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Case 110: Nonossifying Fibroma¹

¹ From the Department of Radiology, University of California, San Francisco, 505 Parnassus Ave, Box L-358, San Francisco, CA 94143-0628. Received March 5, 2004; revision requested May 14; revision received August 7; final version accepted September 2.

Address correspondence to S.W.H. (e-mail: steven.hetts{at}radiology.ucsf.edu).

	HISTORY

TOP HISTORY IMAGING FINDINGS DISCUSSION References

 
A 17-year-old girl with pain and progressive swelling of several months duration in her right medial thigh underwent open surgical biopsy, which revealed alveolar rhabdomyosarcoma. Her symptoms were reviewed and were otherwise negative; they included absence of pain in locations other than her right thigh. Subsequent staging work-up included bone marrow aspiration and right inguinal lymph node biopsy, both of which revealed normal findings. Imaging work-up included whole-body technetium 99m (^99mTc) methylene diphosphonate bone scintigraphy; contrast material–enhanced computed tomography of the chest, abdomen, and pelvis (not shown; findings were normal with the exception of a primary mass in the right superomedial thigh); and whole-body fluorine 18 fluorodeoxyglucose (FDG) positron emission tomography (PET). Magnetic resonance (MR) imaging with and without contrast enhancement was performed principally to evaluate the primary tumor in the patient's right thigh. The results of FDG PET and MR imaging led the orthopedic surgery service to request that the radiology service perform an image-guided percutaneous biopsy of the left distal femoral lesion.

	IMAGING FINDINGS

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Bone scintigraphy revealed diffuse mildly increased FDG uptake in the medial right thigh that was related to the patient's primary rhabdomyoscarcoma and focally increased FDG uptake in the superior aspect of the distal left femoral medial metaphysis (Fig 1). No other areas of abnormal FDG uptake were present. The subsequent FDG PET examination enabled us to confirm the existence of hypermetabolism in both the primary rhabdomyosarcoma and the left femoral metaphyseal lesion, with maximal standardized uptake values (SUVs) of 7.4 and 3.1, respectively (Fig 2). Physiologic FDG uptake was present elsewhere, with the exception of a right inguinal lymph node (SUV, 2.0; not shown), which demonstrated benign histologic findings at subsequent biopsy. To exclude any soft-tissue abnormality and further evaluate the left femoral lesion, MR imaging was also performed. MR images (Figs 3–5) demonstrated an eccentrically located cortically based lesion with a low T1- and T2-weighted signal intensity rim corresponding to the rim of sclerosis seen on conventional radiographs. The central portion of the lesion demonstrated intermediate T1-weighted signal intensity, high T2-weighted signal intensity, and mild enhancement on T1-weighted gadolinium-enhanced images. There was neither an extraosseous extension nor an adjacent soft-tissue abnormality. The remainder of the bone marrow signal intensity was unremarkable.

View larger version (33K): [in this window] [in a new window] [Download PPT slide]   Figure 1a: Anterior 99mTc methylene diphosphonate bone scans of (a) the whole body and (b) the anterior pelvis and proximal legs. Mild increased uptake in the medial right thigh (arrowhead) is associated with the patient's primary soft-tissue rhabdomyosarcoma. Increased uptake is also present in the distal left medial femoral metaphysis (arrow).

View larger version (83K): [in this window] [in a new window] [Download PPT slide]   Figure 1b: Anterior 99mTc methylene diphosphonate bone scans of (a) the whole body and (b) the anterior pelvis and proximal legs. Mild increased uptake in the medial right thigh (arrowhead) is associated with the patient's primary soft-tissue rhabdomyosarcoma. Increased uptake is also present in the distal left medial femoral metaphysis (arrow).

View larger version (93K): [in this window] [in a new window] [Download PPT slide]   Figure 2: Whole-body FDG PET scans. Posterior (left) to anterior (right) adjacent selected coronal images are displayed. Abnormal FDG-avid lesions include primary rhabomyosarcoma in the right thigh (arrowheads; maximum SUV, 7.4) and a smaller lesion in the distal left medial femoral diaphysis (arrows; maximum SUV, 3.1).

View larger version (106K): [in this window] [in a new window] [Download PPT slide]   Figure 3: Coronal T1-weighted spin-echo MR image (repetition time msec/echo time msec, 500/13.86) of the distal left femur. In the posteromedial aspect of the distal left femoral metaphysis, an eccentrically located cortically based lesion (arrow) with a low T1-weighted signal intensity corresponds to a rim of sclerosis seen on conventional radiographs. More central signal intensity characteristics consist of intermediate T1-weighted signal intensity. There is no extraosseous extension. The remainder of the bone marrow signal intensity is normal.

