BACKGROUND
The number of patients diagnosed with thyroid cancer has increased significantly over the last several decades, while death from this cancer has remained consistently low. Part of this is because we have very effective treatments for significant thyroid cancers, meaning those cancers that would cause harm and/or death if not treated. Another part is likely due to the diagnosis of a large number of small thyroid cancers which were found when patients undergo various imaging tests, such as neck ultrasound and CT scans. This is termed “overdiagnosis” of cancer. It is likely that many of these small cancers would never have been diagnosed on their own. As a result, healthy people may undergo unnecessary tests and treatments for small cancers that will never cause any symptoms and will not affect the quality of life or survival of a person. It has been reported that overdiagnosis accounts for up to 60-90% of all cases of thyroid cancer in different countries. This study aimed to evaluate the changes over time in the number significant thyroid cancers that require treatment as well as the number of more aggressive thyroid cancers with worse prognostic factors as defined in the American Thyroid Association (ATA) Guidelines and American Joint Committee on Cancer (AJCC)/TNM staging system.

THE FULL ARTICLE TITLE
Genere N et al 2021 Incidence of clinically relevant thyroid cancers remains stable for almost a century: A population-based study. Mayo Clin Proc 96:2823–2830. PMID: 34736609.

SUMMARY OF THE STUDY
This study that used the Rochester Epidemiology Project database to identify all new cases of thyroid cancer in Olmsted County, Minnesota, U.S.A. from 1935 to 2018. Information regarding the date of cancer diagnosis and type and clinical parameters of the cancer was determined from the patients’ medical records. The Rochester Epidemiology Death Data System was used to assess whether the cause of death was related to thyroid cancer.

Significant thyroid cancers were defined as: (1) high-risk thyroid cancers, including medullary, poorly differentiated, anaplastic, or differentiated thyroid cancers with spread outside of the neck, (2) any thyroid cancer measuring more than 4 cm, and (3) any cancer with significant invasion outside the thyroid into other neck structures or muscle.

A total of 596 cases of thyroid cancer were diagnosed between 1935 and 2018, and among these, 119 cases represented significant thyroid cancers. The average age at diagnosis was 46 years with 71% of cases being women. Most cases (89%) were papillary thyroid cancers and the average cancer size was 1.5 cm.

The sex- and age-adjusted incidence of thyroid cancer increased from 1.3 per 100,000 person-years between 1935 and 1949 to 12.0 per 100,000 person-years between 2010 and 2018, with an overall period percentage change of 24% and an absolute change of 1.4. The data shows a significant increase of 27% in overall thyroid cancer incidence in patients younger than 55 years of age. There was no difference in the trends of thyroid cancer between males and females. Importantly, death from thyroid cancer did not change significantly over time. In addition, there was no significant change in the incidence of each group of significant thyroid cancers.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
This study showed an overall significant increase in the diagnosis of thyroid cancers over the past 80 years, especially in adults younger than 55 years of age, without a corresponding increase in the diagnosis of significant thyroid cancers. In addition, there has not been an increase in death from thyroid cancer. These findings support the fact that most newly diagnosed thyroid cancers represent low risk, small cancers with an excellent prognosis.

— Alina Gavrila, MD, MMSc

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