Abstract
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. A human-chest-size 4D phantom hosting inflatable porcine lungs and corresponding 4D computed tomography (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 (full width at half maximum (FWHM) 8.2 mm). Each frame was delivered five 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 into 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, IDDs were calculated on the average CT. Measured IDDs were compared with simulated ones, calculating the range error for each individual spot. In total, 2025 proton spots were individually measured and analysed. The 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. The determined mean relative range errors justify the use of 3% range uncertainty for robust treatment planning in a clinical setting for thoracic indications.
Highlights
Proton therapy is increasingly employed for the treatment of thoracic indications, such as non-small cell lung cancer (NSCLC) (Macdonald et al 2009)
Proton therapy may reduce radiation-induced complications, such as radiation pneumonitis, which is linked to the lung dose (Palma et al 2013, Appelt et al 2014), dysphagia, which correlates with dose to the oesophagus (Belderbos et al 2005, El Naqa et al 2006, Zhu et al 2010, Gomez et al 2012, Wijsman et al 2015) or a reduced life expectancy, which is correlated with the dose to the heart (Wang et al 2017)
A further on the range error of each specific spot is evaluated relative to the water equivalent path length (WEPL) of this spot through the 4D phantom, which is determined based on multi-layer ionisation chamber (MLIC) measurement
Summary
Proton therapy is increasingly employed for the treatment of thoracic indications, such as non-small cell lung cancer (NSCLC) (Macdonald et al 2009). The adoption of proton therapy for the treatment of thoracic tumours, especially when applied with pencil beam scanning (PBS), has been hampered by numerous sources of uncertainty, which can potentially have severe dose distribution degrading effects. Sources of uncertainty are linked to organ motion, interplay effects (Bert and Rietzel 2007, Paganetti 2012) and range uncertainty, among others. It has been speculated that due to the micro-structure of the lung, which cannot be properly detected with computed tomography (CT) scans, proton beams may be affected by an increased range straggling effect, which would enlarge range uncertainty further (Uries et al 1986, Baumann et al 2019, España and Paganetti 2011, Perles et al 2011, Sell et al 2012, Titt et al 2015)
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