This purpose of this study is to examine the placement of proton spots during pencil beam scanning proton therapy for low and intermediate risk prostate cancer. The researchers will test a unique technique called "Spot Delete" to control the placement of spots during treatment planning. They will also use a special computer model to study how the energy of the proton beam (linear energy transfer) is related to rectal and bladder side effects. The study involves creating a treatment plan based on a CT scan, which helps guide the proton beam in the body. The clinical team uses this CT scan to find the best placement for the protons. The "Spot Delete" method prevents protons from stopping in the rectum, sigmoid, and small bowel, which is thought to be related to acute or late toxicities, such as tenesmus, diarrhea, fecal incontinence, proctitis, and rectal hemorrhage.

Patients will be randomized into two groups of spot placement. In the experimental arm, "Spot Delete" will be used to prohibit proton spots from being placed in the rectum, sigmoid, and small bowel. In the control arm, proton spots are placed by the treatment planning system are not modified. Medical staff will document gastrointestinal toxicities (diarrhea, fecal incontinence, proctitis, and rectal hemorrhage) and genitourinary toxicities (hematuria, urinary incontinence, urinary retention, urinary obstruction, urinary pain, urinary frequency, and urinary urgency) in the electronic medical record.

This study carries the same risks as regular proton therapy. Proton therapy, like other cancer treatments, may cause the aforementioned side effects. It is important to note that the "Spot Delete" technique aims to specifically address the bladder and bowel reactions, so there might be a potential reduction in this particular risk. Dose constraints will be the same for each treatment arm. Though not expected, it is theoretically possible that this method may counterintuitively increase side effects involving the bladder or bowel. Likewise, increased side effects in other organs are not expected, but are theoretically possible. For this reason we believe there is clinical equipoise between the two trial arms.

Inclusion Criteria:

• Pathological proven diagnosis of prostatic adenocarcinoma
• History and physical exam to establish clinical staging
• Clinical stage T1-T2c (AJCC 7th edition)
• Prostate specific antigen (PSA) < 20 ng/mL
• Gleason Score ≤ 7
• Eastern Cooperative Oncology Group (ECOG) Performance status 0-1
• Patients must be 18 years of age or older
• Willingness and ability to complete the International Prostate Symptom Score (IPSS) survey and Expanded Prostate Cancer Index Composite (EPIC) questionnaire

Exclusion Criteria:

• Prior radiotherapy to the pelvic area
• Prior prostate cancer therapy: cryotherapy or hyperthermia
• Prior systemic therapy (chemotherapy) for prostate cancer
• Regional lymph nodes (common iliac, external iliac, internal iliac, presacral) are a target of treatment
• Active diverticulitis, ulcerative colitis, or Crohn's disease

Pathological proven diagnosis of prostatic adenocarcinoma