Gastrointestinal Cancer

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SU_4_2033 - Defining Bowel and Nonsigmoid Bowel Dose Volume Constraints for Pelvic Radiation Therapy in GI Malignancies.

Sunday, October 21
1:15 PM - 2:45 PM
Location: Innovation Hub, Exhibit Hall 3

Defining Bowel and Nonsigmoid Bowel Dose Volume Constraints for Pelvic Radiation Therapy in GI Malignancies.
S. Otter1,2, L. Wedlake3, H. McNair4, R. Lawes3, C. Juman5, J. Andreyev6, A. Stewart2, and S. Gulliford7; 1University of Surrey, Guildford, United Kingdom, 2Royal Surrey County Hospital NHS Foundation Trust, Guildford, United Kingdom, 3The Royal Marsden NHS Foundation Trust, London, United Kingdom, 4The Institute of Cancer Research/The Royal Marsden NHS Foundation Trust, London, United Kingdom, 5University of Birmingham, Birmingham, United Kingdom, 6Department of Gastroenterology, Lincoln County Hospital, Lincoln, United Kingdom, 7The Institute of Cancer Research, London, United Kingdom

Purpose/Objective(s): Up to 50% of patients receiving radical radiotherapy (RT) to the pelvis will develop a chronic change in bowel habit which has a negative impact on quality of life. The aim of this study was to validate previously published constraints for bowel and non-sigmoid (NS) bowel (from bladder, cervical and prostate cancer patients) in patients receiving pelvic RT for gastrointestinal (GI) cancers and to explore deriving further dose constraints.

Materials/Methods: Patients with GI cancers recruited to a randomised controlled trial of dietary fiber in patients undergoing pelvic RT were included. The Inflammatory Bowel Disease Questionnaire – Bowel subset (IBDQB) was completed weekly during RT and at 1-year post-RT. Bowel and NS bowel were retrospectively contoured on the planning CT scans of patients with GI malignancies. The change from baseline IBDQB to nadir and baseline to 1 year were used to assess acute and late toxicity respectively. Patients were excluded if data was missing from these timepoints or if they had an improvement in IBDQB with RT as this was likely due to disease-related symptoms at baseline. Prescribed dose was 45-54Gy in 25-30 fractions. Doses were converted into the equivalent dose in 2Gy per fraction (EQD2) with an α/β ratio of 10 for acute and 3 for late toxicity. Wilcoxon rank sum test was used to compare the current data with previously published dose constraints (Sini, McDonald, Chopra) and to derive dose constraints.

Results: 106 GI patients were recruited. 80 had RT plans available. 13 were removed from the acute analysis and 27 from the late analysis due to improvement in IBDQB. 65 patients were assessable for acute and 38 for late toxicity after exclusion of patients with missing data. The vast majority of patients in this dataset, had dose volume histograms (DVH) below the published datasets and therefore none of those constraints predicted toxicity in these patients. Correlation matrices indicated that there was low correlation between the volumes of bowel and NS bowel receiving higher doses (~30 Gy) and volumes receiving lower doses (~15Gy). For late toxicity for bowel and NS bowel, a higher volume (cc) receiving very low doses was protective leading to smaller changes in IBDQB (e.g. V10bowel>303cc, p=0.041 and V10NS>210cc, p=0.015)). At higher doses, the 75th quartile for V44-V47 were significant for bowel. There were no statistically significant results for acute toxicity.
Toxicity Organ at risk Median change in IBDQB (IQR) Median volume (IQR) cc
Acute n=65 Bowel 13 (7-24) 307.4 (211.4-450.9)
NS Bowel 273.3 (138.4-373.6)
Late n=38 Bowel 6 (3-17) 303.9 (178.5-445.2)
NS Bowel 267.1 (141.0-374.3)

Conclusion: Larger absolute volumes of bowel and NS bowel predicted for less late toxicity as measured by IBDQB. For late bowel toxicity, V44-V47 was predictive of toxicity. The previously published dose constraints were greater than the DVH parameters delivered to the majority of the patients in this dataset and therefore tighter dose constraints are necessary for GI patients.

Author Disclosure: S. Otter: None. L. Wedlake: None. H. McNair: None. J. Andreyev: None. S. Gulliford: None.

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