Prevention of hypothyroidism related to mantle irradiation for Hodgkin's disease: preparative phantom study

To decrease the incidence of hypothyroidism related to mantle irradiation for Hodgkin's disease, we initiated a study designed to protect the thyroid gland using a phantom. A thyroid phantom was filled with technetium-99m. The thyroid phantom was placed inside of its corresponding anterior neck position in a whole body phantom. An anterior scintiscan of the head and neck region demonstrated the radioactivity in the simulated thyroid. A mantle port included a focused block that would shield the thyroid from the anterior port. The phantom was exposed (4 MeV) to 180 cGy (AP-PA) at midplane with lithium fluoride dosimeters in the position of the thyroid. The thyroid received an average of 12 cGy from the anterior field and 48 cGy from the posterior field for a total of 60 cGy per treatment or 30% of the prescribed dose. A complete mantle field course of radiation of 4000 cGy would lead to a thyroid dose of 1200 cGy at a daily fractional dose of 60 cGy. We elected not to block the thyroid from the posterior field to prevent shielding and potential underdosage of involved nodal sites. The present study suggests a method of safe and effective thyroid shielding which needs to be tested clinically to determine whether it would reduce the incidence of chemical and clinical hypothyroidism or simply extend the period until occurrence.