Beam angle comparison for distal esophageal carcinoma patients treated with intensity-modulated proton therapy.

Purpose: To compare the dosimetric performances of intensity-modulated proton therapy (IMPT) plans generated with two different beam angle configurations (the Right-Left oblique posterior beams and the Superior-Inferior oblique posterior beams) for the treatment of distal esophageal carcinoma in the presence of uncertainties and interplay effect. Methods and Materials: Twenty patients' IMPT plans were retrospectively selected, with 10 patients treated with the R-L oblique posterior beams (Group R-L) and the other 10 patients treated with the S-I oblique posterior beams (Group S-I). Patients in both groups were matched by their clinical target volumes (CTVs-high and low dose levels) and respiratory motion amplitudes. Dose-volume-histogram (DVH) indices were used to assess plan quality. DVH bandwidth was calculated to evaluate plan robustness. Interplay effect was quantified using four-dimensional (4D) dynamic dose calculation with random respiratory starting phase of each fraction. Normal tissue complication probability (NTCP) for heart, liver, and lung was calculated, respectively, to estimate the clinical outcomes. Wilcoxon signed-rank test was used for statistical comparison between the two groups. Results: Compared with plans in Group R-L, plans in Group S-I resulted in significantly lower liver D(mean)and lung V(30Gy[RBE])with slightly higher but clinically acceptable spinal cord D-max. Similar plan robustness was observed between the two groups. When interplay effect was considered, plans in Group S-I performed statistically better for heart D(mean)and V-30Gy[RBE], lung Dmean and V-5Gy[RBE], and liver D-mean, with slightly increased but clinically acceptable spinal cord D-max. NTCP for liver was significantly better in Group S-I. Conclusions: IMPT plans in Group S-I have better sparing of liver, heart, and lungs at the slight cost of spinal cord maximum dose protection, and are more interplay-effect resilient compared to IMPT plans in Group R-L. Our study supports the routine use of the S-I oblique posterior beams for the treatments of distal esophageal carcinoma.