Risk estimation of radiation-induced thyroid cancer from treatment of brain tumors in adults and children.

The purpose of this study was to estimate the risk of thyroid cancer induction attributable to brain radiation therapy in adult and pediatric patients. An anthropomorphic phantom was used to simulate treatment of brain tumors with two lateral opposed fields. Thyroid dose was measured using thermoluminescent dosimeters. Phantom measurements were performed for all possible field sizes that may be applied during brain radiotherapy in adults and children. The dependence of the thyroid dose on the distance from the irradiation field and on the presence of beam modifiers in the primary beam was investigated. All phantom exposures were generated with a 6 MV photon beam. Thyroid dose was found to vary from 9.6 to 89.4 cGy and from 8.0 to 194.0 cGy depending upon the field size used and the thyroid location in respect to the field edge for adults and children respectively. The excess relative risk of thyroid cancer induction for exposed children was estimated to be 0.6-14.9. The corresponding excess relative risk for adults was 0.1-1.1. The introduction of lead blocks or wedges into the primary beam may result in a considerable increase of the risk of thyroid cancer due to the increase of the thyroid dose. This study shows that brain radiotherapy during childhood may be associated with an increased risk of secondary thyroid cancer while the risk in adult patients is much smaller.