Assessment of megavoltage imaging dose to organs at risk in breast cancer radiotherapy

Introduction: Advanced radiotherapy require frequent imaging to ensure accurate target coverage. However, imaging verification doses are often unaccounted for, raising questions regarding normal tissue exposure. Daily orthogonal (anterior = 1MU and lateral = 1MU) megavoltage (MV) portal images over 30 fractions may provide 40–400 mSv of dose, according to limited data. Additionally, between 17 % and 19 % of cancer survivors who have undergone radiotherapy, 5 % may acquire secondary malignancies. Daily MV imaging is common during breast cancer radiotherapy at the study site as a departmental protocol, however doses to Organs at Risk (OARs) are unknown. The aim of the study was to assess OAR doses during breast cancer radiotherapy. Methods: A female RANDO® phantom's thoracic region was exposed to anterior and oblique imaging fields simulating four treatment scenarios: left breast, right breast, left chest wall, and right chest wall. Gafchromic EBT2 films were placed in the left lung, right lung, heart, spine, and on the skin to measure absorbed doses during each procedure. Results: Dose measurements across the four scenarios showed minimal variation. Both lungs received the highest daily doses, ranging from 18.4 to 19.8 mGy. The heart received between 6.7 and 10.1 mGy daily, while the skin absorbed the least, between 4.7 and 5.9 mGy, attributed to the skin-sparing effect of the 6 MV X-ray photon beam used. Conclusion: Cumulative imaging doses over a standard fractionation period were significantly below established thresholds for deterministic effects in the assessed organs. Implications for practice: Despite low cumulative doses, the potential increased risk of secondary cancers and other radiation-induced effects in the thoracic region underscores the need to optimize imaging doses and clinical procedures in breast cancer radiotherapy.