46: Factors Affecting Access to Radiotherapy Following Prostatectomy for Prostate Cancer Patients in Ontario in a Contemporary Cohort

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Purpose: Intensity modulated proton therapy (IMPT) allows for the delivery of ionizing radiation over a well-defined range with minimal exit dose compared to photons, and may further improve dose conformality compared to other proton modalities.IMPT is not available for brain cancer treatment in Canada.Instead, patients who would likely benefit from proton over photon therapy are evaluated on a case-by-case basis for referral to US facilities.Improved neurocognitive outcomes would certainly constitute a strong argument.As such, tools were developed to estimate the intelligence quotient (IQ) and the risk of hearing loss post radiotherapy and to compare outcomes of proton against photon in pediatric brain tumours on a case-bycase basis.Methods and Materials: Pediatric patients who had received radical photon intensity modulated radiation therapy (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 contoured structures were used to generate IMPT plans employing a robust optimization procedure.The RBE-corrected dose to brain structures and the cochleas were calculated for both IMPT and IMRT.A dose dependent IQ model was applied to estimate IQ using a Markov chain Monte Carlo technique.Cumulative probability distributions (CDF) were calculated to perform a statistical interpretation and to compare proton versus photon outcomes.The reported incidence of hearing loss as a function of cochlear dose in the literature was used to estimate the probability of occurrence.Results: The average dose to the brain 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 versus photons for the whole cohort at five years post-treatment (p = 0.011).In terms of specific diseases, the gains in IQ points were 0.8, 1.6, 2.3, and 2.7 for medulloblastoma, ependymoma, glioma and craniopharyngioma, respectively.When estimating the IQ using dose to the temporal lobes, these gains increased to 3.1 to 6.0 IQ points.Overall, the probability for IQ deficits ≥ 7.5 points was estimated to be 32% for IMPT compared to 48% for IMRT, an absolute reduction of 16% for the whole cohort (p = 0.014).Hearing loss probability was evaluated on a per-ear-basis and was found to be systematically less for proton versus photon: 2.9% versus 7.2% (p < 10-7).Conclusions: IQ predictions post IMPT and IMRT were found to be very similar, but a modest gain was systematically observed for proton in all patients.Given the uncertainties within the IQ model used and our reinterpretation, the predicted gains may be underestimated.Additional long-term clinical studies are needed to improve our understanding of radiation on the developing brain.

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