DENVER — Hypothyroidism occurs in up to half of patients within 5 years following radiotherapy for head and neck cancer in which the gland remains outside the primary field of radiation but is included in the low-neck field, Anna A. Norris, M.D., reported at the annual meeting of the American Society for Therapeutic Radiology and Oncology.
Endocrinologists agree that it's important to begin thyroid replacement therapy at the first indication of hypothyroidism, so regardless of whether symptoms are even present, it's advisable to screen patients who have undergone radiotherapy which included the thyroid in the low-neck field by regularly measuring TSH levels. A good regimen is to check the TSH every 6 months for the first 2 years and annually thereafter, said Dr. Norris of the University of Florida, Gainesville.
She reported on 390 consecutive patients who underwent radiotherapy for oro-pharyngeal cancer during the 1990s at the university medical center.
None had a history of thyroid disease or surgery. All received radiotherapy with a standard low-neck field that covered the area—including the entire thyroid—with a dose of 50 Gy, with boosts bringing the total dose to selected portions of the field up to 70 Gy. Median follow-up was 6 years.
During the 1990s, the practice at the university was to not routinely monitor TSH in patients who had undergone radiotherapy that included the thyroid only in the low-neck field. But in the subset of 169 patients in whom TSH was monitored, the 5-year incidence of hypothyroidism—as defined by a TSH greater than 4.5 ng/mL, regardless of symptoms—was 54%.
In a retrospective study like this it's impossible to know how many patients had their TSH measured. But even if none of the 221 patients whose TSH was unchecked had hypothyroidism, the 5-year rate following radiotherapy was still 31%, she noted.