A 30-year-old woman with a thyroid nodule undergoes FNA. The cytology report reads 'follicular neoplasm; cannot exclude follicular carcinoma' (Bethesda Category IV). The definitive distinguishing feature between follicular adenoma and follicular carcinoma on histopathology is:

• A Size: >4 cm nodule = carcinoma
• B Capsular invasion or vascular invasion — neither is assessed on FNA ✓
• C Nuclear pleomorphism and mitotic rate
• D RAS mutation on molecular testing

Explanation

Follicular adenoma and follicular carcinoma are architecturally identical on FNA because cytology cannot assess the capsule or vasculature. The histological diagnosis requires intact capsule examination: follicular carcinoma is defined by unequivocal capsular penetration (complete transgression, not just indentation) and/or angioinvasion (intravascular tumor within capsular vessels with fibrin). Without invasion, the lesion is benign adenoma regardless of size or atypia. RAS mutations occur in both adenoma and carcinoma and do not distinguish them. Nuclear pleomorphism characterizes poorly differentiated carcinoma as a secondary finding, not the primary diagnostic criterion.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.