Development of a new model-based radiation dose calculation algorithm for kilovoltage energy x-rays
Pawlowski, Jason Mark
Kilovoltage energy x-rays have long been used in medicine. With the rapid increase in use of x-ray imaging for diagnostics and for imaging guidance during radiotherapy there is an increased interest in determining the imaging dose delivered to patients. Currently, however, there is no accurate model-based dose calculation algorithm applicable for kilovoltage x-rays; existing algorithms are only accurate for megavoltage photon beams. This dissertation presents a model-based algorithm developed to provide accurate dose calculations for kilovoltage x-rays. The new algorithm calculates radiation dose by first considering the dose to water-equivalent media, and then applying a medium-dependent correction factor that accounts for atomic number dependent effects. The dose-to-water is calculated as the sum of primary and scatter components. The scatter dose is calculated by convolving an empirically parameterized scatter kernel with the primary photon fluence. Several approximations are introduced in the scatter dose calculation to increase calculation speed and account for density inhomogeneities. Two methods of calculating medium-dependent correction factors that account for the effects of photoabsorption in bone on the dose distribution are developed in this dissertation. Both approaches correlate correction factors with the distribution of bone and soft-tissues within the body and incorporate the characteristics of the incident radiation beam. The accuracy of the new 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. 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 for all patients. 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 tissue. The new algorithm overcomes the limitations of current model-based dose calculation algorithms and can be implemented in commercial treatment planning systems to extend dose calculations to kilovoltage photon beams resulting from imaging procedures or therapeutic superficial and orthovoltage beams.