What is a characteristic of follicular thyroid carcinoma?

Follicular Thyroid Carcinoma: Key Characteristics

Follicular thyroid carcinoma (FTC) spreads via the hematogenous route, making it distinct from other thyroid cancer types in its metastatic pattern. 1

Epidemiology and Classification

• FTC is the second most common thyroid cancer after papillary thyroid carcinoma (PTC), not the most common 1, 2
• Incidence rates for FTC have remained relatively stable over the past 30 years, unlike the rising rates of PTC 1
• The WHO classifies FTC into three categories based on invasion pattern:
  • Minimally invasive: capsular penetration without vascular involvement (excellent prognosis) 1
  • Angioinvasive: neoplastic emboli involving <4 blood vessels 1
  • Widely invasive: neoplastic emboli involving ≥4 blood vessels or broad areas of transcapsular invasion 1, 3

Diagnostic Challenges

• FTC cannot be readily diagnosed with fine-needle aspiration (FNA) cytology alone 1
• FTC is typically classified as "indeterminate" in thyroid cytology reporting schemes 1
• Definitive diagnosis requires histological evidence of capsular and/or vascular invasion, which can only be determined after surgical excision 1, 4
• Molecular testing may help identify malignancy in indeterminate cytology samples, but has limitations for FTC 1

Metastatic Pattern

• FTC characteristically spreads via the hematogenous (blood vessel) route 5, 3
• This contrasts with PTC, which typically spreads via lymphatic channels 6
• The hematogenous spread pattern explains why FTC more commonly presents with distant metastases to bone, lung, brain, and liver rather than regional lymph node metastases 5, 6
• The degree of vascular invasion is a critical prognostic factor and directly correlates with metastatic potential 5, 3

Multifocality

• FTC is typically unifocal (not multifocal) at presentation 1
• This contrasts with PTC, which is more commonly multifocal 1
• The unifocal nature of FTC influences surgical approach decisions 4

Prognostic Factors

• Key prognostic factors include:
  • Degree of vascular invasion (most important) 5, 3
  • Age (patients <45 years have better outcomes) 5, 6
  • Tumor size 2, 3
  • Presence of distant metastases at diagnosis 2, 3
• Disease-free survival rates vary significantly based on invasion pattern:
  • Minimally invasive (capsular only): 97% 5
  • Angioinvasive: 81% 5, 3
  • Widely invasive: 46% 5, 3

Treatment Implications

• The hematogenous spread pattern and unifocal nature of FTC influence treatment decisions 5, 4
• Total thyroidectomy followed by radioactive iodine ablation is recommended for most FTC cases, especially those with vascular invasion 5, 4
• Patients with minimally invasive FTC without vascular invasion may be candidates for more conservative surgery (lobectomy) 5, 4
• Long-term surveillance should focus on detecting distant metastases rather than regional recurrence 6