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

ABSTRACT

PURPOSE: 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). METHODS AND MATERIALS 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 8 mm and free-breathing proton therapy. The prescribed dose was 70 Gy (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. RESULTS: Gating windows ≤ ±2 mm fulfilled the CTV criteria for all patients (whereas the criteria were not always met for GWs ≥ ±3 mm) and gave an average reduction in V20 of more than 17.2% relative to free-breathing proton therapy (whereas GWs ≥ ±4 mm resulted in similar or increased V20). The average (maximum) irradiation times were 384 seconds (818 seconds) for the ±1-mm GW, but less than 226 seconds (292 seconds) for the ±2-mm GW. The maximum increased considerably at ±1-mm GW. CONCLUSION: Real-time-image gated proton beam therapy with a GW of ±2 mm was demonstrated to be suitable, providing good dose distribution without greatly extending treatment time.