A dosimetric study of kilovoltage cone beam CT image guidance in gynaecological radiation therapy

Cone beam CT (CBCT) is increasingly used in image guided radiation therapy in order to ensure more accurate patient setup and target localization. However, the issue of patient dose due to repeated imaging procedures is an important topic in radiation medicine today. This thesis rigorously explores the organ equivalent doses from kilo-voltage cone beam CT to provide useful data to clinicians for decision making in image protocol development. This work specifically focuses on gynaecological patients undergoing megavoltage radiation therapy using IMRT or RapidArc. A method is established for Monte Carlo simulation of dose distributions from kV CBCT using BEAMnrc/DOSXYZnrc, for an on-board imaging system mounted on a Varian linac. Careful benchmarking was performed to permit further investigation into patient specific dose. In order to account for different biological effects of kV and MV radiation, the investigation focused on equivalent dose, the product of dose and radiation quality factor. Dose mean lineal energy was used to determine beam quality, as opposed to using linear energy transfer, based on microdosimetric principles. Dose mean lineal energy was simulated with a C++ code and the Monte Carlo code NOREC. Quality factor was calculated based on lineal energy by a formula proposed by International Commission on Radiation Units and Measurements (ICRU). Higher relative biological effectiveness of kV CBCT beams compared with MV therapeutic beams was quantified. Finally, the Monte Carlo methods for kV CBCT dose simulation were combined with derived quality factors in a study of organ equivalent dose for gynaecological cancer patients. Using three different dose response models, organ equivalent doses for daily kV CBCT were compared with MV IMRT/RapidArc doses to assess whether patients are at increased risk of radiation induced secondary malignancy due to CBCT. Comprehensive dosimetric information presented in this thesis indicates that patients undergoing IMRT or RapidAcr treatments with daily kV CBCT are not subjected to additional risk due to CBCT imaging dose.Science, Faculty ofPhysics and Astronomy, Department ofGraduat