Question

The difference between follicular carcinoma and follicular adenoma of the thyroid is?

• A. Vascular invasion
• B. Mitosis
• C. Nuclear pleomorphism
• D. Tubule formation

Hint:

Cytology cannot tell them apart; you need to see the tumour breach its capsule and enter vessels.

Answer 1

The Correct Option is A

Step 1: Both follicular adenoma and follicular carcinoma are well-differentiated follicular thyroid lesions; cytologically they can look identical, so cytology (FNAC) cannot distinguish them.
Step 2: The defining feature of follicular carcinoma is invasion, specifically capsular invasion (penetration through the full thickness of the tumour capsule) and/or vascular (angioinvasion) invasion of vessels in or beyond the capsule.
Step 3: A follicular adenoma is a fully encapsulated, non-invasive lesion; it does not breach its capsule and does not invade vessels.
Step 4: Because the diagnosis hinges on demonstrating invasion through the entire capsule and into vessels, the diagnosis can only be made on a complete excised specimen (histopathology), not on FNAC.
Step 5: Mitoses, nuclear pleomorphism and tubule (architecture) formation are not the discriminators here; nuclear features such as ground-glass nuclei and orphan-Annie eyes belong to papillary carcinoma, not the adenoma-versus-follicular-carcinoma distinction.
Conclusion: The correct answer is vascular invasion.