Updated Retrospective Dose Volume Histogram Analysis of High Dose Rate Prostate Brachytherapy Patients With Hydrogel Spacer Implantation

Abstract

A commercially available bioabsorbable hydrogel system—a mixture of a precursor (trilysine buffer solution and polyethylene glycol powder) and accelerator (salt buffer)—is implanted between the prostate and rectum of men undergoing radiation therapy for prostate cancer. Use of this device provides additional separation between the prostate and rectum, ostensibly reducing rectal injury due to proximity of the high dose target. This study investigates rectal sparing achieved in high dose rate (HDR) prostate brachytherapy patients with hydrogel spacer implantation. One hundred and fifty two monotherapy HDR prostate brachytherapy cases were selected from the patient population treated from July 2015 to February 13, 2017. Patients were selected based on a treatment regimen of 13.5 Gy per fraction. Of these cases, 120 fractions were delivered to patients with implanted hydrogel rectal spacers while 32 fractions were treated without the use of this or any other rectal sparing device. For this patient cohort, the dose volume histogram (DVH) from the previously calculated plan was used to obtain D1cc, D2cc, and maximum rectal dose statistics based on rectum contours drawn on the planning CT dataset by the dosimetrist at the time of treatment planning. To compare to conventionally fractionated external beam radiation therapy (EBRT), the equivalent dose in 2 Gy fractions (EQD2) was calculated for each rectal dose statistic. Statistical analysis software was employed for data analysis. The D1cc, D2cc, and maximum dose to the rectum for cases in which the hydrogel spacer was implanted prior to treatment were compared to cases treated without rectal sparing devices. Results with and without hydrogel spacer implantation were as follows: D1cc was 7.9±0.2 Gy and 8.4±0.4 Gy, respectively; D2cc was 7.1±0.2 Gy and 7.5±0.4 Gy, respectively; and the maximum dose to the rectum as 10.7±0.3 Gy and 11.9±0.6 Gy, respectively. Reduction in maximum (p < 0.001) and small volumetric doses (p = 0.027 and 0.113 for D1cc and D2cc) to the rectum is observed in cases implementing the hydrogel rectal spacer. Considering a typical HDR prostate brachytherapy dose prescription of 27.0 Gy (two 13.5 Gy fractions) and an α/β = 3.0 for rectum, the average maximum EQD2 point dose for patients receiving hydrogel implants was 59.9±3.1 Gy compared to 71.9±2.8 Gy (p < 0.001) for those treated without the device. DVH analysis for the patient sampling with hydrogel implantation demonstrated statistically significant reduction in Dmax and D1cc metrics. When compared to plans without hydrogel implants, the average EQD2 of the max point dose to rectum for patients receiving hydrogel implants was lower and fell within the TD5/5 of 60 Gy, as recommended by Emami et al. Future clinical evaluation of these patients may allow for a statistical relationship between the observed dose reduction and clinical toxicity.