Thyroid masses pathology and diagnostic
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Thyroid Masses: Pathology and Diagnostics
Overview
Thyroid nodules are common - with a lifetime incidence of 4-7% and an annual incidence of approximately 0.1% in the United States (~300,000 new nodules/year). The majority are benign (colloid nodules, adenomas, cysts, focal thyroiditis). Only approximately 5% harbor carcinoma, though approximately 1 in 200 nodules is lethal. The diagnostic challenge is separating benign from malignant disease while balancing the morbidity of treatment. - Cummings Otolaryngology, p. 2375
Classification of Thyroid Masses
Benign Lesions
| Type | Key Features |
|---|---|
| Colloid (adenomatoid) nodule | Most common; degenerative change in multinodular goiter |
| Follicular adenoma | Solitary, encapsulated, no capsular/vascular invasion |
| Thyroid cyst | Simple or complex; may result from degeneration |
| Hashimoto thyroiditis | Autoimmune; can form a mass; associated with lymphoma risk |
| Thyroglossal duct cyst | Congenital; midline, moves with swallowing; lined by squamous or respiratory epithelium |
| Subacute (de Quervain) thyroiditis | Painful; granulomatous inflammation |
Malignant Lesions (overview)
| Type | Frequency | Cell of Origin |
|---|---|---|
| Papillary carcinoma | ~85% | Follicular epithelium |
| Follicular carcinoma | 5-15% | Follicular epithelium |
| Oncocytic (Hürthle cell) carcinoma | Subset of follicular | Follicular epithelium |
| Medullary carcinoma | ~5% | Parafollicular C cells |
| Anaplastic carcinoma | <5% | Dedifferentiated |
| Thyroid lymphoma | Rare | B lymphocytes (often on Hashimoto background) |
Malignant Tumors: Detailed Pathology
1. Papillary Thyroid Carcinoma (PTC)
Epidemiology: Most common thyroid cancer (~85%). Peak age 25-50 years. Strongly associated with prior ionizing radiation exposure. More common in women.
Macroscopy: May be solitary or multifocal. Some well-circumscribed, others infiltrative with ill-defined margins. Often cystic; may contain fibrosis and calcification.
Histopathology (Fig. 24.19):
Fig. 24.19 - Papillary carcinoma of the thyroid. (A) Gross - papillary structures visible on cut surface. (B) Branching papillae with fibrovascular cores. (C) Crowded cells with characteristic nuclear features. (D) "Orphan Annie eye" nuclei with nuclear pseudo-inclusions (arrow). (Robbins & Cotran Pathologic Basis of Disease)
Key microscopic hallmarks:
- Branching papillae with fibrovascular stalks covered by cuboidal epithelial cells
- "Orphan Annie eye" (ground-glass) nuclei - finely dispersed chromatin, optically clear appearance
- Nuclear grooves (coffee-bean shape) and nuclear pseudoinclusions (cytoplasmic invaginations)
- Psammoma bodies - concentrically calcified structures in papillary cores/stroma; virtually pathognomonic (absent in follicular and medullary carcinoma)
- Lymphatic invasion common; blood vessel invasion uncommon in smaller lesions
Spread: Lymph node metastases to cervical nodes in up to 50% of cases. Hematogenous metastasis (to lung) less common.
Molecular drivers: BRAF V600E point mutation (most common) or fusion genes (RET/PTC, NTRK, BRAF fusions).
Prognosis: Excellent - 10-year survival >95%. 5-20% local/regional recurrence; 10-15% distant metastases. Prognosis worsens with age >55, extrathyroidal extension, distant metastases.
2. Follicular Carcinoma
Epidemiology: 5-15% of thyroid cancers (up to 25-40% in iodine-deficient regions). Peak age 40-60 years.
Macroscopy:
Fig. 24.20 - Follicular carcinoma. (A) Cut surface showing tan appearance with foci of hemorrhage. (B) Glandular lumens containing recognizable colloid. (Robbins & Cotran)
Solitary nodule; may be circumscribed or widely infiltrative. Gray-tan-pink on cut surface; may be translucent (large colloid-filled follicles). Larger lesions may penetrate the capsule.
Histopathology: Uniform cells forming small follicles with colloid - resembles normal thyroid. No ground-glass nuclei, no psammoma bodies. A subset shows oncocytic (Hürthle cell) features with abundant granular eosinophilic cytoplasm.
Critical distinction from adenoma: Requires demonstration of capsular invasion and/or vascular invasion on histology. FNA alone cannot distinguish follicular adenoma from minimally invasive follicular carcinoma - extensive capsular sampling is mandatory.
- Vascular invasion involves venous vessels within and just beyond the capsule
- Widely invasive follicular carcinoma: obvious infiltration of thyroid parenchyma and extrathyroidal soft tissues
Spread: Unlike PTC, hematogenous spread predominates (lung, bone). Lymphatic spread is uncommon.
