DENVER — Up to half of all patients become hypothyroid within 5 years following radiotherapy for head and neck cancer in which the thyroid remains outside the primary field of radiation but is included in the low-neck field, Dr. Anna A. Norris reported at the annual meeting of the American Society for Therapeutic Radiology and Oncology.
As a result, such patients should undergo TSH screening. A good regimen is to check the TSH levels every 6 months for the first 2 years after radiotherapy and annually thereafter, said Dr. Norris of the University of Florida, Gainesville.
She reported on 390 consecutive patients who underwent radiotherapy for oropharyngeal cancer during the 1990s at the university. 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 6.1 years.
During the 1990s, the practice at the university was to not routinely monitor TSH levels in such patients. 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—was 54%.
In a retrospective study, it's impossible to know how many patients had their TSH measured because of symptoms or physician preference. But even if it is assumed none of the 221 untested patients had hypothyroidism, the 5-year rate following radiotherapy was still 31%, she noted.