas, and malignant peripheral nerve sheath tumour CK, Cytokeratin; DCIS, Ductal carcinoma in situ; ERG, Erythroblast transformation- specific related gene; IHC, Immunohistochemistry; MBC, Metaplastic breast carcinoma; SMA, Smooth muscle actin. *The diagnosis of high-grade sarcomatoid MBC is based on the demonstration of an epithelial origin by the presence of a morphologically or immunohistochemically proven carcinomatous component and/or the presence of DCIS. The only difference between MBC and gradematched breast sarcoma resides in potential nodal metastases in sarcomatoid MBC, which are extremely rare in sarcomas 67 (node biopsy in MBC), and the use of systemic therapy in MBC. occur. In a retrospective study of 24 cases diagnosed at the University of Texas MD Anderson Cancer Center between 1987 and 2000 with tumours ranging in size between 10 and 50 mm, two of the patients who underwent modified radical mastectomy developed lung metastases within 2 years after the initial diagnosis. 27 In our practice, we have noted that most of the large-size spindle cell MBCs diagnosed as fibromatosis-like MBCs show focal areas of moderate atypia, which may explain the development of metastasis in those cases treated with mastectomy in that study. 27 As the behaviour of bland-looking fibromatosis-like MBC remains to be further characterized, 25,27 currently this diagnosis mandates management in a way similar to that for low-grade conventional-type mammary carcinoma. Recognition of this lesion is important, but the morphological overlap with fibromatosis and other bland BSCLs (Figures 1 and 2), including nodular fasciitis, makes the diagnosis potentially challenging, particularly on NCB. Recognition of areas of epithelial differentiation on morphology or with IHC with a panel of epithelium-associated biomarkers often results in a definitive diagnosis. 6. Although low-grade angiosarcoma, dermatofibrosarcoma protuberans (DFSP) and low-grade fibromyxoid sarcoma and myofibrosarcoma 23,28 may show bland cytological features, these tumours typically show some degree of cytological atypia with distinct architecture sufficient for their identification, typically on surgical specimens. 23,24 A B C D Figure 1. Similar fat infiltration patterns in the different breast spindle cell lesions. A, Fibromatosis (no atypia, nuclear spacing and perivascular inflammatory cuffing are seen). B, Myofibroblastoma (often defined; no entrapped normal breast parenchyma; immunohistochemistry is characteristic). C, D, Lowgrade spindle cell metaplastic breast carcinoma (some degree of cytological atypia is often seen, at least focally; focal epithelioid areas can be seen).

Spindle cell lesions of the breast 39 A B Figure 2. The morphological overlap between different entities of bland-looking spindle cell lesions of the breast. A, Low-grade metaplastic spindle cell breast carcinoma with an entrapped benign breast duct. B, Part of a cellular myofibroblastoma. C, A case of fibromatosis. D, This is from a cellular area of nodular fasciitis showing mitotic figures and vesicular nuclei. There is often a focal tissue culture-like appearance. C D MALIGNANT-LOOKING BSCLS Lesions within this category show overt cytological atypia in addition to other features of malignancy, such as mitoses and necrosis, and their identification on NCB results in a malignant invasive diagnosis (B5b; malignant invasive). 29 However, further classification on NCB and on surgical specimens is needed to guide management. The most important, and probably the most common, entity in this category is high-grade spindle cell MBC. These tumours show a wide morphological spectrum, and may have a fascicular, storiform or haphazard growth pattern with infiltrative borders. These tumours are typically hormone receptor-negative and HER2-negative (triple negative). Spindle cells of MBC can be pure, comprising a mesenchymal-like spindle cell malignant proliferation, or mixed with a conventional mammary-type carcinomatous component. 30,31 The presence of a carcinomatous component in the mixed tumours makes the diagnosis relatively straightforward. In contrast, the diagnosis of MBC composed of malignant spindle cells with no morphological evidence of epithelial differentiation is often challenging, even in surgical specimens. Diligent examination of the sections and thorough sampling is often useful. In-situ carcinoma or small cohesive epithelial foci, as discussed above, should be sought. Immunohistochemical expression of epithelial differentiation markers combined with an absence of the characteristic leaflike architecture of malignant phyllodes tumours helps in the diagnosis of MBC. In the absence of these diagnostic features, other malignant soft tissue spindle cell tumours should be considered. A previous history of conventional-type breast carcinoma treated with radiotherapy raises the possibility of post-irradiation primary breast sarcoma. If the previous breast carcinoma was spindle cell MBC, the lesion is likely to represent a recurrence. Malignant osseous or lipomatous differentiation is more likely to be a component of malignant phyllodes tumour or MBC with heterologous elements, rather than a primary osteosarcoma or a liposarcoma of the breast. 