Gynecological Cancer

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TU_20_3515 - Variable Impact of Bowel Space Contouring Technique on Radiation Dose and Volume Assessments in Treatment Planning for Gynecologic Cancers

Tuesday, October 23
2:45 PM - 4:15 PM
Location: Innovation Hub, Exhibit Hall 3

Variable Impact of Bowel Space Contouring Technique on Radiation Dose and Volume Assessments in Treatment Planning for Gynecologic Cancers
A. Hoover1, A. Tejwani1,2, M. J. Tennapel1, M. Smith3, and A. M. Chen1; 1Department of Radiation Oncology, University of Kansas School of Medicine, Kansas City, KS, 2Cotton O'Neil Cancer Center - Stormont Vail Health, Topeka, KS, 3University of Kansas Medical Center, Kansas City, KS

Purpose/Objective(s): The small bowel is a relatively radiosensitive organ that can be associated with significant acute and late radiation toxicity during the treatment of gynecologic cancers. For radiation treatment planning, this normal structure is often contoured on computed tomography (CT) as “bowel space”, defined anatomically as the volume of peritoneal cavity that can be potentially occupied by small bowel. This definition is supported by the Quantitative Analysis of Normal Tissue Effects in the Clinic (QUANTEC) review of radiation dose/volume effects and used in recent cooperative group trials. Unlike other pelvic organs at risk (OAR), anatomic boundaries for the bowel space can be challenging to define leading to a degree of subjectivity. We hypothesized that variations in bowel space delineation among providers would be significantly greater than those for rectal and bladder contours, leading to inconsistent and/or inaccurate estimates of radiation dose/volume measures.

Materials/Methods: Ten board-certified radiation oncologists at a single academic medical center were asked to provide OAR contours in accordance with RTOG 1203 protocol guidelines for bowel space, bladder, and rectum on two distinct CT datasets (identified as A and B below) representing post-hysterectomy endometrial cancer patients. Dose volume histogram data was obtained for OAR contours based on a single intensity modulated radiation therapy plan, generated according to RTOG 1203 guidelines, for each respective CT data set. Cubic centimeters (cc) of bowel space, rectum, and bladder receiving 45 Gy or greater (V45) were obtained for each provided OAR contour. The Wilcoxon Rank Sum Test was used to explore differences among normalized means. Calculated p-values were one-tailed with <0.05 considered significant. Statistical tests were performed using SAS 9.3 (SAS Institute Inc., Cary, NC).

Results: Variations in OAR contours and DVH parameters were significantly greater for bowel space than for bladder or rectum. For dataset A, V45 for bowel space ranged from 164 to 385 cc (mean 245 ± 87.4 cc), V45 for bladder from 110 to 126 cc (mean 117 ± 6.3 cc), and V45 for rectum from 35 to 38 cc (mean 36 ± 1.4 cc). For dataset B, the V45 for bowel space ranged from 109 to 449 cc (mean 237 ± 120 cc), V45 for bladder from 95 to 122 cc (mean 122 ± 9.0 cc), and V45 for rectum from 17 to 19 cc (mean 17.6 ± 0.8 cc). Variations in normalized means of V45 from dataset A were significantly greater for bowel space when compared to bladder (p=0.02) and rectum (p=0.01). Similarly, significantly greater variation in V45 was observed for bowel space compared to bladder (p=0.01) and rectum (p<0.01) for dataset B.

Conclusion: Significant variability exists in how the bowel space OAR is practically delineated during treatment planning for gynecologic cancers. These discrepancies may potentially result in misleading radiation dose and volume assessments for the small bowel. The clinical impact of these findings warrant further study.

Author Disclosure: A. Hoover: None. A. Tejwani: None. M.J. Tennapel: None. M. Smith: None.

Andrew Hoover, MD

University of Kansas Medical Center

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