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Purpose: To assess acute gastrointestinal (GI) and genitourinary (GU) toxicities of intensity-modulated proton therapy (IMPT) targeting the prostate/seminal vesicles and pelvic lymph nodes for prostate cancer. Materials and Methods: A prospective study (ClinicalTrials.gov: NCT02874014), evaluating moderately hypofractionated IMPT for high-risk or unfavorable intermediate-risk prostate cancer, accrued a target sample size of 56 patients. The prostate/seminal vesicles and pelvic lymph nodes were treated simultaneously with 6750 and 4500 centigray radiobiologic equivalent (cGyRBE), respectively, in 25 daily fractions. All received androgen-deprivation therapy. Acute GI and GU toxicities were prospectively assessed from 7 GI and 9 GU categories of the Common Terminology Criteria for Adverse Events (version 4), at baseline, weekly during radiotherapy, and 3-month after radiotherapy. Fisher exact tests were used for comparisons of categorical data. Results: Median age was 75 years. Median follow-up was 25 months. Fifty-five patients were available for acute toxicity assessment. Sixty-two percent and 2%, respectively, experienced acute grade 1 and 2 GI toxicity. Grade 2 GI toxicity was proctitis. Sixty-five percent and 35%, respectively, had acute grade 1 and 2 GU toxicity. The 3 most frequent grade 2 GU toxicities were urinary frequency, urgency, and obstructive symptoms. None had acute grade ≥ 3 GI or GU toxicity. The presence of baseline GI and GU symptoms was associated with a greater likelihood of experiencing acute GI and GU toxicity, respectively. Of 45 patients with baseline GU symptoms, 44% experienced acute grade 2 GU toxicity, compared with only 10% among 10 with no baseline GU symptoms (P = 0.07). Although acute grade 1 and 2 GI and GU toxicities were common during radiotherapy, most resolved at 3 months after radiotherapy. Conclusion: A moderately hypofractionated IMPT targeting the prostate/seminal vesicles and regional pelvic lymph nodes was well tolerated with no acute grade ≥ 3 GI or GU toxicity. Patients with baseline GU symptoms had a higher rate of acute grade 2 GU toxicity.

Background and purpose: Intensity modulated proton therapy (IMPT) allows for modulation parameterized for individual beamlets by position, intensity, and depth. This modulation capability is ideally suited for sparing organs at risk intermediate of the radiation target, such as hippocampal volumes within the whole brain. This work compared IMPT relative to volumetric modulated arc therapy (VMAT) during hippocampal avoidance whole brain radiation therapy (HA WBRT). Materials and methods: Ten adult and ten pediatric patients previously treated for central nervous system malignancies were identified. IMPT and VMAT treatment plans employing HA WBRT were generated for each patient, delivering 30 GyE (Gray Equivalent) in 10 fractions for adults and 36 GyE in 20 fractions for pediatrics. Dose indices, including dose volume histogram metrics and homogeneity index HI = [D5% − D95%]/[Dmean] × 100, were used to assess plan quality and describe target coverage and normal-tissue sparing. Results: IMPT offered significant benefits relative to VMAT for hippocampal sparing. Hippocampal mean dose was reduced from 13.7 ± 0.8 Gy with VMAT to 5.4 ± 0.3 GyE using IMPT for pediatrics, and was reduced from 11.7 ± 0.9 Gy with VMAT to 4.4 ± 0.2 GyE using IMPT for adults. IMPT similarly lowered left hippocampal mean dose. Dose to 95% of the clinical target volume was statistically equivalent for both groups; however IMPT reduced the homogeneity index by roughly half. Conclusion: This manuscript demonstrates that HA IMPT can match or exceed dosimetric benefits offered with modulated X-rays. Inclusion of IMPT in future prospective studies is warranted. Keywords: Intensity modulated proton therapy, Hippocampal sparing, Whole brain radiation therapy, RTOG 0933