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

Abstract

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, which can be a very challenging situation 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 blasts-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, and hence a PET-CT was performed, which 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. We report this case to highlight this rare presentation of embryonal RMS in an adult and to emphasize that a multidisciplinary diagnostic approach is needed to correctly diagnose metastatic RMS in an adult.