Polypoid angiomyofibroblastoma-like tumor of the oral cavity: a hitherto unreported soft tissue tumor mimicking embryonal rhabdomyosarcoma.

We report on a previously unrecognized fibro-myofibroblastic tumor in the oral cavity of a 15-year-old girl. Morphologically, the tumor mimicked a rhabdomyosarcoma, botryoid variant. It was composed of mitotically active small- to medium-sized, vimentin+/desmin+, round- to oval- to epithelioid-shaped cells embedded in an alternating fibrous to myxoid/edematous stroma. These cells were separated from the overlying squamous epithelium by a rim of fibrous stroma. The tumor contained abundant small- to medium-sized, thin-walled blood vessels without hyalinization. Frequently, neoplastic cells condensed around these vessels. An unusual and striking feature was the presence of numerous hyalinized collagen mats, including "amianthoid-like fibers", similar to those observed in myofibroblastomas. The presence of these collagen mats and the expression of desmin, in association with no immunoreactivity to myogenin and MyoD1, were in keeping with the fibro-myofibroblastic nature of the tumor, excluding the diagnosis of embryonal rhabdomyosarcoma. Regarding fibro-myofibroblastic tumors, we believe that the present case falls within the wide spectrum of benign stromal tumors, originally described in the lower female genital tract, but potentially occurring also at extragenital sites. As morphological and immunohistochemical features were reminiscent of, but not identical with, angiomyofibroblastoma, the term "polypoid angiomyofibroblastoma-like tumor" is proposed. Awareness and recognition of this tumor is crucial to avoid a diagnosis of malignancy.