– Irregular curvilinear bony trabeculae (of woven bone), set in a hypocellular fibrous stroma.
– Fibrous stroma shows bland fibroblastic spindle cells.
– Bony trabeculae show variable size and shape with barely visible osteoblasts lining.
– Significant atypia is not seen (if seen, it is not a fibrous dysplasia).
Practical points of Pathoclinics:
– Fibrous dysplasiacan develop at any age but is common in the first three decades of life, and that age group overlaps with that of osteosarcoma.
– Two forms of fibrous dysplasia: 1. Monostotic fibrous dysplasia: The most common form and affects one bone site; while 2. Polyostotic fibrous dysplasia: less common, affecting multiple bones, and presents at an earlier age. Polyostotic fibrous dysplasia is commonly associated with McCune-Albright syndrome (fibrous dysplasia, with endocrine abnormalities especially precocious puberty and cutaneous café au lait spots) and Mazabraud syndrome (fibrous dysplasia and intramuscular myxomas). All forms of fibrous dysplasia are associated with activating missense mutations in the GNAS gene.
– Common sites: Most common sites are craniofacial bones and the femur. Other sites are tibia, ribs, and pelvis.
– Radiology: well-defined expansile mass, with a ground-glass opacity in the intramedullary region of bone and a sclerotic rim. Cortical involvement and periosteal reaction are not seen.
– On histology, fibrous dysplasia is a well-defined lesion. Some cases may show rounded psammomatous or cementum-like ossification and hyaline cartilage in addition to the irregular, curvilinear branching and anastomosing bony trabeculae of woven bone in a hypocellular bland fibrous stroma. Fibrous stroma may show moderate cellularity with a storiform pattern. Cement lines are not seen. Aneurysmal bone cyst-like change, collection of foam cells, variable number of osteoclast-like giant cells, or myxoid change can be seen.
– Related close differential diagnosis is osteofibrous dysplasia, which also occurs in the same age group but involves the cortex of the tibia (not in the medullary location). Bowing deformity of the tibia with lucent lesion within the cortex and surrounding sclerosis are helpful to clinch the diagnosis. On histology, in addition to the features of fibrous dysplasia, the presence of irregular trabeculae of woven and lamellar bone with prominent osteoblastic rimming favours osteofibrous dysplasia.
– Another important differential is parosteal Osteosarcoma, whichdevelops on the surface of the bone (not prominent in the medullary location) as a broad-based mass attached to the cortex and wrap around the bone. Focal involvement of the intramedullary space may be seen. Classic histologic findings are parallel bony trabeculae of lamellar bone with intervening bland low-grade fibroblastic cell proliferation. Cytologic atypia is minimal like that of fibrous dysplasia. Parosteal Osteosarcoma is positive for MDM2 and CDK4 on immunohistochemistry, while these markers are negative in fibrous dysplasia. Strict co-relation with radiology findings is necessary to avoid a pitfall.
– Conventional well-Differentiated fibroblastic osteosarcoma shows a lytic bone lesion with periosteal reaction and permeative growth pattern. High cellularity, more atypia, mitoses, and lacelike new osteoid formation by neoplastic cells favours osteosarcoma.
– The usual management of this benign lesion is curettage with bone grafting. Incomplete excision may end up with a recurrence. Malignant changes are very rare.