The increased sparing of normal tissues in intensity modulated proton therapy (IMPT) compared to photon intensity modulated radiotherapy (IMRT) in brain tumor treatments should translate into improved neurocognitive outcomes. Models were used to estimate the intelligence quotient (IQ) and the risk of hearing loss 5 years post radiotherapy and to compare outcomes of proton against photon in pediatric brain tumors.
Patients who had received radical IMRT were randomly selected from our retrospective database: 10 cases each of craniopharyngioma, ependymoma and medulloblastoma, and 20 cases of glioma. The existing planning CT and contours were used to generate IMPT plans. The RBE-corrected dose to brain structures and cochleas were calculated for both IMPT and IMRT. A model was applied to estimate IQ using a Markov chain Monte Carlo technique. The reported incidence of hearing loss as a function of cochlear dose was used to estimate the rate of occurrence.
The average brain dose was less in all IMPT plans compared to IMRT: ranging from a 6.7% reduction (P=0.003) in the case of medulloblastoma to 38% (P=0.007) for craniopharyngioma. This dose reduction translated into a gain in IQ of 1.9 points on average for protons vs photons for the whole cohort at 5 years post-treatment (P=0.011). In terms of specific diseases, the gains in IQ ranged from 0.8 points for medulloblastoma, to 2.7 points for craniopharyngioma. Hearing loss probability was evaluated on a per-ear-basis and was found to be systematically less for proton versus photon: overall 2.9% versus 7.2% (P
A novel method was developed to predict neurocognitive outcomes in pediatric brain tumor patients on a case-by-case basis. A modest gain in IQ was systematically observed for proton in all patients. Given the uncertainties within the model used and our reinterpretation, these gains may be underestimated.
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