What causes eggshell calcification of lymph nodes on chest radiography and CT?
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Answer:
Eggshell calcification is a thin, peripheral, shell-like rim of calcium in enlarged hilar or mediastinal lymph nodes, classically defined as a ring of calcification up to 2 mm thick in at least two nodes, one of which measures more than 1 cm. The centre of the node remains uncalcified, so the rim stands out as a discrete circumferential ring rather than the dense homogeneous calcification of a healed granuloma. It is most characteristic of silicosis and coal workers’ pneumoconiosis, where it accompanies upper-zone nodularity and progressive massive fibrosis, and its presence in an occupational context is strong supportive evidence for pneumoconiosis. Other reported causes are sarcoidosis, previously irradiated Hodgkin lymphoma, scleroderma, amyloidosis, blastomycosis and histoplasmosis. CT is far more sensitive than the radiograph and shows the peripheral ring clearly, alongside the nodal enlargement and any parenchymal disease.
Why is it called so?
The thin calcified rim surrounding a non-calcified nodal centre resembles the shell of an egg.
Pathophysiology
Inhaled silica or coal dust is carried by macrophages to the draining hilar and mediastinal nodes, where it excites a chronic fibrotic and granulomatous reaction. Fibrosis and dystrophic calcification begin at the periphery of the node, in and beneath the capsule where the dust-laden macrophages and the fibrotic reaction are concentrated, while the more cellular central node is spared. The result is a peripheral shell of calcium enclosing a still-uncalcified centre. The same peripheral-first pattern of dystrophic calcification explains the appearance after irradiation of lymphoma and in the other reported causes.
Alternative names: Shell-like calcification, rim calcification of lymph nodes
Other associated named signs: Progressive massive fibrosis, popcorn calcification of a hamartoma, and Erasmus syndrome (silicosis with systemic sclerosis)
References
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