# Thyroid Pathology: Follicular Patterned Lesions and Papillary Carcinoma

## Introduction

Thyroid nodules are extremely common, found in up to 50-60% of adults on ultrasound. The pathologist plays a central role in distinguishing benign from malignant thyroid lesions, classifying tumor types according to WHO criteria, and providing prognostic information that guides surgical and medical management.

## Follicular Patterned Lesions: A Diagnostic Spectrum

### The Diagnostic Challenge

Follicular patterned thyroid lesions range from benign (adenomatoid nodule, follicular adenoma) to malignant (follicular carcinoma, follicular variant of PTC). **Nuclear features** and **capsular/vascular invasion** are the critical distinguishing criteria. Fine needle aspiration (FNA) cannot reliably distinguish follicular adenoma from follicular carcinoma, as both are classified as Bethesda IV ("follicular neoplasm/suspicious for follicular neoplasm"). Molecular testing on FNA material is increasingly used to guide management.

### Follicular Adenoma

Follicular adenoma is a **benign** encapsulated neoplasm with follicular cell differentiation. It has a complete, intact capsule with no capsular or vascular invasion. The architecture may be microfollicular, normofollicular, or macrofollicular. Nuclei are round and uniform, without PTC-type nuclear features. Variants include oncocytic (Hurthle cell) adenoma and hyalinizing trabecular tumor (which is controversial and may represent an adenoma or a low-risk neoplasm).

### Follicular Carcinoma

**Minimally invasive** follicular carcinoma shows capsular penetration and/or limited vascular invasion (fewer than 4 vessels). **Encapsulated angioinvasive** tumors demonstrate extensive vascular invasion (more than 4 vessels) and carry a significantly worse prognosis. **Widely invasive** tumors show broad invasion beyond the capsule into surrounding tissue. Diagnosis requires **complete capsule examination** with multiple sections through the capsule-tumor interface (10 or more sections for encapsulated tumors). Vascular invasion is defined as tumor within endothelial-lined vascular spaces **within or beyond the capsule**, and tumor must be adherent to the vessel wall or covered by thrombus to be diagnostic.

### Noninvasive Follicular Thyroid Neoplasm with Papillary-Like Nuclear Features (NIFTP)

NIFTP was reclassified in 2016 from "encapsulated follicular variant of PTC" to a **low-risk neoplasm** rather than a carcinoma. Diagnostic criteria include encapsulation or well-demarcation, follicular growth pattern, PTC-type nuclear features (score 2-3), no invasion, no papillae (more than 1%), no psammoma bodies, no solid/trabecular/insular pattern (more than 30%), no tumor necrosis, and no high mitotic activity. Because it is **not called carcinoma**, the reclassification avoids unnecessary radioactive iodine therapy and lifelong cancer surveillance. Excision alone is curative, and the recurrence rate approaches zero.

![Follicular thyroid carcinoma showing capsular and vascular invasion at the tumor-capsule interface](images/follicular-carcinoma-invasion.jpg)

## Papillary Thyroid Carcinoma

### Diagnostic Nuclear Features

**PTC nuclear features** are the defining diagnostic criterion. These include nuclear enlargement and overlapping, **optically clear nuclei** ("Orphan Annie eyes") with ground-glass chromatin clearing, **nuclear grooves** (longitudinal folds in the nuclear membrane), **intranuclear pseudoinclusions** (cytoplasmic invaginations into the nucleus), and irregular nuclear contours. These features are scored systematically across 3 categories (2 points each) for a total nuclear score of 0-6.

### Histologic Features

**True papillae** consist of fibrovascular cores lined by neoplastic follicular cells with PTC nuclear features. **Psammoma bodies** are concentric laminated calcifications that are highly specific for PTC and rarely seen in other thyroid tumors. **Multinucleated giant cells** and a **lymphocytic infiltrate** are common. Multifocality and bilateral involvement are frequently encountered. Lymph node metastases via lymphatic spread are present in **30-80%** of cases at diagnosis.

### Major Variants of PTC

**Classic PTC** displays papillary architecture with characteristic nuclear features. The **follicular variant** has predominantly follicular architecture with PTC nuclear features and must be distinguished from NIFTP (noninvasive) and the invasive follicular variant. The **tall cell variant** features cells three times as tall as wide in more than 30% of the tumor, is associated with more aggressive behavior, BRAF V600E, and older age. The **hobnail variant** has cells with apical nuclei bulging into the lumen and is aggressive. The **diffuse sclerosing variant** involves one or both lobes diffusely, with prominent squamous metaplasia, psammoma bodies, lymphocytic infiltrate, and is seen in young patients. The **columnar cell variant** has pseudostratified columnar cells and is aggressive when widely invasive. The **cribriform-morular variant** is associated with FAP/APC germline mutations and tends to be bilateral.

