Purpose: Pancreatic cancer is a radioresistant malignancy with few treatment options. Proton therapy may serve as an efficacious treatment due to its high conformality and increased RBE. We investigated the feasibility of hypofractionated pancreas radiation treatment using intensity modulated proton therapy (IMPT).
Methods: We identified 10 patients with locally advanced pancreatic cancer previously treated with hypofractionated stereotactic body radiation therapy using 5-field IMRT plans (67.5 Gy in 15 fractions, high dose PTV). IMPT plans were generated in Raystation using Monte Carlo dose calculation algorithm. IMPT plans used a 3-field design (2 posterior oblique and 1 lateral oblique fields). Robustness optimizations was performed with 3-mm isocenter positional uncertainty and ±3.5% range uncertainty to cover the GTV (21 perturbations). A high (67.5 Gy) and low dose PTV (either 37.5 or 45 Gy) approach was used with robustness evaluations (±3.5% range uncertainty). Note that high-dose PTV coverage was sacrificed for luminal OAR sparing, when necessary. Target coverage (D99% of GTV & D95% PTVs) and OAR constraint doses (stomach, duodenum, small bowel) of the IMPT plan were compared to the IMRT plan.
Results: The median GTV D99% dose difference between IMPT and IMRT plans was 0.36 Gy (range:-1.36 to 4.45 Gy), indicating higher coverage on most IMPT plans. The median PTV D95% dose difference for the high- and low-dose PTVs were 4.67 Gy and -1.22 Gy, respectively. IMPT plans resulted in lower doses to OARs, with median D1.0cm3 dose differences of 16.30 Gy, 1.35 Gy, and 0.47 Gy for the stomach, duodenum, and small bowel, respectively.
Conclusion: Hypofractionated IMPT is feasible for pancreatic cancer treatment. Future studies are required to understand how daily GI anatomical changes affect IMPT dose distributions.