Gastrointestinal Cancer

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SU_16_2158 - Visceral Adipose Ratio as a Novel Predictor for Acute Bowel Toxicity in Patients Receiving Pelvic Radiation

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

Visceral Adipose Ratio as a Novel Predictor for Acute Bowel Toxicity in Patients Receiving Pelvic Radiation
R. G. Ganju, M. J. Tennapel, A. M. Chen, and A. Hoover; Department of Radiation Oncology, University of Kansas School of Medicine, Kansas City, KS

Purpose/Objective(s): The optimal method of contouring bowel as an organ at risk is unknown. The “bowel bag” method delineates the entire peritoneal space that may be occupied by small bowel. One limitation of this method is that it does not distinguish visceral adipose tissue from bowel. As a result, inconsistent dose and volume relationships have been reported. We evaluated the relative volume of visceral adipose tissue within the peritoneal space (visceral adipose ratio), and applied this as a correction factor to bowel bag dosimetric measures; we hypothesized this would correlate more strongly with acute bowel toxicity than current constraints using bowel bag alone.

Materials/Methods: Twenty consecutive patients receiving external beam radiation to the pelvis for diagnoses of gynecologic and gastrointestinal cancer at a single institution from July 2017 to December 2017 were included. Pelvic radiation was delivered with intensity modulated or 3D conformal techniques. Patients receiving para-aortic radiation were excluded. Simulation computed tomography (CT) scans were retrieved and bowel bags were retrospectively contoured to comply with RTOG contouring guidelines. Dosimetric data regarding V45 (cc) and V40 (%) cutoffs used by recent protocols, was collected. Visceral adipose tissue was auto-contoured using published Hounsfield Unit criteria. Visceral adipose ratio was defined as the volume of non-adipose tissue within the peritoneal space divided by the total bowel bag volume. This was multiplied by the bowel bag dosimetric values to correct for the volume of adipose tissue within the bowel bag. Bowel toxicity was assessed using EPIC Bowel Assessment Scores collected from the patient medical records comparing baseline scores from initiation of treatment to the 4th or 5th week of radiation. Spearman correlation coefficients were used to identify significant declines in EPIC Bowel Scores.

Results: Seventeen patients met criteria for inclusion. Median age was 60 (range, 46 to 78). Seventy-one percent of patients were receiving treatment for gynecologic cancers. Nine patients (53%) were treated adjuvantly, 6 patients (35%) definitively, and 2 patients (12%) preoperatively. Median decline in EPIC Bowel Score was 18 points from baseline (range -45 to +7). When evaluating V45 (cc), standard bowel bag dosimetric values did not correlate with acute toxicity (p=0.73), though there was a trend with corrected values (p=0.06). When evaluating V40 (%), the corrected bowel bag values significantly correlated with acute toxicity (p=0.03), while standard bowel bag values did not (p=0.09).

Conclusion: The incorporation of visceral adipose ratio into bowel bag dosimetric measures results in significantly more accurate means of predicting for acute bowel toxicity than current standard practice. Longer follow-up is needed to assess whether similar findings are observed with late toxicity.

Author Disclosure: R.G. Ganju: None. M.J. Tennapel: None. A.M. Chen: None.

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