Monte Carlo Validation of Dose, Quality Assurance Protocols and Shielding in Radiation Therapy

Abstract

This work is a two-fold Monte Carlo study of radiation transport through medical linear accelerator treatment heads and the shielded rooms that house them. Firstly, a Monte Carlo model of the Elekta Agility treatment head in use at the Walker Family Cancer Center was created and validated using experimental data for three different treatment energies (6 MV, 10 MV and 18 MV). This model was used for an in-depth study of incident beam deviations and their impact on dose collected at patient level. Standard quality assurance tests of beam flatness, profile constancy, and TPR20/10 were used to characterize how these deviations presented themselves in the treatment beam. Deviations in the beam energy, position, and direction showed, for the first time, a direct link of the sensitivity of current quality assurance tests to beam parameters. A novel quality assurance metric, known as the Wasserstein distance, was implemented to determine if the two deviations were distinguishable at patient level.

The second study focused on neutron transport throughout the patient treatment room and surrounding areas. While the treatment beam used at the Walker Family Cancer Center is a photon beam, photo-neutrons are an unwanted secondary particle created in the treatment head and must be shielded for in accordance with the Canadian Nuclear Safety Commission. The Walker Family Cancer Center uses a new style of doorless bunker, the design of which was commissioned on theoretical predictions - based solely on empirical data - alone. We use our validated Monte Carlo study of the treatment head to simulate the photo-neutron production at the highest treatment energy. This information was then used in a secondary Monte Carlo simulation of the treatment room to predict the yearly dose at key radiation survey points around the room. On top of these specific points, the dose was also determined for various other locations inside of the treatment room, allowing for a more in-depth study of the theoretical equations that govern radiation transport through the room.