Thyroid cancer fulfills the criteria for overdiagnosis by having a reservoir of indolent cancers and practice patterns leading to the diagnosis of incidental cancers from the reservoir. The occurrence of overdiagnosis is also supported by population-based data showing an alarming rise in thyroid cancer incidence without change in mortality. Because one of the activities leading to overdiagnosis is the workup of incidental thyroid nodules detected on imaging, it is critical that radiologists understand the issue of overdiagnosis and their role in the problem and solution. This article addresses 1) essential thyroid cancer facts, 2) the evidence supporting overdiagnosis, 3) the role of radiology in overdiagnosis, 4) harms of overdiagnosis, and 5) steps radiologists can take to minimize the problem.
Overdiagnosis is the detection of a disease that is not destined to cause symptoms or result in death. Thyroid cancer represents one of the best examples of overdiagnosis. It has become one of the most rapidly increasing cancer diagnoses in the United States and now ranks as the fifth most common cancer in women . The recent rate of growth in incidence is particularly alarming. The incidence of papillary thyroid cancer has doubled in the last decade, whereas before 2002, the incidence of papillary thyroid cancer doubled over a 30-year period . And yet, these trends are dwarfed by the experience in South Korea where opportunistic ultrasound screening for thyroid cancer is offered as an inexpensive “add-on” during screening for other cancers. This has resulted in a 15-fold increase in thyroid cancer incidence in an 8-year period with no change in mortality .
One of the activities leading to overdiagnosis of thyroid cancer is the workup of incidental thyroid nodules detected on imaging . Thus, radiologists are a key part of the problem and solution. In March 2012, the National Cancer Institute (NCI) convened a meeting to evaluate the problem of overdiagnosis . An important recommendation made by the working group was that the physician, patient, and public need to be aware that overdiagnosis is common. For radiologists, this means being informed about the issue of overdiagnosis and interpreting incidental findings on imaging studies with consideration of cancer biology and epidemiology. This article describes thyroid cancer overdiagnosis for radiologists in the format of five questions. We address 1) essential thyroid cancer facts, 2) the evidence supporting overdiagnosis, 3) the role of radiology in overdiagnosis, 4) harms of overdiagnosis, and 5) steps radiologists can take to minimize the problem.
What are the essential facts about thyroid cancer?
Thyroid cancer ranks as the ninth most common cancer in the United States and the fifth most common cancer in women . It is estimated that there will be more than 62,000 new cases of thyroid cancer in 2014, but the number of deaths in 2014 will be much lower at 1890 . In fact, thyroid cancer has the lowest mortality rate among the top 10 cancers, and as a result, more than half a million people in the United States are currently living with thyroid cancer .
There are four main types of thyroid carcinomas. Papillary and follicular carcinomas arise from the follicular epithelial cells and are known as differentiated thyroid carcinomas. Differentiated thyroid cancers have an excellent prognosis, with a 10-year survival rate >95% for papillary carcinoma and 85% for the follicular type . Medullary thyroid carcinoma arises from neuroendocrine “C” cells and has a survival rate of 75% at 10 years . Anaplastic carcinoma is an aggressive undifferentiated tumor typically occurring in the elderly with a median survival of 9 weeks and a 5-year survival of 7% . The most common histology is papillary carcinoma, which represents 88% of all thyroid malignancies .
Radiologists may have a skewed view of the aggressiveness of thyroid malignancy because cross-sectional imaging with computed tomography (CT) and magnetic resonance imaging (MRI) is only performed for suspected locally invasive cancers or patients with recurrence . However, these “bad actors” represent the minority of thyroid cancers. Only 4% of thyroid cancers have distant metastases, and the rate of thyroid cancer recurrence is low, ranging from 7% to 14% . Although the 5-year survival for thyroid cancer patients with distant metastases is poor at 55%, patients with regional (nodal) metastases still have excellent survival at 98% .
Are we really seeing overdiagnosis of thyroid cancer?
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A Mismatch in Trends in Incidence and Mortality
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A Reservoir of Clinically Silent Cancers
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How have radiologists contributed to thyroid cancer overdiagnosis?
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Incidental Thyroid Nodules on Imaging
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Easy Target for Biopsy
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Incidental Thyroid Cancer on Pathology Specimens
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How can thyroid cancer overdiagnosis be harmful?
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What can radiologists do about thyroid cancer overdiagnosis?
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Conclusions
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