Quantifying and assessing the dosimetric impact of changing gas volumes throughout the course of VMAT radiation therapy of upper gastrointestinal tumors

Purpose: This retrospective patient study assessed the consistency of abdominal gas presence throughout radiation therapy for patients with upper gastrointestinal cancer and determined the impact of variations in gas volume on the calculated dose distribution of volumetric modulated arc therapy. Methods and Materials: Eight patients with pancreatic cancer were included for analysis. A plan library consisting of 3 reference plans per patient (Ref0.0, Ref0.5, and Ref1.0) was created based on planning computed tomography (CT) with density overrides of 0.0, 0.5, and 1.0 applied to gas volumes, respectively. Corresponding cone beam CT (CBCT) data sets were obtained and density overrides were applied to enable fractional dose calculation. Variation in gas volume relative to initial volume determined from CT was assessed. Dose metrics for targets and organs at risk were compared between the accumulated CBCT dose and the planned dose of the 3 reference plans for each patient. Results: There was a significant decrease in gas present from CT to treatment CBCT, with a mean decrease in volume of 48.6% for the entire cohort. Dosimetrically, all accumulated target and organ-at-risk parameters, aside from the kidneys, exhibited the smallest mean deviation from the Ref0.0 plan and largest mean deviation from the Ref1.0 plan. A statistically significant difference in mean accumulated dose to Ref0.0 and Ref1.0 was observed for the dose delivered to 95% of the planning target volume. Conclusions: Significant variation in gas volumes from CT to treatment can occur throughout volumetric modulated arc therapy for pancreatic cancer. Through the use of a plan library, it was determined that initial assessment of a patient’s treatment plan with an assigned gas density of 0.0 provided the most accurate prediction of the accumulated dose.