Purpose: An injectable rectal hydrogel spacer (RHS) is used in both external beam and HDR treatments of the prostate to create spatial separation between the target and the rectum. In a re-planning study, we evaluated the dosimetric impact of the presence of RHS on prostate HDR plan quality.
Methods: Eighteen patients with RHS who recently underwent prostate HDR treatment, were re-planned with modified objectives in order to simulate the absence of the spacer. Patients with three types of treatments were included in this study: monotherapy (9 patients; 27Gy in 2 fractions), salvage therapy (7 patients; 24Gy in 2 fractions), and boost (2 patients; 15Gy in one fraction, following external beam treatment of 46Gy in 23 fractions). The clinical plans were dosimetrically compared to the post-treatment re-plans, which were optimized with modified goals aiming at sparing the urethra, bladder, and RHS (as a surrogate for rectum). The trends in target coverage as well as organ-at-risk sparing were statistically analyzed.
Results: The analysis of 33 plan pairs showed that the presence of RHS significantly reduced the rectal volume receiving >75% of the prescription dose: from mean V75%=0.63cc without RHS to 0.02cc with RHS. Without RHS, the mean prostate coverage was reduced by 1.5% and the urethral volume receiving >125% of the prescription dose on average increased by 0.1cc (from 0.02cc to 0.12cc). No change in the bladder dose was observed. The plan heterogeneity index (V150%/V100%) was slightly worsened for the plans without RHS: from 0.35(5) to 0.36(4).
Conclusion: This dosimetric study clearly demonstrated the benefits of injectable RHS in prostate HDR treatments. Excellent rectal sparing was achieved as well as improved target coverage and marginally better urethral sparing. Injectable RHS was shown to be indispensable in both salvage and boost HDR treatments, where organ-at-risk sparing is of critical importance.