Stereotactic Ablative Radiotherapy (SABR) for Low Risk Prostate Cancer With Injectable Rectal Spacer

The purpose is to determine if use of rectal spacers are effective at improving protection of rectum from high dose radiation, using rate of rectal ulceration as a surrogate measure of acute effects. It is also to determine whether it provides sufficient dosimetric benefits to warrant further clinical investigation in future SABR (Stereotactic Ablative Body Radiation) related clinical studies.

A phase II study to assess safety and efficacy of the spacer injection process, ability of the spacer to effectively provide the space necessary to reduce acute events in the rectum, and also meet the SABR based rectal constraints, and to monitor stability of this process during SABR. Unlike IMRT, which uses smaller dose/fraction, when using such high dose/fraction, even a few mm of shift in spacer positioning may impact the dose that the rectum receives, and therefore, a rigorous study of stability of material during the SABR treatments will need to be determined. If there is some shift, by doing this study, we may be able to determine the margin of error that will be necessary in considering rectal organ dosimetry, based on the possible shift in positiong that may occur with the spacer over time. As the SABR therapy is strictly local, we will select for patients with prostate cancer locally confined to the prostate gland. As such, we will select eligibility criteria of low risk patients to minimize risk of extraprostatic spread, seminal vesicle invasion, and nodal spread. Hormonal therapy may also be used to shrink prostates that are massively enlarged as this may also help further reduce length of rectum that will be irradiated. As the primary toxicity will likely be mucosal damage, we will avoid enrolling patients with pre-existing mucosal dysfunction (including those with previous radiation, TURP, very large prostate glands, inflammatory bowel disease) and immunosuppressed individuals based on our phase I experience\[13\]. In this way, patients will be uniformly selected in a fashion that would identify patients likely to receive benefit from the therapy.