Solitary Cutaneous Xanthogranuloma With DNMT3A Mutation Arising Postallogenic Transplant in a Patient With T-Cell Acute Lymphoblastic Leukemia.

The development of a mature histiocytic neoplasm after an immature lymphoproliferative disorder is rare but well documented. Previous studies have demonstrated clonal relationships between these entities-often through T- and B-cell receptor clonality analyses-supporting the concept of transdifferentiation. We report a unique case of a solitary, subtle xanthogranuloma identified during routine comprehensive skin examination in a patient with a history of T-cell acute lymphoblastic lymphoma, postallogeneic stem cell transplant. Next-generation sequencing of the skin lesion revealed the presence of a DNMT3A mutation identical to that found in the patient's prior lymphoma, despite the patient being in both morphologic and molecular remission in the peripheral blood and bone marrow. This case underscores the importance of vigilant surveillance and highlights a potential mechanistic link through clonal evolution or transdifferentiation.