32 A previous history of soft tissue sarcoma or a history of metastatic sarcomatoid tumour, including renal carcinoma, sarcomatoid squamous cell carcinoma, and melanoma, 33 may help to narrow down the differential diagnosis. A pure spindle cell malignancy lacking features of epithelial differentiation should lead to consideration of a differential diagnostic list including malignant phyllodes, primary breast sarcomas such as angiosarcoma, melanoma, and metastatic sarcomatoid tumour. Other types of sarcoma that are exceedingly rare include leiomyosarcoma, rhabdomyosarcoma, fibrosarcoma, synovial sarcoma, dendritic cell sarcoma, and malignant peripheral nerve sheath tumour; their histological features remain similar to these tumours occurring elsewhere. The differential diagnosis of malignant spindle cell tumours with pleomorphic morphology may also include anaplastic large-cell lymphoma 34 and pleomorphic undifferentiated sarcoma; the so-called malignant fibrous histiocytoma. 35

40 E A Rakha et al. High-grade malignant myoepithelial spindle cell tumours of the breast, such as those arising in adenomyoepithelioma, are rare, and often show a mixed epithelial and myoepithelial immunophenotype. These are best regarded as MBCs arising in a background of adenomyoepithelioma. 36 Pure malignant spindle cell myoepithelioma with a predominant myoepithelial immunophenotype and a component of benign myoepithelioma is extremely rare. IHC in the differential diagnosis of BSCLs IHC is a very useful ancillary tool in the diagnosis of BSCL. It is best to use a panel of antibodies, and not attach too much importance to any individual result, as no marker is completely sensitive or specific. Any IHC results must be interpreted in the light of the morphology. When spindle cell carcinoma is in the differential diagnosis, it is important to use a panel of antibodies against epithelial markers, including both low and high molecular weight cytokeratins (CKs), because no individual antibody stains all metaplastic carcinomas. 37 It is important to note that spindle cell MBC may only show focal (or no) immunoreactivity for CKs, and often expresses one or more mesenchymal markers, such as vimentin and smooth muscle actin (SMA). Occasionally, a core biopsy will only sample the stromal component of a phyllodes tumour; this is particularly seen in malignant phyllodes tumours. It is important to be aware that focal expression of CKs can be seen in the stroma of phyllodes tumour. 38 CD34 is a useful marker of phyllodes tumour: expression is less strong in higher-grade tumours, but, nevertheless, the majority of malignant phyllodes tumours are CD34-positive. CD34 expression is not seen in spindle cell MBC or fibromatosis. p63 is frequently expressed by MBC, but it is less frequently positive in high-grade MBC showing no squamous differentiation, and it can also be seen in some malignant phyllodes tumours. 39 Breast stromal spindle cell lesions can be classified, according to the predominant cell component, as: fibroblasts in solitary fibrous tumour, myofibroblasts in myofibroblastoma, or adipocytes in spindle cell lipoma. These lesions are frequently positive for CD34, bcl-2, and CD99. Myofibroblastoma is typically positive for SMA and desmin. The adipose tissue component of BSCL is positive for S100, whereas malignant lipomatous tissue is positive for MDM2 and CDK4. Hamartoma may show focal spindle cell proliferation in the form of PASH, which is CD34-positive or is smooth muscle. Nuclear b-catenin expression is seen in most fibromatoses, but it is important to exclude spindle cell MBC with a panel of CK antibodies. Focal weak nuclear staining of b-catenin can be seen in some (23%) spindle cell MBCs. 40 IHC can also be useful for some rare diagnoses such as melanoma (S100, melan-a, and HMB-45) and angiosarcoma (CD31 and CD34). Genetics and molecular pathology of BSCLs Despite the morphological overlap of some BSCLs, they have differences in histogenesis and molecular profiles. The pathogenesis of spindle cell MBC and the mechanisms underlying the spindle cell transformation of the neoplastic mammary epithelial cells remain poorly defined. A number of theories explaining the phenotypic diversity of MBCs have been proposed. For instance, it has been classically hypothesized that the initial oncogenic events occur in a multipotent progenitor or stem cell with myoepithelial characteristic as an origin of these tumours. 