However, a 2001 study[6] followed by numerous reports[1] found tumors with the microscopic histopathology and other key features of mammary MFB in a wide range of organs and tissues.

Finally, studies suggest that spindle cell lipoma and cellular angiofibroma are variants of MFB.

Extramammary MFB occurs about equally in males and females of both sexes and has a broad age distribution that includes children.

Mammary MFB likewise occurs about equally in both sexes but has a decided predominance in middle-aged and older adults.

[9] The neoplastic cells commonly: 1) have acquired a gene chromosome abnormality in which a small part of chromosome 13 is deleted;[9] 2) fail to express the retinoblastoma protein (pRb) due to this deletion;[11] and 3) contain key tumor marker proteins.

[6] Milk line MLB tumors occur in areas around the arm pit, anus, vulva, and testicles.

[12] In a study of 128 patients with extramammary MFB tumors (average size 6.6 cm, range 1–22 cm), 65 occurred in the inguinal/groin region, 18 in the leg, 17 in the trunk, 14 in the abdominal cavity, retroperitoneal space, or organs in these areas, 7 in the chest, 3 in the head-neck areas, 2 in the vagina, and 2 in the arm.

[1][6] The tentatively classified spindle cell variant of extramammary MFB typically presents as a well-defined heterogeneous subcutaneous mass in men (10:1 male-to-female ratio) aged 45–70 years (mean age 54 years) with a predilection for arising in the shoulder, posterior neck, and upper back.

[15] The tentatively classified cellular angiofibroma variant of MFB typically occurs in adults (women 40–50 years old, men 60–70 years old with rare cases in children), is usually a small tumor but can be as large as 25 cm,[16] and frequently develops in the groin-scrotum or vulva-vaginal regions.

The diagnosis of MFB depends on the microscopic histopathology (i.e. appearances after proper tissue preparation and staining) of its pre-surgery biopsied issues.

[7] This leads to a loss in expression of pRb, a protein made by the Rb gene located at the deletion site.

[4] The presence of two marker proteins in MFP neoplastic cells, also detected by immunohistochemical analyses, support the diagnosis of MFB.

[16] Neoplastic cells in mammary and extramammary MMB commonly express the estrogen, progesterone and androgen receptors.

Well-differentiated liposarcoma (WDL), also termed atypical lipomatous tumor (ATL) or ATL/WDL, is an extremely rare tumor which unlike MFB is composed of mature, morphologically homogenous fat and stromal cells that frequently have atypical nuclei.

This tumor characteristically has neoplastic cells with an extra, abnormally-shaped small supernumerary marker chromosome(s).

[24] Dermatoid fibromatosis tumors consist of uniform myofibroblasts in a background of abundant collagenous stroma and vascular tissue.

It consists of interconnecting, angulated, slit-like spaces lined by slender spindle cells and surrounded by dense collagenous stroma.