Morphologic changes in blast cells of children with acute lymphoblastic leukemia in relapse.

Relapse in children with acute lymphoblastic leukemia (ALL) on therapy may be due to development of a resistant clone of blast cells. Seven children who presented initially with the "common"-type, L1 lymphoblast relapsed with a morphologically different and more undifferentiated blast cell. All were male, with a median age of 12 years at initial presentation. One child who relapsed while off therapy was successfully reinduced and remains in hematologic remission on therapy. The remaining 6 children died within 10 months of relapse. Selection of a resistant clone of lymphoblasts by chemotherapy may be responsible for relapse in children with ALL and should be studied in hopes of controlling the disease.