Molecular drivers: Oncogenic RAS mutations and PAX8:PPARG fusions.
3. NIFTP (Noninvasive Follicular Thyroid Neoplasm with Papillary-Like Nuclear Features)
A reclassified entity - formerly "encapsulated follicular variant of PTC." Encapsulated, no invasion. Has PTC-like nuclear features but behaves as a benign/borderline lesion with excellent prognosis after surgical resection alone. This reclassification was made to avoid overtreatment.
4. Medullary Thyroid Carcinoma (MTC)
Epidemiology: ~5% of thyroid neoplasms. Derived from parafollicular C cells (neuroendocrine). 70% sporadic; 30% familial (MEN2A, MEN2B, familial MTC) - all caused by germline RET proto-oncogene mutations.
Macroscopy: Sporadic = solitary nodule. Syndromic = bilateral and multicentric. Firm, pale gray to tan, infiltrative. Areas of necrosis and hemorrhage in larger lesions.
Histopathology:
Fig. 24.22B - Medullary carcinoma. Nested architecture with round nuclei, stippled (neuroendocrine) chromatin, and intercellular amyloid (pink material) derived from calcitonin peptides. (Robbins & Cotran)
- Polygonal to spindle-shaped cells in nests, trabeculae, or follicles
- Amyloid deposits in stroma (calcitonin-derived) - Congo red positive
- Immunohistochemistry positive for calcitonin, CEA, synaptophysin, chromogranin
- Electron microscopy: membrane-bound electron-dense secretory granules
- Syndromic cases: C-cell hyperplasia in surrounding thyroid (precursor lesion) - often in contralateral lobe
Biomarkers:
- Serum calcitonin - key diagnostic and postoperative follow-up marker
- Serum CEA - useful especially in calcitonin-negative tumors
- Also may secrete: serotonin, corticotropin (ACTH), VIP
Clinical presentations: Neck mass; may have diarrhea (VIP), Cushing syndrome (ACTH). Hypocalcemia is NOT prominent despite elevated calcitonin.
Genetics: All MEN2 patients carry germline RET mutations. Prophylactic thyroidectomy offered to asymptomatic carriers. Treatment of advanced disease: RET tyrosine kinase inhibitors.
5. Poorly Differentiated and Anaplastic Thyroid Carcinoma
Anaplastic carcinoma is one of the most aggressive cancers known. It typically arises from dedifferentiation of pre-existing well-differentiated carcinoma. Associated with TP53 and TERT promoter mutations (in addition to RAS/BRAF as initiating events).
Macroscopy/Clinical: Rapidly enlarging bulky neck mass. At presentation, disease has usually spread beyond the thyroid or metastasized to the lungs. Symptoms of compression/invasion (dyspnea, dysphagia, hoarseness, cough) are common. Death occurs within 1 year in most cases.
Histopathology: Three main patterns - spindle cell, pleomorphic giant cell, and squamoid. Extensive necrosis. Marked nuclear pleomorphism and mitoses.
Diagnostic Approach
Step 1 - Clinical Assessment
High-risk history features:
- Age <20 years (20-50% malignancy rate for solitary nodule)
- Men >40 years, women >50 years with rapid growth
- Prior head/neck radiation exposure
- Family history of thyroid cancer, MEN2, pheochromocytoma, hyperparathyroidism
- Associated syndromes: Gardner syndrome, Cowden disease
High-risk physical exam features (from Cummings):
- Hard, fixed lesion
- Rapid growth
- Associated lymphadenopathy
- Vocal cord paralysis (perform laryngoscopy in all cases)
- Aerodigestive compromise (dysphagia, stridor)
- Pemberton sign positive (arms raised - facial suffusion/venous engorgement = substernal involvement)
Step 2 - Laboratory Studies
| Test | Indication/Role |
|---|---|
| TSH | First-line screening; hypothyroid/hyperthyroid shifts workup away from cancer |
| Serum calcitonin | If family history MTC, MEN2, or FNA suspicious for MTC |
| CEA | Useful in MTC monitoring |
| Thyroglobulin (Tg) | NOT useful at initial presentation; used postoperatively after total thyroidectomy for differentiated cancer |
| Calcium | Exclude hyperparathyroidism if MEN2 suspected |
| 24h urine metanephrines/catecholamines + abdominal MRI | If RET mutation present (exclude pheochromocytoma before surgery) |
Step 3 - Imaging
Ultrasound (US) - First-line imaging modality for all thyroid nodules.
Suspicious sonographic features:
- Hypoechoic solid nodule
- Irregular/lobulated margins
- Microcalcifications (correlate with psammoma bodies in PTC)
- Taller-than-wide shape
- Absent halo
- Increased intranodular vascularity
- Associated suspicious lymphadenopathy
The ACR TI-RADS (Thyroid Imaging Reporting and Data System) provides a standardized risk stratification to guide FNA decisions.
