Bone Marrow Metastasis of Embryonal Rhabdomyosarcoma in an Adult, Confounding as Acute Leukemia: A Diagnostic Challenge.

Rhabdomyosarcoma (RMS) is a primitive mesenchymal malignancy of childhood and is very rare in adults. In the presence of a soft tissue mass, bone marrow metastasis of RMS is easy to diagnose. However, RMS can rarely present with extensive marrow replacement, without an obvious primary-a situation that is very challenging for the pathologist as well as the clinician. Owing to the classical round cell morphology, metastatic RMS is morphologically indistinguishable from acute leukemia, especially when the cells are singly scattered, rather than in clusters. We report a rare presentation of bone marrow metastasis of embryonal RMS in an adult, who presented only with pancytopenia and leucoerythroblastic blood picture. Bone marrow aspiration revealed near-total replacement of the hematopoietic elements by atypical blast-like cells, which were singly scattered as well as arranged in loose clusters focally. The case was worked up as acute leukemia. Flow cytometric immunophenotyping ruled out acute leukemia. The trephine biopsy was more in favor of a metastatic tumor; hence, a PET-CT was performed. The scan revealed diffuse uptake in the axial and appendicular skeleton, along with enlarged supraclavicular lymph nodes. An excision biopsy of the lymph node clinched the diagnosis of metastatic RMS, which was confirmed by immunohistochemistry for desmin and Myo-D1. This report highlights a rare case of adult embryonal RMS and emphasizes the importance of a multidisciplinary diagnostic approach for metastatic disease in this population.