Pancreatic cancer treated with SBRT: Effect of anatomical interfraction variations on dose to organs at risk.

BACKGROUND AND PURPOSE: Interfraction shape and position variations of organs at risk (OARs) may increase uncertainty in dose delivery during stereotactic body radiotherapy (SBRT), potentially leading to overirradiation or concessions in planned tumor dose and/or coverage to prevent clinical constraints violation. The aim of our study was to quantitatively analyze the impact of anatomical interfraction variations on dose to OARs in pancreatic cancer (PC) treated by SBRT using a CyberKnife with integrated CT-on-rails.

MATERIALS AND METHODS: Thirty-five PC patients treated with SBRT (40 Gy/5 fractions) underwent a CT-scan in treatment position before each of the first three fractions using the CT-on-rails system. OARs (stomach, duodenum, bowel) were manually delineated and concatenated to one structure (Gastro-Intestinal Organ, GIO). To overlay the planned dose distribution, fiducial-based alignment of the fraction CT with the planning CT was performed. Planned DVH parameters of the OAR were compared to the parameters calculated in the fractions CTs.

RESULTS: Compared to the treatment plan, the median V35, D2, D5, D10 and Dmax of the fraction CTs in the GIO was increased by 1.0 (IQR: 0.2-2.6), 4.4% (0.4-10.8), 2.3% (0.2-7.5), 3.3% (-0.4 to 7.1), and 12.0% (5.0-18.9) respectively. Median increase was statistically significant for all parameters in GIO and for V35 in all critical structures at Wilcoxon test.

CONCLUSIONS: Anatomical interfraction variations increase OAR dose during SBRT for pancreatic cancer daily imaging using integrated CT/CyberKnife may allow to implement strategies to reduce the risk of OAR overirradiation during pancreatic SBRT.