Calcification.

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Pathologic Calcification

Pathologic calcification is the abnormal tissue deposition of calcium salts (primarily calcium phosphate/hydroxyapatite), together with smaller amounts of iron, magnesium, and other mineral salts. There are two major forms - dystrophic and metastatic - plus a third special category of ectopic ossification.

Robbins, Cotran & Kumar Pathologic Basis of Disease, p. 76

1. Dystrophic Calcification

Definition: Deposition of calcium in necrotic or dying tissue, occurring despite normal serum calcium levels and no systemic disturbance of calcium metabolism.

Sites

Areas of coagulative, caseous, or liquefactive necrosis
Enzymatic fat necrosis (e.g., pancreatitis)
Atheromatous plaques of advanced atherosclerosis (almost invariably present)
Aging or damaged heart valves (e.g., calcific aortic stenosis - see image below)
Tuberculous lymph nodes (may be virtually converted to stone)

Dystrophic calcification of the aortic valve - the semilunar cusps are thickened and fibrotic with irregular masses of piled-up calcification behind each cusp

Fig. Dystrophic calcification of the aortic valve - the cusps are thickened and fibrotic, with irregular masses of calcific deposits causing calcific aortic stenosis.

Morphology (Histology)

On H&E stain: basophilic, amorphous granular (sometimes clumped) deposits
Can be intracellular, extracellular, or both
Psammoma bodies: Lamellated (concentric ring) configurations formed when single necrotic cells act as seed crystals that become progressively encrusted. Named for resemblance to grains of sand (psammos = sand). Seen in:
- Papillary thyroid carcinoma
- Meningioma
- Papillary serous ovarian carcinoma
Asbestos bodies: In asbestosis, calcium and iron salts gather around long slender asbestos spicules in the lung, creating beaded dumbbell forms
Heterotopic bone may eventually form at the focus of calcification
Serum calcium is normal

Clinical significance

May simply mark previous injury - or may cause significant organ dysfunction:
- Calcific valvular disease (aortic stenosis)
- Atherosclerotic plaque instability/stenosis

2. Metastatic Calcification

Definition: Deposition of calcium salts in otherwise normal tissues, almost always due to hypercalcemia from systemic calcium/phosphate metabolic disturbance.

Four Principal Causes of Hypercalcemia (Robbins)

Increased PTH secretion - primary hyperparathyroidism (parathyroid tumors), or ectopic PTH-related protein (PTHrP) secretion by malignant tumors
Bone resorption - multiple myeloma, diffuse skeletal metastases (e.g., breast cancer), Paget disease, immobilization
Vitamin D-related disorders - vitamin D intoxication; sarcoidosis (macrophages activate vitamin D precursor); Williams syndrome (idiopathic hypercalcemia of infancy)
Renal failure - phosphate retention leads to secondary hyperparathyroidism; also aluminum intoxication (chronic dialysis), milk-alkali syndrome (excessive calcium + absorbable antacids)

Sites Predominantly Affected

Tissues that excrete acid have an internal alkaline compartment that predisposes them:

Gastric mucosa (parietal cells pump H⁺ → alkaline interstitium)
Kidneys (nephrocalcinosis)
Lungs (may cause respiratory compromise)
Systemic arteries and pulmonary veins

Key rule from Harrison's: Calcifications in hypercalcemic states with normal or low phosphate preferentially affect kidney, lungs, and gastric mucosa. Hyperphosphatemia with normal/low calcium preferentially affects kidney and arteries.

Calcium-phosphate precipitation is especially likely when serum Ca × PO₄ concentration product >75.

Morphology

Morphologically similar to dystrophic calcification - amorphous noncrystalline deposits or hydroxyapatite crystals
Usually causes no clinical dysfunction, but massive pulmonary involvement can cause respiratory compromise; massive renal deposits (nephrocalcinosis) can cause renal damage

3. Mixed Calcification

A combination of metastatic and dystrophic mechanisms. Calcium dysregulation initiates the process (metastatic component), and trauma then propagates it (dystrophic component). Seen in disorders like dermatomyositis.

Dermatology 2-Volume Set 5e

4. Ectopic Ossification / Tumoral Calcinosis

A third mechanism described in Harrison's: true bone formation in soft tissues (as opposed to calcium salt deposition). Examples:

Myositis ossificans - heterotopic bone in muscle after trauma
Fibrodysplasia ossificans progressiva (FOP) - rare genetic disorder
Tumoral calcinosis - rare, caused by FGF23/GALNT3/α-Klotho gene mutations → reduced FGF23 activity → increased renal phosphate reabsorption → hyperphosphatemia → periarticular calcium phosphate masses (shoulders, hips, ankles). Labs show elevated serum phosphate and 1,25-(OH)₂ vitamin D; PTH and calcium are normal.

Summary Table

Feature	Dystrophic	Metastatic
Site	Necrotic/injured tissue	Normal tissue
Serum calcium	Normal	Elevated (usually)
Serum phosphate	Normal	Often elevated
Mechanism	Local cell death → phosphatase release → nucleation	Systemic hypercalcemia/hyperphosphatemia
Examples	Atherosclerosis, calcific aortic stenosis, TB node, fat necrosis	Hyperparathyroidism, renal failure, sarcoidosis, vitamin D toxicity
Preferred sites	Anywhere necrosis occurs	Gastric mucosa, lungs, kidneys, arteries
Clinical significance	May cause valve dysfunction, atherosclerosis	Usually asymptomatic; nephrocalcinosis, pulmonary calcification if severe

Sources:

Robbins, Cotran & Kumar Pathologic Basis of Disease, pp. 76-77 (Pathologic Calcification)
Harrison's Principles of Internal Medicine, 22E (2025), p. 3362 (Extraskeletal Calcification and Ossification)
Dermatology 2-Volume Set 5e (Mixed Calcification)

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