This is not a universally accepted finding, however, as some researchers point to environmental radiation as a cause of the increased rate of thyroid cancer.
In recent decades, thyroid cancer has reportedly been one of the fastest-growing cancers, even while many other head and neck cancers were declining.
Increasingly sensitive diagnostic techniques -- including ultrasonography, and fine-needle aspiration -- have simplified early detection of subclinical thyroid cancer, before symptoms have appeared.
The investigators evaluated data to determine if patterns over the past three decades show a true increase in the rate of thyroid cancer, or increasing numbers due to better detection. The data, spanning 1973-2002, came from the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) program and the National Vital Statistics System.
The researchers found:
- Thyroid cancer rose from an incidence rate of 3.6 per 100,000 in 1973 to 8.7 per 100,000 in 2002 -- a 2.4-fold increase.
- Almost all of the increased incidence was due to the papillary form of thyroid cancer.
- There was no observed increase in the other, less common forms of thyroid cancer, which include follicular, medullary, and anaplastic thyroid cancer
- Most of the increased incidence were increased detection of small cancers, and almost half of the increase was in cancers less than 1 cm. 87% were cancers smaller than 2 cm
- The death rate from thyroid cancer has remained stable
"Given the known prevalence of small, asymptomatic papillary thyroid cancers at autopsy, we believe this suggests that increased diagnostic scrutiny has caused an apparent increase in incidence of cancer rather than a real increase. Because many of these cancers would likely never have caused symptoms during life, epidemiologists have labeled the phenomenon 'overdiagnosis' -- a term perhaps most familiar in the setting of prostate cancer..."The researchers also suggested that a more cautious approach to thyroid cancer diagnosis -- basically, consisting of followup -- may be called for, and that potentially, "papillary cancers smaller than one centimeter could be classified as a normal finding."
What This Means for Patients
Thyroid nodules are quite common, and it's estimated that as many as 70% of the population will develop nodules in our lifetime. Thyroid nodules are more common with increased age, and more common in women. Only a small percentage of the nodules, however, are actually large enough for a physician or patient to feel externally, and only a small percentage of nodules are cancerous. This research raises the issue that practitioners may not feel a need to evaluate small (less than 1 cm) nodules for potential cancer, or periodic monitoring may be recommended, rather than treatment.
In an accompanying editorial titled "Managing Small Thyroid Cancers." Ernest L. Mazzaferri, MD, of the University of Florida, Gainesville expressed concern about this potential interpretation, saying that not all small papillary thyroid cancers are simply a result of overdiagnosis, and not all of them are inactive or slow to develop.
Instead, Mazzaferri recommends that practitioners continue to follow American Thyroid Association Guidelines for nodule management: