The Risk of Second Primary Malignancies up to Three Decades after the Treatment of Differentiated Thyroid Cancer

TLDR

Differentiated thyroid cancer has a 10‑year survival rate exceeding 90%. The study determined the risk of nonthyroid second primary malignancies in 30,278 differentiated thyroid cancer patients diagnosed between 1973 and 2002. The analysis followed patients for a median of 103 months and evaluated risk by radioisotope therapy, gender, latency, and age at diagnosis. Patients had a 9% higher overall risk of nonthyroid second primary malignancies (O/E = 1.09, AER = 6.39 per 10,000 person‑years), with irradiated patients showing a 20% higher risk (O/E = 1.20) than unirradiated (O/E = 1.05) and a relative risk of 1.16; risk was greatest within five years of diagnosis and higher in younger patients, while gender had no effect.

Abstract

The 10-yr survival rate of patients with differentiated thyroid cancer exceeds 90%. These patients may be at elevated risk for secondary cancers.The risk of nonthyroid second primary malignancies after differentiated thyroid cancer was determined in 30,278 patients diagnosed between 1973 and 2002 from centers participating in the National Cancer Institute's Surveillance, Epidemiology, and End Results program. Median follow-up was 103 months (range, 2-359 months). Risk was further assessed for the addition of radioisotope therapy, gender, latency to development of secondary cancer, and age at thyroid cancer diagnosis.There were 2158 patients who developed a total of 2338 nonthyroid second primary malignancies, significantly more than that expected in the general population [observed/expected (O/E) = 1.09; 95% confidence interval (CI), 1.05-1.14; P < 0.05; absolute excess risk per 10,000 person-years (AER) = 6.39]. A significantly greater risk of second primary malignancies over that expected in the general population was for patients treated with radioisotopes (O/E = 1.20; 95% CI, 1.07-1.33; AER = 11.8) as well as for unirradiated patients (O/E = 1.05; 95% CI, 1.00-1.10; AER = 3.53). However, the increased risk was greater for the irradiated vs. the unirradiated cohort (relative risk = 1.16; 95% CI, 1.05-1.27; P < 0.05). Gender did not affect risk. The greatest risk of second primary cancers occurred within 5 yr of diagnosis and was elevated for younger patients.The overall risk of second primary malignancies is increased for thyroid cancer survivors and varies by radioisotope therapy, latency, and age at diagnosis.