August 2003 Case of the Month

History: Bone Lesion. Evaluate for etiology in a 16-year-old.

Compiled by: Zachary Kilpatrick, MD, 07/23/03

Technical Factors: Long and short axis fat- and water-weighted sequences were obtained.

Findings: A large eccentric and subtly expansile mass is present in the distal medial femoral metaphysis.

The lesion has sharp margination.

The internal character of the lesion is mixed, and, therefore, calcification, ossification, or hemorrhage within the lesion is suspected as a complex fibroxanthoma or fibrous medullary inclusion lesion.

A subtle pattern of perilesional hyperintensity may represent a low-grade stress reaction.

Based on the lesion's character but extremely large size, serial follow-up to assess whether the progression of an insufficiency phenomenon occurs might be considered at nine months to one year. However, the likelihood that this lesion represents an aggressive process is so low that biopsy would not be considered as a diagnostic algorithm at this juncture.

The differential diagnosis is limited. Other considerations might include ossifying fibroma and focal fibrous dysplasia.

Coronal T2	Plain Film

Axial T1 Arrow showing lesion	Axial T1 showing focus of intralesional marrow fat

Axial T1 with Arrows showing lesion & intact cortex	Axial T2 arrow showing lesion

Contrast T1 Coronal	Coronal T1 arrow showing lesion.

DIAGNOSIS: Eccentric fibrous or fibroxanthomatous complex inclusion lesion of large size but demonstrating complexity but characteristics of a nonaggressive lesion. DISCUSSION: Nonossifying fibroma or fibrous cortical defect is an entity that has been described in the literature for just over 50 years. Taxonomy of this most common of non-aggressive fibrous lesions of bone remains controversial, but commonly used terms include fibroxanthoma, nonossifying fibroma, NOF and fibrous cortical defect. Size of the lesion has been proposed as a method to distinguish fibrous cortical defect, nonossifying fibroma and fibroxanthoma with 2 to 3 cm being the threshold measurement, depending on the author. Nonossifying fibroma typically occurs in children and adolescents with a male predominance to be approximately two to one. No racial or ethnic predilection has been reported. It reportedly occurs in up to 20% of children and usually spontaneously regresses. There is no reported case of malignant degeneration although multiplicity and association with other diseases and syndromes have been reported. These lesions are usually discovered incidentally and almost always asymptomatic unless large when stress changes and pathologic fractures can develop. The typical location for nonossifying fibroma or fibrous cortical defect is emanating from the metaphyseal cortex of a long bone, usually about the knee, with the distal femur being the most common site. Radiographically, these lesions are slow growing, cortically based, heterogeneously lucent and slightly expansile. They classically demonstrate a thin, sclerotic and well-demarcated scalloped border with no periosteal reaction present. Even when large, plain film findings are widely considered pathognomonic with no further imaging required. Fibrous cortical defects usually undergo spontaneous 'healing' by bony sclerosis or remodeling. CT and/or MRI are not usually indicated unless an atypical presentation warrants. CT demonstrates the eccentric location, overall central lucency and the sclerotic rim quite well. The appearance on MRI is variable and depends not only on the subtype as listed in Table 1, but also whether or not hemorrhage, stress change and/or fracture is present. An uncomplicated NOF should never demonstrate surrounding marrow edema. On both CT and MRI, an apparent interruption of the cortex can be seen which merely represents replacement of the cortex by fibrous tissue and should not be mistaken for cortical destruction. Treatment for nonossifying fibroma is usually expectant. Surgical curretage and bone grafting has been proposed for lesions that involve more than 50% of the transverse diameter of a weight-bearing bone or if transverse diameter approaches approximately 3.5 centimeters or if a pathologic fracture develops. Table 1: Nonossifying Fibromas: Signal Intensities & Subtypes 1. Cystic type: Hypointense T1, hyperintense T2, fibrous circumferential rim usually associated with lytic lesion on conventional radiography 2. Fibrous type: Intermediate to hypointense T1 and T2, usually lytic on conventional radiography 3. Xanthomatous type: Hyperintense TI, hyperintense T2, circumferential hypointense rim, usually lytic on conventional radiography 4. Mixed type: Any combination of above signal intensities usually associated with a mixture of fibrous signal (gray T2) and cyst signal (punctate hyperintense T2), either lytic (50%) or sclerotic (50%) on plain film 5. Healed type: Hypointense T1, intermediate or hypointense T2, uniform sclerosis on conventional radiography References: 1. Pomeranz, Stephen J: Gamuts and Pearls in MRI, 2nd Ed. MRI-EFI Publications 1993 2. Greenspan, Adam: Orthopedic Radiology: A Practical Approach, Raven Press; 2nd edition (July 1992), pp. 599-601 3. Brant W, Helms C. Fundamentals of Diagnostic Radiology. Baltimore: Williams & Wilkins Co., 1998; pp 969-971. 4. Kransdorf MJ, Utz JA, Gilkey FW: MR appearance of fibroxanthoma. J Comput Assist Tomogr 1988 Jul-Aug; 12(4): 612-5 5. Resnick D, Greenway G: Distal femoral cortical defects, irregularities, and excavations. Radiology 1982 May; 143(2): 345-54 6. Resnick D, Kyriakos M, Greenway GD: Tumors and tumor like lesions of bone: imaging and pathology of specific lesions. In: Diagnosis of Bone and Joint Disorders. 3rd ed. 1995: 3628-938. 7. Smith SE, Kransdorf MJ: Primary Musculoskeletal Neoplasms of Fibrous Origin. Semin Musculoskeletal Radiology 2000; 4 (1): 73-88 8. Dorfman, Howard D: Bone Tumors, Mosby; 1st edition (January 1998); pp. 492-555