There was no significant difference in the decline in intelligence quotient (IQ) score over time between pediatric brain tumor patients treated with either photon radiation (XRT) or proton beam radiation therapy (PBRT), according to a new study. On a multivariate analysis, XRT was associated with a significant decline in IQ score, while PBRT was not; however, the slopes of the two were no different, and further study is needed to untangle potential differences in cognitive effects.
“Cranial radiation therapy (RT) is associated with declines of two to four IQ points per year among pediatric patients with brain tumors,” wrote study authors led by Lisa S. Kahalley, PhD, of Texas Children’s Hospital in Houston. Because PBRT reduces the total radiation dose to patients, “there is optimism that PBRT may protect against cognitive sequelae.”
The study compared 60 XRT patients with 90 PBRT patients. Gliomas were more common in the PBRT group, while medulloblastomas and ependymomas were more common in the XRT group. Craniotomy and ventriculoperitoneal shunts were also more commonly used in the XRT group. Results of the study were published in the Journal of Clinical Oncology.
On a multivariate analysis including several factors significantly associated with RT group or IQ, there was no significant decline in IQ scores over time with PBRT; the rate was found to be −0.7 IQ points per year since RT (95% CI, −1.6 to −0.2; P = .130). In contrast, there was a significant decline with XRT, at −1.1 IQ points per year (95% CI, −1.8 to −0.4; P = .004).
In spite of that difference, the slopes of the two IQ curves did not significantly differ (−0.7 vs −1.1; P = .509). Overall, the IQ scores in the XRT group were 8.7 points lower than in the PBRT group, which the authors described as a “persistent” difference because the slopes did not differ.
A subgroup analysis was done for patients who received craniospinal radiation. This found no significant IQ decline in either the PBRT or XRT groups, and again the slopes did not differ between RT groups (P = .890).
A further analysis of patients who received focal RT found a significant decline in IQ scores with XRT (−1.6 points per year; P = .026) but not with PBRT (−0.6 points per year; P = .401). Once again, however, the slopes of the IQ declines did not significantly differ (P = .342).
“This study does not provide clear evidence that PBRT results in clinically meaningful sparing of global IQ significantly exceeding that of modern XRT protocols,” the authors concluded. Longer follow-up on larger sample sizes will be needed to further describe any differences in cognitive outcomes in these patients.
In an accompanying editorial, Stephen A. Sands, PsyD, of Columbia University Medical Center in New York, noted that even larger, rigorous studies on this question are using different batteries of neuropsychological tests and at different time points in non-randomized patients, making interpretation very difficult. Only with careful studies using standardized batteries of tests, he wrote, will allow researchers “to better understand the anticipated late effects that may arise from our continued efforts to minimize the toxicity of curative radiation therapies for pediatric patients with brain tumors.”