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

<p dir="ltr">Myofibroblastic <u>fibrosarcoma</u> (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 <u>temporal bone</u>. 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.</p><h2>Other Information</h2><p dir="ltr">Published in: Journal of Comparative Pathology<br>License: <a href="http://creativecommons.org/licenses/by/4.0/" target="_blank">http://creativecommons.org/licenses/by/4.0/</a><br>See article on publisher's website: <a href="https://dx.doi.org/10.1016/j.jcpa.2025.01.003" target="_blank">https://dx.doi.org/10.1016/j.jcpa.2025.01.003</a></p>