View larger version (82K): [in this window] [in a new window] [Download PPT slide]   Figure 4: Coronal short inversion time inversion-recovery (5000/56) MR image of the distal left femur. Short inversion time inversion-recovery MR examinations of the musculoskeletal system are performed routinely at our institution to obtain relatively T2-weighted images. In the posteromedial aspect of the distal left femoral metaphysis, there is an eccentrically located cortically based lesion (arrow) that demonstrates a low T2-weighted signal intensity rim corresponding to a rim of sclerosis seen on conventional radiographs. More central signal intensity characteristics consist of high T2-weighted signal intensity. There is no extraosseous extension. The remainder of the bone marrow signal intensity is normal.

View larger version (91K): [in this window] [in a new window] [Download PPT slide]   Figure 5: Coronal T1-weighted gadolinium-enhanced (Omniscan; Amersham Health, Princeton, NJ) spin-echo MR image (750/13.86) obtained with frequency-selective fat saturation of the distal left femur. In the posteromedial aspect of the distal left femoral metaphysis, there is an eccentrically located cortically based lesion (arrow) demonstrating mild internal enhancement. There is no extraosseous extension. The remainder of the bone marrow signal intensity is normal.

View larger version (122K): [in this window] [in a new window] [Download PPT slide]   Figure 6: Conventional frontal radiograph of the left femur. In the distal left femoral medial metaphysis, there is a smoothly marginated cortically based lucency (arrow) with a thin sclerotic margin. There is no associated periostitis or fracture.

	DISCUSSION

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Given the patient's known rhabdomyosarcoma and the depiction of a hypermetabolic focus at FDG PET that also accumulated methylene diphosphonate at bone scintigraphy, metastatic disease and separate primary malignant bone tumor headed the list of differential diagnoses for the distal left metaphyseal lesion. Other considerations included eosinophilic granuloma, primary benign bone tumor, and infection. Lack of pain in the distal left femur made separate primary bone malignancy or eosinophilic granuloma highly unlikely diagnoses. Similarly, lack of pain and fever reduced the likelihood of osteomyelitis (1–3). Although pain is typical in patients with bone metastases, the lack of pain in this patient did not substantially reduce the possibility of metastasis, and further work-up seemed warranted.

Radiography provided the most specific depiction in this case: A regular thin sclerotic rim and lack of aggressive features were most consistent with a benign bone tumor. The appearance of the lesion in this skeletally immature patient was virtually pathognomonic for nonossifying fibroma (NOF) (2). Metastasis was confidently excluded on the basis of the absence of aggressive features on the radiograph. Although hypermetabolism appeared to be limited to bone on the FDG PET scan, blooming artifact related to the high SUV could have obscured a subtle adjacent soft-tissue mass. MR imaging enabled us to confirm the absence of a nearby soft-tissue lesion that might contribute to the hypermetabolism identified at FDG PET. MR imaging also indicated a benign process with an eccentrically located lesion that had an overlying intact cortex and was separated from the medullary space by a thin rim of low signal intensity, consistent with sclerosis (3). This constellation of findings raised the possibility of an NOF.

NOF or fibroxanthoma, which is referred to as fibrous cortical defect when smaller than 3 cm in size, is the most common type of benign fibrous bone lesion (2,4). Sometimes considered a developmental defect arising in the trabeculae of tubular bones, NOF consists of spindle-shaped fibroblasts, multinucleated giant cells, and foamy histiocytes (1–3). NOF is present most commonly in skeletally immature patients, and its natural course is involution with progressive ossification over several years. NOFs typically occur in the metaphyses of long bones, and they are asymptomatic and demonstrate a thin sclerotic border that is usually scalloped and slightly expansile on conventional radiographs. Given the diagnostic radiographic appearance of the benign lesions, they have been given a "don't touch" distinction, as biopsy is both unnecessary and invasive. In addition, a lesion examined at biopsy may occasionally be misdiagnosed as a giant cell tumor at histologic analysis, while the clinical and radiographic findings would allow us to eliminate this as a diagnostic dilemma (2,3,5).

The appearance of NOF on MR images is hypointense to skeletal muscle on T1-weighted images, is more often hypointense than hyperintense on T2-weighted images, and is often associated with avid contrast enhancement (6,7). Sometimes, septa are also apparent on T2-weighted images. Low-signal-intensity regions of NOFs on T1- and T2-weighted images have been correlated pathologically with hemosiderin and fibrous tissue elements. The overall appearance of the NOF depends on the relative amounts of hypercellular fibrous tissue, collagen, foamy histiocytes, hemorrhage, hemosiderin, and bone trabeculae. Similar to the findings on radiographs, however, the lobular contour, eccentric location, and sclerotic margin are helpful features in the identification of NOF (7).

