Case #7

• Plain film shows a large, elongated, well-defined intramedullary lesion of the proximal femur with "shepherd's crook" deformity (lateral bowing) due to a healed pathologic fracture. The lesion is partially surrounded by a sclerotic rim and has a complex appearance with lytic areas, multiple foci of "ground glass" density, and radiopaque areas.

• Curettage specimen consisted of pieces of firm, white-gray, gritty tissue. Microscopic examination showed moderately cellular fibrous tissue with a whorled arrangement of bland spindle cells and interspersed, slender, curved trabeculae of focally calcified woven bone ("chinese characters").

• The majority of the trabeculae did not show an osteoblastic rimming or osteoclastic activity.

Diagnosis: Fibrous Dysplasia

• Fibrous dysplasia is a common benign fibro-osseous lesion, which occurs sporadically during the period of skeletal growth (ages 10 to 25). It is a hamartoma and is characterized by the intramedullary location. There are two forms of the disease: monostotic (80% of cases) and polyostotic. Polyostotic involvement may be a part of McCune-Albright syndrome (fibrous dysplasia, patchy cutaneous pigmentation, and precocious puberty), or Mazabraud's syndrome (fibrous dysplastic lesions in close proximity to soft tissue myxomas).
• Most common locations include the long bones (femur, tibia and humerus), the ribs, cranio-facial bones and pelvis. In the long bones, the lesion is found in the metaphysis or diaphysis.
• The hallmark of fibrous dysplasia is inability of tissue at the affected site to produce mature lamellar bone. The maturation is arrested at the level of woven bone. The difference between the two types of bone is best appreciated by using polarizing light.

	Woven bone (immature bone) is characterized by random arrangement of the collagen fibers. In adults, this type of bone is only seen in pathologic conditions such as fracture repair or bone-forming tumors. Keep in mind that in benign conditions the bone trabeculae are surrounded by a fibrovascular stroma. In malignant tumors, those spaces are often occupied by malignant cells.
	In lamellar (mature) bone, the collagen fibers are orderly arranged forming layers, known as lamellae.

• Clinical behavior. Pathologic fractures and bone deformities are common complications. Occasionally, lesions may show rapid enlargement due to secondary cystic changes including aneurysmal cyst formation. Malignant transformation is a rare complication (less than 1%), more common with polyostotic disease. Secondary malignancies are high-grade sarcomas (MFH, osteosarcoma, chondrosarcoma).
• Differential diagnosis. Two important differential diagnoses include well-differentiated intramedullary osteosarcoma and, in the tibia, osteofibrous dysplasia. Low-grade osteosarcoma is characterized by infiltrative growth pattern and mild cellular atypia. Bone spicules are often rimmed by mildly atypical osteoblasts. Osteofibrous dysplasia can be ruled out based on the intracortical location and orderly osteoblastic rimming of bone spicules.