A multivariate survival analysis including gender, age, log white blood cell (WBC) count, liver and spleen size at diagnosis, mean log WBC count during maintenance therapy, and the prescribed cumulative doses of 6-mercaptopurine (6-MP), methotrexate (MTX), vincristine (VCR), and prednisone (PDN) during maintenance therapy was performed on 53 children with average-risk acute lymphoblastic leukemia (ALL). The 6-MP cumulative dose prescribed during maintenance therapy resulted in the most important statistically significant independent prognostic factor. Patients who received less than the median cumulative dose of 6-MP (86% of planned protocol dose) fared significantly worse than the other patients, regardless of WBC count at diagnosis, gender, age, and other factors studied. Therefore, 6-MP cumulative dose during maintenance therapy may be the critical factor for effective maintenance therapy in childhood ALL.

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