SU-E-T-745: Convolution-Superposition Model for Photon Dose Calculations of Finite Size Cobalt-60 Radiation Source

Abstract

Purpose: The collapsed cone convolution (CCC) superposition method is commonly used to calculate dose in intensity modulated radiation therapy(IMRT), particularly for beams originating from a point source of radiation. Here we propose and present a validation of a modified version of this method that can be used for dose calculations of intensity modulated beams from finite size radiationsources such as Cobalt‐60 (Co‐60) source. Methods: The CCC method computes 3D dose by convolving the TERMA (total energy released in a medium per unit mass), which is dependent on energy fluence, with a photondose kernel. In our method, TERMA is calculated using an approach that takes into account the effective source diameter. Specifically, the energy fluence depends on a sourcedistribution function, which is based on calculating the effective source diameter for a particular field size. The calculations of the modified convolution model were compared with the GafChromic film measurements and EGSnrc Monte Carlo (MC) calculations. The studies were done on a clinical Theratronics 780C Co‐60 unit with a 2cm diameter cylindrical source. Results: The energy fluence for various field sizes ranging from 1×1cm2 to 30×30cm2 was calculated and compared with the MC simulations. The results showed agreement better than 1.4% for fields centered on the central‐axis and 3% for those centered off‐axis. The dose was calculated by convolving energy fluence with the MC based pre‐calculated dose kernels. The dose comparisons to film measurements showed agreement to 2% in high dose regions and 3.8% in low dose penumbral regions. Conclusions: The results of this study show that the modified convolution‐superposition method can provide an acceptable accuracy when calculating dose for finite size sources. The implementation of this model to the current treatment planning systems can be useful for treatment planning of Co‐60 based IMRT and tomotherapy. Ontario government funding through Ontario Consortium for Adaptive Interventions in RadiationOncology (OCAIRO), which has an industrial component of matched support from Best Theratronics (Kanata, ON).