| PTC Variant | Key Histologic Features | Molecular/Clinical Associations | Behavior |
|---|---|---|---|
| Classic | Papillary architecture, PTC nuclei | BRAF V600E (~50%) | Indolent |
| Follicular | Follicular architecture, PTC nuclei | RAS mutations | Indolent (if encapsulated/NIFTP) |
| Tall cell | Cells 3× taller than wide, >30% | BRAF V600E, older age | Aggressive |
| Hobnail | Apical nuclei bulging into lumen | BRAF, TP53 | Aggressive |
| Diffuse sclerosing | Diffuse involvement, squamous metaplasia, psammoma bodies | Young patients, RET/PTC | Intermediate |
| Columnar cell | Pseudostratified columnar cells | CDX2+ | Aggressive (if widely invasive) |
| Cribriform-morular | Cribriform architecture, morules | FAP/APC germline mutation | Bilateral; generally favorable |

### Molecular Features

**BRAF V600E** is the most common mutation, present in approximately 45-50% of PTC, and is associated with classic and tall cell variants. It generally confers good prognosis but correlates with less iodine avidity. **RAS mutations** (NRAS, HRAS, KRAS) are more common in the follicular variant. **RET/PTC rearrangements** occur in approximately 10-20% of cases, are associated with radiation exposure and younger patients. **TERT promoter mutations** confer adverse prognosis, particularly when combined with BRAF V600E. Molecular testing on FNA includes ThyroSeq v3 and the Afirma Genomic Sequencing Classifier, used for indeterminate cytology (Bethesda III-IV).

![Papillary thyroid carcinoma showing classic nuclear features including grooves, clearing, and pseudoinclusions](images/ptc-nuclear-features.jpg)

## The Bethesda System for Reporting Thyroid Cytopathology

### Categories and Risk of Malignancy

**Bethesda I** is nondiagnostic/unsatisfactory with 5-10% risk of malignancy (ROM). **Bethesda II** is benign with 0-3% ROM. **Bethesda III** is atypia of undetermined significance/follicular lesion of undetermined significance with 6-18% ROM. **Bethesda IV** is follicular neoplasm/suspicious for follicular neoplasm with 10-40% ROM. **Bethesda V** is suspicious for malignancy with 45-75% ROM. **Bethesda VI** is malignant with 94-99% ROM. The ROM values have been adjusted downward with the NIFTP reclassification.

### Molecular Testing for Indeterminate Nodules

**Bethesda III and IV** nodules benefit from molecular testing to guide surgical decision-making. **Rule-out tests** with high NPV (such as the Afirma GSC) allow a benign result to avoid surgery. **Rule-in tests** with high PPV include ThyroSeq v3 and specific mutation panels (BRAF V600E, RET/PTC, RAS). Molecular testing reduces diagnostic surgery rates by approximately **50%** for indeterminate nodules.

## Grading and Staging

### WHO 2022 Updates

The WHO 2022 classification introduced **tumor grading** for differentiated thyroid carcinomas. **Low-grade** tumors show no necrosis and fewer than 5 mitoses per 10 HPF. **High-grade** tumors demonstrate necrosis and/or 5 or more mitoses per 10 HPF. This grading applies to both papillary and follicular carcinomas. High-grade differentiated carcinomas have worse prognosis, approaching poorly differentiated carcinoma outcomes. **AJCC staging** is based on tumor size (T), lymph node involvement (N), distant metastasis (M), and patient age (under 55 versus 55 and older).

![Thyroid FNA cytology showing Bethesda VI category with papillary carcinoma cells displaying nuclear grooves and inclusions](images/thyroid-fna-ptc.jpg)

## Clinical Pearls

NIFTP reclassification removed thousands of patients from a cancer diagnosis; the key requirements are encapsulation, follicular growth, PTC nuclear features, and absence of invasion, papillae, or adverse histologic features. The diagnosis of follicular carcinoma requires histologic demonstration of capsular and/or vascular invasion; FNA cytology alone cannot make this distinction, and thorough capsular sampling is mandatory. BRAF V600E is the most common mutation in papillary thyroid carcinoma and, when combined with TERT promoter mutation, confers significantly worse prognosis. Molecular testing on Bethesda III/IV FNA specimens can reduce unnecessary diagnostic thyroidectomies by approximately 50% by reliably identifying benign nodules.

## References

1. Nikiforov YE, et al. Nomenclature revision for encapsulated follicular variant of papillary thyroid carcinoma: a paradigm shift to reduce overtreatment of indolent tumors. *JAMA Oncol*. 2016;2(8):1023-1029.
2. Ali SZ, Cibas ES, eds. *The Bethesda System for Reporting Thyroid Cytopathology*. 3rd ed. Springer; 2023.
3. WHO Classification of Tumours Editorial Board. *WHO Classification of Tumours: Endocrine and Neuroendocrine Tumours*. 5th ed. IARC; 2022.
4. Steward DL, et al. Performance of a multigene genomic classifier in thyroid nodules with indeterminate cytology: a prospective blinded multicenter study. *JAMA Oncol*. 2019;5(2):204-212.