Mammary myofibroblastoma (spindle cell tumour of the breast)

This is a benign myofibroblastic tumour of the breast. Extra-mammary cases have also been reported.

Clinical features

This tumour seems to occur more commonly in older men and may be associated with gynaecomastia and androgen ablation for prostatic carcinoma. It also occurs in postmenopausal women. In women it is often picked up on mammographic screening.

Extra-mammary cases are more common in men and have a predilection for the groin but have also been reported at other sites (posterior vaginal wall, buttock, anterior abdominal wall and mid-back).

Macroscopic appearances

These tumours are generally well circumscribed but not encapsulated.

Histopathology

Spindle cell fascicles are embedded in a hyalinised stroma with thick hyalinised bands of collagen. Mast cells are usually plentiful. There is a fatty component which makes up a variable proportion of the tumour, composed of adipocytes of variable size but lacking lipoblasts. Some cases show smooth muscle or cartilaginous differentiation. A minority of cases variously show focal myxoid stroma, epithelioid foci, nuclear enlargement and multinucleation, which may resemble a pleomorphic lipoma.

Immunohistochemistry

	mammary cases	extra-mammary cases
desmin	usually positive	9/9
CD34	usually positive	8/9
SMA	some cases positive	3/9, focal positivity
oestrogen receptors	variably positive (both sexes)
progesterone receptors	variably positive (both sexes)
S-100		1/9 (in a case with epithelioid foci)
cytokeratin		0/9

Differential diagnosis: see differential of spindle cell tumours of the breast

	shared features	differentiating features
spindle cell lipoma	Positive for CD34 SCL and myofibroblastoma are probably related entities with shared cytogenetic abnormalities (partial deletions of 13q and of 16q)	Restricted to shoulder/head/neck. Spindle cells are more delicate and are more haphazardly arranged. Usually more fat. Lack thick, keloid-like bands of collagen. Usually negative for desmin and SMA
cellular angiofibroma (angiomyofibroblastoma-like tumour) (occurs in the groin)	Scattered intralesional adipocytes common. CD34 may be positive.	This lesion is distinguished by striking hyalinised vessels. Desmin is negative or weak; SMA is negative.
angiomyofibroblastoma (Mainly affects females)	May have intralesional adipocytes.	Prominent small blood vessels with small rounded, usually perivascular, cells which are desmin-positive.
nodular fasciitis	CD34 may be focally positive	The myofibroblastic cells are plumper. There is usually focal microcystic myxoid change and red cell extravasation. Usually diffusely positive for SMA. Desmin is usually negative.
soft tissue perineuroma	Fat is rarely present. CD34 may be positive	Long spindle cells show a storiform pattern. There is positivity for EMA and negativity for desmin.
spindle cell well-differentiated liposarcoma	CD34 is likely to be positive.	Less well circumscribed. There is nuclear atypia and lipoblasts are present. S-100 are likely to be positive. Desmin is negative except in a few dendritic cells.
solitary fibrous tumour	CD34 is positive.	Prominent haemangiopericytomatous vessels are present. Desmin is usually negative.
lipomatous haemangiopericytoma (this may be a variant of solitary fibrous tumour)	Admixed fatty component. CD34 is positive.	Prominent haemangiopericytomatous vessels are present. Desmin is usually negative.
low-grade MPNST:		Desmin is negative. In 50% of cases, S-100 is positive.
dermatofibrosarcoma protuberans	Positive for CD34	More superficial and more infiltrative. Negative for desmin.

Management

Marginal excision appear adequate.

References

McMenamin, M. E., Fletcher, C. D. Mammary-type myofibroblastoma of soft tissue: a tumor closely related to spindle cell lipoma. Am J Surg Pathol 2001;25:1022-1029.

This page last revised 26.12.2001.