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Dosimetric and radiobiological impact of abdominal compression on adjacent gastro-intestinal critical structures for patients treated with upper and mid-abdominal stereotactic body radiotherapy
Soumyajit Roy, Svetlana Kuznetsova, Kundan Thind, Petra Grendarova, Nicolas Ploquin and Richie S. Sinha

Objectives: We evaluated the dosimetric and radiobiological impact of abdominal compression (AC) on nearby gastrointestinal critical structures (GI-CS) and reported toxicities of patients treated with non-hepatic abdominal stereotactic body radiotherapy (SBRT).

Methods: Two sets of CT scans, planning scans with AC and pre-treatment diagnostic scans without AC (non-AC) were compared for patients treated with a prescription dose to planning target volume (PTV) ≥25 Gy/5-fractions at a single institution. Target volumes were delineated on both sets of scans and PTV was expanded isotropically by 2 cm (PTV+2) and 4 cm (PTV+4). All GI-CS were summated to create a composite CS (GI-lumen). Rigid registration of AC and non-AC scans was done using Velocity AI (Varian Medical Systems) to obtain dose distribution information. Lymann- Probit and logit models were used for radiobiological calculations. Toxicity scores were obtained from prospectively collected clinical data.

Results: A total of 12 patients were included. Mean PTV volumes were 190.3cc and 196.4cc with AC and non-AC (p=0.95). Significant improvement in V30 of GI-lumen was seen with AC (0.11cc vs. 4.97cc, p=0.04). There were no differences in the normal tissue complication probabilities of the individual GI-CS or the summary indices except a notable trend towards better NTCP for small bowel late effects with AC (0.21% vs. 2.45%; p=0.055). Three patients had acute grade-1 anorexia, one patient had acute grade-2 gastritis. There was no grade ≥3 GI toxicity. At a median follow-up of 2.6 years, total of 8/12 (66.7%) patients developed local recurrence of whom 4 (33.3%) had isolated local recurrence.

Conclusion: Use of AC did not result in any dosimetric or radiobiological inferiority for GI-CS. The current cohort completed their treatment with minimal toxicity.

Keywords: stereotactic body radiotherapy, abdominal compression, NTCP, gastrointestinal toxicity, abdominal SBRT

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