CASE 55

By: Dr Subramaniyan Ramanathan MD, Dr. Mahmoud Alheidous and Dr. Nizar Al-Nakshabandi. MD, FRCPC.

HISTORY: 29-year-old female with incidental abnormality detected on chest radiograph.

What are your findings?

What is the differential diagnosis?

What are the causes?

FINDINGS:

Plain chest radiograph shows a focal expansile lytic lesion with sclerotic rim involving the anterior part of left 9th rib. No fracture. No perisoteal reaction.

Limited CT cuts show the fusiform enlargement of left 9th rib with internal bubbly cystic and ground glass appearance with thick sclerotic rind. No fracture or periosteal reaction. No associated soft tissue.

DIAGNOSIS:

Mono-ostotic fibrous dysplasia of rib.

PEARLS AND DISCUSSION:

Fibrous dysplasia is a hamartomatous fibro-osseous metaplastic disorder caused by a defect in the proliferation and maturation of fibroblasts and the marrow becomes replaced by fibrous tissue. Fibrous dysplasia is found predominantly in children and young adults, with ~75% of patients presenting before the age of 30 years. In polyostotic form, patients usually present by 10 years old. There is no recognized gender predilection. Patients with polyostotic disease frequently have cafe-au-lait spots (30% to 50%) and may present with endocrine dysfunction, particularly precocious puberty (referred to as McCune-Albright syndrome). The condition is often an incidental finding and is usually painless. Alternatively, it may present due to bony expansion or remodeling. Morbidity may arise from compression and displacement of adjacent structures.

About 90% of patients who have polyostotic disease be involved in only one limb. Ribs are the most common site of mono-ostotic fibrous dysplasia followed by proximal femur. The characteristic appearance is uniform ground glass density lytic lesion with sclerotic rim called rind sign. Expansion of the bone is common as are bowing deformities. Variable amount of sclerosis can be seen depending on the degree of fibroblast maturation. Periosteal reaction is typically absent unless there is associated pathological fracture. CT can demonstrate the bubbly cystic nature of the lesion with intact and thickened cortex. MRI appearance is variable with varying degrees of contrast enhancement making it difficult to distinguish from bone tumours.

FURTHER READING:

1. Fitzpatrick KA, Taljanovic MS, Speer DP et-al. Imaging findings of fibrous dysplasia with histopathologic and intraoperative correlation. AJR Am J Roentgenol. 2004;182 (6): 1389-98

2. Shak ZK, Peh WC, Koh WL, Shek TW: Magnetic resonance imaging appearances of fibrous dysplasia. Br J Radiol 78:1104–1115, 2005.

3. Malloy PC, Scott WW Jr, Hruban RH: Fibrous dysplasia. Skeletal Radiol 22:66–69, 1993.