The dependency of beam-on-time efficiency on respiration period and layer switching time for respiratory gating proton therapy of liver cancer

Background: Patient's respiratory motion can significantly affect the treatment accuracy and efficiency of radiotherapy, making effective respiratory control essential. One option is respiratory gating therapy, which allows treatment only during specific phases of the respiratory cycle but typically increases treatment time. Therefore, optimizing beam-on-time (BoT) efficiency in respiratory gating therapy is needed. Purpose: This study examines the BoT efficiency of respiratory gating proton line-scanning therapy for liver cancer patients, with the aim of minimizing treatment time. Methods: We calculated the BoT of proton beams for 101 liver cancer patients, by using a simulated respiratory signal. BoT efficiency was defined as the ratio of BoT to the total treatment time, which is the sum of BoT, layer switching times (T_LS) and gating-off times for each layer. We varied three parameters to measure their effect on BoT efficiency: respiratory period (T_R), T_LS, and gating window level. Results: We observed that BoT efficiency reached a maximum at a specific respiratory period, T_R^M, that showed a linear correlation with T_LS. With very short T_LS values, BoT efficiency did not vary with changes in T_R. However, when T_LS exceeded 1 s, the respiratory period that yielded peak BoT efficiency, T_R^M, was closely aligned with T_LS. Furthermore, this alignment displays a slope close to 1. By comparing results across different simulated respiratory signals, we found our conclusions were insensitive to the specific shape of the respiratory signal. Conclusions: This study presents two key strategies for reducing total treatment time: (1) for the proton therapy machine with very short T_LS, BoT efficiency is less of a concern; and (2) for machines with a T_LS of around 2 s, optimal BoT efficiency is achieved with a respiration period between 2.5 and 3 s. It is important to note that the exact optimal value may vary depending on patient-specific factors such as target size.