Post-transplant BRAF/MEK inhibitor maintenance sustains complete remission in BRAF V600E-mutant histiocytic sarcoma

Histiocytic sarcoma is a rare and aggressive malignancy with poor outcomes under conventional chemotherapy. Activating MAPK pathway alterations, including BRAF V600E mutations, provide a rationale for targeted therapy; however, the durability of responses and the role of post-transplant maintenance remain unclear. We report a patient with refractory BRAF V600E-mutant histiocytic sarcoma who achieved complete remission following BRAF/MEK inhibition and subsequently underwent allogeneic stem cell transplantation. Post-transplant maintenance therapy with dabrafenib and trametinib was initiated early after engraftment. The patient achieved sustained complete remission with no radiological or molecular evidence of disease during follow-up exceeding one year. Circulating tumor DNA analysis remained negative for previously detected mutations. This case suggests that an integrated strategy combining targeted therapy, allogeneic transplantation, and post-transplant maintenance may enable durable disease control in high-risk BRAF-mutant histiocytic sarcoma and warrants further evaluation.