Reducing Acute Rectal Toxicity for Hypofractionated Prostate Radiotherapy - An Multi-institution Pilot Study of Reduced PTV Margin

Hypofractionated radiotherapy is being adopted as standard practice for intermediate risk prostate cancer patients. The objective of this project is to retrospectively validate whether reducing the Planning Target Volume (PTV), a margin placed around the prostate to account for movement and variability in patient placement, will have reduced acute rectal toxicity. The project will focus on patients with intermediate risk prostate cancer treated with hypofractionated Volumetric Modulated Arc Therapy (VMAT) from multiple institutions. The investigators intend to prospectively demonstrate that patients planned and treated with the reduced margin, along with using advanced image-guidance and consistent bladder and rectum preparation will have reduced acute rectal toxicity.

The project consists of two phases: phase one is a retrospective planning study and phase two is a prospective pilot clinical trial.

Phase one aims to validate the benefits of the proposed decrease in PTV margin by assessing whether the investigators can continue to deliver the intended radiation dose to the prostate while further sparing the rectum. This will be accomplished by evaluating the daily delivered dose on Cone-Beam Computed Tomography (CBCT), a daily scan performed during treatment that determines the location of the prostate and rectum. Using these CBCT scans, the investigators can reconstruct the radiation dose delivered to any organs of interest which was affected by their relative positions each day. There are two parts in this phase one retrospective planning study. Part one will focus on 30 intermediate risk prostate cancer cases already treated with hypofractionation (60 Gy in 20 fractions) using the standard PTV margins from all three participating institutions. All 30 cases will then be re-planned using the reduced PTV margin and dose reconstruction will be performed on daily CBCT to confirm adequate target coverage of the prostate. Part two aims to demonstrate that the reduced PTV margin can help enable hypofractionation. Thirty cases that were originally intended to receive a hypofractionated regimen of 60 Gy in 20 fractions but had to either switch to a standard fractionation treatment of 78 Gy in 39 fractions (a regimen used when 60Gy in 20 fractions is not achievable), or where there had to be compromised target coverage of the prostate, due to Organs at Risk (OAR) receiving unacceptable doses of radiation, will be selected. These cases will be re-planned with the validated smaller PTV margin to illustrate that they would now be eligible to receive hypofractionation due to the reduced dose to OARs.

Phase two aims to demonstrate that patients treated with the reduced PTV margin will have reduced acute rectal toxicity as evaluated by the Cancer Care Ontario (CCO) Expanded Prostate Cancer Index Composite for Clinical Practice (EPIC-CP) Quality of Life (QOL) questionnaire. Two groups of patients will be accrued: one group of 25 patients will be planned and treated with the current standard PTV margin as the control group, while the second group of 25 patients will be planned and treated with the reduced PTV margin validated from the phase one retrospective study. A comparison of rectal toxicity scores between these two groups will then be carried out using the EPIC-CP questionnaire.