Radiotherapy did not increase thyroid cancer risk among women with breast cancer: A nationwide population-based cohort study

The aim of this study was to evaluate whether an increased risk of thyroid cancer exists among women with breast cancer in Taiwan, particularly among those receiving RT. We used data from the National Health Insurance system of Taiwan for the investigation. The breast cancer cohort contained 55,318 women (including 28,187 who received RT and 27,131 who received no RT), each of whom was randomly frequency matched according to age and index year with three women without breast cancer from the general population. Cox's proportion hazards regression analysis was conducted to estimate the effects of breast cancer with or without RT treatment on subsequent thyroid cancer risk. We found that women with breast cancer exhibited a significantly higher risk of subsequent thyroid cancer (adjusted hazard ratio [aHR]=1.98, 95% confidence interval [CI]=1.60-2.44). The two groups (with or without RT) in the breast cancer cohort exhibited significantly increased risks. However, in the breast cancer cohort, the risk of thyroid cancer among women who received RT was not significantly higher than that of women who received no RT (aHR=1.28, 95% CI=0.90-1.83). Stratified analysis according to age revealed that only younger women with breast cancer (20-54 y) had a significantly higher risk of developing thyroid cancer. This study determined that Taiwanese women with breast cancer had a higher risk of developing thyroid cancer; however, RT seems to not play a crucial role in this possible relationship. What's new? Women who survive breast cancer are at increased risk of thyroid cancer, possibly because of treatment-related factors, such as the use of radiotherapy to combat breast tumors. This study suggests, however, that radiotherapy has no impact on subsequent thyroid cancer risk in breast cancer patients. In a Taiwanese study cohort, risk of thyroid cancer was found to be significantly elevated among women with breast cancer, regardless of whether or not patients received radiotherapy. Risk of thyroid cancer was highest among relatively young breast cancer survivors (ages 20-54).