Experimental validation of 4D log file‐based proton dose reconstruction for interplay assessment considering amplitude‐sorted 4DCTs

Abstract

Purpose The unpredictable interplay between dynamic proton therapy delivery and target motion in the thorax can lead to severe dose distortions. A fraction‐wise four‐dimensional (4D) dose reconstruction workflow allows for the assessment of the applied dose after patient treatment while considering the actual beam delivery sequence extracted from machine log files, the recorded breathing pattern and the geometric information from a 4D computed tomography scan (4DCT). Such an algorithm capable of accounting for amplitude‐sorted 4DCTs was implemented and its accuracy as well as its sensitivity to input parameter variations was experimentally evaluated. Methods An anthropomorphic thorax phantom with a movable insert containing a target surrogate and a radiochromic film was irradiated with a monoenergetic field for various 1D target motion forms (sin, sin4) and peak‐to‐peak amplitudes (5/10/15/20/30 mm). The measured characteristic film dose distributions were compared to the respective sections in the 4D reconstructed doses using a 2D γ‐analysis (3 mm, 3%); γ‐pass rates were derived for different dose grid resolutions (1 mm/3 mm) and deformable image registrations (DIR, automatic/manual) applied during the 4D dose reconstruction process. In an additional analysis, the sensitivity of reconstructed dose distributions against potential asynchronous timing of the motion and machine log files was investigated for both a monoenergetic field and more realistic 4D robustly optimized fields by artificially introduced offsets of ±1/5/25/50/250 ms. The resulting dose distributions with asynchronized log files were compared to those with synchronized log files by means of a 3D γ‐analysis (1 mm, 1%) and the evaluation of absolute dose differences. Results The induced characteristic interplay patterns on the films were well reproduced by the 4D dose reconstruction with 2D γ‐pass rates ≥95% for almost all cases with motion magnitudes ≤15 mm. In general, the 2D γ‐pass rates showed a significant decrease for larger motion amplitudes and increase when using a finer dose grid resolution but were not affected by the choice of motion form (sin, sin4). There was also a trend, though not statistically significant, toward the manually defined DIR for better quality of the reconstructed dose distributions in the area imaged by the film. The 4D dose reconstruction results for the monoenergetic as well as the 4D robustly optimized fields were robust against small asynchronies between motion and machine log files of up to 5 ms, which is in the order of potential network latencies. Conclusions We have implemented a 4D log file‐based proton dose reconstruction that accounts for amplitude‐sorted 4DCTs. Its accuracy was proven to be clinically acceptable for target motion magnitudes of up to 15 mm. Particular attention should be paid to the synchronization of the log file generating systems as the reconstructed dose distribution may vary with log file asynchronies larger than those caused by realistic network delays.

Experimental validation of a 4D dynamic dose calculation model for proton pencil beam scanning without spot time stamp considering free‐breathing motion

“…Moreover, Spautz et al. showed that the gamma scores were maintained at more than 95% at 3%/3 mm for the time stamp shifts within 5 ms from 10 to 30 mm motion 36 . The spot response time results from the acceptance test conducted by the vendor showed that our 4DDD calculation system could be completely controlled in less than 20 ms, including the delay of the trigger signal.…”

“…Besides the dose calculation accuracy and divided spot assignment in the TPS, uncertainties arise from the deformation accuracy of the DIR. Factors affecting the dose deformation of the reference CT include deformation grid sizes, time resolution of the 4DCT phases for dividing spots, CT value variations between 4DCT phases, and the deformation algorithm 36,37 . Ribeiro et al.…”

“…20 Moreover, Spautz et al showed that the gamma scores were maintained at more than 95% at 3%/3 mm for the time stamp shifts within 5 ms from 10 to 30 mm motion. 36 The spot response time results from the acceptance test conducted by the vendor showed that our 4DDD calculation system could be completely controlled in less than 20 ms, including the delay of the trigger signal. We assumed that time stamp shifts from several tens to several hundreds of milliseconds would have occurred in our system under both scan time differences and time stamp shifts because the Anzai output files have a time resolution of 10 ms. We have shown that the validation results are in good agreement, with a 10% response time error (80 ms).…”

“…Factors affecting the dose deformation of the reference CT include deformation grid sizes, time resolution of the 4DCT phases for dividing spots, CT value variations between 4DCT phases, and the deformation algorithm. 36,37 Ribeiro et al reported that the maximum error was 11% for the clinical target volume V 95 at the corresponding dose points among several DIR algorithms. 38 They reported that the variability of 4DDD accumulation results due to deformation is not small.…”

Limitations of phase-sorting based pencil beam scanned 4D proton dose calculations under irregular motion

“…In addition, 4DDCs are clearly indispensable for 4D optimization (Keall et al 2006, West et al 2007, Eley et al 2014, Graeff 2014, Bernatowicz et al 2017, Graeff 2017. Different 4DDC approaches have also been experimentally validated (Bert et al 2009, Testa et al 2013, Krieger et al 2018, Pfeiler et al 2018, Kostiukhina et al 2020, Spautz et al 2022.…”