The treatment of thyroid cancer — surgery followed by post-operative radiation, typically with radioactive iodine — has been extremely successful in treating thyroid cancer, with a very low cancer-related mortality rate. Comes now, however, is a very disturbing study from Stanford University Medical School showing that people so treated have a much higher than expected likelihood of getting malignant melanoma in the area where the thyroid resided before it was removed. The authors tracked seventeen separate databases from the SEER database, which tracks cancers from local cancer registries all over the US (these registries are typically linked together in a network; each originates in the pathology department of a local hospital). They looked at over 174,000 cases of thyroid cancer and analyzed which patients received radiaton and which did not. They found an excess incidence of 25%. Interestingly, although the overall incidence of melanoma was higher, only 20% of the melanomas occurred in the region of the previous thyroid. The rest of the excess is unexplained, other than to blame the effects of ionizing radiaion on cancer risk. Radioiodine is administered as a drink; the iodine migrates anywhere there is thyroid tissue but travels throughout the body first. It works because removal of the thyroid invariably is not total; some thyroid tissue is left behind.
So what is the take home message? There is no free lunch in cancer care. Even without radiation most patients with thyroid cancer are cured with surgery. Among all the irradiated patients there were almost 500 melanmoas. Is it worth it? Physicians and epidemiologists will be struggling with this for a while.