Women who have had thyroid cancer have a significant increase in the risk of breast cancer, especially for women ages 30 to 34, according to a report in the journal Cancer (Vol. 92, No. 2: 225-31).

Amy Chen, MD, and her colleagues from the departments of head and neck surgery, biomathematics, epidemiology, and endocrinology at the M. D. Anderson Cancer Center in Houston, wanted to find out whether or not there was a higher incidence of breast cancer in women who were previously treated for thyroid cancer, or if women who had breast cancer had a higher incidence of subsequent thyroid cancer. Both cancers are most frequently found in women, but only a single study had suggested there might be some relationship between the two.

The researchers examined information from the National Cancer Institute’s SEER (Surveillance, Epidemiology, and End Results) database (which collects cancer information from 14 different regions of the country and covers about 14% of the US population). They examined the records of 365 women with both thyroid and breast cancer diagnosed between January 1973 and December 1994.

The authors report that when the first cancer diagnosed was breast cancer, there was no subsequent increase in thyroid cancer. However, when the situation was reversed, and thyroid cancer was the first cancer diagnosed and breast cancer the second, there was a significant increase in the risk of breast cancer, in particular in women aged 30 to 34 where the risk was 1.9 times that expected for women in the same age group who had not had thyroid cancer.

The researchers also noted that this increased risk was found only in white, premenopausal women. Similar increases were not noted in African-American women (although the numbers of cases in this group was small). The greatest increased risk of breast cancer was found 15 to 20 years after the diagnosis and treatment of the thyroid cancer.

Why this relationship exists is uncertain, according to the authors, who note their concern regarding the potential role of radioactive iodine in the treatment of the thyroid cancer as a possible factor.

Another possibility, the authors suggest, is that similar exposures to something in the environment may cause the relationship, but this is less likely since women diagnosed first with breast cancer did not later develop increased numbers of thyroid cancers.

What is important, the authors conclude at this time, is that these women and their physicians remember the importance of lifetime monitoring and cancer screening.