PP05 Presentation Time: 8:56 AM

Purpose A range of standard dose-fractionation schedules for high dose rate (HDR) vaginal cuff brachytherapy (VBT) following hysterectomy is recommended by published guidelines including 3, 4, 5 and 6 fraction (Fx) regimens with prescription varying from the surface to a depth of 5 mm. Currently, there are no dose constraints for organs at risk (OARs) in VBT. Given the presumed relatively low OAR doses, these OAR doses are simply recorded with minimal to no dosimetric adjustment during treatment planning. However, frequently these OARs are in close proximity to the target with bowel draping over the vaginal cuff. The purpose of this study is to evaluate acute and chronic gastrointestinal (GI) or genitourinary (GU) toxicity following HDR VBT with 21 Gy in 3 Fx prescribed to 5 mm depth and to investigate if certain dosimetric parameters predict for toxicity. Methods We conducted a retrospective review of endometrial cancer patients treated at University of Virginia Health with adjuvant single-channel HDR VBT and CT-based 3D planning between 2015 and 2022. All patients received 21 Gy in 3 Fx prescribed to 5 mm depth twice weekly. The dose (D2cc, D1cc, and D0.1cc) to OARs (bladder, rectum, sigmoid colon, small bowel) were recorded. Acute and chronic GI and GU toxicities were graded according to the Common Terminology Criteria for Adverse Events. Boxplot and Wilcoxon-Mann-Whitney tests were used to compare dose thresholds between those with and without toxicity. Results 177 patients with stage I - IV endometrial cancer received mean D2cc to rectum, sigmoid, small bowel, and bladder of 5.7 Gy (range: 2.8 - 8.7), 3.3 Gy (0.6 - 9.3), 3.8 Gy (0 - 9.08), and 5.4 Gy (2.0 - 8.2). Overall, 6.2% (n = 11) of patients experienced new or worse acute GI and 7.3% (n = 13) acute GU toxicities. 18.6% (n = 33) of patients experience chronic GI and 15.8% (n = 28) chronic GU toxicities. The most common acute/chronic GI toxicities were diarrhea (3.4%/2.8%), abdominal pain (1.7%/9.0%), and constipation (1.1%/2.8%). The most common acute/chronic GU toxicities were urinary incontinence (2.3%/6.8%), dysuria (2.3%/1.1%), and urinary frequency (1.7%/4.0%). There were no acute grade 3 GI toxicities. The 1 (0.6%) patient with acute grade 3 GU symptoms including urinary incontinence resolved at follow up. There were no chronic grade 3 GU toxicities. The 1 (0.6%) patient with grade 3 chronic GI symptoms including constipation had baseline of grade 2. This study demonstrates there is no statistically significant difference (p>0.05) in dose levels between the group that experienced symptoms following HDR VBT and the group that did not experience symptoms (Figure 1). Conclusions This retrospective study of adjuvant HDR VBT with 21 Gy in 3 Fx prescribed to 5mm depth for endometrial cancer shows overall low rates of GI and GU toxicity and no evidence that acute or late toxicity events are associated with higher dosimetric OAR values in rectum, sigmoid, small bowel, or bladder. Our single institution experience confirms that high point-doses to the OARs at the vaginal cuff when prescribing to 5 mm depth are safe and well tolerated. The optimal dose fractionation schedule for VBT that preserves high local control and minimizes toxicity is yet to be determined. A range of standard dose-fractionation schedules for high dose rate (HDR) vaginal cuff brachytherapy (VBT) following hysterectomy is recommended by published guidelines including 3, 4, 5 and 6 fraction (Fx) regimens with prescription varying from the surface to a depth of 5 mm. Currently, there are no dose constraints for organs at risk (OARs) in VBT. Given the presumed relatively low OAR doses, these OAR doses are simply recorded with minimal to no dosimetric adjustment during treatment planning. However, frequently these OARs are in close proximity to the target with bowel draping over the vaginal cuff. The purpose of this study is to evaluate acute and chronic gastrointestinal (GI) or genitourinary (GU) toxicity following HDR VBT with 21 Gy in 3 Fx prescribed to 5 mm depth and to investigate if certain dosimetric parameters predict for toxicity. We conducted a retrospective review of endometrial cancer patients treated at University of Virginia Health with adjuvant single-channel HDR VBT and CT-based 3D planning between 2015 and 2022. All patients received 21 Gy in 3 Fx prescribed to 5 mm depth twice weekly. The dose (D2cc, D1cc, and D0.1cc) to OARs (bladder, rectum, sigmoid colon, small bowel) were recorded. Acute and chronic GI and GU toxicities were graded according to the Common Terminology Criteria for Adverse Events. Boxplot and Wilcoxon-Mann-Whitney tests were used to compare dose thresholds between those with and without toxicity. 177 patients with stage I - IV endometrial cancer received mean D2cc to rectum, sigmoid, small bowel, and bladder of 5.7 Gy (range: 2.8 - 8.7), 3.3 Gy (0.6 - 9.3), 3.8 Gy (0 - 9.08), and 5.4 Gy (2.0 - 8.2). Overall, 6.2% (n = 11) of patients experienced new or worse acute GI and 7.3% (n = 13) acute GU toxicities. 18.6% (n = 33) of patients experience chronic GI and 15.8% (n = 28) chronic GU toxicities. The most common acute/chronic GI toxicities were diarrhea (3.4%/2.8%), abdominal pain (1.7%/9.0%), and constipation (1.1%/2.8%). The most common acute/chronic GU toxicities were urinary incontinence (2.3%/6.8%), dysuria (2.3%/1.1%), and urinary frequency (1.7%/4.0%). There were no acute grade 3 GI toxicities. The 1 (0.6%) patient with acute grade 3 GU symptoms including urinary incontinence resolved at follow up. There were no chronic grade 3 GU toxicities. The 1 (0.6%) patient with grade 3 chronic GI symptoms including constipation had baseline of grade 2. This study demonstrates there is no statistically significant difference (p>0.05) in dose levels between the group that experienced symptoms following HDR VBT and the group that did not experience symptoms (Figure 1). This retrospective study of adjuvant HDR VBT with 21 Gy in 3 Fx prescribed to 5mm depth for endometrial cancer shows overall low rates of GI and GU toxicity and no evidence that acute or late toxicity events are associated with higher dosimetric OAR values in rectum, sigmoid, small bowel, or bladder. Our single institution experience confirms that high point-doses to the OARs at the vaginal cuff when prescribing to 5 mm depth are safe and well tolerated. The optimal dose fractionation schedule for VBT that preserves high local control and minimizes toxicity is yet to be determined.