Regrowth resistance in leukemia and lymphoma: the need for a new system to classify treatment failure and for new approaches to treatment.

Treatment failure in drug sensitive malignancies is a complex phenomenon resulting from both drug resistance and from the rapid regrowth of malignant cells ('regrowth resistance'). Attempts to overcome regrowth resistance during the treatment of the aggressive lymphomas by increasing the frequency of cytotoxic therapy appears to have failed. An alternative approach of significant potential would be to administer biologically active agents to directly slow the regrowth of neoplastic cells between courses of full dose cytotoxic therapy. To facilitate this approach a new system for classifying treatment failure in the leukemias and lymphomas is needed so that the extent of regrowth resistance and the effects of treatment on regrowth resistance can be directly assessed. Accordingly, a new classification system is proposed.