Shiva Rahbar Yazdi - Department of Medical Physics, Shahid Sadoughi University of Medical Sciences and Health Services, Yazd, Iran
Mohammad Hosein Zare - Department of Medical Physics, Shahid Sadoughi University of Medical Sciences and Health Services, Yazd, Iran
Mohammad Ali Broomand - Department of Clinical Oncology, Shahid Ramazan Zadeh Clinic, Shahid Sadoughi University of Medical Sciences, Yazd, Iran

Background: Radiation therapy, the most common form of cancer treatment, can result in late complications, such as secondary breast and thyroid cancers.Objective: This study aimed to evaluate the risk of secondary cancers using two radiobiological models of Excess Absolute Risk (EAR) and Excess Relative Risk (ERR) in patients with brain cancer undergoing radiotherapy for improved survival rates of cancer patients.Material and Methods: In this expository cross-sectional study, ۴۵ patients under the age of ۴۰ years underwent Whole Brain Radiotherapy (WBRT) using a compact accelerator in Shahid Ramezanzadeh Hospital, Yazd, Iran. Out-of-field organ dose measurement was performed using a Thermoluminescent Dosimeter (TLD) to determine the dose to thyroid and breast tissues. The risk of secondary cancers in these organs was calculated ۳, ۵, ۱۰, ۱۵, and ۲۰ years after radiation therapy.Results: The mean values of thyroid cancer risk in men and women were ۰.۴۱۸±۰.۵۰۹ and ۰.۲۷۴±۰.۳۰۶, respectively. ERR values of breast cancer in ۳-, ۵-, ۱۰-, ۱۵-, and ۲۰-year women undergoing radiation therapy were ۱.۰۸۴±۲.۹۳۸, ۰.۵۹۴±۱.۴۰۷, ۰.۲۴۸±۰.۴۹۷, ۰.۱۳۸±۰.۲۴۸, and ۰.۰۹۱±۰.۱۴۸, respectively. EAR values of breast cancer in ۳-, ۵-, ۱۰-, ۱۵-, and ۲۰-year women following radiation therapy were ۰.۰۶۴±۰.۰۶۰, ۰.۰۷۷±۰.۰۷۱, ۰.۱۱۹±۰.۱۰۰, ۰.۱۷۸±۰.۲۴۸, and ۰.۲۵۹±۰.۱۷۸, respectively. Conclusion: After irradiation, the risk of secondary cancer is affected by factors, such as the patient’s age and gender. The secondary thyroid cancer is higher than that of other organs, such as the breast, in the patients undergoing WBRT.

Second Cancer Risk, Out-of-Field Dose, Dosimetry, Radiotherapy, Breast, Thyroid