New radiation precautions for patients with thyroid cancer

A better iodine biokinetic model and more appropriate radiation precautions may be offered to patients with thyroid cancer who are undergoing 131I therapy by using actual biokinetic measurements, according to a study published by Radiology.

Given the need for radiation protection in targeted radionuclide therapy, radiation precautions are needed for patients with thyroid cancer. Patient-specific iodine biokinetics play a large role in determining cumulative doses and creating precautions. Lead author Bin Liu, MD, Phd, of Sichuan University in Chengdu, China, and colleagues sought to develop an iodine biokinetic model and generate radiation precautions based on actual biokinetic measurements for patients with thyroid cancer who undergo thyroid hormone withdrawal-induced hypothyroidism and 131I therapy.

Seventy-seven patients with differentiated thyroid cancer who had been prepared for 131I ablation treatment or 131I treatment for metastatic disease during follow-up were included in the study. The researchers made calculations based on deduced whole-body retention and measured iodine biokinetics in thyroidal tissue were derived to establish the thyroidal and extrathyroidal compartment uptake fractions and effective half-lives.

The actual dose rates from patients after therapeutic 131I administration were greatly lower than those described in the American Thyroid Association (ATA) and Nuclear Regulatory Commission (NRC) models. The mean initial dose rate at 0.3 m for patients with ablation treatment was 28 percent and 36 percent of that described in the respective ATA and NRC models. The equivalent values for patients with follow-up treatment were 30 percent and 38 percent, respectively.

The actual mean effective 131I half-life in the thyroid remnant tissue was greatly lower than that described in the ATA and NRC models, at 47.6 versus 175.2 hours.

“For typical administered activities of 3.7 GBq to a patient with ablation treatment or 7.4 GBq to a patient with follow-up treatment, 3 days of sleeping apart may be enough for keeping the doses to pregnant women and children below 1 mSv, while no precautions are needed for nonpregnant adult family members who do not sleep with the patient,” concluded Liu and colleagues.