Verification of Dose Calculation Algorithms in Treatment Planning Systems for External Radiation Therapy: A Monte Carlo Approach

Abstract: This thesis presents a new verification concept, the virtual accelerator, for dose calculation algorithms used in treatment planning systems (TPSs) for external beam radiotherapy. The algorithm input data required to implement a treatment unit in the TPS are generated by Monte Carlo simulations as are the beam reference data needed for the subsequent evaluation of the dose calculation algorithm. The virtual accelerator and its corresponding unit in the TPS can thus be used for comprehensive verification of dose calculation algorithms in the TPS. The virtual accelerator concept provides a new means of verifying dose calculation algorithms in TPSs. Properties that are difficult or even impossible to assess using conventional measurements can be studied. Problems associated with conventional measurements, e.g., detector limitations and accelerator stability, can be circumvented. The flexibility of the virtual accelerator is high since additional beam reference data can be acquired without compromising the consistency of the data. The feasibility of the virtual accelerator concept has been demonstrated by the successful implementation of a virtual photon accelerator and a virtual electron accelerator in commercial TPSs. The success of the implementations was determined by the ability of the dose calculation algorithms to reproduce the algorithm input data, and in most cases the agreement was within +-2%. The advantages and usefulness of the virtual photon accelerator have been illustrated in a mediastinum and a hip-prostheses-like geometry. The ability of the virtual photon accelerator to generate both total dose and the primary and phantom-scattered components was used to study the performance of two dose calculation algorithms in the presence of metallic implants. The virtual electron accelerator has been used to study the performance in homogeneous and inhomogeneous phantoms. Studies of the beam model and the handling of patient-specific inserts in the dose calculation algorithm were possible due to the ability of the virtual accelerator to separate the total dose into beam model components. Another advantage of the virtual accelerator that has been utilized for both photons and electrons is the possibility of evaluating the accuracy achievable in anthropomorphic phantoms based on patient X-ray computed tomography data. This feature has been used for photon algorithms in the case of tangential breast treatment and for the electron algorithm in the cases of nose, parotid gland, thorax wall and spinal cord treatment. For the electron cases, an elliptical gamma-evaluation was performed in three dimensions. For the 0.02 Gy/2 mm criteria 92% of the volume receiving more than 0.85 Gy per 100 monitor units (MU) has gamma-values less than one in the worst case. The corresponding value for the volume receiving more than 0.10 Gy/100 MU is 98%.