Assessment of range uncertainty in lung-like tissue using a porcine lung phantom and proton radiography.

Purpose: Thoracic tumours are increasingly considered indications for pencil beam scanned proton therapy (PBS-PT) treatments. Conservative robustness settings have been suggested due to potential range straggling effects caused by the lung micro-structure. Using proton radiography (PR) and a 4D porcine lung phantom, we experimentally assess range errors to be considered in robust treatment planning for thoracic indications. Methods and Materials: A human-chest-size 4D phantom hosting inflatable porcine lungs and a corresponding 4DCT were used. Five PR frames were planned to intersect the phantom at various positions. Integral depth-dose curves (IDDs) per proton spot were measured using a multi-layer ionisation chamber (MLIC). Each PR frame consisted of 81 spots with an assigned energy of 210 MeV (FWHM 8.2 mm). Each frame was delivered 5 times while simultaneously acquiring the breathing signal of the 4D phantom, using an ANZAI load cell. The synchronised 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 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 with simulated ones, calculating range error for each individual spot. Results: In total 2025 proton spots were individually measured and analysed. Range error of a specific spot is reported relative to its 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. Conclusions: Determined mean relative range errors justify the use of 3% range uncertainty for robust treatment planning in clinical setting for thoracic indications.