2983 - Radiation Doses to Small Bowel, Sigmoid and Anal Canal during Preoperative Radiation for Locally Advanced Rectal Cancer: Do They Impact Long-Term Bowel Function?

T. J. Forbes¹, M. K. Rooney², J. Niedzielski³, R. M. Salazar⁴, and E. Holliday⁵; ¹University of Texas Houston School of Medicine, Houston, TX, ²Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, ³The University of Texas MD Anderson Cancer Center, Houston, TX, ⁴Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, ⁵Department of Gastrointestinal Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX

Purpose/Objective(s): Low anterior resection syndrome (LARS) can occur after surgical resection of rectal cancer and is more frequent after preoperative radiation (RT). We hypothesized RT dose to the sigmoid colon and anal sphincter complex, in addition to the small bowel, could impact the function of the postoperative anatomy. In this study, we retrospectively evaluated radiation plans of patients who did and did not experience LARS syndrome after multimodality therapy for locally advanced rectal cancer and sought to identify potential dosimetric predictors of LARS. Materials/

Methods: We identified patients with rectal cancer treated with long-course chemoRT (LC-CRT; 50.4Gy in 28 fractions) at our institution from 2016–2020 who were alive without disease. We administered a patient-reported outcomes (PRO) survey that included the LARS instrument for patients without an ostomy at the time of the survey. We then accessed their RT treatment plans and contoured the sigmoid colon, small bowel and the anal sphincters (approximated as two centimeters proximal to the anal verge). We collected mean dose, maximum dose and V5-V50 Gy at 5 Gy increments for each structure. We then performed a logistic regression to evaluate the potential relationships between the dosimetric variables and overall LARS score, as well as the individual LARS symptom items for flatus, leakage, frequency, clustering and urgency.
Results: Of 110 patients treated with preoperative-intent LC-CRT who did not have an ostomy at the time of survey, 57 responded (51.8%). Sixteen had a complete clinical response and did not have surgery. Thirty-nine were treated with a low anterior resection and were included in this analysis. The median [IQR] interval between RT and survey was 13.5 [29.7-49.0] months. The median [IQR] age of patients was 55 [49.5-62.5], 24 (61.5%) were male, 31 (79.5%) had T3 tumors, 35 (89.7%) had N+ disease, 34 (87.2%) were treated with 3D conformal RT and 36 (92.3%) were treated in the prone position. The median [IQR] LARS score was 34 [29-39], and 27 (69.2%) qualified as Major LARS (score 30-42). Twenty-four (61.5%) patients had incontinence of flatus at least once a week, 27 (69%) experienced incontinence/leakage of liquid stool, 18 (46.2%) had 4-7 bowel movements (BM) per day, 5 (12.8%) had >7 BMs per day, 31 (79.5%) had to move their bowels within an hour of the last BM. All but one patient (97.4%) had urgent BMs such that they have to rush to the toilet, and 23 (59.0%) had these urgency episodes more than once per week. None of the dosimetric variables significantly correlated with numeric LARS score, Major LARS category, or any of the LARS symptom items.
Conclusion: We were unable to identify small bowel, sigmoid or anal sphincter dosimetric predictors of LARS score or major LARS category in this cohort of patients treated with preoperative LC-CRT. Further study is warranted to explore the potential relationship between radiation dose to key pelvic OARs and patient-reported functional outcomes.