Increasing incidence of thyroid cancer together with improved prognosis raises a concern regarding development of second primary cancers. Moreover, little is known about the incidence of thyroid cancer as a second primary neoplasm after other cancers.
A cohort of 39002 people (356034.5 person years) with primary thyroid cancer was identified in 13 population-based cancer registries in Europe, Canada, Australia and Singapore and followed for second primary cancers over 25 years. We used cancer incidence rates of first primary cancer to calculate standardized incidence ratios (SIRs). Risk of second primary thyroid cancer after other cancer types was also calculated.
During the observation period there were 2821 second primary cancers after primary thyroid cancer, SIR 1.31 (95% CI 1.26-1.36). The overall increased risk was irrespective of sex, type of thyroid cancer, age at first cancer or follow up period. Following primary thyroid cancer there was increased risk (SIRs) of developing cancer of adrenal (8.34), soft tissue sarcoma (3.63), bone (2.62), kidney (2.33), leukaemia (2.26). Conversely there was an increased incidence of subsequent thyroid cancer following a primary cancer at these sites [adrenal (13.0), soft tissue sarcoma (2.94), bone (3.36), kidney (2.91), leukaemia (2.49)]. There was an increased incidence of breast cancer and lymphoma after thyroid cancer and of thyroid cancer after cancer of lung and larynx.
In conclusion, the pooled data from 13 worldwide cancer registries shows that there is an excess of second primary cancer following primary thyroid cancer. Common environmental and genetic factors may explain the concurrence at those sites where a primary tumour was also associated with an increased incidence of subsequent thyroid cancer. The increased incidence of breast cancer and lymphoma after thyroid cancer and of thyroid cancer after cancer of lung and larynx could be a consequence of therapy for the first primary.