Thyroid cancer: Rising incidence or overdiagnosis?
The reported incidence of thyroid cancer in Australia is steadily increasing. However, evidence suggests that there is not a true increase in the incidence of thyroid cancer, rather there is an increase in the number of people being diagnosed. Professor Anthony Gill explains that, despite a dramatic increase in diagnosis, there has been no change in the number of people dying of thyroid cancer.
“About 3,000 people are diagnosed clinically with thyroid cancer in Australia every year. However, in a large number of people thyroid cancer will be subclinical, meaning it will not cause any symptoms during their lifetime. In fact, around 10% of all people who die from other causes will be shown to have thyroid cancer if the thyroid is examined by a pathologist at autopsy.
“Thyroid cancers are increasingly encountered incidentally due to the widespread use of imaging and are often discovered when a patient is undergoing tests for unrelated conditions. This has contributed to the increase in incidence of thyroid cancer even though many of these cancers would not have caused the patient any problems throughout their lives, even if they received no treatment,” said Prof Gill.
Despite dramatically increasing incidence of thyroid cancer, there has been no change in the number of people dying of thyroid cancer. For example, although the rate of diagnosis of thyroid cancer in Australia has increased 8-fold in the last 45 years, there has been no increase in the number of people dying of thyroid cancer, despite no new treatments being introduced in that time. In addition, the rate of thyroid cancer being diagnosed at autopsy has stayed the same over the last 80 years (4 to 11%, averaging about 10% in all studies). This is strong evidence that the true incidence of thyroid cancer is not increasing, only the number of people being diagnosed.
In 1999, a government-funded national cancer screening program in South Korea led to the widespread use of ultrasound to screen for thyroid cancer in people without any symptoms. Between 1993 and 2011 the country saw a 15-fold increase in the diagnosis of thyroid cancer associated with routine use of neck ultrasounds as part of physical examinations, but no change in mortality. Although it is unclear whether this rise represents over diagnosis due to increased screening, it cannot be ruled out.
“The main harm with overdiagnosis is ‘overtreatment’ which is when patients receive treatment for a cancer that would not have caused problems otherwise. In addition to the risks associated with receiving unnecessary treatment, a cancer diagnosis often causes patients to worry even if no treatment is required,” said Prof Gill.
Differentiating between a benign and malignant nodule can be challenging, and community guidelines have standardised investigation, management and follow-up procedures. The key tests for risk stratification of thyroid nodules include serum thyroid-stimulating hormone testing, ultrasonography and fine-needle aspiration.
“Overdiagnosis can be reduced by using ultrasounds only when there are symptoms, rather than as part of a routine physical. If an ultrasound finds a small nodule in the thyroid, most experts now recommend that these usually do not need further tests,” said Prof Gill.