Congenital myofibroblastic fibrosarcoma of the temporal bone in a newborn Arabian foal.

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Myofibroblastic fibrosarcoma (MFS) is a malignant soft tissue tumour that is rare in domestic animals. Here we report a case of congenital MFS in an Arabian foal at birth. A large solitary subcutaneous mass arising in the left temporal region was found in the foal. On cut section, an unencapsulated hemispheric, white-yellowish, elastic fibrous mass was attached to the temporal bone. Histologically, the mass was composed of a neoplastic proliferation of spindle-shaped fibroblasts arranged in fasciculate or interwoven streams and bundles, with low nuclear atypia and mitotic rate, local invasion of the surrounding soft tissues and temporal bone, non-metastasis and abundant collagenous stroma. Immunohistochemically, vimentin, calponin, desmin and alpha-smooth muscle actin were strongly and diffusely labelled, S100 was mildly labelled and h-caldesmon was weakly labelled in the cytoplasm of the neoplastic cells; however, the neoplastic cells were immunonegative for pancytokeratin AE1/AE3 and p63. Based on the histological and immunohistochemical findings, a diagnosis of MFS was made. To our knowledge, this is the first report of congenital MFS in animals.