Abstract
Histiocytic lesions arising in the setting of hematologic malignancies, though rare, are a well-described phenomenon. A 2-year-old female was diagnosed with a malignant histiocytic sarcoma (HS) and a genetically related non-Langerhans cell histiocytic process with involvement of the scalp 9 months after treatment of T-cell acute lymphoblastic leukemia (T-ALL). This is the first report of biopsy proven scalp involvement of a disseminated histiocytic process arising in the setting of T-ALL. Molecular and genetic characterization of the T-ALL, HS, and scalp lesion provide evidence of a trans-differentiation process or a clonally related but genetically distinct precursor with a common germline mutation in this case. Discussion will focus on common genetic abnormalities in these uncommon lesions with application to the proposed mechanisms in the pathogenesis of histiocytic sarcoma, highlighting the varied morphologic and histologic appearances of these lesions.