Isolated extramedullary relapse of acute lymphoblastic leukaemia after allogeneic bone marrow transplantation.

Isolated extramedullary relapse of acute lymphoblastic leukaemia (ALL) with sparing of the marrow after allogeneic bone marrow transplantation (BMT) is a rare occurrence, and the mechanisms underlying the selective involvement of extramedullary sites remain undefined. These might be due to relapse in sanctuary sites where the leukaemic cells are resistant to chemotherapy, or a stronger putative graft-versus-leukaemia (GVL) effect in the marrow as compared with peripheral tissues. We report two ALL patients with repeated episodes of extramedullary relapse after BMT in whom both mechanisms might be operating. In the first patient, the marrow was in morphologic and molecular remission before isolated leukaemic relapse in the central nervous system (CNS) occurred. Subsequent secondary infiltration of leukaemic cells into the marrow was only evident molecularly but not morphologically, implying that the relapse had arisen in a sanctuary CNS site. In the second patient, a first relapse in the marrow, which was induced into morphologic and molecular remission by chemotherapy and donor lymphocyte infusion, was followed by extramedullary relapses without any subsequent involvement of the marrow. This suggested that factors, likely to be due to a GVL effect, were stronger in the marrow than in peripheral tissues.