The possible association between breast and thyroid cancer has been a topic of research for several decades. For their meta-analysis, a team of physicians from Chicago was able to draw on data from a good million women with primary breast cancer and from about 45,000 patients with primary thyroid carcinoma. Is the likelihood of a corresponding second tumor actually increased? And if so, why?
Several times it has been observed that women with a history of breast cancer are more likely to develop thyroid carcinoma. At the same time, the rate of breast cancer is also increased in patients with primary thyroid carcinoma. The so-called detection bias was frequently mentioned as an explanation. In fact, the chance of detecting a second tumor appears to be higher if a patient is regularly followed up for her primary tumor and is therefore in closer contact with the medical system than the general population. Nevertheless, it cannot be excluded that genetic predisposition, treatment effects, or common hormonal risk factors also influence the genesis of a secondary malignancy.
Do physicians need to specifically consider this association and keep it more in mind given the increasing number of tumor survivors? The answer is yes, if one believes the results of a large-scale meta-analysis from the USA, which also took into account data from Europe.
Risk increased by up to 55
In total, the researchers evaluated 19 studies of primary breast cancer and 18 studies of primary thyroid cancer. Median follow-up periods of up to 15 years were found. What came out was a clear correlation between the two types of tumors:
- Women with primary breast cancer had a 55% increased risk of subsequent thyroid cancer (odds ratio 1.55; 95% CI 1.44-1.67).
- In turn, women with primary thyroid cancer had an 18% increased risk of subsequent breast cancer (odds ratio 1.18; 95% CI 1.09-1.26).
Why is that?
The authors consider pathophysiological explanations more likely than bias effects as the cause. Both estrogens and thyroid-stimulating hormone (TSH) are discussed as common causes. Another factor in carcinogenesis could be obesity, which leads to elevated estrogen levels. According to the authors, this results in a call for treating physicians to point out the increased risk of thyroid cancer to obese breast cancer patients in particular and to encourage them to lose weight. Conversely, in women with a history of thyroid cancer but no other risk factors, earlier initiation of breast cancer screening is discussed.
Furthermore, future findings on pathophysiologically relevant gene mutations are expected to further elucidate the relationship.
Source: Nielsen SM, et al: The Breast-Thyroid Cancer Link: A Systematic Review and Meta-analysis. Cancer Epidemiology, Biomarkers & Prevention 2016; 25(2): 231-238.
InFo ONCOLOGY & HEMATOLOGY 2016; 4(4): 2
