PO24

Purpose To evaluate the dosimetric impact of bladder fill change between the time of simulation and treatment delivery. Materials and Methods This dosimetric study was completed with 10 individual high-dose-rate brachytherapy implants for cervical cancer patients (7 Gy/fx). Eight implants were performed with the Venezia and two with the Geneva (Elekta Brachy, Veenendaal, The Netherlands). An average of 8.0±1.8 interstitial needles were used. Each implant was imaged at two time point (T0 & T1): once the implant was completed (CT, T0) and approximately an hour later (MR, T1). For this study, optimized treatment plans were generated using the T0 scan. Organs at risk were also contoured on the T1 scan and the unmodified original plan was then applied to the new anatomy (re-digitized, but same dwell times) to assess the impact of anatomical changes to the dosimetrics. EQD2 D2cc for all OARs was calculated assuming an alpha-beta ratio of 3 Gy and a prescription dose of 7 Gy per fraction. Results The mean ± SD bladder fill volume at time, T0, was 222±113 cm3. The bladder fill increased between -100 and +225 cm3 at T1. Changes in EQD2 D2cc to the bladder, rectum, sigmoid and bowel between T0 and T1 are plotted in Figure 2. The impact on EQD2 D2cc changes due to increases in bladder fill volume correlates highly for sigmoid (-0.75), and weakly for bladder (+0.31) and bowel (-0.20). For the rectum, EQD2 D2cc changes are negligibly correlated with respect to bladder fill changes. Conclusions Increases in bladder volume tend to decrease GI (rectum, sigmoid, bowel) OAR doses while increasing dose to the bladder. Ensuring the bladder fill does not decrease at the time of treatment is paramount for protecting GI OARs which have much lower dose limits. Increases in bladder volume should be weighed against the remaining dose tolerance budget for the bladder. Future work will involve acquire more data which may enable the development of quantitative model for predicting patient-specific dosimetric changes based on bladder fill changes. To evaluate the dosimetric impact of bladder fill change between the time of simulation and treatment delivery. This dosimetric study was completed with 10 individual high-dose-rate brachytherapy implants for cervical cancer patients (7 Gy/fx). Eight implants were performed with the Venezia and two with the Geneva (Elekta Brachy, Veenendaal, The Netherlands). An average of 8.0±1.8 interstitial needles were used. Each implant was imaged at two time point (T0 & T1): once the implant was completed (CT, T0) and approximately an hour later (MR, T1). For this study, optimized treatment plans were generated using the T0 scan. Organs at risk were also contoured on the T1 scan and the unmodified original plan was then applied to the new anatomy (re-digitized, but same dwell times) to assess the impact of anatomical changes to the dosimetrics. EQD2 D2cc for all OARs was calculated assuming an alpha-beta ratio of 3 Gy and a prescription dose of 7 Gy per fraction. The mean ± SD bladder fill volume at time, T0, was 222±113 cm3. The bladder fill increased between -100 and +225 cm3 at T1. Changes in EQD2 D2cc to the bladder, rectum, sigmoid and bowel between T0 and T1 are plotted in Figure 2. The impact on EQD2 D2cc changes due to increases in bladder fill volume correlates highly for sigmoid (-0.75), and weakly for bladder (+0.31) and bowel (-0.20). For the rectum, EQD2 D2cc changes are negligibly correlated with respect to bladder fill changes. Increases in bladder volume tend to decrease GI (rectum, sigmoid, bowel) OAR doses while increasing dose to the bladder. Ensuring the bladder fill does not decrease at the time of treatment is paramount for protecting GI OARs which have much lower dose limits. Increases in bladder volume should be weighed against the remaining dose tolerance budget for the bladder. Future work will involve acquire more data which may enable the development of quantitative model for predicting patient-specific dosimetric changes based on bladder fill changes.