Follicular Thyroid Cancer Terminology
Follicular thyroid cancer is commonly abbreviated as FTC in medical literature and clinical practice. 1
Classification and Characteristics
Follicular thyroid carcinoma (FTC) is the second most common histological type of thyroid cancer after papillary thyroid carcinoma (PTC), accounting for approximately 10% of all thyroid cancer cases 2, 3.
The World Health Organization (WHO) classification (updated in 2017) categorizes FTC into several subtypes based on invasion patterns:
- Minimally invasive FTC: Shows capsular penetration without vascular involvement (associated with excellent prognosis)
- Angioinvasive FTC: Shows neoplastic emboli involving fewer than 4 blood vessels
- Widely invasive FTC: Shows neoplastic emboli involving 4 or more blood vessels 1
Diagnostic Challenges
FTC presents unique diagnostic challenges:
- Preoperative distinction between FTC and follicular adenoma (FA) is difficult due to similar cytoarchitectural features 2
- FTC is typically classified as "indeterminate" in various thyroid cytology reporting schemes 1
- Fine-needle aspiration (FNA) cannot reliably diagnose FTC preoperatively, as capsular or vascular invasion must be identified on surgical specimens 1
- Molecular testing may assist with risk stratification but is not universally available 2
Prognostic Factors
Several factors influence FTC prognosis:
- Degree of vascular invasion (most important histological prognostic factor)
- Distant metastasis
- Patient age (>50 years carries worse prognosis)
- Tumor size
- TERT promoter mutation status
- Histological subtype 2, 4
Clinical Implications
Understanding FTC terminology and classification is important because:
- FTC has different biological behavior than PTC, with higher rates of hematogenous spread and lower rates of lymphatic metastasis 4
- FTC requires distinct staging approaches from PTC due to different prognostic factors 4
- Certain FTC variants (oxyphilic/Hürthle cell and insular) may be associated with increased morbidity and mortality 3
- The diagnostic reclassification of historical FTC cases has revealed that many were actually PTC, follicular adenoma, or poorly differentiated carcinoma, which impacts survival statistics 5
Treatment Considerations
Treatment approaches for FTC include:
- Complete surgical tumor removal as primary treatment
- Levothyroxine therapy for TSH suppression
- Radioactive iodine (RAI) for higher-risk cases and metastatic disease
- Multi-tyrosine kinase inhibitors (sorafenib, lenvatinib) for RAI-refractory cases 2, 3, 6
Understanding the specific FTC subtype is crucial for determining appropriate treatment intensity and follow-up protocols.