PO64

Purpose When utilizing a hydrogel spacer for HDR prostate brachytherapy, hydrogel can be inserted at time of HDR catheter implantation or on a separate visit prior to HDR. The insertion of gel at time of HDR can be more difficult due to interference from the perineal template with imbedded catheters. To assess whether time of hydrogel placement impacted its insertion geometry, we compared patients who had hydrogel placed by a single provider at either the time of HDR brachytherapy (templated insertion - TI) or in advance of prostate stereotactic body radiation therapy (non-templated insertion - NTI). The ultimate aim of this study was to determine whether patients undergoing HDR should have hydrogel placed prior to catheter implantation for improved rectal dosimetry. Materials and Methods The last consecutive 25 patients treated with HDR brachytherapy with hydrogel (TI) and the last consecutive 25 patients treated with prostate SBRT with hydrogel (NTI) in 2022 were included for analysis. CT planning scans for all patients were reviewed and insertion geometry was recorded as represented by measurements of the anteroposterior rectoprostatic separation at the gland apex, mid, and base. Prostate clinical target volume (CTV) measurements for all patients were recorded. Additionally, rectal D0.1cc, D1cc, and D2cc measurements were also noted for the 25 HDR TI patients. Data were analyzed using a one-way MANOVA to determine significance of templated insertion. Subsequently multiple regression analyses were performed to evaluate the impact of insertion geometry and CTV measurements on rectal dosimetry. Results The differences in AP separations between TI and NTI were nonsignificant. The mean TI and NTI separations (cm) were 1.08 vs. 1.18 for apex, p=0.40; 1.40 vs. 1.42 for mid, p=0.84; and 1.52 vs. 1.47 for base, p=0.77. In HDR patients with templated insertion, AP separations at the apex and mid gland were significant in predicting rectal D0.1cc (β -0.49 and -0.51, p<0.001), D1cc (β -0.46 and -0.56, p<0.001) and D2cc (β -0.45 and -0.55, p<0.001). The base separations were not significant. CTVs also did not significantly predict for rectal dosimetry. Conclusions Placement of hydrogel spacer at time of HDR brachytherapy does not appear to adversely affect hydrogel insertion geometry and consequently rectal dosimetry when compared to placement in advance. We will continue our practice of inserting hydrogel at time of HDR brachytherapy as this method is efficient and also more convenient for patients. When utilizing a hydrogel spacer for HDR prostate brachytherapy, hydrogel can be inserted at time of HDR catheter implantation or on a separate visit prior to HDR. The insertion of gel at time of HDR can be more difficult due to interference from the perineal template with imbedded catheters. To assess whether time of hydrogel placement impacted its insertion geometry, we compared patients who had hydrogel placed by a single provider at either the time of HDR brachytherapy (templated insertion - TI) or in advance of prostate stereotactic body radiation therapy (non-templated insertion - NTI). The ultimate aim of this study was to determine whether patients undergoing HDR should have hydrogel placed prior to catheter implantation for improved rectal dosimetry. The last consecutive 25 patients treated with HDR brachytherapy with hydrogel (TI) and the last consecutive 25 patients treated with prostate SBRT with hydrogel (NTI) in 2022 were included for analysis. CT planning scans for all patients were reviewed and insertion geometry was recorded as represented by measurements of the anteroposterior rectoprostatic separation at the gland apex, mid, and base. Prostate clinical target volume (CTV) measurements for all patients were recorded. Additionally, rectal D0.1cc, D1cc, and D2cc measurements were also noted for the 25 HDR TI patients. Data were analyzed using a one-way MANOVA to determine significance of templated insertion. Subsequently multiple regression analyses were performed to evaluate the impact of insertion geometry and CTV measurements on rectal dosimetry. The differences in AP separations between TI and NTI were nonsignificant. The mean TI and NTI separations (cm) were 1.08 vs. 1.18 for apex, p=0.40; 1.40 vs. 1.42 for mid, p=0.84; and 1.52 vs. 1.47 for base, p=0.77. In HDR patients with templated insertion, AP separations at the apex and mid gland were significant in predicting rectal D0.1cc (β -0.49 and -0.51, p<0.001), D1cc (β -0.46 and -0.56, p<0.001) and D2cc (β -0.45 and -0.55, p<0.001). The base separations were not significant. CTVs also did not significantly predict for rectal dosimetry. Placement of hydrogel spacer at time of HDR brachytherapy does not appear to adversely affect hydrogel insertion geometry and consequently rectal dosimetry when compared to placement in advance. We will continue our practice of inserting hydrogel at time of HDR brachytherapy as this method is efficient and also more convenient for patients.