Diffuse Large B‐Cell Lymphoma Transdifferentiating Into Histiocytic Sarcoma: Case Report + Systematic Review

A 68-year-old man with a history of diffuse large B-cell lymphoma (DLBCL) with cutaneous involvement presented with a new, rapidly growing, exophytic, bleeding tumor. Histopathologic and immunophenotypic characterization was consistent with a diagnosis of histiocytic sarcoma (HS). A subsequent lymph node biopsy also yielded a diagnosis of HS. Genetic analysis of this patient's original DLBCL bone marrow specimen and subsequent lymph node HS specimen identified identical p.G13D KRAS mutations. This case highlights, to our knowledge, the first case of systemic DLBCL with cutaneous involvement linked to transdifferentiated cutaneous HS by identical KRAS mutations. We also include a systematic review of cutaneous HS cases, identifying only two cases with underlying hematologic transdifferentiation. The distinct clinical morphology, histopathologic characteristics, and immunohistochemical markers associated with each of these entities highlight a very rare and unique example of histiocytic transdifferentiation in the context of a hematologic malignancy.