EWSR1::ATF7 fusion driven mesenchymal neoplasm with rhabdomyoblastic differentiation

We report a case of a 28-year-old man who presented with a slowly enlarging intramuscular mass in left posterior neck. Imaging demonstrated a relatively well-circumscribed mass with focal scattered calcifications. Histologic examination revealed a lobulated neoplasm composed of monotonous round to ovoid to spindle shaped tumor cells arranged in large nodules, loose fascicles, and solid nests within a sclerotic fibrous stromal background. Focal metaplastic bone formation and rare calcification were identified. No significant cytologic atypia, increased mitotic activity or necrosis was identified. The tumor showed immunophenotypic evidence of rhabdomyoblastic differentiation. Molecular studies identified a novel EWSR1::ATF7 fusion and methylation profiling yielded no matching classes. Overall, the findings favor classification as a fusion-associated spindle cell rhabdomyosarcoma. However, additional studies involving a larger number of cases will be required to confirm its classification as such and to better define its biologic behavior.