Original contributionCharacterization of the leiomyomatous variant of myofibroblastoma: a rare subset distinct from other smooth muscle tumors of the breast☆,☆☆
Introduction
Tumors of smooth muscle derivation are exceedingly rare in the breast, largely in part due to the absence of indigenous smooth muscle in this anatomic site, with the exception of the nipple. Still, reports of benign smooth muscle dominant tumors such as leiomyomas and myoid (or “muscular”) hamartomas have been described in the literature [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. In addition, other benign mammary entities are known to occasionally demonstrate smooth muscle differentiation to varying degrees such as fibroadenomas, phyllodes tumors, and myofibroblastomas. Because of the rarity of these tumors, a critical review of benign mammary smooth muscle tumors has not been performed. This invariably perpetuates the ambiguity of not only classifying them in practice but also understanding them as part of distinct subgroup possessing specific clinical, morphologic, immunohistochemical, and/or molecular features.
Mammary myofibroblastoma is a benign stromal tumor characterized by bland spindle cells growing in fascicles with intervening bands of collagen and variable amounts of fat. Various morphologic variants have been described, including cellular, infiltrating, epithelioid, and myxoid, among others, and some show heterologous differentiation in the form of mature chondroid, osseous, or leiomyomatous components [12], [13]. In recent years, myofibroblastoma and morphologically similar nonmammary CD34-positive tumors including (extra) mammary-type myofibroblastoma, spindle cell lipoma, and cellular angiofibroma of vulva have been found to share a deletion of 13q14, which includes RB1 and FOX01A genes [14], [15], [16], [17], [18]. In reported cases of mammary myofibroblastomas specifically, deletion of 13q14 has been found in 78.5% (11/14) of examples studied [14], [16], [18], [19], [20], [21]. Identifying this common deletion in CD34-positive tumors motivated us to better characterize a rare variant of myofibroblastoma that can be nonimmunoreactive for CD34—considered a hallmark of mammary myofibroblastomas, which in its absence may pose a diagnostic pitfall. Moreover, we aimed to better understand how the leiomyomatous variant of myofibroblastomas fits in the overall context of benign smooth muscle tumors in the breast.
Section snippets
Case selection
This study was conducted under an institutional review board—approved protocol. Four cases of mammary myofibroblastoma with leiomyomatous differentiation were identified from our surgical pathology and breast consultation files. Slides were reviewed by 2 breast pathologists (T. D., S. S.). Clinical data were obtained from electronic patient records and from submitting pathologists and clinicians.
Immunohistochemistry
Immunohistochemical staining was accomplished using the Bond III Autostainer (Leica Microsystems,
Results
A summary of the clinicopathological, immunohistochemical, and cytogenetic features of studied cases is outlined in Table 2.
Discussion
After its initial characterization nearly 3 decades ago by Wargotz et al [13], several morphologic variants of mammary myofibroblastoma have been described [12], [13]. A relatively less common variant of mammary myofibroblastoma is one that demonstrates smooth muscle differentiation. The differential diagnosis of leiomyomatous myofibroblastoma includes other benign tumors such as leiomyoma, myoid hamartoma, and fibroepithelial tumors with prominent stromal myoid metaplasia. The list of
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