The Potential Benefit of Adaptive Intensity Modulated Proton Therapy in Nasopharyngeal Carcinoma: Planning Comparison Study

The potential advantages of adaptive intensity-modulated proton therapy(A-IMPT) when compared with adaptive intensity-modulated X-ray therapy (A-IMXT) for nasopharyngeal carcinomas (NPC) has not been reported. The purpose of this study is to investigate this using simulation and deformable image registration (DIR) with the data of patients who had been treated with A-IMXT. The actual A-IMXT plans and simulation A-IMPT plans were compared for 6 patients with NPC. In the actual A-IMXT, the 1st plan was generated using the CT taken before treatment and the 2nd plan using the CT at the third or fourth week of radiation treatment. In the 1st plan, 46 Gy was to be delivered to the CTV46 (low risk region including GTV and elective lymph node region) in 23 fractions. In the 2nd plan, 24 Gy was to be delivered to the CTV70 (high risk region including GTV) in 12 fractions. Seven fields were used, the CTV to PTV margin was 3mm, and 95% of the volume of the PTV received the prescribed dose both in the 1st and 2nd plans in the A -IMXT. In the present simulation study, the A-IMPT plans were generated using an RTP system in the same way as the A-IMXT. Setup uncertainties of 3 mm and a range uncertainty of 3.5% of the nominal ranges of the beams were used in a robust optimization of the same CTV as of the A-IMXT. Both in 1st and 2nd A-IMPT plans, three fields were used and 99% of the volume of the CTV received the prescribed dose. The DIR software was used for dose accumulation in the 1st and 2nd plans with both A-IMXT and A-IMPT. To evaluate the CTV coverage, the Conformity Index (CI) and the Homogeneity Index (HI) were calculated for the CTV70 in both plans. The mean dose to the oral cavity and larynx were also evaluated. There were no significant differences in the mean HI (A-IMPT; mean±SD =1.02±0.01 vs A-IMXT; 1.03±0.01) (p=0.060) and a statistically significant difference in CI (A-IMPT; 1.57±0.05 vs A-IMXT; 2.13±0.42) of the CTV70 suggesting better conformity in A-IMPT(p=0.009). The mean dose to the oral cavity of A-IMPT (13.0±5.0Gy) was statistically lower than A-IMXT(44.5±5.4Gy)(p<0.001), The mean dose to the larynx of A-IMPT (26.3±7.7Gy) was also statistically lower than that in A-IMXT(41.1±6.7Gy)(p<0.001). Compared with A-IMXT, A-IMPT has the potential benefit of sparing the oral cavity and larynx better and with better conformity and equivalent homogeneity in the CTV of NPC.