This is a single arm phase II study of image-guided pencil beam scanning proton SBRT (40Gy RBE in 5 fractions delivered every other day) for patients with low- and intermediate-risk prostate cancer. The primary aim is to assess GU/GI toxicity of proton SBRT and compare this to historic outcomes associated with photon-based prostate SBRT. The primary endpoint is 2-year grade 3+ GU/GI toxicity free rate by CTCAE v5.0, which is expected to be ≥95%. Toxicity will be evaluated by the treating radiation oncologist at least once during SBRT, then following SBRT at 1, 3, 6, 12, 18, and 24 months. The treatment will be considered safe if grade 3 or higher GU/GI toxicity free rate at 2 years is >85% (95% rate expected with a 10% non-inferiority margin). The accrual goal is 61 patients over 3 years. To ensure that unexpected significant toxicity is identified, all grade 3 or higher toxicities will be reported to the study PI and the trial will stop accruing if at any point 4 or more patients experience a grade 3 or higher toxicity after completing SBRT. This is felt to be conservative given the vast experience with photon SBRT at this dose with an expected G3+ toxicity of ~5%.

Secondary objectives are to examine patient-reported urinary, gastrointestinal, sexual, and financial outcomes using IPSS, EPIC-26, and COST questionnaires at the same follow-up timepoints as above. Baseline measures of these domains will be obtained prior to treatment as well. Clinical outcomes will also be evaluated with PSA measured at each follow-up, as well as prostate MRI and biopsies at 2 years. Patients will be followed for at least 2 years to determine rates of PSA relapse, salvage treatment, development of metastases, death from prostate cancer, and overall survival. A dosimetric comparison will be performed where each patient will be planned for proton and photon SBRT to determine possible advantages of proton SBRT.

Inclusion Criteria:

• Biopsy-proven NCCN low- or intermediate-risk prostate cancer, which includes patients with:

• Gleason score ≤7 disease

• PSA ≤20 ng/ml

• Clinical T1-T2c disease

  • Note: Patients who only have radiographic evidence of possible or probable T3 disease (i.e., extracapsular extension or seminal vesical invasion) will not be excluded.

• KPS ≥ 80%

• Prostate size as determined on MRI to be < 100 cc. Prostate size can be determined on CT scan if MRI is not available.

• Male 18 years of age or older

• IPSS ≤ 20

• Patient must be a candidate for and agree to placement of intraprostatic fiducial markers and a hydrogel rectal spacer

• Patient must be available for at least 2 years of follow-up

Exclusion Criteria:

• Prior prostate surgery (including cryosurgery)

  • Note: Patients who underwent TURP or greenlight PVP are eligible if it was > 12 weeks prior to the anticipated start date of SBRT

• Prior history of chronic prostatitis or urethral stricture

• Currently active cancer(s) other than non-melanoma skin cancers. Patients are not considered to have currently active cancers if they have completed therapy and are considered by their physicians to be at <5% risk of relapse within 2 years.

• Life expectancy of < 2 years