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Monostotic Fibrous Dysplasia

Thomas F Gudas, MD
Madeleine D Kraus, MD
Nancy D Baker, MD

March 16, 1994

Presentation

A 64-year-old woman presented with pain in the right lower chest.

Imaging Findings

Computed Tomography
Bone Scintigraphy
Gross Pathology
Microscopic Pathology

On CT scan an expansile lesion (arrow) in a right lower rib exhibits several benign features: sharp margins, no aggressive periosteal reaction, and no associated soft tissue mass. No cartiliginous tumor matrix is evident.

Bone scintigraphy (posterior and right posterior oblique) demonstrates diffuse radiotracer uptake involving most of the right 11th rib (arrows). No other focus of abnormal activity is shown.

Differential Diagnosis

Fibrous dysplasia is a benign fibro-osseous developmental anomaly, which can have this appearance. Ribs are the most frequent site in the more common monostotic form. Primary cartilaginous bone tumor, such as enchondroma or chondrosarcoma, can present as a rib lesion in this age group. Cartilaginous tumor matrix in a costochondral or costovertebral location is more typical of these tumors. Metastatic disease may involve ribs, but multiple abnormal foci would be expected in a patient with known malignancy. Plasmacytoma and aneurysmal bone yst are expansile lesions, but rib involvement is rare. Paget's disease is often "hot" on bone scan but is usually accompanied by increased uptake in the pelvis, spine, or skull.

Diagnosis

Monostotic fibrous dysplasia

Discussion

Radiology

Monostotic fibrous displasia, the most common form of fibrous displasia, is the most common benign lesion of the rib case. The lesion is usually asymptomatic and discovered incidentally on a bone scan or radiograph. Malignant transformation is extremely rare. Pathologic or stress fractures may cause pain. Most monostotic lesions are diagnosed in the second or third decade and remain unchanged throughout life. Comparison with a prior chest radiograph is the most efficient diagnostic aid, eliminating the need for biopsy.

Pathology

Fibrous dysplasia, which can occur in isolated fashion or as part of Albrightıs syndrome, is a benign disorder of bone formation that most ommonly occurs in the jaw and cranium, though is often seen in the ribs and femur as well. The medullary cavity is replaced by firm white fibrous tissue with a rim of sclerotic bone at the periphery. Microscopically, the tumor is composed of a haphazard array of spindled cells intermixed with metaplastic (but benign) bone. When myxoid change is present (it is not in this case) the differential diagnosis includes fibromyxoma and chondromyxoid fibroma.

ARROWS:Long arrow - tumor. Medium arrow - sclerotic rim. Short arrow - normal hematopoietic tissue.

References

1. Resnick D. Bone and joint imaging. Philadelphia: Saunders, 1989:4033-73.

2. Palmer EL, Scott JA, Strauss HW. Practical nuclear medicine. Philadelphia: Saunders, 1992.

3. Park. Osteofibrous Dysplasia: a clinicopathologic study of 80 cases. Hum Path 1993;24(12): 1339-1347.

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