Radionuclide Scintigraphy (¹²³I or ⁹⁹ᵐTc)
- "Cold" (non-functioning) nodule: higher malignancy risk - warrants FNA
- "Hot" (hyperfunctioning) nodule: almost never malignant; warrants investigation for thyrotoxicosis
- FDG PET-CT is of limited value for pre-operative assessment (both benign and malignant lesions may be FDG-avid). However, it has an important role in post-treatment surveillance of differentiated thyroid cancer, particularly in patients with elevated thyroglobulin but negative ¹³¹I scan (pooled sensitivity 95%, specificity 81%). - Scott-Brown's Otorhinolaryngology, p. 597
CT/MRI - Used for:
- Substernal goiter assessment
- Evaluation of tracheal displacement/compression
- Staging of known malignancy and lymph node mapping
- Suspected extrathyroidal extension
Step 4 - Fine-Needle Aspiration Cytology (FNAC) - Cornerstone of Diagnosis
FNAC is the procedure of choice for evaluating thyroid nodules. It has dramatically reduced unnecessary surgeries by 35-75% and nearly tripled the malignancy yield in operated patients. Accuracy of PTC diagnosis by FNAC: 99% with false-positive rate <1%. - Cummings, p. 2375
Indications (general thresholds):
- Nodules ≥1 cm with any suspicious sonographic features
- All nodules ≥2 cm (due to higher false-negative rate for larger lesions)
- Nondiagnostic aspirates: repeat with ultrasound guidance; persistent nondiagnostic = surgical excision for solid nodules
The Bethesda System for Reporting Thyroid Cytopathology (TBSRTC):
| Bethesda Category | Diagnosis | Risk of Malignancy | Management |
|---|---|---|---|
| I | Nondiagnostic | 5-10% | Repeat US-guided FNA |
| II | Benign | 0-3% | Clinical follow-up |
| III | Atypia of undetermined significance (AUS/FLUS) | 10-30% | Repeat FNA, molecular testing |
| IV | Follicular neoplasm / Suspicious for follicular neoplasm | 25-40% | Surgical lobectomy |
| V | Suspicious for malignancy | 50-75% | Near-total thyroidectomy or lobectomy |
| VI | Malignant | 97-99% | Total thyroidectomy |
Limitation: FNAC cannot distinguish follicular adenoma from follicular carcinoma - this requires surgical resection and histologic demonstration of capsular/vascular invasion.
Step 5 - Molecular Testing
When FNA cytology is indeterminate (Bethesda III-IV), molecular testing significantly improves diagnostic accuracy:
- BRAF V600E mutation - highly specific for PTC malignancy (>99%)
- RET/PTC rearrangements, NTRK fusions - indicate papillary carcinoma
- RAS mutations - associated with follicular neoplasms (adenoma or carcinoma)
- PAX8:PPARG fusion - associated with follicular carcinoma
- TERT promoter mutations - marker of aggressive behavior/dedifferentiation
- TP53 mutations - associated with anaplastic transformation
- Germline RET mutation testing - all patients diagnosed with MTC and their first-degree relatives
Commercial panels (e.g., ThyroSeq, Afirma) combine multiple molecular markers for risk stratification of indeterminate nodules.
Summary Table: Key Distinguishing Features
| Feature | Papillary | Follicular | Medullary | Anaplastic |
|---|---|---|---|---|
| Frequency | 85% | 5-15% | ~5% | <5% |
| Origin | Follicular cells | Follicular cells | C cells | Dedifferentiated |
| Nuclear features | Ground-glass, grooves, pseudoinclusions | Normal/bland | Neuroendocrine (stippled) | Pleomorphic |
| Psammoma bodies | Yes | No | No | No |
| Amyloid in stroma | No | No | Yes | No |
| Spread | Lymphatic | Hematogenous | Both | Rapid local invasion |
| Key biomarker | Thyroglobulin (post-op) | Thyroglobulin (post-op) | Calcitonin + CEA | None reliable |
| Molecular marker | BRAF V600E | RAS, PAX8:PPARG | RET (germline/somatic) | TP53, TERT |
| FNA diagnosis | Reliable | Cannot distinguish from adenoma | Reliable | Reliable |
| Prognosis | Excellent (>95% 10-yr) | Good (capsular) / Guarded (widely invasive) | Intermediate | Very poor (<1 yr median) |
Sources: Robbins, Cotran & Kumar Pathologic Basis of Disease (9780443264528); Cummings Otolaryngology Head and Neck Surgery (9780323612173); Scott-Brown's Otorhinolaryngology Head & Neck Surgery Vol 1 (9781138094611)
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