Understanding the 2022 WHO Thyroid Cancer Classification

The diagnosis and classification of cancer is essential for providing the best care to individual patients, as well as for researching the cause, prevention, diagnosis and treatment of cancer. In the past, cancer classification was based on the consensus of histopathological opinion, with limited consideration of molecular pathology. However, new technologies are revolutionizing the field of pathology and it is becoming increasingly clear that the traditional approach to cancer classification is inadequate. Our understanding of cancer at the molecular level has advanced to the point where this information should be included in diagnoses.

Digital pathology and image analysis are also providing new insights, offering quantitative justification for many existing diagnostic criteria and challenging others. With rapid improvements in computer technology, including artificial intelligence, clinically applicable diagnostic aids are becoming available, and this trend is likely to accelerate. The International Agency for Research on Cancer (IARC) has been responsible for the WHO Tumor Classification, also known as the WHO Blue Books, since the third edition (2000-2001), which covered all organ sites in 10 volumes. The WHO document divides the molecular classification of the atlas7 PC genome into two large groups (BRAF type and Ras type), and also includes the main molecular characteristics of the different PC subtypes.

Although NIFTP shares the molecular alterations of thyroid tumors that show a follicular pattern and exhibits a high prevalence of mutations in the RAS gene family, it can sometimes be associated with fusions of the PPARG and THADA genes. A number of tumors remain whose cell lineage is unclear and are listed as such; these include sclerosing mucoepidermoid carcinoma with eosinophilia and cribriform-morular thyroid carcinoma. This review summarizes the changes in the fifth edition of the WHO Classification of Endocrine and Neuroendocrine Tumors related to the thyroid gland. Anaplastic thyroid carcinoma remains the most undifferentiated form; squamous cell carcinoma of the thyroid is now considered a subtype of anaplastic carcinoma.

The current classification also emphasizes the value of biomarkers that can aid diagnosis and provide prognostic information. The IARC WHO Tumor Classification Group is responsible for publishing the WHO Tumor Classification series, which is currently in its fifth edition. Although there is debate as to whether cribiform-morular variant is a form of PC, it represents the form of thyroid cancer found in patients with familial adenomatous polyposis. Papillary thyroid carcinomas (PTC), with many morphological subtypes, represent BRAF-type malignancies, while invasive encapsulated follicular variant PTC and follicular thyroid carcinoma represent RAS-type malignancies.

Several unusual neoplasms that occur in the thyroid have been placed in new sections based on their cytogenesis. It is essential to integrate these facets of diagnosis into international cancer classification systems and to update the WHO Tumor Classification regularly. The association between adenolipomas and hamartoma tumor syndrome PTEN is highlighted, particularly when they occur in young individuals with multiple thyroid nodules. This tumor variant is characterized by a high proliferation index (Ki-67 ≈ 10%), with frequent spread beyond the thyroid gland, relapses and metastases to distant lymph nodes. The 2022 WHO Tumor Classification will be an important milestone in understanding how to diagnose and classify thyroid cancer accurately.

It will provide clinicians with valuable information about biomarkers that can be used to diagnose and prognosticate patients with thyroid cancer more effectively. Furthermore, it will help researchers gain a better understanding of how different types of thyroid cancer develop and progress.

Greta Rulnick
Greta Rulnick

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