Comparison of hydrogel spacer and rectal immobilization on intra-fraction motion equivalence using image guidance prostate proton therapy

Presented by Rachel Rendall, BS (Northwestern Medicine Chicago Proton Center) at the PTCOG-NA October 2016

Background:  Proton therapy treatment plans are sensitive to internal and external motion due to the specific calculation of range to the target volume. Because of this, localization and immobilization of the prostate is important. At our center, the standard of prostate immobilization is through the use of an endorectal balloon (ERB) placed into the rectum. SpaceOAR (Augmenix) is a hydrogel made of polyethylene glycol intended to reduce the high dose to the rectum by acting as a spacer between the rectum and the prostate during prostate radiotherapy. In this study we evaluate the efficacy of hydrogel acting a prostate stabilizer and the efficiencies of its use.

Material and Methods:  Data was collected from 79 prostate cancer patients treated with proton therapy. All patients had 3 fiducial markers implanted perineally into the prostate to aid in target alignment for 2D kV portal imaging. 45 of the patients received 10cc SpaceOAR hydrogel as a rectal spacer. 34 patients were simulated and treated daily with a 90cc endorectal balloon.

Results:  2020 images gained during 1010 fractions were analyzed: 487 fractions in the hydrogel group and 523 fractions in the 90cc ERB group. The mean magnitude (in mm) for the hydrogel group was 1.28 (SD=0.95). The mean magnitude for the 90cc ERB group was 1.1 (SD=0.55). An ANOVA was performed to test the difference in the mean magnitude between the two groups, and this yielded statistically insignificant results (p=0.344).

Conclusions:  Based on our study, hydrogel can be used as a prostate stabilizer for proton therapy. Hydrogel proved to be just as effective as endorectal balloons to reduce intra-fraction motion. The additional benefit of hydrogel is that it is a onetime injection prior to treatment. This saves time for therapists by eliminating daily endorectal balloon preparation as well as the cost of having a balloon for each fraction. These efficiencies in conjunction to its actual use of reducing rectal dose show hydrogel as a good choice for prostate immobilization for proton therapy.