Hydrogel Spacer Reduces Rectal Dose during Proton Therapy for Prostate Cancer: A Dosimetric Analysis

Abstract

Purpose: Proton therapy for prostate cancer may reduce bowel dose and risk of bowel symptoms relative to photon-based methods. Here, we determined the effect of using a biodegradable, injectable hydrogel spacer on rectal dose on plans for treating prostate cancer with intensity-modulated proton therapy (IMPT) or passive scattering proton therapy (PSPT). Materials and Methods: Pairs of IMPT and PSPT plans for 9 patients were created from fused computed tomography/magnetic resonance imaging scans obtained before and after spacer injection. Calculated values of rectal V40, V60, V70, V80, and maximum dose (Dmax) were compared with Wilcoxon signed rank tests. Displacements at the base (BP), midgland (MP), and apex (AP) of the prostate relative to the anterior rectal wall with the spacer in place were averaged for each patient and correlated with V70 by using linear regression models. Results: The presence of a spacer reduced all dosimetric parameters for both PSPT and IMPT, with the greatest difference in V70, which was 81.1% lower for PSPT-with-spacer than for IMPT-without-spacer. Median displacements at BP, MP, and AP were 12 mm (range 7-19), 2 mm (range 0-4), and 1 mm (range 0-5) without the spacer and 19 mm (range 12-23), 10 mm (range 8-16), and 7 mm (range 2-12) with the spacer. Modest linear trends were noted between rectal V70 and displacement for IMPT-with-spacer and PSPT-with-spacer. When displacement was ≥8 mm, V70 was ≤5.1% for IMPT-with-spacer and PSPT-with-spacer. Conclusion: Use of biodegradable hydrogel spacers for prostate cancer treatment provides a significant reduction of radiation dose to the rectum with proton therapy. Significant reductions in rectal dose occurred in both PSPT and IMPT plans, with the greatest reduction for IMPT-with-spacer relative to PSPT alone. Prospective studies are ongoing to assess the clinical impact of reducing rectal dose with hydrogel spacers.

Preliminary analysis of prostate positional displacement using hydrogel spacer during the course of proton therapy for prostate cancer

“…As a result, we confirmed that rectal dose was significantly reduced in the HS-inserted group compared with the non-inserted group as in a previous report [ 13 ]. In addition, both groups showed a significant correlation with changes in rectal volume in the AP prostate displacement, but the absolute amount tended to be smaller in the HS-inserted group.…”

Stability of daily rectal movement and effectiveness of replanning protocols for sparing rectal doses based on the daily CT images during proton treatment for prostate cancer

“…The use of hydrogel rectal spacers for proton prostate treatment is considered to be effective to reduce the rectal dose, resulting in lower toxicity by making space between the rectum and prostate. The use of this technique can be expected to increase, but the stability of implantation as well as the absorption rate of a biodegradable compound over the treatment period must be studied by using several imaging modalities to adjust treatment plans appropriately 14,15 . In particular, the implantation stability around the SV region is questionable, since the daily movement of the rectum around this region was found to be significant herein.…”

“…The use of this technique can be expected to increase, but the stability of implantation as well as the absorption rate of a biodegradable compound over the treatment period must be studied by using several imaging modalities to adjust treatment plans appropriately. 14,15 In particular, the implantation stability around the SV F I G . 8.…”

Expanding the Utilization of Rectal Spacer Hydrogel for Larger Prostate Glands (>80 cc): Feasibility and Dosimetric Outcomes

“…Several prospective multi-institutional studies 10 , 11 and a number of retrospective clinical or dosimetric studies have demonstrated the feasibility of rectal spacer application with improvement in rectal sparing. 13 , 14 , 15 , 16 , 29 , 30 , 31 , 32 , 33 …”

“…Several prospective multi-institutional studies [10,11] as well as a number of retrospective clinical or dosimetric studies have demonstrated the feasibility of rectal spacer application with improvement in rectal sparing. [13][14][15][16][29][30][31][32][33] Song et al [10] reported the results of a multi-institutional study where hydrogel resulted in > 7.5-mm prostate-rectal separation in 95.8% of patients with 95.7% achieving a reduction in rV70 of > 25%. Eligibility was limited to prostate glands < 80 cc.…”