To date, no prospective study has demonstrated improved disease control or side effects with proton beam radiation therapy (PBRT) compared to intensity-modulated radiotherapy (IMRT). Proponents of PBRT have offered that it has dosimetric advantages compared to IMRT. That is, while the target volume for both techniques includes the prostate and a margin of normal tissue (bladder and rectum) that is irradiated to the prescribed dose, proton beam delivers lower integral doses and mean doses to normal tissues compared to IMRT.244 However, this dosimetric difference has not been shown to result in fewer side effects or better patient reported QOL. Indeed, the existing peer-reviewed literature suggests that clinical outcomes (e.g., complications, patient-reported QOL) are similar.245 Comparative effectiveness studies have been published to evaluate relative toxicity and oncologic outcomes between proton and photon therapies. Three studies comparing patients treated with proton therapy or photon therapy reported similar toxicity rates and patient reported outcomes. A prospective comparison of patients treated with IMRT (n=204) and patients treated with proton therapy (n=1,234) with regard to patient-reported outcomes measured using the EPIC instrument concluded that “no differences were observed in summary score changes for bowel, urinary incontinence, urinary irritative/obstructive, and sexual domains between the 2 cohorts” after up to 2 years of follow-up.246 Meanwhile, a retrospective analysis of Medicare data from 421 patients treated with proton therapy and a matched cohort of 842 patients treated with IMRT showed less genitourinary toxicity at 6 months with proton therapy, although the difference disappeared after 1 year.247 No other significant differences were seen between the groups.  A more recent large observational study of 772 low- or intermediate-risk group prostate cancer patients treated with either PBT or IMRT compared patient-reported gastrointestinal and genitourinary toxicities.248 Authors found no significant difference in late gastrointestinal or genitourinary toxicities between the two treatment modalities at 12 and 24 months. After adjusting for factors such as age and risk group, both PBT and IMRT demonstrated low rates of toxicity. A report compared PBRT to IMRT using a case-matched approach.249 This observational study of 177 patients found that oncological outcomes (long-term biochemical failure-free survival, prostate cancer-specific survival, and OS) were similar between PBT and IMRT (all p>0.05).   Randomized trials are ongoing comparing IMRT and PBRT using long-term side effects and QOL as the primary endpoints. The PARTIQoL trial, which has a primary endpoint of bowel function at 24 months, has three preliminary reports including a description of baseline characteristics of the study participants.250-252