Congenital Rapidly Progressive Perianal Embryonal Rhabdomyosarcoma With Spindle Cell Morphology: A Rare Presentation.

This report describes a 2-day-old male infant who presented with a perianal swelling since birth. The swelling was painless, exhibited a soft to firm consistency, and progressively increased in size over a period of 2 months. The perianal region is an uncommon location for rhabdomyosarcoma, accounting for approximately 2% of all tumours. Diagnostic workup included MRI of the abdomen, which revealed a 4.7 × 2.7 × 2.5 cm heterogeneous soft tissue mass in the perineum extending into the pelvis. Histopathological examination demonstrated lesional spindle and round cells with striations in the spindle cells. Immunohistochemical analysis showed diffuse positivity for desmin and MYOD1, with focal positivity for myogenin and a MKI67 proliferation index of 30%, leading to a final diagnosis of embryonal rhabdomyosarcoma with spindle cell morphology (ERMS). This study highlights the rarity of this presentation and underscores the critical role of immunohistochemical markers in the accurate diagnosis of ERMS at unusual locations.