4D Dose Calculation
Dynamic dose reconstruction
The interplay between respiratory tumour motion and dose application by means of complex, intensity modulated photon radiotherapy techniques can potentially lead to undesirable and non-intuitive deviations from the planned dose distribution. We developed a 4D Monte Carlo (MC) dose recalculation framework to precisely simulate the dose distribution for a moving target volume. For the assessment of interplay effects, we implemented a dose accumulation tool that enables dose recalculations of arbitrary breathing curves including also the actual breathing curve of the patient. This MC dose recalculation framework is based on linac log-files, facilitating a high temporal resolution up to 0.1 s. By statistical analysis the interplay sensitivity to different treatment parameters, such as MU rate, treatment techniques or dose per fraction can be accessed and evaluated. This universality allows a comprehensive assessment of interplay effects in retrospective and prospective clinically relevant scenarios.
4D CBCT correction
Analogous to the above-mentioned deviations in photon therapy, proton therapy faces even more challenges regarding tumour motion and inter-fractional anatomical changes due to its characteristic Bragg-peak. To address these issues, cone-beam computed tomography (CBCT) scanners, installed in state-of-the-art gantry-equipped proton therapy facilities, are currently utilised to correct inter-fractional patient errors by setup corrections. We have developed the ability to compute proton dose distributions on intensity corrected 3D CBCT images, supporting the future implementation of adaptive radiation therapy (ART). To further tackle intra-fractional errors due to respiratory motion, a four-dimensional CT scan (4D-CT), capturing average breathing cycles, is used for treatment planning. The planning accuracy is degraded by changes in the breathing pattern from fraction to fraction and thus 4D imaging at the day of treatment would be desirable. However, the image quality of 4D-CBCT is insufficient for proton dose calculation and consequently the 3D correction methods have to be enhanced to 4D. For this purpose, we will develop a 4D-CBCT correction protocol, including optimised CBCT acquisition, image registration, iterative reconstruction and correction algorithms, that allow time-resolved proton dose calculation. This project is the topic of the DFG-funded project “4D cone beam computed tomography as a novel tool for accurate time-resolved dose calculation in particle therapy” (project #399148265).