3169 - Characterization of Involuntary Mobility Motion-Induced Dosimetric Variation in Gastrointestinal Organs in High-Dose Rate Gynecologic Brachytherapy

Purpose/Objective(s): This study aims to characterize dose variation in gastrointestinal (GI) organs caused by involuntary mobility motion in High-Dose Rate (HDR) Gynecologic (GYN) Brachytherapy. Involuntary mobility motion may lead to time-varying near-contact or overlapping between the highly radiosensitive GI organs, such as sigmoid and rectum, and target volume; however, the resulting dosimetric impact has not been fully characterized. We present an innovative framework to thoroughly characterize the interplay between time-dependent involuntary mobility motion and discrete spatiotemporal dose accumulation in HDR delivery, as well as the resulting dose variations on the GI organs.

Materials/Methods: We selected six patients with GYN cancer who underwent combined radiotherapy consisting of 45Gy of external beam radiation therapy to the pelvis, followed by HDR brachytherapy prescribed at 28Gy in 4 fractions. We generated a continuous and topology-aware representation of three involuntary mobility motions (Peristalsis, High Amplitude Propagating Contractions, and Rhythmic segmentations) of the rectum and sigmoid, and simulated a stochastic sequence of these motion events encompassing 300 consecutive frames over a 10-min duration. We developed a dose reconstruction framework that simulated the synchronized interplay between the involuntary motion sequence and HDR dose delivery, identified dwell positions/times and calculated TG43 dose on each motion frame, and created accumulated dose on planning CT via our in-house deformable image registration pipeline. We performed 20 simulations for each patient and calculated the D90 of the HRCTV and D2cc of sigmoid and rectum and the EQD2 of all three structures.

Results: We observed the coverage loss for HRCTV and dose increase in OARs. For rectum and sigmoid, D2cc rose on average by 0.5 Gy/fraction and 0.4 Gy/fraction, respectively, with 75th percentile increases of 0.6 Gy/fraction and 0.7 Gy/fraction. Similar patterns were observed for EQD2, the mean EQD2 of rectum and sigmoid were elevated by 5.1 and 3.2Gy, respectively, with 75^th percentile increases of 7.7Gy and 4.3Gy. For the HRCTV, D90 and its corresponding EQD2 also demonstrated coverage loss compared with the original plan. The mean and 75^th percentile of D90 were reduced by 0.5Gy/fraction and 0.2Gy/fraction. The corresponding EQD2s were reduced by 4.1 Gy and 1.3 Gy,

Conclusion: We demonstrated that involuntary mobility motion of GI organs may lead to significant variation in near-maximum dose of OARs in GYN HDR brachytherapy.