Clinical Behavior of Radiation-Induced Thyroid Cancer: Factors Related to Recurrence

Background: Whether thyroid cancer is more aggressive in radiation-exposed patients is not resolved. The frequency of aggressive features in post-Chernobyl patients suggests this may be the case. Our aim was to address this question by re-examining the pattern of risk factors for recurrence of thyroid cancers found in a cohort exposed to external radiation. Methods: The study population was drawn from a cohort of 4296 people, followed since 1974, who were treated before the age of 16 with conventional external radiation for benign conditions of the head and neck between 1939 and the early 1960s. The study group consisted of 390 patients who had surgically verified thyroid cancer. Potential risk factors for recurrence were evaluated by proportional hazards analysis. Results: Fifty patients had recurrences an average of 8.7 years after diagnosis while the other 340 patients were followed for an average of 19.7 years. The sooner after radiation exposure the cancer occurred, the more likely it was to recur (hazard ratio, 0.96/year; 95% confidence interval [CI] 0.91-0.99). Taking into account the effect of the onset of screening in 1974, the features predictive of recurrence were younger age at the initial diagnosis (hazard ratio, 0.95/year; 95% CI, 0.91-0.99) and the size of the thyroid cancer (hazard ratio, 1.2/cm; 95% CI, 1.0-1.6). Conclusion: Although not based on a direct comparison, we conclude that thyroid cancers following external radiation exposure are not, on average, more aggressive than other thyroid cancers. The similarity of risk factors for recurrence suggests that they should be treated and followed in the same way as non-radiation-induced thyroid cancers.