PO37

Abstract

Purpose Treating multiple fractions of cervical HDR BT with a single implant can reduce the overall package time of the procedure and reduces patient anesthesia and radiation oncology personnel resources due to reduced numbers of plans generated. Intracavitary (IC) and Intracavitary/Interstitial (IC/IS) applicators can shift between brachytherapy fractions, and bladder and bowel filling can cause interfraction variations of OAR dosimetry. Interfraction dosimetry variations of single implants have not been well-quantified and it is unclear whether an adaptive replan is required for each fraction. Methods and Materials 10 patients with locally advanced cervical cancer treated with HDR BT were included in this study: 5 consecutive IC (T&R applicator) and 5 consecutive hybrid IC/IS (T&R with interstitial needles). Each patient received 45Gy in 25 fractions of EBRT followed by 28Gy in 4 fractions of HDR MR-IGABT with Alatus Vaginal Balloon packing (Radiadyne, Latham, NY). BT was administered using a two-implant technique. After the first implant, fraction 1 was planned and delivered using CT and MRI. After overnight admission, a new CT was obtained prior to fraction 2 and rigidly registered to the fraction 1 CT using the applicator and cervix as landmarks. The bladder, rectum, and sigmoid colon were contoured and the treatment plan for fraction 1 was copied onto the fraction 2 CT. The same procedure was applied for fractions 3 and 4 with a new implant followed by a CT/MR-based plan generated for fraction 3, and an evaluation CT for fraction 4. The absolute interfraction change in EQD2 and cumulative EBRT+BT was calculated using an alpha/beta value of 3; all doses are reported in EQD2. Differences in interfraction variability between IC and IC/IS plans were investigated. Results The mean HRCTV volume was 17.8cc (8.1-27cc) for IC plans and 32.6cc (19.6-44.4cc) for IC/IS plans. All IC/IS plans had an average fraction of total dwell time of less than 20% in needles. The D2cc for bladder, rectum, and sigmoid were below the soft/hard EMBRACE II guidelines for all plans (80/90Gy, 65/75Gy, 65/75Gy, respectively). The mean single BT fraction D2cc to bladder, rectum, and sigmoid were 7.9Gy, 2.9Gy and 4.2Gy. The bladder D2cc displayed the greatest interfraction variability, with an average absolute interfraction difference of 1.8±1.3Gy, while rectum and sigmoid were 0.6±0.5Gy and 0.7±0.7Gy. Differences between IC and IC/IS plans were also compared. The mean single fraction D2cc to bladder was lower for IC/IS plans (7.2±1.0Gy) as compared to IC plans (8.7±0.4Gy). The mean single fraction D2cc to rectum and sigmoid were similar between the two methods (2.8±0.3Gy and 4.3±0.3Gy for IC/IS versus 2.9±0.3Gy and 4.1±0.3Gy for IC). The absolute interfraction D2cc difference was minimal with both applicators (Figure 1). The interfraction D2cc difference to bladder, rectum, and sigmoid was similar for IC/IS plans (1.8±1.1Gy, 0.6±0.5Gy and 0.7±0.8Gy) as compared to IC plans (1.8±1.4Gy, 0.5±0.4, 0.6±0.7Gy). Conclusion Overall, the dose to the organs at risk was within the acceptable limits for all fraction plans. The dose differences between the two applicators were minimal, demonstrating that an overnight treatment procedure using IC and IC/IS plans is a viable option. Treating multiple fractions of cervical HDR BT with a single implant can reduce the overall package time of the procedure and reduces patient anesthesia and radiation oncology personnel resources due to reduced numbers of plans generated. Intracavitary (IC) and Intracavitary/Interstitial (IC/IS) applicators can shift between brachytherapy fractions, and bladder and bowel filling can cause interfraction variations of OAR dosimetry. Interfraction dosimetry variations of single implants have not been well-quantified and it is unclear whether an adaptive replan is required for each fraction. 10 patients with locally advanced cervical cancer treated with HDR BT were included in this study: 5 consecutive IC (T&R applicator) and 5 consecutive hybrid IC/IS (T&R with interstitial needles). Each patient received 45Gy in 25 fractions of EBRT followed by 28Gy in 4 fractions of HDR MR-IGABT with Alatus Vaginal Balloon packing (Radiadyne, Latham, NY). BT was administered using a two-implant technique. After the first implant, fraction 1 was planned and delivered using CT and MRI. After overnight admission, a new CT was obtained prior to fraction 2 and rigidly registered to the fraction 1 CT using the applicator and cervix as landmarks. The bladder, rectum, and sigmoid colon were contoured and the treatment plan for fraction 1 was copied onto the fraction 2 CT. The same procedure was applied for fractions 3 and 4 with a new implant followed by a CT/MR-based plan generated for fraction 3, and an evaluation CT for fraction 4. The absolute interfraction change in EQD2 and cumulative EBRT+BT was calculated using an alpha/beta value of 3; all doses are reported in EQD2. Differences in interfraction variability between IC and IC/IS plans were investigated. The mean HRCTV volume was 17.8cc (8.1-27cc) for IC plans and 32.6cc (19.6-44.4cc) for IC/IS plans. All IC/IS plans had an average fraction of total dwell time of less than 20% in needles. The D2cc for bladder, rectum, and sigmoid were below the soft/hard EMBRACE II guidelines for all plans (80/90Gy, 65/75Gy, 65/75Gy, respectively). The mean single BT fraction D2cc to bladder, rectum, and sigmoid were 7.9Gy, 2.9Gy and 4.2Gy. The bladder D2cc displayed the greatest interfraction variability, with an average absolute interfraction difference of 1.8±1.3Gy, while rectum and sigmoid were 0.6±0.5Gy and 0.7±0.7Gy. Differences between IC and IC/IS plans were also compared. The mean single fraction D2cc to bladder was lower for IC/IS plans (7.2±1.0Gy) as compared to IC plans (8.7±0.4Gy). The mean single fraction D2cc to rectum and sigmoid were similar between the two methods (2.8±0.3Gy and 4.3±0.3Gy for IC/IS versus 2.9±0.3Gy and 4.1±0.3Gy for IC). The absolute interfraction D2cc difference was minimal with both applicators (Figure 1). The interfraction D2cc difference to bladder, rectum, and sigmoid was similar for IC/IS plans (1.8±1.1Gy, 0.6±0.5Gy and 0.7±0.8Gy) as compared to IC plans (1.8±1.4Gy, 0.5±0.4, 0.6±0.7Gy). Overall, the dose to the organs at risk was within the acceptable limits for all fraction plans. The dose differences between the two applicators were minimal, demonstrating that an overnight treatment procedure using IC and IC/IS plans is a viable option.