Characteristics of Follicular Thyroid Carcinoma
Follicular thyroid carcinoma (FTC) spreads via the hematogenous route, not through lymphatics, which is answer choice (c). 1
Key Characteristics of Follicular Thyroid Carcinoma
- FTC is the second most common thyroid cancer after papillary thyroid carcinoma (PTC), not the most common thyroid cancer, making option (a) incorrect 1, 2
- FTC cannot be readily diagnosed with fine-needle aspiration (FNA) cytology alone, as it typically yields "indeterminate" results in thyroid cytology reporting schemes, making option (b) incorrect 1
- Definitive diagnosis of FTC requires histological evidence of capsular and/or vascular invasion, which can only be determined after surgical excision 1
- FTC is typically unifocal (not multifocal) at presentation, contrasting with PTC which is more commonly multifocal, making option (d) incorrect 1
Metastatic Pattern and Spread
- FTC characteristically spreads via the hematogenous route (blood vessels) rather than lymphatic vessels 1, 3
- This hematogenous spread pattern explains why FTC more commonly presents with distant metastases to bones, lungs, brain, and liver compared to PTC 2, 4
- The degree of vascular invasion (angioinvasion) is a critical prognostic factor that directly correlates with the risk of distant metastasis 5, 3
Classification and Prognostic Implications
- The WHO classifies FTC into three categories based on invasion pattern: minimally invasive, angioinvasive, and widely invasive 1
- Some experts recommend a three-tier classification system: minimally invasive (capsular invasion only), moderately invasive (angioinvasion with or without capsular invasion), and widely invasive 3
- Survival rates vary significantly based on invasion pattern: 98% 5-year survival for minimally invasive FTC, 80% for angioinvasive FTC, and only 38% for widely invasive FTC 3
- The presence of vascular invasion is one of the most powerful predictors of distant metastasis 5
Diagnostic Challenges
- Unlike papillary thyroid carcinoma, FTC cannot be diagnosed by cytologic features alone 6
- The definitive diagnosis requires histological demonstration of capsular and/or vascular invasion 1, 6
- This diagnostic limitation explains why FTC is often classified as "indeterminate" on FNA, requiring surgical excision for definitive diagnosis 1