Initial Experience with Real-Time Gated Proton Therapy (RGPT) in the Definitive Treatment of Prostate Cancer

Abstract

<h3>Purpose/Objective(s)</h3> RGPT has the potential to provide instantaneous feedback for intrafraction target motion to maximize patient safety and inform optimal treatment planning. Our purpose was to report our early experience with RGPT and to summarize intrafraction target motion and implications on treatment delivery, including added time, for a pilot population of prostate cancer patients before expansion to other sites with greater motion. <h3>Materials/Methods</h3> RGPT with a proton beam therapy system was commissioned and implemented at our proton center, representing the first experience in the United States. Patients being treated definitively with proton therapy to the prostate ± seminal vesicles-only and who had successful placement of platinum fiducial markers and hydrogel spacer were assessed on a prospective IRB-approved registry. Orthogonal fluoroscopic imaging was taken at a pulse rate of one image per second during fractionated proton therapy to assess positioning of a pre-selected fiducial marker. Thresholds to guide beam-on and beam-off gating were determined for a matching score component (based on quality of the image tracking and the guiding template) and for a shift tolerance component (based on our typical set-up uncertainty robustness of 3mm). The shift tolerances were defined as 2mm for left-right and anterior-posterior, and 3mm for superior-inferior. Each patient was treated with single-field optimization planning and a two lateral beam setup. <h3>Results</h3> To date, 8 prostate cancer patients have been treated with a total of 152 RGPT-guided fields. The table details the mean, maximum, standard deviation, and estimated upper 90^th percentile of directional shifts for the entire dataset for 1) the total RGPT-tracked treatment time and 2) the beam-on time as gated by RGPT tracking. On average, the initial imaging to set up the RGPT system required 2.5 seconds per field. The average total treatment time during RGPT guidance, including periods of both "on" and "off" gating, was 41.3 seconds per field. RGPT tracking triggered "beam-off" for an average of 7.8 seconds per field, increasing the total treatment delivery time by 20.2% across the 152 fields. <h3>Conclusion</h3> RGPT successfully tracked intrafraction fiducial marker motion during proton therapy for prostate cancer, while having minimal impact on total treatment time. Further aggregated RGPT-derived data may help to inform optimal treatment planning parameters such as robust optimization.