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Considerations in Lung SABR IMPT Using Two Different PBS Systems

Z Su1*, W Hsi1, E Brooks1, (1) Univerisity Florida Health Proton Therapy Institute, Jacksonville, Florida

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PO-GePV-T-127 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: Stereotactic Ablative Body Therapy (SABR) is an effective treatment for early-stage lung cancer. To effectively implement Lung SABR IMPT, effective spot size, plan dosimetric quality, and delivery time needed for practical and effective clinical treatment were evaluated for two different proton systems.

Methods: A compact proton therapy delivery system (Proteus-one, P1) was configured to deliver large spot IMPT similar to Proteous-plus with universal nozzle (P+). Total of 9 patients with 10 tumors were selected for this study. Seven targets were prescribed 60Gy in 10 fractions; three targets were 48Gy in 4 fractions. Raystation 8A were used to generate treatment plans for each nozzle type with exact same optimization objectives. P1 plans used 4cm range-shifter placed at 45cm from isocenter to create large in-air beam spots, which were compared to the P+ in-air spots without range-shifter. P+ plans used 7cm range-shifter placed 4cm away from patients, to minimize spot size degradation. Target coverage was optimized to prescription dose covers 100-percent GTV with 90-percent covers PTV. Dose to the OARs were compared to QUANTEC metrics. Three volumetric dose-repainting was performed. Plan beam-on times were obtained to evaluate treatment delivery efficiency.

Results: The in-air beam spots were comparable between P+ without and P1 with range shifter. All target coverage are acceptable. OAR doses were comparable and within the QUANTEC limits. The beam-on time of P+ plans were 3 to 6 time of those of P1 plans. When combined with patient setup and image guidance time, patient discomfort could lead to higher uncertainty in P+ treatment.

Conclusion: P1 system range-shifter configuration can effectively simulate a large spot P+ system. For the two proton systems, plan qualities were dosimetrically comparable; volumetric dose-repainting were applicable to reduce interplay effect. However, P1 delivery was significantly shorter than that of P+.

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