ALK-positive anaplastic large cell lymphoma of skeletal muscle masquerading as soft tissue sarcoma.

Anaplastic large cell lymphoma (ALCL) is a subtype of T cell non-Hodgkin's lymphoma and can present as an extranodal disease. Primary ALCL of skeletal muscle is rare. We report a case of ALK-positive ALCL involving the left thigh and anterior chest wall in a 12-year-old male child. The fine needle aspiration cytology from the lesion showed cellular smears with singly scattered and occasional clusters of large pleomorphic atypical cells. A diagnosis of poorly differentiated malignant neoplasm was rendered. On core biopsy, the histomorphological features posed a diagnostic challenge with a myriad of morphological mimickers. The diagnosis was established by excluding specific entities by relevant immunostains and confirming the diagnosis by strong expression of CD30 and ALK on immunohistochemistry. Fluorescence in-situ hybridisation confirmed the characteristic t(2:5) translocation. Presentation of ALCL with skeletal muscle involvement is uncommon, and the diagnosis relies on broadening the diagnostic possibilities and judicious use of immunohistochemical markers.