Relationship between hydrogel spacer distribution and dosimetric parameters in linear-accelerator-based stereotactic body radiotherapy for prostate cancer

Purpose: To explore the potential of quantitative parameters of the hydrogel spacer distribution as predictors for separating the rectum from the planning target volume (PTV) in linear-accelerator-based stereotactic body radiotherapy (SBRT) for prostate cancer. Methods: Fifty-five patients underwent insertion of a hydrogel spacer and were divided into groups 1 and 2 of the PTV separated from and overlapping with the rectum, respectively. Prescribed doses of 36.25-45 Gy in five fractions were delivered to the PTV. The spacer cover ratio (SCR) and hydrogel-implant quality score (HIQS) were calculated. Results: Dosimetric and quantitative parameters of the hydrogel spacer distribution were compared between the two groups. For PTV, D-99% in group 1 (n = 29) was significantly higher than that in group 2 (n = 26), and D-max, D-0.03cc, D-1cc, and D-10% for the rectum were significantly lower in group 1 than in group 2. The SCR for prostate (89.5 +/- 12.2%) in group 1 was significantly higher (p < 0.05) than that in group 2 (74.7 +/- 10.3%). In contrast, the HIQS values did not show a significant difference between the groups. An area under the curve of 0.822 (95% confidence interval, 0.708-0.936) for the SCR was obtained with a cutoff of 93.6%, sensitivity of 62.1%, and specificity of 100%. Conclusions: The SCR seems promising to predict the separation of the rectum from the PTV in linear-accelerator-based SBRT for prostate cancer.