THYROID DOSE IN CHILDREN UNDERGOING PROPHYLACTIC CRANIAL IRRADIATION

Purpose: To determine the radiation dose received by the thyroid gland as a result of prophylactic cranial irradiation (PCI) in childhood le ukemia and the factors influencing that dose. Methods and Materials: T he dose to the thyroid resulting from simulated cranial irradiation wi th parallel opposed lateral fields of an adult anthropomorphic (ART) p hantom with both 6 MV X-rays and Cobalt-60 gamma-rays was measured usi ng thermoluminescent dosimeters (TLDs). The dependence of thyroid dose on the distance of the field from the thyroid and the proportions of thyroid dose from stray radiation (leakage, scatter from jaws, etc.) a nd tissue scattered radiation were measured. The effects of a shadow t ray and shielding blocks mere also determined. Calculation of thyroid dose using the Clarkson scatter integration method was performed for 6 MV X-rays to compare with the measured doses. In vivo thyroid dose es timates were made using TLD measurements for three children receiving PCI with 6 MV X-rays. Results: Using open, unshielded fields, the thyr oid region of the phantom received 1.2-1.4% of the prescribed cranial dose for 6 MV X-rays and 1.5-1.7% for Cobalt-60. For both treatment un its, stray radiation accounted for approximately two thirds of the thy roid dose and tissue scatter accounted for the remaining one third. Th e thyroid dose increased as the field moved closer to the thyroid, wit h an increasing proportion of the dose due to tissue scatter. Placemen t of a thyroid shielding block on a shadow tray reduced the thyroid do se by only 20% compared with the open, unshielded setup. Thyroid dose from 6 MV using open fields was affected by the orientation of the col limator. When the inferior field edge was defined by the lower jaw, th e dose was reduced by 27% compared with the upper jaw. Good correlatio n of dose to the thyroid region was obtained between phantom measured doses, in vivo measured doses and calculation of dose using the Clarks on method. Conclusion: For PCI doses of 1800 or 2400 cGy in the adult phantom, the dose to the thyroid was 20-40 cGy (1-2%). For small child ren this could rise to approximately 5% of the prescribed dose, of whi ch half was due to stray radiation. As the thyroid in children is very sensitive to radiation and the dose-response curve for thyroid tumor induction is linear, attempts to shield the thyroid during cranial irr adiation are mandatory. Cobalt-60 units should not be used, as the thy roid dose was higher than using 6 MV X-rays. Collimator orientation an d the use of shadow trays and shielding were important factors in dete rmining thyroid dose. (C) 1998 Elsevier Science Inc.