Processing speed, working memory, and IQ: a developmental model of cognitive deficits following cranial radiation therapy.

IQ decrements following cranial radiation therapy (CRT) for acute lymphoblastic leukemia (ALL) are most apparent years after treatment. The authors examined a developmental model for delayed deficits by evaluating the relationship between processing speed, working memory, and IQ in long-term survivors of childhood ALL (n = 27) compared with demographically matched controls (n = 27). The ALL group treated with CRT showed deficits in IQ, working memory, and processing speed relative to controls. Differences in IQ between the CRT group and controls were mediated by differences in working memory. Processing speed did not fully account for the working memory deficit in the CRT group. Participants with ALL treated only with chemotherapy showed similar working memory and processing speed as matched controls. Data suggest that deficits in processing speed and working memory following CRT may underlie declines in IQ.