Session Item

The goal of this study was to quantify the dosimetric benefit of SpaceOAR Hydrogel (SOH), in the setting of simulated unilateral and bilateral hip prostheses, in particular with regard to rectal V55Gy, the hypofractionated equivalent of rectal V70Gy, regardless of the VMAT prosthesis avoidance technique used.

An anonymized dataset of 13 prostate cancer patients’ radiation planning scans taken before and after SOH placement were used to create radiation plans with simulated unilateral and bilateral hip prostheses as well as no hip prosthesis. The shape and were based on DuPey Pinnacle acetabular shell, Pinnacle Marathon 35ID/58, Summit 7 titanium femoral component, with Biolox delta ceramic head. Hypo-fractionated RT plans (60Gy in 20 fractions) were created, using photon optimizer in Eclipse (v15.6.06) by a single planner using a highly prescribed approach derived from previous SOH work and an assigned titanium alloy material for the prostheses. All plans were required to meet the CHiiP trial goals in addition to local defined goals. Two VMAT prosthesis avoidance techniques were investigated, a commercial approach, Avoid Structure – Entry (ASE) and a user-defined approach, Upper Constraint on Prosthesis PRV (UCPP). The pre-SOH plans were compared to the post-SOH plans, using V37Gy, V46Gy, V50Gy, V55Gy, and V60Gy for rectum, the primary structure of interest, and various metrics for bladder, penile bulb, CTV, PTV and BODY-PTV.

SOH resulted in statistically significant reductions, as per Wilcoxon signed rank test, in all rectal metrics, except V37Gy, for both unilateral and bilateral prostheses scenarios, regardless of avoidance technique. In the unilateral prosthesis setting, 11/13 patients had 50%+ reduction in V55Gy, while 10/13 patients had a 50%+ drop in V55Gy in the setting of bilateral prostheses. Furthermore, the mean and standard deviation of the rectal metrics for all scenarios simulated are in line with the values reported in the pivotal trial by Mariados et al. No significant change in bladder, penile bulb, CTV and PTV doses was noted between the pre- and post-SOH plans. Both avoidance techniques resulted in a statistically significant increase in bladder dose compared to baseline plans (no prosthesis), however, the increase was statistically less with the ASE compared to UCPP technique. However, the ASE technique resulted in a statistically significant increase in all rectal metrics, except V60Gy and PTV V63Gy in the bilateral hip setting.

The use of SOH appears to result in statistically significant benefits in terms of rectal dose despite the presence of simulated unilateral and bilateral hip prostheses, regardless of avoidance technique. However, both avoidance techniques resulted in a statistically higher bladder dose compared to baseline while the UCPP resulted in statistically higher rectal doses, except V60Gy, in the context of bilateral hip prostheses, making ASE a better avoidance technique for VMAT treatment.