Brown Tumor

Brown tumors, also known as osteoclastomas, represent a reparative cellular process with localized accumulations of fibrous tissue and giant cells that can replace bone and even produce osseous expansion. In the setting of hyperparathyroidism, bone resorption by osteoclasts results in holes in the bone that become confluent and filled with hemorrhage, reparative granulation tissue, and active, vascular, proliferating fibrous tissue. The name brown tumor comes from the brownish color of the mass due to its vascularity, hemorrhage, and hemosiderin deposition.

Brown tumors are seen in 3% of patients with hyperparathyroidism and 1.5% of patients with secondary hyperparathyroidism, but due to the higher incidence of secondary hyperparathyroidism, when a brown tumor is encountered it is more likely to be associated with secondary hyperparathyroidism.

Brown tumors of hyperparathyroidism may mimic a true tumor on imaging and histology. Radiographs reveal central, slightly expansile, lightly septated, lucent lesions with distinct geographic margins. Cortical thinning and fracture may be seen; however, cortical destruction should not be present.

CT reveals a lytic lesion with an associated soft-tissue attenuation component Conventional angiography reveals a hypervascular mass, a feature that is seen as intense activity on nuclear medicine bone scan

The appearance of brown tumors on MRI depends on the relative proportion of its tissue components. The lesions 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. Contrast administration reveals enhancement of the solid component and septa. Cortical destruction and soft-tissue extension, not readily apparent on radiographs, can be seen in solid lesions with MRI.

Brown tumors involving the spine and cranium tend to be solid, while lesions in the extremities and pelvis tend to be mixed or cystic.

With correction of the underlying metabolic abnormality, rapid healing with sclerosis is typical. Cyst formation may also be seen.

Differential considerations for a solitary brown tumors include: aneurysmal bone cyst, giant cell tumor, solitary bone cyst, and giant cell reparative granuloma.

With mutliple brown tumors, osteolytic metastases, multiple myeloma, Langerhans cell histiocystosis, leukemia, multiple bone cysts, multiple non-ossifying fibromas, and fibrous dysplasia may be considered; however, MRI can narrow the differential, as few lesions are osteolytic on radiographs and T2-hypointense on MRI. Seeking out radiographic findings seen in hyperparathyroidism is also helpful.

The images above show small kidneys (yellow arrows) and a lytic lesion with sclerotic geographic margins in the left femoral head. No cortical destruction is seen. The appearance is nonspecific, but in a patient with hyperparathyroidism, the diagnosis of brown tumor was suggested.

References

• Chew FS, Huang-Hellinger F. Brown tumor. AJR Am J Roentgenol. 1993 Apr;160(4):752.
• Hong WS, Sung MS, Chun KA, Kim JY, Park SW, Lee KH, Lim HW, Lim YS, Yoo WJ, Chung MH. Emphasis on the MR imaging findings of brown tumor: a report of five cases. Skeletal Radiol. 2011 Feb;40(2):205-13.