Intrafractional and Interfractional Range Variations in Charged Particle Therapy of Lung Cancer

Background:To quantify water equivalent pathlength (WEL) variations due to both intrafractional and interfractional changes in anatomy and physical properties through analysis of weekly serial four-dimensional (4D) CT scans.\nMaterials and Methods:Serial 4DCT scans were performed under free breathing conditions during 6 weeks of radiotherapy. Evaluation metrics were variations in WEL, lung density, CT number and chest wall thickness. Metrics for the intrafractional variations were evaluated as a function of 10 respiratory phases using 4DCT data sets. For the interfractional changes, metrics from the initial CT scan (week 0) were compared with those from subsequent serial weekly CT scans (weeks 1 to 5) at peak exhalation as the reference.\nResults:Mean WEL variations over the internal target volume region for all patients were -7.1mm and -15.5 mm due to intrafractional and interfractional changes, respectively. Chest wall thickness variations of < -3.0 mm, -3.6 mm, -3.6 mm were observed during respiration in the upper, middle and lower lung regions respectively. WEL variations of -7.2 mm, -6.5 mm, -4.3 mm in the upper, middle and lower lung were observed in the interfractional data. Lung density and soft tissue CT numbers did not show significant changes. Chest wall thickness showed variation from week to week; these variations could be attributed to patient setup in particular, maintaining the arm-up position can be a challenge for physically weak cancer patients. In addition, the WEL values decreased gradually through the treatment course. This trend could be explained by weight loss, muscle atrophy, lymphadenopathy, etc.\nConclusions:Chest wall thickness change was identified as the greatest common factor among all patients affecting range variations, potentially resulting in beam overshoot in charged particle therapy. Replanning and redesigning the compensating bolus may be necessary to better conform the prescribed dose to the target and minimize normal tissue irradiation. In addition, immobilization and better reproducibility of the arm position may be important to minimize the changes in chest wall thickness, especially the upper chest region.PTCOG4