Optimization of Field Design in the Treatment of Rectal Cancer with Intensity Modulated Proton Beam Radiation Therapy: How Many Fields Are Needed to Account for Rectal Distension Uncertainty?

Abstract

Purpose: Preoperative chemoradiation represents the standard of care in patients with locally advanced rectal cancer. Robustness is often compromised in the setting of proton beam therapy owing to the sensitivity of proton particles to tissue heterogeneity, such as with intestinal gas. The ideal beam arrangement to mitigate the anatomic uncertainty caused by intestinal gas is not well defined. Methods and Materials: We developed pencil beam scanning plans using (1) 1-beam posteroanterior (PA) plans, (2) 2-beam with right and left posterior oblique (RPO and LPO) plans, (3) 3-beam with PA and opposed lateral plans, and (4) 5-beam with PA, RPO, LPO, and opposed lateral plans. We created 12 plans with robustness optimization and ran a total of 60 plan evaluations for varying degrees of intestinal gas distension to evaluate which plans would maintain clinical goals to the greatest degree. Results: A single PA beam resulted in considerable loss of target coverage to the clinical target volume prescribed 50 Gy (volume receiving 100% of the prescribed dose [V100%] 90% even with collapsed rectum to as low as 1 cm, simulating a patient treatment scenario without any rectal gas. Conclusions: A single PA beam should be avoided when using proton beam therapy for rectal cancer. RPO/LPO and PA/opposed lateral arrangements may both be considered; RPO/LPO is favored to reduce integral dose and avoid beams traversing the hips. In patients for whom the plan CT has rectal distension of ≥3 cm, resimulation or strategies to reduce intestinal gas should be strongly considered.