Recurrent extramedullary relapse of acute promyelocytic leukemia after allogeneic stem cell transplantation: successful treatment by arsenic trioxide in combination with local radiotherapy.

Isolated extramedullary relapse is rare in patients with acute promyelocytic leukemia (APL) after allogeneic stem cell transplantation (SCT), and an optimal therapy for it has not been established. We describe a patient with APL who developed serially occurring extramedullary disease (EMD) after SCT. We confirmed that EMD had arisen from the recipient's APL blasts by detecting t(15;17) and PML/RARalpha from the tumor cell suspension. The patient displayed EMD 4 times at different sites. Administration of all-trans retinoic acid with local radiotherapy and with chemotherapy for the first to third EMDs resulted in regression of the tumors. However, these regimens did not prevent the subsequent occurrence of new EMD. For the fourth EMD, intravenous administration of arsenic trioxide followed by local radiotherapy resulted in the disappearance of EMD, and no further EMD has developed to date. In the present case, the bone marrow was in morphologic and molecular remission during the course of recurrent EMD. The accumulation of detailed cases is needed to elucidate the pathogenesis, predisposing factors, and optimal therapy for EMD in APL after SCT.