Abstract

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). 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. 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. 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.

Highlights

  • Patients receiving radiation therapy for prostate cancer are at risk of developing treatment-related rectal toxicity, as hypofractionated and stereotactic ablative approaches have become more prominent

  • We present a dosimetric analysis of rectal dose sparing when using biodegradable, injectable hydrogel spacers combined with either passive scattering proton therapy (PSPT) or intensity-modulated proton therapy (IMPT)

  • Our results suggest that a prostate-to-rectum displacement of at least 8 mm provides significant reductions in clinically relevant rectal dose parameters

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Summary

Introduction

Patients receiving radiation therapy for prostate cancer are at risk of developing treatment-related rectal toxicity, as hypofractionated and stereotactic ablative approaches have become more prominent. Use of such a spacer led to a relative reduction in mean rectal V70 of 74% [17]. These studies suggest that significant dosimetric benefit requires at least 7- to 15-mm separation between the prostate and rectal wall [13, 18,19,20,21,22]

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