Risk estimation of second primary cancer in patients undergoing radioactive iodine therapy for treatment of differentiated thyroid carcinoma

The differentiated thyroid carcinoma is the most common endocrine cancer, and its incidence is growing. The therapy with radioactive iodine (¹³¹I) is a well-established method used to treat the disease. However, its use in patients at low risk of recurrence after surgery is controversial because of the risk of causing a radiation-induced second primary cancer, especially in younger patients. In order to estimate the risk of carcinogenesis in patients treated with radioactive iodine, the absorbed doses, after a single administration of Na¹³¹I, were calculated in four critical organs (salivary glands, stomach, kidneys and bone marrow) of 416 patients from two hospitals in the city of Rio de Janeiro. The average age of patients in the study was 47.7 ± 15.4 years, and 352 of them (85%) were women. The average absorbed doses in the critical organs were: 1.567 ± 0.431 Gy in the salivary glands, 1.022 ± 0.738 Gy in the stomach, 0.315 ± 0.090 Gy in the kidney and 0.185 ± 0.051 Gy in the bone marrow. These dose values were well below the thresholds for deterministic effects, and younger patients received absorbed doses similar to those of older people. Then, these data were used to estimate the lifetime attributable risk (LAR) using the mathematical models of BEIR VII report. This calculation was made by dividing patients by gender and average age to four age groups. The radiation-induced cancer estimates ranged from the upper extreme of 948 cases per 100,000 people for stomach cancer in 21-year-old female patients, to 16 cases per 100,000 people for kidney cancer in female patients with 75 years of age. The average absorbed dose of 0.18 Gy observed in the bone marrow of female patients up to 25 years of age is related to a LAR of leukemia of 151 cases per 100,000 people. The risk of carcinogenesis was considerably higher in younger patients compared to older ones. 20 years women treated with 3.7 GBq of Na¹³¹I had an estimated risk of leukemia of 0.54 in 1,000 and a risk of stomach cancer induction of about 3.5 in 1,000. The risk of stomach cancer induction for these same woman decreases to 1.75 in 1,000 for an administered activity of 1.85 GBq, and about 1 in 1,000 for an activity of 1.11 GBq. This study indicates that low-risk young patients should not receive an activity of Na¹³¹I higher than 1.85 GBq, if the treatment with radioactive iodine is considered necessary. (author)