Thyroid nodule classification

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Mahshid Mir, M.D. [2]

Bethesda System for Reporting Thyroid Cytopathology

• 19888858

classification	FNA cytology	Predicted risk of malignancy
Benign	Macrofollicular Adenomatoid/hyperplastic nodules Colloid adenomas (most common) Nodular goiter Lymphocytic thyroiditis Granulomatous thyroiditis	0–3 %
Follicular lesion of undetermined significance	Mixed macro- and microfollicular nodules	5–15 %
Atypia of undetermined significance	Atypical cells
Follicular neoplasm	Microfollicular nodules Hurthle cell lesions	15–30 %
Suspicious for a follicular neoplasm	Suspicious for Hurthle cell neoplasm
Malignant	PTC (most common) MTC Anaplastic carcinoma High-grade metastatic cancers	97–99 %

The risk of malignancy development based on the FNA result in case of indefinite diagnosis are as below:

• Nondiagnostic or Unsatisfactory: 1–4 % predicted risk of malignancy.
• Suspicious for malignancy: 60–75 % predicted risk of malignancy

Thyroid carcinomas are classified according to the cell type from which they develop

Classification of neoplastic thyroid nodules based on their origin:

		Origin	histologic subtypes
nonmedullary thyroid cancers (NMTCs)	95% of tumors	thyroid epithelial cells	papillary (85%)	95% are sporadic tumors 5% may be related to inherited genetics due to familial origin
			follicular (11%)
			Hürthle cell (3%)
			anaplastic (1%)
Medullary thyroid cancers (MTCs)	5% of all thyroid malignancies	calcitonin-producing parafollicular cells		20% they are familial and occur as part of the multiple endocrine neoplasia (MEN) syndromes