41 This is supported by the morphological observations 42,43 and by the expression of several myoepithelial-associated markers in MBC. 44 These findings have been reinforced by gene expression profiling (GEP) studies. 45 In contrast, some authors have hypothesized that oncogenic events transform normal breast epithelial cells into carcinoma cells with more primitive features that proceed through epithelial mesenchymal transition (EMT) to transform an epithelial/carcinomatous component into sarcomatous/spindle cells, giving MBC its characteristic mesenchymal-like morphology. 46 The EMT theory has also been supported by GEP studies of MBC, which have reported deregulation of genes encoding proteins of cellular adhesion, motility, migration, and extracellular matrix formation. 47 Gene signatures reflecting EMT include up-regulation of the EMT triggers Snail, Twist, and transforming growth factor-b, and down-regulation of E-cadherin, the salient feature reflecting EMT occurrence. 48 In line with this finding, it was reported that E-cadherin loss had an inverse relationship with nuclear Snail expression in metaplastic chondroid cells in a series of MBCs. 49 In addition, IHC for Snail has been reported as a sensitive, but not specific, diagnostic marker of MBC, as it is also seen in other BSCLs, including phyllodes tumour and myofibroblastoma. 50 Currently, data supporting both hypotheses exist and continue to evolve, and there is no consensus regarding the underlying pathogenesis of MBC. Although microarray-based GEP studies have demonstrated that MBC is part of basal-like triple-neg-

Spindle cell lesions of the breast 41 ative breast carcinoma (BLBC) at the transcriptome level, 45 patients with MBC do not have the same good response to neoadjuvant chemotherapy as those with more typical BLBC. 45 This observation has prompted some authors to consider MBC as a unique subtype of breast carcinoma independent of the BLBC, and to be more closely related to the claudin-low tumour subtype. 51 In MBC, the same p53 mutations were identified in the morphologically distinct carcinomatous and sarcomatous components, suggesting that p53 mutation is an early event and is maintained during tumour progression, supporting the monoclonal origin of both components. 52,53 Epidermal growth factor receptor (EGFR) overexpression has been reported in up to 80% of MBCs, with substantial proportions of these cases showing EGFR amplification. 54,55 Although the mechanisms for EGFR overexpression are largely unknown, 56 and no activating mutations in EGFR have been found in MBC, 54 Gilbert et al. 57 demonstrated that a high copy number of EGFR is primarily attributable to aneusomy, which was particularly found in tumours with spindle cell or squamous differentiation. In breast fibromatosis, mutations in the b-catenin gene (CTNNB1) have been reported frequently, 58 and a high level of nuclear b-catenin staining has been identified in >80% of cases. 58 b-catenin is a cadherin-binding protein involved in cell cell adhesion, and also acts as a transcriptional activator, whose regulation of cytosolic levels occurs via a multiprotein complex. 59 Interestingly, mutations in exon 3 of CTNNB1, as detected by direct DNA sequencing, were much more frequent in breast fibromatoses. These mutations, along with nuclear expression of b-catenin, provide a useful tool with which to distinguish breast fibromatoses from morphologically similar reactive and neoplastic processes. 60 However, it is important to note that, despite the fact that most fibromatoses express nuclear b-catenin, 23% of MBCs also show focal b-catenin nuclear expression. 40 In benign and malignant phyllodes tumours, stromal cells express nuclear b-catenin in 94% and 57% of cases, respectively. 40 However, mutations in CTNNB1 were not identified in phyllodes or MBCs. 40 Fibromatosis can be associated with familial adenomatous polyposis, which is the result of germline mutations in the adenomatous polyposis coli gene, whose protein regulates b-catenin. Some cytogenetic findings have been specifically identified in mammary myofibroblastoma. For instance, partial monosomy of 13q and 16q with deletion of the 13q14 region, harbouring RB and FKHR, has been reported in >50% of cases. 61 Although these alterations were not detected in solitary fibrous tumours (STAT6-positive), 62 rearrangements affecting 13q and 16q typically occur in spindle cell lipomas. 