A 45-year-old woman undergoes thyroidectomy for a thyroid nodule. Histopathology shows a malignant tumour with follicular architecture, moderate nuclear pleomorphism, and scattered mitotic figures. What is the most common type of thyroid cancer in this patient?

Explanation

## Most Common Thyroid Malignancy **Key Point:** Papillary thyroid carcinoma (PTC) is the most common thyroid cancer, accounting for 80–85% of all thyroid malignancies. ### Epidemiology & Frequency | Thyroid Cancer Type | Frequency | 5-Year Survival | | --- | --- | --- | | Papillary | 80–85% | 93% | | Follicular | 10–15% | 85% | | Medullary | 3–5% | 81% | | Anaplastic | 1–2% | 5–10% | | Lymphoma | <1% | Variable | **High-Yield:** Despite being the most common, PTC has the *best* prognosis because it grows slowly, spreads to lymph nodes (not distant organs early), and responds well to radioactive iodine therapy and TSH suppression. ### Histological Features of PTC 1. **Nuclear features** (diagnostic hallmark): - Overlapping nuclei ("crowded" appearance) - Grooved nuclei ("coffee-bean" shape) - Intranuclear pseudoinclusions - Fine, dispersed chromatin ("Orphan Annie eye" nuclei) 2. **Architecture**: - Papillary growth pattern (not always present) - Follicular variant (most common variant) - Solid, trabecular, or mixed patterns 3. **Spread pattern**: - Early lymph node metastasis (30–50% at diagnosis) - Late distant metastasis (lung, bone) - Encapsulated variants have better prognosis **Clinical Pearl:** Even with lymph node involvement, PTC prognosis remains excellent. Distant metastases (especially to lungs) occur in only 5–10% of cases and are often slow-growing. ### Why PTC is Most Common - **Radiation sensitivity**: PTC arises from follicular epithelium and is the predominant cancer after radiation exposure (Chernobyl, atomic bombs, childhood head/neck radiation). - **Female predominance**: 3:1 female-to-male ratio (like most thyroid diseases). - **Age**: Can occur at any age but peak is 30–50 years. **Mnemonic — PTC Features: COFFEE** - **C**rowded nuclei - **O**rphan Annie eye appearance - **F**ollicular variant (most common) - **F**ine chromatin - **E**arly lymph node spread - **E**xcellent prognosis **Warning:** The stem describes "follicular architecture" — this does NOT mean follicular carcinoma. The follicular variant of PTC is the most common PTC subtype and still shows nuclear features of PTC (grooved, overlapping nuclei) that distinguish it from true follicular carcinoma. [cite:Robbins 10e Ch 24]