Room: Track 2
Purpose: Thoracic tumors are increasingly considered indications for pencil beam scanned proton therapy (PBS-PT). Conservative robustness settings have been suggested due to potential range straggling effects caused by the lung micro-structure. With the help of proton radiography (PR) and a 4D porcine lung phantom, we experimentally assessed range errors to be considered in robust proton treatment planning for thoracic indications.
Methods: A human-chest-size 4D phantom hosting inflatable porcine lungs and a corresponding 4DCT were used. Five PR fields were planned to intersect the 4D phantom at various positions. Integral depth-dose curves (IDDs) per proton spot were measured using a multi-layer ionization chamber. Each PR field consisted of 81 spots with an assigned energy of 210 MeV. Each PR field was delivered 5 times while simultaneously acquiring the breathing signal of the 4D phantom, using an ANZAI load cell. The synchronized ANZAI and delivery log file information was used to retrospectively sort spots to their corresponding breathing phase. Based on this information, IDDs were simulated by the treatment planning system (TPS) Monte Carlo dose engine on a calculation dose grid of 1 mm. In addition to the time-resolved TPS calculations on the 4DCT phases, IDD were calculated on the average CT. Measured IDDs were compared against simulated ones, obtaining relative range errors for each spot.
Results: In total 2025 proton spots were individually measured and analyzed. Range errors of spots are reported relative to their water equivalent path length (WEPL). The mean relative range error was 1.2 % (1.5 SD 2.3 %) for the comparison with the time-resolved TPS calculations and 1.0 % (1.5 SD 2.2 %) when comparing to TPS calculations on the average CT.
Conclusion: Determined mean relative range errors justify the use of 3% range uncertainty for robust treatment planning in clinical setting for thoracic indications.
Funding Support, Disclosures, and Conflict of Interest: University of Groningen, University Medical Centre Groningen, Department of Radiation Oncology has active research agreements with RaySearch, Philips, IBA, Mirada, Orfit