Stereotactic MR-guided Adaptive Radiation Therapy for Localized Prostate Cancer

Rationale: This prospective study investigates the outcomes of daily online stereotactic MR-guided adaptive radiation therapy (SMART) in patients with localized prostate cancer (cT1c-T3bN0M0). Visualization of the prostate, rectum and bladder prior to and during radiation delivery can be used to deliver "gated" treatment (beam-on only when the prostate is in the predetermined position) using small uncertainty margins. The novel MRIdian treatment delivery system (ViewRay, USA), which will be used for this study, allows for the immediate generation of an optimal radiotherapy plan based on the current anatomy of the prostate and surrounding normal organs prior to each fraction. These major advances will (i.e. dosimetrically) allow for an optimisation of normal tissue radiation doses, which should theoretically decrease toxicity to surrounding organs such as the rectum or bladder. Another advantage of this approach is that online MR-based prostate imaging does not rely on implanted gold markers, avoiding an invasive procedure to insert such markers. If proven feasible, this approach could set a new standard of care for patients with localized prostate cancer. The main goal of this phase II study of SMART for prostate cancer is to evaluate the early and early-delayed toxicity, i.e. within the first year after treatment. An established 5-fraction hypofractionated radiation scheme will be used in this trial. At VUmc, experience with this scheme has been obtained as a result of participation in a recently concluded multicenter randomized phase II trial \[METc NL4181402912; 20012/398\]. Main outcome parameters will include gastro-intestinal- (GI), genitourinary- (GU) and sexual symptoms, which will be monitored at fixed time points using CTCAE criteria. In addition, patient-reported outcomes will be evaluated using EORTC-QOL questionnaires. Objective: To investigate the early and early-delayed toxicity profile of SMART in patients with localized prostate cancer. Study design: A phase II observational study Study population: 100 consecutive patients with localized prostate cancer (cT1c-T3bN0M0). Study intervention: Study patients will be treated with an online MR-guided hypofractionated course of radiotherapy in 5 fractions of 7.25 Gy per fraction delivered on the prostate with a simultaneous integrated sparing (SIS) of the urethra with a dose of 32.5 Gy in 5 fractions (6.5 Gy per fraction). This stereotactic radiation scheme was also used in \[METc NL4181402912; 20012/398\]. Main study parameters: Early and early-delayed toxicity (CTCAE v. 4.0); prostate specific symptom score (IPSS) and Qol (EORTC-QoL C30 \& QLQ Prostate Cancer module (QLQ-PR25)). Secondary endpoint will be the offline evaluation of the dosimetric benefit of SMART by comparing cumulative doses to organs at risk. Nature, extent of the burden and risks associated with participation, benefit and group relatedness: This novel SMART approach could set a new standard of care for patients with localized prostate cancer by limiting radiation doses to surrounding normal organs and thereby potentially radiation-induced toxicity. Also, implantation of gold markers would become unnecessary using MR-guided "gated" radiotherapy. Disadvantages for patients include the need to be positioned within the MRI bore during radiation delivery, and a prolonged time per treatment fraction (estimated at 30 minutes per fraction), which has to be weighed against the use of a total of only five fractions. As the radiation fractionation scheme that is used in this study has been evaluated in prior trials, no further patient-related risks are anticipated.