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Produced: 04/17/2026 7:22 PM
Question 20260003
Inquiry Details
References:
#1: Solid Tumor Rules. Other Sites, 2026 Update
#2: WHO Class Endocrine Tumors. 5th edition, online version
Question:
Solid Tumor Rules/Histology--Thyroid: What is the correct histology for invasive encapsulated follicular variant of papillary thyroid carcinoma (IEFVPTC)? The 2026 Solid Tumor Rules (STR) Manual, Other Sites Table 12, conflicts with the ICD-O-3.2. See Discussion.
Discussion:
STR Manual, Table 12, Thyroid Histologies, includes "Invasive encapsulated follicular variant of papillary thyroid carcinoma" as histology 8340/3 and is on its own row from other papillary thyroid carcinomas (PTC). A new footnote was added which states, "IEFVPTC and Infiltrative follicular variant of papillary thyroid carcinoma (a PTC subtype) share a histology code, but they are distinctly different histologies. They are on different rows of the table and are different primaries." However, IEFVPTC (and its synonyms) are listed in the ICD-O-3.2 as 8343/3, and 8343/3 was listed as a subtype/variant of papillary thyroid carcinoma in previous versions of the STR Manual.
Answer:
Assign histology as 8340/3 for IEFVPTC using the STR Manual, 2026 Update. Rule M18, Note 2, of the Other Sites STR state: Invasive encapsulated follicular variant of papillary thyroid carcinoma and Infiltrative follicular variant of papillary thyroid carcinoma share a histology code (8340) but are distinctly different entities. They are on separate rows of the table.
WHO Classification of Endocrine and Neuroendocrine Tumors, 5th ed., indicates that after classic PTC, the follicular variant of PTC (FVPTC) is the second most common histological subtype of PTC. Two major forms are known, infiltrative FVPTC and invasive encapsulated FVPTC. The majority of follicular PTCs are encapsulated FVPTCs, whereas infiltrative FVPTC is quite rare and clinically behaves like classic PTC.
