Myofibroblastoma entraps fat cells as it grows. Tthe lesional cells show variable fibro-myofibroblastic differentiation, and they may adopt marked intralesional and interlesional variability in morphology.
The mass grows as an unencapsulated tumor with bland-looking, slender, spindle-shaped cells, closely packed in short, straight, haphazardly intersecting fascicles or clusters of cohesive cells. The cells are interrupted by thick, hyalinized collagen bundles.
Importantly, there is no atypia or mitotic activity, although some reported cases do show a low mitotic count. Mast cells are also a frequent feature,
This different case is composed of fibroblastic-like cells with elongated nuclei to cells with overt myoid features consisting of abundant palely to deeply eosinophilic cytoplasm.
Ocassionally, the neoplastic cells may exhibit a palisading of nuclei, closely mimicking a benign peripheral nerve sheath tumor.
This example is composed of a somewhat less cellular spindle cell proliferation.
Myofibroblastoma of the breast is an uncommon benign tumor, which has the potential to show a wide variety of cytomorphologic features and architectural patterns. Several histologic variants (cellular, infiltrative, epithelioid, deciduoid-like, collagenized/ fibrous, lipomatous, myxoid variants), have been recognized in the last 2 decades.
Myofibroblastoma occurs mainly in older men and postmenopausal women as a solitary, unilateral, painless, freely movable, usually firm in consistency, nontender nodule. Patients may report that the nodule has been growing slowly over several months to years.
There has been no evidence of recurrence or metastasis after a follow-up of 15 years after excision (Magro, 2002)
Magro G. Mammary myofibroblastoma: a tumor with a wide morphologic spectrum. Arch Pathol Lab Med. 2008 Nov;132(11):1813-20.
Magro, G. , M. Bisceglia , M. Michal , and V. Eusebi . Spindle cell lipoma-like tumor, solitary fibrous tumor and myofibroblastoma of the breast: a clinicopathological analysis of 13 cases in favor of a unifying histologic concept. Virchows Arch 2002. 440:249–260