63 Cytogenetic studies have also demonstrated that nodular fasciitis perhaps represents a transient neoplasia, with most cases carrying the balanced translocation t (17;22)(p13;q13), resulting in MYH9 USP6 fusion. 16 Therefore, these specific cytogenetic alterations could be explored as an ancillary tool for supporting the diagnosis of these tumours. Diagnosis of BSCLs on core biopsy Diagnosis of BSCL on NCBs is based on awareness of the clinical setting, careful examination of the haematoxylin and eosin-stained sections, and IHC. The commonest spindle cell lesion of the breast, in our experience, is a scar. Usually, a definite diagnosis of a scar can be made, and the lseion can be categorized as B2 benign. 29 The presence of haemosiderinladen macrophages or fat necrosis is an important clue to this diagnosis. Sometimes, making a definite diagnosis is difficult, because the fibroblasts show atypia or there is no fat necrosis or there are no haemosiderin-macrophages to support the diagnosis. In such cases, it is prudent to categorize the lesion as B3 (lesions of uncertain malignant potential 29 ) and recommend further core biopsy or diagnostic excision. At the malignant end of the spectrum, MBC is the commonest diagnosis. A malignant diagnosis (B5b 29 ) can be made if there is definite epithelial differentiation on morphology or IHC. In such cases, sentinel node biopsy and systemic therapy can be considered when indicated. However, focal epithelial differentiation based purely on IHC should be cautiously interpreted. In challenging cases, the full differential diagnosis, including malignant phyllodes tumour, sarcoma, and metastasis, needs to be considered. Clinical history and IHC may enable a definite diagnosis. The IHC panel in such cases should include epithelial differentiation markers, including multiple low and high molecular weight CKs, and other markers to help exclude other entities in the differential diagnosis based on morphology and clinical history. Occasionally, the features are clearly malignant, but a definite diagnosis is not possible. Recommendation for node biopsy or consideration of neoadjuvant therapy is not possible in such cases, and a definite diagnosis should be attempted on the surgical specimen. The diagnosis of low-grade lesions may be difficult. MBC must be considered and appropriate IHC

42 E A Rakha et al. requested if appropriate. The diagnosis of low-grade angiosarcoma may be challenging. A vasoformative growth pattern and expression of endothelial markers, together with the presence of endothelial atypia, will help in diagnosis. There is evidence suggesting that histological grade may not be of prognostic value in angiosarcoma. 64 Some vascular lesions cannot be definitively classified on core biopsy, and should be categorized as B3. DFSP may show low-grade atypia, but the location, infiltrative pattern, cellularity and CD34 positivity are useful features in making the diagnosis. The diagnosis of fibromatosis is primarily based on the morphological features of fascicles of spindle cells with an infiltrative margin. Pleomorphism is usually minimal, with few or no mitoses. The degree of cellularity can be variable. b-catenin is typically expressed in the nuclei, but interpretation of staining can be difficult, and MBC and phyllodes tumour can also express this marker. The features can overlap with those of nodular fasciitis. If there is difficulty in making a diagnosis, the opinion of a specialist soft tissue pathologist may be helpful. Diagnostic excision of fibromatosis should be discouraged to avoid leaving lesional tissue behind. Multiple surgical interventions are associated with higher recurrence rate. A conservative approach or therapeutic excision (excision with margins) is recommended. 65 Sometimes, the distinction between MBC and other spindle cell lesions of the breast is extremely difficult in NCB material, and a definite diagnosis is not possible. 66 Such lesions should be categorized as lesions of uncertain malignant potential (B3 29 ), as most of these lesions will require surgical excision, either to confirm the diagnosis or for further management of the patient. Myofibroblastoma, nodular fasciitis, spindle cell lipoma, neurofibroma, schwannoma and leiomyoma may be amenable to simple excision. Conclusions BSCLs constitute a wide spectrum of morphologically overlapping diseases, ranging from reactive processes to aggressive malignant tumours. It is important to consider a wide differential diagnosis. In addition to morphology, clinical history and IHC are most helpful in their diagnosis. BSCLs can generally be classified as bland-looking and malignant-looking. 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