Radiation-induced cancer risk derived from dosimetry of image guided breast radiation therapy

Image guidance is an integral component of modern radiation therapy, enabling the visualisation and verification of the tumour volume and organ motion prior to and during treatment delivery. This thesis represents a body of work examining the additional radiation dose and associated risk of image guidance during breast radiation therapy. There are three main areas within the thesis. The first was the dosimetric assessment of breast radiation therapy treatment techniques and associated imaging. Radiation dose was measured within an anthropomorphic phantom with thermoluminescent dosimeters and metal-­‐oxide-­‐semiconductor-­‐field effect-­‐transistor dosimeters (skin only). Tissue dose values were obtained from the average of dosimetric points within the anthropomorphic phantom at the corresponding organ locations. Three radiation therapy treatment techniques for left-­‐sided breast cancer were compared: standard tangents delivered with a conventional linear accelerator, static tomotherapy, and helical tomotherapy. Overall, helical tomotherapy delivered higher doses to organs and tissues surrounding the target volume. Comparable dose values were measured for the tangential radiation therapy treatment and the static tomotherapy treatment. Image guidance modalities assessed were megavoltage cone-­‐beam CT (MV-­‐CBCT), megavoltage fan-­‐beam CT (MV-­‐FBCT), and kilovoltage cone-­‐beam CT (kV-­‐CBCT). A single MV-­‐CBCT, MV-­‐FBCT, and kV-­‐CBCT acquired for breast radiation therapy patient position verification delivered 5.5 cGy, 1.7 cGy, and 0.4 cGy to the contralateral breast, 5.3 cGy, 1.3 cGy and 0.3 cGy to the contralateral lung, and 6.4 cGy, 1.1 cGy and 0.4 cGy to the heart. The imaging dose values reported in this work will supplement the current literature. The phantom dose measurements were then used to predict the additional dose burden to the patient when imaging is utilised for position verification during breast radiation therapy. Assessment of the magnitude of imaging dose relative to treatment delivery dose was the second area of this thesis. For organs outside the treatment field-­‐of-­‐view, such as the contralateral breast, which would not receive high doses from treatment alone, imaging was shown to considerably increase the total organ dose due to inclusion of the organ within the imaging field-­‐of-­‐view. For a weekly imaging protocol during breast radiation therapy, image verification with a MV-­‐CBCT, MV-­‐FBCT and kV-­‐CBCT contributes 16.7 %, 9.5 % and 2.4 %, respectively to the total contralateral breast dose. For a daily imaging protocol during breast radiation therapy, image verification with a MV-­‐CBCT, MV-­‐ FBCT, and kV-­‐CBCT contributes 75.1 %, 49.6 %, and 19.5 %, respectively to the total contralateral lung dose. The dose increases reported by image verification protocols utilising MV-­‐CBCT, MV-­‐FBCT, and kV-­‐CBCT, as presented in thesis, are of significance and may assist clinicians make informed decisions about the type image modality and the frequency of imaging for image-­‐guided breast radiation therapy. The third area of this thesis was the evaluation of the potential detriment from image guidance during breast radiation therapy and different treatment techniques with existing radiation-­‐induced cancer risk models. Using the Biological Effects of Ionising Radiation committee model, the lifetime attributable risk of developing a contralateral breast cancer after static tomotherapy with no imaging is 0.6 %. Daily imaging increases this by an absolute 1.0 %, 0.3 % and 0.1 % for the MV-­‐CBCT, MV-­‐FBCT, and kV-­‐CBCT respectively. These radiation-­‐induced secondary cancer risk increases should be considered but kept in perspective considering the low absolute values and the benefits of image-­‐guided radiation therapy setup. A model incorporating dose inhomogeneity and a number of dose-­‐response relationships was applied to the tangential radiation therapy, static tomotherapy, and helical tomotherapy dose distribution. From this a range of possible radiation-­‐induced secondary cancer risk values attributable to radiation therapy treatment of breast cancer was estimated. The ratio of these calculated risk values highlighted that tangential breast radiation therapy and static breast tomotherapy result in comparable radiation-­‐induced secondary cancer risk for all organs considered. The radiation dose from image guidance for breast radiation therapy patients was explored in this thesis. The findings presented ensure a quantitative assessment of dose and risk from these procedures can be used to better inform the appropriate frequency of image guidance procedures in the radiotherapy process