A 28-year-old woman is found to have thyroid nodules on ultrasound. Fine-needle aspiration cytology (FNAC) reveals malignant cells. Which is the most common type of thyroid cancer?

Explanation

## Most Common Type of Thyroid Cancer **Key Point:** Papillary thyroid carcinoma (PTC) accounts for 80–85% of all thyroid cancers, making it by far the most common type. It arises from thyroid follicular epithelium and has an excellent prognosis with appropriate treatment. ### Epidemiology and Classification | Type | Frequency | Origin | Prognosis | 10-Year Survival | |------|-----------|--------|-----------|------------------| | **Papillary** | 80–85% | Follicular epithelium | Excellent | >95% | | Follicular | 10–15% | Follicular epithelium | Good | 85–90% | | Medullary | 3–5% | Parafollicular C cells | Intermediate | 70–80% | | Anaplastic | 1–2% | Follicular epithelium (dedifferentiated) | Poor | <10% | | Lymphoma | 1–2% | Lymphoid tissue | Variable | Variable | ### Key Features of Papillary Thyroid Carcinoma **High-Yield:** PTC characteristics: - **Slow growth** — often indolent course - **Lymph node metastasis** — common (30–50% at presentation) but does NOT worsen prognosis significantly - **Distant metastasis** — rare (<10%); when present, typically to lungs and bone - **Radioiodine sensitivity** — responds well to radioactive iodine (I-131) therapy - **TSH suppression** — standard treatment includes levothyroxine to suppress TSH ### Histopathology **Clinical Pearl:** PTC is characterized by: 1. **Papillary architecture** — finger-like projections with fibrovascular cores 2. **Orphan Annie eye nuclei** — pale, empty-appearing nuclei with fine chromatin (pathognomonic) 3. **Intranuclear pseudoinclusions** — nuclear grooves and inclusions 4. **Ground-glass chromatin pattern** ### Why PTC is Most Common **Mnemonic:** **P**apillary = **P**erfect (prognosis), **P**revailing (most common), **P**roliferation (slow) The reasons for PTC's high frequency are: 1. Lower malignant potential compared to other types 2. Slower growth rate allows detection at earlier stages 3. Better differentiation allows survival of tumor cells 4. Radiation sensitivity (including from natural background radiation and medical exposures) ### Molecular Basis PTC is often driven by: - **RET/PTC rearrangements** (fusion oncogenes) - **BRAF V600E mutations** (associated with aggressive variants) - **TERT promoter mutations** (less common) ### Treatment and Prognosis **Key Point:** Standard management includes: 1. Total thyroidectomy (or near-total) 2. Radioactive iodine ablation (for high-risk cases) 3. TSH suppression with levothyroxine 4. Long-term follow-up with thyroglobulin monitoring The excellent prognosis of PTC means that even patients with lymph node involvement at presentation have >90% 10-year survival with appropriate treatment.