Radiation Therapy Does Not Increase Thyroid Cancer Risk

In an important study published in the journal Cancer, a team of Canadian researchers has determined that women who are treated with radiation therapy for their breast cancer do not appear to be at increased risk of developing thyroid cancer due to scattered radiation.

Lead researcher William Mackillop and colleagues from Kingston General Hospital in Ontario concluded that women who undergo radiation therapy for breast cancer therefore do not require special surveillance of their thyroid gland after their treatment.

Over the past 20 years, the combination of breast conservation therapy plus radiotherapy has been increasingly used to treat early-stage breast cancer. However, some previous studies have suggested that when patients receive radiation therapy, areas adjacent to the affected area also might receive a small dose of radiation.

To determine the potential risk, if any, the researchers studied data from the National Surveillance, Epidemiology, and End Results (SEER) program of the National Cancer Institute. A total of 194,798 women diagnosed with invasive breast carcinoma between 1973-1993 were included in their analysis. Of these women, 48,495 received radiation therapy within the first four months after diagnosis; the remainder of the women (146,303) received no radiation therapy.

The authors then recorded the total number of patients from the study population who were subsequently diagnosed with invasive thyroid cancer. Cases of thyroid cancer that occurred within two years of the breast cancer diagnosis were excluded because the latency period for radiation-induced cancer in the thyroid gland is considered to be at least several years.

Importantly, the researchers did not observe any significant increase in the risk of developing thyroid cancer in either group of women. In fact, they found that the incidence of thyroid cancer in overall study group was comparable to that found in the general population.

"These data should be reassuring to women considering radiation therapy for the treatment of their breast carcinoma," they wrote. "The occurrence of thyroid carcinoma among women with breast carcinoma is not associated with radiation therapy."

SOURCE:

Cancer, September 15, 2001; 92(6):1411-1418

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