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A 65-year-old presented with a 5.0 cm painless scrotal mass. Grossly, the lesion was well-circumscribed and composed of lobulated adipose tissue with intervening fibrous areas, without evidence of hemorrhage or necrosis.
Archive Case and Diagnosis:
This case first appeared as Performance Improvement Program in Surgical Pathology (PIP) 2008, Case 33, and is mammary-type myofibroblastoma of soft tissue.
Criteria for Diagnosis and Comments:
The lesion is composed of an admixture of mature adipose tissue and bland, often parallel-arranged, spindle cells many of which are present within fascicles of hyalinized collagenous stroma. Vessels are not conspicuous. The spindled component expresses both CD34 and desmin. These features are characteristic of (extra-mammary) mammary-type myofibroblastoma.
This tumor was originally described in the breast of older men and called mammary spindle cell tumor. It was later further characterized by Wargotz et al in 1987 and renamed mammary myofibroblastoma. In 2001, McMenamin and Fletcher described nine cases with identical histological features that were located in extra-mammary soft tissue sites. This case is such an example.
As in the breast, this lesion is considered a benign, non-recurring mesenchymal neoplasm with a predilection for older men (mean age 55 years). The histological appearance is identical to that seen in the mammary variant. It consists of a variable admixture of fat, fascicles of often wavy spindle cells, intersecting coarse hyalinized stroma, small inconspicuous vessels often cuffed by lymphocytes, and prominent mast cells. Nuclear atypia is rare and mitotic activity is typically absent. The characteristic immunoprofile includes both CD34 and desmin positivity. Interestingly, the appearance is also very similar, if not identical, to spindle cell lipoma of soft tissue with rare examples resembling pleomorphic lipoma. These lesions all share rearrangements involving chromosomes 13q and 16q.
This lesion is typically well-circumscribed, with median size of 5-6 cm, and the cut surface is usually described as nodular or whorled. It is most often asymptomatic and discovered incidentally during surgery such as herniorrhaphy. All reported tumors follow a benign clinical course following treatment by simple surgical excision. The predilection of this tumor for the inguinal and scrotal region suggests that these lesions may arise along the embryonic milk lines and possibly originate in hormonally-responsive mesenchymal tissue (although residual mammary tissue is usually not identified). However, its occurrence in other locations including abdominal wall, buttock, back and vaginal wall casts some doubt on this theory.
The differential diagnosis is lengthy but principally includes those entities listed above. As opposed to liposarcoma, MTMFB is generally smaller, more circumscribed and does not demonstrate infiltration into adjacent tissues. While adipocytes may vary in size, true lipoblasts are not seen in MTMFB. Liposarcoma also contains at least scattered atypical cells that are S100 (+) and desmin (-). Spindle cell lipoma and MTMFB are closely related entities and probably form a continuum. Therefore they are difficult to separate from each other. Nonetheless, because SCL is usually desmin-negative and typically located on the neck, back or shoulder, it is still probably best diagnosed as a separate entity at this time. Those reported outside of back/neck/should region may indeed represent MTMFB.
Both cellular angiofibroma and angiomyofibroblastoma are also benign lesions. CA, which may be related to AMFB, occurs in this general perineal location in both males and females and consists of spindle cells and a prominent vascular component. Although relatively circumscribed like MTMFB, the presence of distinctive perivascular hyalinized fibrosis, general absence of desmin positivity, relative lack of fat, presence of wispy rather than ropy collagen and presence of mitotic activity separate it from MTMFB. AMFB, also a circumscribed lesion, mainly occurs in females, typically vulvovaginal region, but is well-described, albeit rarely, in males. In contradistinction from MTMFB, it is a vascular lesion with a characteristic component of spindled to rounded desmin-positive cells preferentially cuffing prominent small and medium-sized ectatic vessels within a loosely textured stroma. These features are absent in MTMFB. Finally, aggressive angiomyxoma, originally described in women, can be rarely seen in males. In contrast to MTMFB, DAA is poorly delineated, less cellular, with bland spindled to stellate-shaped cells and variable-calibered, often clustered vessels, some with mural hyalinization loosely distributed in a myxoid matrix. It also lacks the densely-arranged spindle cells and ropy collagen of MTMFB. DAA is typically a bulky lesion and importantly, poorly marginated. Microscopically it is infiltrative, as well as more richly vascular compared to MTMFB. Unlike the other entities (except for liposarcoma), this lesion has a marked tendency to recur with incomplete excision.