Follicular Thyroid Carcinoma Characteristics

Follicular thyroid carcinoma (FTC) spreads via the hematogenous route (option c is correct). 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 (option a is incorrect) 1, 2
The incidence rates of FTC have remained relatively stable over the past 30 years 1
There are geographical variations in the relative proportions of FTC and PTC, with higher rates of FTC in iodine-deficient areas 3

FTC cannot be readily diagnosed with fine-needle aspiration (FNA) cytology alone (option b is incorrect) 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
The cytoarchitectural features of FTC are similar to those of follicular adenoma, making preoperative distinction challenging 2

FTC characteristically spreads via the hematogenous route, leading to distant metastases in organs such as lungs and bones 1
This contrasts with papillary thyroid carcinoma, which typically spreads via lymphatics 1
The propensity for hematogenous spread correlates with the degree of vascular invasion 5, 6

FTC is typically unifocal (not multifocal) at presentation (option d is incorrect) 1
This contrasts with PTC, which is more commonly multifocal 1

The WHO classifies FTC into three categories based on invasion pattern: minimally invasive, angioinvasive, and widely invasive 1
Prognosis varies significantly based on this classification 6:
- Minimally invasive FTC (capsular invasion only): 5-year survival rate of 98% 6
- Moderately invasive FTC (angioinvasive): 5-year survival rate of 80% 6
- Widely invasive FTC: 5-year survival rate of 38% 6
Age and vascular invasion are powerful predictors of distant metastasis 5

The inability to diagnose FTC by FNA necessitates surgical excision for definitive diagnosis 1, 4
Treatment approach should be based on the degree of invasion 5:
- For patients <45 years with minimally invasive FTC without vascular invasion, hemithyroidectomy may be adequate 5
- All other patients should undergo total thyroidectomy and radioactive iodine ablation 5
Careful histopathological examination is crucial to determine the extent of capsular and vascular invasion, which directly impacts prognosis and treatment decisions 6

Misdiagnosing FTC as a benign follicular adenoma based on cytology alone 2, 4
Underestimating the risk of distant metastasis in cases with vascular invasion 5, 6
Failing to recognize that FTC is typically unifocal rather than multifocal, which can affect surgical planning 1
Not considering the degree of vascular invasion when determining prognosis and treatment approach 6