Investigation of absorbed dose and estimation the risk of secondary thyroid cancer in whole brain radiotherapy

After the surgery, radiation therapy is the most important part of treating brain tumors. Thyroid cancer is the second most prevalent malignancy, as a result of radiation therapy of head and neck. Therefore, in whole-brain radiation therapy, determining the absorption dose of thyroid and the risk of secondary thyroid cancer is important. The aim of this study was to calculate the absorption dose of thyroid and the risk of secondary thyroid cancer in whole-brain radiation therapy.
The study included 50 patients undergoing whole-brain radiation therapy, with a mean age of less than 30 years old, in Isfahan Milad Hospital, Iran, using the ONCOR accelerator and 6-MV photon. Thermoluminescence dosimeter (TLD) was used to determine the dose received by the thyroid. Then, to calculate the risk of secondary thyroid cancer, the method of calculating risk in Biological Effected of Ionizing Radiation Committee (BEIR VII) report and the suggested quantity of International Commission on Radiological Protection (ICRP), which means excessive relative risk (ERR), were used.
Findings: The calculated mean absorbed dose was 1.976 ± 0.611 and 2.213 ± 1.041 cGy in women and men, respectively. The mean risk of secondary cancer of the thyroid 3, 5, 10, 15 and 20 years after whole-brain radiation therapy was 4.601 ± 1.614, 1.559 ± 0.595, 0.176 ± 0.063, 0.050 ± 0.018, and 0.022 ± 0.009, in women and 3.253 ± 1.721, 1.150 ± 0.516, 0.125 ± 0.066, 0.035 ± 0.019, 0.016 ± 0.008 in men, respectively. The absorption dose of thyroid was not related to age (P = 0.743).
The risk of secondary cancer of the thyroid is associated with sex and is 1.4 times more in women. In addition, by increasing the age after radiation the mean risk of induced cancer had a reducing behavior.