Dosimetric analysis of thyroid doses from total cranial irradiation

The purpose was to estimate the radiation dose to the thyroid from total cranial irradiation with parallel opposed lateral fields of 1.25 MeV gamma rays of 60Co, to represent the current treatment modality, and with 6 MV photon beams derived from a linear accelerator. The mean thyroid doses were measured using thermoluminescence dosemeters in the Rando phantom with two cranial irradiation set-ups: (1) the lateral beams were angled by rotating the collimator to avoid irradiation of the lens and healthy tissues and (2) the lateral beams were not angled, and shielding blocks were used to protect the lens and healthy tissues. For a cranial dose of 18 Gy, thyroid doses for child patients for 60Co with the open and blocked fields were 0.27 and 0.32 Gy, respectively. With 6 MV, the thyroid dose was less than the doses for 60Co open and blocked cranial fields by 17 and 10%, respectively. Unblocked cranial fields and a high-energy photon beam should be preferred for cranial irradiation of children. (authors)