Brown tumors are tumors of bone that arise in settings of excess osteoclast

Osteoclast

An osteoclast is a type of bone cell that removes bone tissue by removing its mineralized matrix and breaking up the organic bone . This process is known as bone resorption. Osteoclasts were discovered by Kolliker in 1873...

 activity, such as hyperparathyroidism

Hyperparathyroidism

Hyperparathyroidism is overactivity of the parathyroid glands resulting in excess production of parathyroid hormone . The parathyroid hormone regulates calcium and phosphate levels and helps to maintain these levels...

, and consist of fibrous tissue, woven bone and supporting vasculature, but no matrix. They are radiolucent

Radiodensity

Radiodensity refers to the relative inability of electromagnetic radiation, particularly X-rays, to pass through a particular material. Radiolucency indicates greater transparency or "transradiancy" to X-ray photons...

 on x-ray. The osteoclasts consume the trabecular bone that osteoblasts lay down and this front of reparative bone deposition followed by additional resorption can expand beyond the usual shape of the bone, involving the periosteum

Periosteum

Periosteum is a membrane that lines the outer surface of all bones, except at the joints of long bones. Endosteum lines the inner surface of all bones....

thus causing bone pain. The characteristic brown coloration results from hemosiderin deposition into the osteolytic cysts. Also characteristic of giant cell tumors of the bone.