Dosimetric analysis of organs at risk during expiratory gating in stereotactic body radiation therapy for pancreatic cancer.

PURPOSE

To determine how the respiratory phase impacts dose to normal organs during stereotactic body radiation therapy (SBRT) for pancreatic cancer.

METHODS AND MATERIALS

Eighteen consecutive patients with locally advanced, unresectable pancreatic adenocarcinoma treated with SBRT were included in this study. On the treatment planning 4-dimensional computed tomography (CT) scan, the planning target volume (PTV), defined as the gross tumor volume plus 3-mm margin, the duodenum, and the stomach were contoured on the end-expiration (CTexp) and end-inspiration (CTinsp) phases for each patient. A separate treatment plan was constructed for both phases with the dose prescription of 33 Gy in 5 fractions with 95% coverage of the PTV by the 100% isodose line. The dose-volume histogram (DVH) endpoints, volume of duodenum that received 20 Gy (V20), V25, and V30 and maximum dose to 5 cc of contoured organ (D5cc), D1cc, and D0.1cc, were evaluated.

RESULTS

Dosimetric parameters for the duodenum, including V25, V30, D1cc, and D0.1cc improved by planning on the CTexp compared to those on the CTinsp. There was a statistically significant overlap of the PTV with the duodenum but not the stomach during the CTinsp compared to the CTexp (0.38 ± 0.17 cc vs 0.01 ± 0.01 cc, P=.048). A larger expansion of the PTV, in accordance with a Danish phase 2 trial, showed even more overlapping volume of duodenum on the CTinsp compared to that on the CTexp (5.5 ± 0.9 cc vs 3.0 ± 0.8 cc, P=.0003) but no statistical difference for any stomach dosimetric DVH parameter.

CONCLUSIONS

Dose to the duodenum was higher when treating on the inspiratory than on the expiratory phase. These data suggest that expiratory gating may be preferable to inspiratory breath-hold and free breathing strategies for minimizing risk of toxicity.