Some tumors previously designated as well-differentiated follicular variants of papillary carcinoma of the thyroid are being downgraded to a borderline or benign tumor status.
Alternate name: follicular adenoma with nuclear atypia
Features:
(1) follicular pattern of growth (less than 1% papillary)
(2) nuclear features as seen in papillary carcinoma (3 or more foci per 3 mm of linear tumor growth observed with 40x objective)
(3) well-encapsulated
(4) absence of invasion (both capsular and/or lymphovascular)
(5) absence of tumor necrosis
(6) low mitotic rate (<= 3 per 10 high power fields)
(7) absence of metastases
Molecular findings:
(1) RAS mutation or PAX8/PPARG rearrangement
(2) absence of BRAF V600E
(3) absence of RET/PTC rearrangement
where:
• Capsular invasion was defined as penetration through the capsule greater than 50% of capsule thickness,
• Vascular invasion was defined as a tumor deposit within a vascular channel within or immediately beyond the tumor capsule.
• Papillary-like nuclear features include: nuclear enlargement, crowding and overlapping, loss of polarity, elongation, ovoid shape, grooves or folds, pseudoinclusions, nuclear chromatin clearing, nucleoli on nuclear membranes, nuclear margination.
