Empirical shielding design data for facilities administering I131 for thyroid carcinoma.

Retrospective review of the records for 434 post thyroidectomy patients receiving I131 therapy for thyroid carcinoma revealed approximately 75% of the patients were discharged within 48 hours and 90% within 72 hours. Criterion for discharge was an external radiation dose below 25 muSv/hr, measured at one metre anterior to the patient's neck. The time-averaged average dose rate one metre anterior to the neck of a typical patient during the isolation period was 72 muSv/hr, with 90% of the patients below 82 muSv/hr. After correcting for the effects of patient size and scatter, the effective design dose rate from a patient in an isolation room treating two or three patients/week is 105 muSv.m2.hr-1, or 75 muSv.m.hr-1 where only one patient is treated each week. Concrete is the most economical shielding material, with 190 mm filled concrete block walls and 150 mm concrete floors as the minimum recommended shielding for a radioiodine therapy suite. Additional shielding will be required if the suite adjoins (including areas immediately above and below) areas with a high occupancy factor.