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We report rare case of myofibroblastoma (MFB) of the breast comprised predominantly of a mature fatty component, representing approximately 70% of the entire tumour area. This tumour, designated "lipomatous MFB", should be interpreted as the morphological result of an unbalanced bidirectional differentiation of the precursor mammary stromal cell, with the adipocytic component overwhelming the fibroblastic/myofibroblastic one. Lipomatous MFB is a rare variant of mammary MFB, which can mimic malignancy because of the close juxtaposition of fibroblasts/myofibroblasts with mature adipocytes, resulting in a finger-like infiltrative growth pattern of the former towards the latter. Histogenetic considerations and differential diagnostic problems with other bland-looking spindle cell tumours containing infiltrating fat are provided.

We report a rare case of angiomyofibroblastoma (AMFB) of the vulva, composed predominantly of a mature fatty component, representing approximately 60% of the entire tumour. The tumour, designated as "lipomatous AMFB", should be interpreted as the morphological result of an unbalanced bidirectional differentiation of the presumptive precursor stromal cell resident in the hormonally-responsive stroma of the lower genital tract, with the adipocytic component overwhelming the fibroblastic/myofibroblastic one. The close admixture of adipocytes with spindled/epithelioid cells of the conventional AMFB resulted, focally, in a pseudo-infiltrative growth pattern and pseudo-lipoblast-like appearance, raising problems in differential diagnosis, especially with well-differentiated lipoma-like liposarcoma and spindle cell liposarcoma. Awareness of the possibility that vulvo-vaginal AMFB may contain large amount of lipomatous component is crucial to avoid confusion with other bland-looking spindle cell tumours containing infiltrating fat.