Out-Of-Field Dose Measurement and Second Cancer-Risk Estimation Following External Beam Radiotherapy and Brachytherapy for Cervical Cancer Treatment: A Phantom Study

Introduction: The present study aimed to measure the scatter and leakage dose received by out-of-field organs while delivering Radiotherapy (RT) treatment of cervical cancer. Moreover, this study estimated the risk of second cancer (SC). The doses to out-of-field organs were measured using a lithium fluoride (TLD 100) dosimeter while delivering External Beam Radiotherapy (EBRT) by 6 MV photon beam with Brachytherapy Boost (BB) treatment in the humanoid phantom. Material and Methods: The excess absolute risk of SC for the stomach, colon, liver, lung, breast, and kidney, as well as excess relative risk for the thyroid, were estimated based on Biological Effects of Ionizing Radiation VII report. Results: The out-of-field organ doses varied with respect to distance between organs. The colon (3DCRT-282.13 cGy and IMRT-381.24 cGy in 25 fractions) and kidney (70.65 cGy in 3 fractions) received the highest doses with EBRT and BB, respectively. For most of the aforementioned organs, the calculated dose was 0.2 Gy/fraction according to the treatment planning system. With the age at exposure (i.e., 30 years) as a reference, the highest LARs were associated with the colon (0.74%) and breast (2.76%) in 3DCRT plus BB and IMRT plus BB, respectively. The lifetime attributable risk of SC was also shown to decrease with increasing the age at exposure for all the organs. Conclusion: Although all the evaluated out-of-field organs in this study showed some levels of risk, the risk was more frequently reported for the colon, stomach, and breast with IMRT technique than that in 3DCRT.