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) i s a rare occurrence, and the mechanisms underlying the selective involvemen t of extramedullary sites remain undefined. These might be due to relapse i n 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 mole cular remission before isolated leukaemic relapse in the central nervous sy stem (CNS) occurred, Subsequent secondary infiltration of leukaemic cells i nto the marrow was only evident molecularly but not morphologically, implyi ng that the relapse had arisen in a sanctuary CNS site. In the second patie nt, a first relapse in the marrow, which was induced into morphologic and m olecular remission by chemotherapy and donor lymphocyte infusion, was follo wed by extramedullary relapses without any subsequent involvement of the ma rrow. This suggested that factors, likely to be due to a GVL effect, were s tronger in the marrow than in peripheral tissues.