Impact of Real-Time Image Gating on Spot Scanning Proton Therapy for Lung Tumors: A Simulation Study

Abstract

To investigate the effectiveness of real-time-image gated proton beam therapy for lung tumors and to establish a suitable size for the gating window (GW). A proton beam gated by a fiducial marker entering a preassigned GW (as monitored by 2 fluoroscopy units) was used with 7 lung cancer patients. Seven treatment plans were generated: real-time-image gated proton beam therapy with GW sizes of ±1, 2, 3, 4, 5, and 8mm and free-breathing proton therapy. The prescribed dose was 70Gy (relative biological effectiveness)/10 fractions to 99% of the target. Each of the 3-dimensional marker positions in the time series was associated with the appropriate 4-dimensional computed tomography phase. The 4-dimensional dose calculations were performed. The dose distribution in each respiratory phase was deformed into the end-exhale computed tomography image. The D99 and D5 to D95 of the clinical target volume scaled by the prescribed dose with criteria of D99>95% and D5 to D95<5%, V20 for the normal lung, and treatment times were evaluated. Gating windows≤±2mm fulfilled the CTV criteria for all patients (whereas the criteria were not always met for GWs≥±3mm) and gave an average reduction in V20 of more than 17.2% relative to free-breathing proton therapy (whereas GWs≥±4mm resulted in similar or increased V20). The average (maximum) irradiation times were 384seconds (818seconds) for the ±1-mm GW, but less than 226seconds (292seconds) for the ±2-mm GW. The maximum increased considerably at ±1-mm GW. Real-time-image gated proton beam therapy with a GW of ±2mm was demonstrated to be suitable, providing good dose distribution without greatly extending treatment time.