SU-E-T-555: A Model-Based Dose Calculation Algorithm for Kilovoltage X-Rays

Abstract

Purpose: Current model-based dose calculation algorithms implemented in commercial radiotherapy treatment planning systems are inaccurate for kilovoltage photon beams. This work presents a new model-based dose calculation algorithm designed to provide accurate, patient-specific dose distributions for kV x-rays. Methods: This algorithm calculates the radiation dose as the sum of primary and scatter dose components. The scatter dose is calculated by convolving an empirically parameterized scatter kernel with the primary photon fluence. Several approximations are introduced to increase calculation speed: (1) the photon energy spectrum is approximated as monoenergetic; (2) density inhomogeneities are accounted for by implementing a global distance scaling factor in the scatter kernel; (3) kernel tilting is ignored. These approximations allow for efficient calculation of the scatter dose convolution with the fast Fourier transform. Medium atomic number dependent effects are accounted for by implementing a Medium-Dependent Correction, which correlates the effects of photoabsorption in bone with a surrogate quantity that is determined for an x-ray source associated with an imaging procedure. Results: The accuracy of this algorithm was tested by comparing dose distributions resulting from kV-CBCT scans and radiographs of real patients with those calculated by the Monte Carlo method. Imaging sites studied include the head-and-neck, thorax, and pelvis. For all patients studied, the differences of mean dose between the Monte Carlo Method and the model-based algorithm for kV-CBCT were within 4% for bone, and 3% for soft tissues. This is in contrast to current model-based methods used in commercial treatment planning systems that Result in underestimations exceeding 100% for bone and overestimations of 8% for soft tissues. Conclusion: This new algorithm overcomes the limitations of current model-based photon beam calculation algorithms and can be implemented in commercial treatment planning systems to extend dose calculations to kV photon beams resulting from imaging guidance procedures or therapeutic orthovoltage beams.