Treatment of non-small cell lung cancer patients with proton beam-based stereotactic body radiotherapy: Dosimetric comparison with photon plans highlights importance of range uncertainty

Purpose: Proton beam radiotherapy has been proposed for use in stereotactic body radiotherapy (SBRT) for early-stage non-small-cell lung cancer. In the present study, we sought to analyze how the range uncertainties for protons might affect its therapeutic utility for SBRT. Methods and Materials: Ten patients with early-stage non-small-cell lung cancer received SBRT with two to three proton beams. The patients underwent repeat planning for photon SBRT, and the dose distributions to the normal and tumor tissues were compared with the proton plans. The dosimetric comparisons were performed within an operational definition of high- and low-dose regions representing volumes receiving >50% and <50% of the prescription dose, respectively. Results: In high-dose regions, the average volume receiving ≥95% of the prescription dose was larger for proton than for photon SBRT (i.e., 46.5 cm ³ vs. 33.5 cm ³; p =.009, respectively). The corresponding conformity indexes were 2.46 and 1.56. For tumors in close proximity to the chest wall, the chest wall volume receiving ≥30 Gy was 7 cm ³ larger for protons than for photons (p =.06). In low-dose regions, the lung volume receiving ≥5 Gy and maximum esophagus dose were smaller for protons than for photons (p =.019 and p <.001, respectively). Conclusions: Protons generate larger high-dose regions than photons because of range uncertainties. This can result in nearby healthy organs (e.g., chest wall) receiving close to the prescription dose, at least when two to three beams are used, such as in our study. Therefore, future research should explore the benefit of using more than three beams to reduce the dose to nearby organs. Additionally, clinical subgroups should be identified that will benefit from proton SBRT.