Abstract

A 50-year-old man with no pertinent history presented with a 14 cm scalp and 4 cm cervical mass; CT-scan showed no osseous extension.  Histology revealed hypercellular sheets, cords, and thin loose fascicles of small, monomorphic spindled to epithelioid cells in a fibromyxoid stroma, extending from the papillary dermis to the base of the biopsy. Cells had abundant, eosinophilic cytoplasm, round to oval or angulated nuclei, open to hyperchromatic chromatin, variably prominent nucleoli, and scattered mitoses and single cell necrosis. Some nuclei were eccentrically located with dense cytoplasm.  Positivity for pancytokeratin, desmin, MyoD1, and ALK was observed. NGS revealed a EWSR1::TFCP2 fusion; no ALK fusion or amplification was identified.  A diagnosis of cutaneous rhabdomyosarcoma (RMS) with TFCP2 fusion (RMSTF) was made. RMSTF is an emerging entity in the spindle cell/sclerosing RMS category, and the subset of intraosseous RMS with EWSR1/FUS:-TFCP2 or MEIS::NCOA2 fusions.  ALK overexpression is thought to result from a truncated isoform originated from intragenic ALK deletion. Homozygous CDKN2A deletions commonly occur.  RMSTF has striking predilection for craniofacial bones, with only four extraosseous cases reported:  peritoneal, inguinal, posterior cervical, and maxillary gingiva/vestibule. RMSTF has a poor prognosis with 74 and 35% 1- and 2- year disease-specific survival respectively. Median age at diagnosis is just 25 years (range 11-74) with 37% of cases occurring in pediatric patients.  Treatment regimens typically involve combination chemo- and radiotherapy with ALK inhibitors.   We present this extremely rare case to raise awareness of this emerging entity, and prevent diagnostic confusion given the unique immunohistochemical findings.