Assessing proton plans with 3 different beam lines vs photon plans for early-stage lung cancer

To compare proton plans (IMPT) to VMAT plans and intercompare proton plans using 3 different spot sizes with robustness: cyclotron-generated proton beams (CPB) (σ: 2.7-7.0 mm), linear accelerator proton beams (LPB) (σ: 2.9-5.5 mm), and linear accelerator proton mini beams (LPMB) (σ: 0.9-3.9 mm) for the treatment of early-stage lung cancer. Twenty-two lesions from a total of twenty patients with early-stage lung cancer, originally treated with SBRT, were replanned using CPBs, LPBs, LPMBs, and VMAT using the same treatment planning system and dose calculation algorithm. The average intensity projected CTs (AIP-CT) were used for planning and 3D robust optimization was used for all proton plans. Conformity index (CI), homogeneity index (HI), R₅₀, lung V_20 Gy, and mean lung dose were compared among all proton plan types and with VMAT plans. Set-up uncertainties of ±5 mm and ±3.5% range uncertainty were included in the IMPT robust optimization and evaluation, using V_100%Rx > 98% of the ITV. The Wilcoxon signed-rank test was used to evaluate statistical differences between VMAT plans and all proton plan types. When compared to VMAT plans, all proton plans generally show improvement in CI, HI, Lung V_20 Gy, Mean lung dose, and R₅₀. The LPMB plans showed the most improvement from VMAT plans. Comparison between CPB and linear accelerator proton plans showed statistical significance (p < 0.05). R₅₀ and mean lung dose for the CPB, LPB and LPMB plans were 3.6 ± 0.9, 3.1 ± 0.8 and 2.6 ± 0.6; 2.2 ± 1.1 Gy, 1.9 ± 1 Gy and 1.6 ± 0.9 Gy, respectively (p < 0.05). The mean R₅₀ and mean lung dose from the VMAT plans were 4.1 ± 0.4 and 3.8 ± 2 Gy, respectively. The V_20 Gy (%) of lung and mean lung dose were improved across all proton plans when compared with those of VMAT plans. When evaluated for robustness in the worst-case scenario at V_100%Rx of the ITV > 98%, average ITV coverage of 98.6 ± 0.3%, 98.6 ± 0.6%, and 98.9 ± 0.6% were achieved for CPB plans, LPB plans, and LPMB plans, respectively. With decreased spot size, the LPB and LPMB plans are excellent alternatives to VMAT and cyclotron-generated proton plans with reduced dose to normal tissue and improved plan quality for early-stage lung cancer treatments.