The clinical use of volumetric modulated arc therapy (VMAT) has augmented rapidly since it became commercially available. As a result, the need for comprehensive quality assurance (QA) has increased. Current practices in Sweden normally compare the delivered dose with the planned dose based on measurements with different methods, for instance: portal dosimetry, point dose verification and/or dose distribution verification performed in a homogenous phantom. One drawback is the difficulty to interpret deviations between predicted and measured doses, if the deviation is caused by failures of the accelerator performance or the theoretical patient dose distributions. Another weakness is that the recalculations are performed for a homogenous phantom and not for the real patient. Furthermore, there are a resolution limitations caused by the distance between the detectors in dose verification phantoms.
The project objective was to develop a Monte Carlo (MC) patient-specific pre-treatment QA system for VMAT technology. The system ought to be able to evaluate dose-planning distributions in the patient geometry. Objectives in the system include the establishing of tools for MC calculations of VMAT dose distributions in patient geometries, the developing of tools for quantitative analysis and the performing of retrospective calculations and analysis. The Swedish Radiation Safety Authority's intention with the project is to contribute to a safer radiotherapy. Furthermore, the authority would gain knowledge and support for formulation of regulations and supervision. This project result in new methods for quality assurance for these kinds of patient treatments.
The project developed a MC patient-specific pre-treatment QA system. The system allows the calculation and evaluation of dose distributions for VMAT plans in patient geometry based on CT images. The QA system was tested by performing retrospective analysis of radiation therapy plans created for treatment of cancer. An advanced software tool was developed for generation of MC compliant voxel phantoms from patient DICOM data. A stand-alone analysis module was also developed to perform quantitative calculation of deviations between planned and MC dose distribution. Gamma evaluation is implemented as well as the more advanced normalized dose difference method.