What is the pathophysiology of brown tumors in advanced hyperparathyroidism?

• A. Osteoblastic overactivity
• B. Focal bone resorption with fibrous tissue replacement and hemorrhage
• C. Adipose replacement
• D. Normal bone remodeling

Answer: (C)

Brown tumors reflect osteitis fibrosa cystica from advanced hyperparathyroidism. The driving force is excess parathyroid hormone, which ramps up osteoclastic bone resorption. PTH stimulates osteoblasts and stromal cells to express RANKL, activating osteoclasts and tipping the balance toward bone breakdown. The resorption leaves cavities that are then filled by fibrous tissue and hemorrhagic material, and multinucleated giant cells can be present. The brown color comes from hemosiderin deposited by the surrounding hemorrhage. In this setting, the lesions are focal areas of resorption with fibrous tissue and blood products, not adipose replacement. Adipose replacement isn’t the characteristic mechanism of brown tumors in hyperparathyroidism.