thyroid cancer
Summary
Thyroid cancer includes several histologic subtypes: papillary (most common, ~80%, excellent prognosis, associated with RET/PTC and BRAF mutations, spreads via lymphatics), follicular (hematogenous spread, associated with RAS mutations), medullary (arises from parafollicular C cells, secretes calcitonin, associated with MEN 2A/2B and RET mutations), and anaplastic (rare, aggressive, poor prognosis, elderly patients). Presents as a thyroid nodule, often found incidentally; diagnosis via fine-needle aspiration (FNA) biopsy.
Detail
Thyroid cancer arises from follicular epithelial cells (papillary, follicular, anaplastic) or parafollicular C cells (medullary). Papillary carcinoma is the most common type, often seen in younger patients with history of radiation exposure; histology shows 'Orphan Annie eye' nuclei, psammoma bodies, and papillary projections; it spreads via lymphatics to cervical nodes but has excellent prognosis (>95% 10-yr survival). Follicular carcinoma is more common in iodine-deficient areas, spreads hematogenously (bone, lung), and requires biopsy showing capsular/vascular invasion to differentiate from follicular adenoma (FNA cannot distinguish). Medullary carcinoma originates from neural crest-derived C cells, produces calcitonin (tumor marker) and can cause hypocalcemia symptoms; associated with MEN2A/2B syndromes (RET proto-oncogene mutation) — prophylactic thyroidectomy is recommended in RET mutation carriers. Anaplastic carcinoma is highly aggressive, undifferentiated, occurs in older patients, presents with rapidly enlarging neck mass with local invasion (dysphagia, dyspnea), and has very poor prognosis. Diagnostic workup: thyroid nodule evaluation with TSH, ultrasound, and FNA biopsy; 'cold' nodules on radioiodine scan are more likely malignant. Treatment: surgery (lobectomy/thyroidectomy) +/- radioactive iodine ablation for papillary/follicular; TSH suppression with levothyroxine; medullary and anaplastic do not respond to radioactive iodine since they don't arise from follicular cells. Serum thyroglobulin used to monitor for recurrence after treatment of differentiated thyroid cancers; calcitonin and CEA used for medullary carcinoma surveillance.
Sources
- First Aid for the USMLE Step 1
- Robbins Basic Pathology
- UpToDate: Thyroid nodules and cancer
- Goldman-Cecil Medicine
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