Brown tumour, also known as osteitis fibrosa cystica and rarely as osteoclastoma, is one of the manifestations of hyperparathyroidism. It represents a reparative cellular process, rather than a neoplastic process. Histologically brown tumours are identical to giant cell tumour (both are osteoclastomas), and therefore, this entity can easily be misdiagnosed as such if elevated blood calcium and/or parathyroid hormone levels are not assessed for and identified.
Brown tumours have a slightly greater frequency in primary than in secondary hyperparathyroidism (3% versus 2%). However, secondary hyperparathyroidism is much more common than primary hyperparathyroidism, therefore most brown tumours that are seen are associated with secondary hyperparathyroidism.
In chronic renal disease, continual and excessive urinary calcium excretion can lower serum calcium level and lead to a rise in parathyroid hormone secretion. This results in mobilisation of skeletal calcium through rapid osteoclastic turnover of bone to maintain normal serum calcium levels.
In localised regions where bone loss is particularly rapid, haemorrhage, and reparative granulation tissue, with active, vascular, proliferating fibrous tissue may replace the normal marrow contents, resulting in a brown tumour.
Haemosiderin imparts the brown colour (hence the name of the lesions).
Well-defined, purely lytic lesions that provoke little reactive bone. The cortex may be thinned and expanded, but will not be penetrated.
Attenuation values on CT will be in the range of blood and fibrous tissue.
The MRI appearance depends on the relative proportion of its components. The lesions, therefore, may be solid, cystic, or mixed. Solid components are intermediate to low intensity on T1- and T2-weighted images, while the cystic components are hyperintense on T2-weighted images and may have fluid-fluid levels.
- T1 C+ (Gd): there can be enhancement of the solid component and septa
- lesions are usually hypervascular
Bone scan often shows intense uptake.
- mnemonic for the differential diagnosis of a lucent/lytic bone lesion: FEGNOMASHIC
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