Spot-scanned pancreatic stereotactic body proton therapy: A dosimetric feasibility and robustness study

Purpose: We explored the dosimetric potential of spot-scanned stereotactic body proton therapy (SBPT) for pancreatic cancer.

Methods: We compared SBPT to stereotactic body intensity-modulated radiotherapy (SB-IMRT) in 10 patients. We evaluated 3 variables in SBPT planning: (1) 4 and 6 mm spot size; (2) single vs. multi-field optimization (SFO vs. MFO); and (3) optimization target volume (OTV) expansion. Robustness analysis was performed with unidirectional isocenter shifts of ±3 mm in x, y, and z and ±3% stopping power uncertainties.

Results: SBPT plans had lower V10Gy for the stomach and small and large bowels. Under static robustness, a 5 mm OTV and SFO-6 mm spot size represented the best compromise between target and normal structure. A 4-mm spot-size and 3 mm OTV resulted in significant target underdosing with deformable dose accumulation analysis.

Conclusions: This study provides a critical basis for clinical translation of spot size, optimization technique, and OTV expansion for pancreatic SBPT.