Release: Embargoed until September 24, 2000 Contact: Kenneth Satterfield(202) 371-4517 (9/23-9/27) 703-519-1563[email protected]


Thyroid cancer treatment may be the cause of the link between the two cancers

Washington, DC -- Standard treatment of thyroid carcinoma includes surgery and the post-operative use of therapeutic doses of radioactive iodine (131I). Postoperative use of 131I is recommended because 131I improves the disease free interval and survival in patients with well-differentiated thyroid carcinoma. While 131I is primarily concentrated in the thyroid, exposure of other tissues that have a sodium/ iodine transporter mechanism, e.g., breast, is also possible.

Epidemiologic evidence points to the long latency period between exposure to radiation and the development of carcinoma. A retrospective study from the University of Texas MD Anderson Cancer Center suggested that an increased risk exists for women with an index thyroid carcinoma to develop subsequent breast carcinoma This finding introduces the idea that treatment for thyroid carcinoma may play a role in the subsequent development of breast cancer. In order to examine this association further, the authors conducted a systematic analysis to examine the association between thyroid and breast carcinomas.

The authors of the study, "The Development of Breast Cancer in Women with Thyroid Cancer," are Amy Y. Chen, MD, MPH, Larry Levy, Helmuth Goepfert, MD, Barry Brown, PhD, Margaret Spitz, MD, MPH, Rena Vassilopoulou-Sellin, MD, all from the The University of Texas MD Anderson Cancer Center Houston, TX. Their findings will be presented Tuesday, September 26, 2000, at the American Academy of Otolaryngology--Head and Neck Surgery Foundation Annual Meeting/Oto Expo, being held at the Washington, DC Convention Center.

Methodology: Between January 1973 and December 1994, 299,828 cases of breast carcinoma and 23,080 cases of thyroid carcinoma were identified in the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) database. Of the two groups, 612 women were diagnosed with both thyroid and breast cancer. A minimum latency of two years between the diagnoses of these two cancers in each patient was selected so as to identify patients with sequential diagnoses. Two hundred, forty-seven cases reported were based only on autopsy or death certificate information, whose index cancer was not breast or thyroid, whose pathology was not microscopically confirmed "in situ" were excluded. Accordingly, a total of 365 women were included in the study.

The SEER database from 1973-1996 was utilized to calculate age-specific and calendar year-specific incidence rates for each year for thyroid and breast cancers. The expected number of second cancers for each age group, ethnicity, calendar year, and follow-up period was determined by multiplying these incidence rates by the age-specific and calendar year-specific number of person-years at risk. They assessed risk for women with breast carcinoma followed by thyroid carcinoma as well as for women with thyroid carcinoma followed by breast cancer.

Results: No women were less than 20 years of age at index diagosis. The average age at diagnosis for 113 women with breast cancer as the index malignancy was 56.8 years. There were 252 women with thyroid cancer as the index malignancy with an average age of 48.6 years. The average interval between diagnoses of the two primaries was 6.7 years for index breast cancer and 9.0 years for index thyroid cancer. The average age at diagnosis for breast cancer was 62 years, and the average age at diagnosis for thyroid cancer was 47 years.

A total of 1,333,115 person-years with breast and thyroid cancer were available for analysis. Some 114.7 subsequent thyroid cancer cases were expected and compared to 113 observed for a risk ratio of 0.99 (p=0.576). Analysis of five-year follow-up periods did not reveal a significant increase in risk. There were no increased risks for either pre-menopausal (20-49 years) or post-menopausal (50+ years) women. At no time during follow-up (2-5 years, 5-10 years, 15-20 years, 20+ years) were premenopausal nor postmenopausal women with index breast cancer at increased risk of developing thyroid cancer. Premenopausal Black women were at significantly increased risk (OR = 2.5). However, this estimate was based only on five observed cases. Assessed were 214.4 subsequent expected breast cancer cases compared with 252 observed cases for a risk ratio of 1.18 (p=0.007). Analysis of risk by five-year intervals suggested that young women (30-34 years) exhibited the greatest risk of breast cancer (RR=1.90, p=0.001). Women who were between the ages of 40 and 44 at initial diagnosis of thyroid cancer were also at significantly elevated risk (RR=1.39, p=0.04).

A comparison was conducted for pre-menopausal women (20-49 years) and post-menopausal (50+ years) women with index thyroid carcinoma by ethnicity. Caucasian premenopausal women with a diagnosis of index thyroid carcinoma exhibited significantly increased risk of developing subsequent breast carcinoma (RR=1.41, p=0.001). The risk for premenopausal Black women was elevated but not statistically significantly (RR=1.54, p=0.155). There was no evidence of increased risk in postmenopausal women of either ethnicity (RR=0.97, p=0.622). Although significantly increased risk appears as early as 5-10 years following index thyroid carcinoma for Caucasian premenopausal women, the data suggest that the greatest risk appears 15-20 years after the index thyroid carcinoma.

Conclusions: The study's analysis of SEER data demonstrated that premenopausal adult Caucasian women who are treated for differentiated thyroid cancer are at increased risk to develop breast cancer five to 20 years later. That the diagnosis of breast cancer does not increase the risk of subsequent thyroid cancer suggests that the susceptibility to breast cancer after thyroid cancer is related to thyroid cancer treatment.

Consequently, the authors recommend regular follow-up of all women patients with thyroid cancer and a judicious use of radioactive iodine as a treatment regimen.

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