Calibrating and adapting a deterministic proton transport algorithm to perform patient-specific quality assurance

Abstract

To reduce the probability of complications in proton therapy, while maintaining a high tumor control probability, adaptive treatment can be applied. In adaptive treatments, the treatment plans are frequently assessed and adjusted to, for example, account for anatomical changes. Ideally, daily adaptive treatment would be implemented, where every day a repeat scan is made and the treatment plan is adjusted. However, due to computational limitations, daily adaptive treatment is not possible yet. Quality assurance (QA) is a vital part of the proton therapy workflow. Intending to implement daily adaptive treatment, fast-working quality assurance tools are necessary. The goal of this thesis is to calibrate and adapt a deterministic proton transport algorithm to reconstruct the delivered dose using log files, which can be compared to the planned dose as a QA tool.

To achieve this, Yet anOther Dose Algorithm (YODA) was calibrated to the popular planning software RayStation, by optimizing the input parameters of YODA to minimize the difference in dose between YODA and RayStation for a 0° gantry angle single spot in a homogeneous water phantom. From this calibration, beam data library (BDL) files were created for cases without a range shifter and with range shifters of thicknesses 2 cm, 3 cm and 5 cm and planned spot energies ranging from 70 MeV to 190 MeV. An approximation to improve the inclusion of the effect of nuclear interactions was added to YODA. The lowest passing rate found was 99.47% for a 190 MeV spot without a range shifter, therefore the calibration was successful. The passing rate decreased for high energies, likely due to the crude approximation to deal with nuclear interactions. Other ways to include
the effect of nuclear interaction should be investigated to further improve the calibration results. After the calibration was complete, three experiments were performed.

First, simple treatment plan comparisons were performed using the BDL files and a homogeneous water phantom, including plans containing a single spot irradiated from gantry angles ranging from 10° to 90°. To account for the angled beams, a beam splitting algorithm was used. It was observed that the BDL files are specific for the isocenter to CT volume surface distance, as the boundary conditions of YODA are at the CT volume surface while the treatment plans define spots at the isocenter in air. To include this effect, the BDLs should contain beamlet parameters at the isocenter, from which the beamlet parameters at the CT volume surface can be calculated. The calibration procedure for the spatial spread, angular spread and correlation needs to be adjusted. Alternatives to the beam splitting algorithm should be investigated, as this algorithm inadequately reflects reality when a beam enters the CT volume under an angle.

Second, YODA assumes that spots are laterally symmetrical, however, in reality, spots are ellipse-shaped. To solve this, the asymmetrical spot solution to the Fermi-Eyges equation was derived. The difference in integrated Fermi-Eyges flux between the symmetrical and asymmetrical spot was calculated for realistic combinations of lateral spread, angular spread and correlation. The mean error in integrated Fermi-Eyges flux, relative to the maximum, induced by assuming lateral spot symmetry, around the central beam axis is significant, as it ranges from 0.6853% to 3.071% at the entrance of the CT volume and from 0.4710% to 4.957% at the Bragg peak. Therefore the asymmetrical spot solution should be implemented, after which, YODA should be re-calibrated

Third, the error induced by systematic errors in the log file was investigated. The plan contains a target cube centered in the homogeneous water phantom irradiated by a 0° gantry angle beam. By perturbing each energy layer randomly using a uniform distribution, where the maximum perturbation was 0.8 mm in spot position, 1% for MU and 0.1% for energy, the magnitude of the dose difference was calculated. The mean differences in dose in the target cube induced by randomly perturbing each energy layer were 0.1204% ± 0.0408% in x, 0.1700% ± 0.0324% in z, 0.2592% ± 0.1595% in MU and 0.4802% ± 0.1558% in energy. These differences are small compared to the error induced by assuming symmetrical spots, note that the differences in spatial spread were an order of magnitude bigger than the perturbation in spot position. Before this investigation on the effect of systematic errors can continue, the asymmetrical spot solution needs to be implemented and the problems discovered with angled beams should be solved.

The calibration was successful, and additional extensions and alterations have been identified to further improve YODA before it can be used as the described patient-specific quality assurance tool.