Mapping glucose metabolism with FDG PET is often useful in the discrimination of malignant tumors from a variety of benign tumors, including neoplastic, inflammatory, or infectious lesions. Although PET is a sensitive method used to identify osseous metastases, visually apparent uptake of FDG within an osseous lesion does not guarantee that the lesion is indeed malignant. The SUV is an additional parameter that may aid in the discrimination of benign from malignant lesions. The SUV is a ratio of the concentration of FDG within the abnormality in question to the calculated average concentration of FDG within other tissues of the body. A study by Feldman et al (8) revealed that benign and malignant musculoskeletal abnormalities could be separated by using an SUV of 2.0 as a cutoff point. (Benign lesions had SUVs of less than 2.0, whereas malignant lesions had SUVs greater than or equal to 2.0.) Other studies have revealed that although SUVs may correlate with the aggressiveness of bone tumors, there may be substantial overlap between benign and malignant lesions. For example, in studies by Aoki and colleagues (9,10), all three NOFs had SUVs of 2.5 or less, all five giant cell tumors (low-grade malignant lesions) had SUVs of 3.0 or more, chrondrosarcomas had SUVs that ranged from less than 2.0 to approximately 4.0, and osteosarcomas had SUVs that ranged from approximately 2.0 to approximately 5.0. In addition, Ewing sarcomas and malignant lymphomas had SUVs of more than 4.0. High FDG avidity can be observed in fibroblastic, histiocytic, and some neurogenic lesions, regardless of whether they are benign or malignant (5,8). Analysis of FDG kinetics may eventually offer better discriminatory specificity than current methods, which currently are most useful in the exclusion of malignant lesions when the SUV is low (11).

NOF is among the benign bone lesions that may be FDG avid at FDG PET; in patients undergoing metastatic work-up, correlation with radiographic, MR imaging, and clinical findings is essential to making the correct diagnosis. The current case demonstrates the importance of remembering that many benign bone lesions accumulate FDG and can be false-positive findings when one is searching for metastases in a patient with cancer. Confirmation of lesion appearance with other imaging modalities can lead to more accurate oncologic staging, and in cases such as this, it can obviate biopsy in patients already destined to undergo numerous other interventions. Despite the SUV of 3.1 (which was slightly higher than the SUV reported in NOFs in a small case series [9]) in the left femoral lesion in the current case, radiographic and MR imaging findings enabled us to confirm the diagnosis.

	FOOTNOTES

 
Authors stated no financial relationship to disclose.

Part one of this case appeared 4 months previously and may contain larger images.

 

	References

TOP HISTORY IMAGING FINDINGS DISCUSSION References

 

1. Brant W, Helms C. Fundamentals of diagnostic radiology. Baltimore, Md: Williams & Wilkins, 1998; 969–971.
2. Resnick D. Tumors arising from or forming fibrous connective tissue, benign tumors, nonossifying fibroma. In: Resnick D, ed. Diagnosis of bone and joint disorders. 3rd ed. Vol 6. Philadelphia, Pa: Saunders, 1995; 3767–3775.
3. Dorfman H, Czerniak B. Bone tumors: fibrous and fibrohistiocytic lesions. In: Bone tumors. St Louis, Mo: Mosby, 1998; 20–21, 492–506.
4. Selby S. Metaphyseal cortical defects in the tubular bones of growing children. J Bone Joint Surg Am 1961;43:395.
5. Smith SE, Kransdorf MJ. Primary musculoskeletal tumors of fibrous origin. Semin Musculoskelet Radiol 2000;4:73–88.[CrossRef][Medline]
6. Jee WH, Choe BY, Kang HS, et al. Nonossifying fibroma: characteristics at MR imaging with pathologic correlation. Radiology 1998;209:197–202.[Abstract/Free Full Text]
7. Kaplan PA, Helms CA, Dussault R, Anderson MW, Major NM. Tumors. In: Muskuloskeletal MRI. Philadelphia, Pa: Saunders, 2001; 125–150.
8. Feldman F, van Heertum R, Manow C. ¹⁸FDG PET scanning of benign and malignant musculoskeletal lesions. Skeletal Radiol 2003;32:201–208.[Medline]
9. Aoki J, Watanabe H, Shinozaki T, et al. FDG PET of primary benign and malignant bone tumors: standardized uptake value in 52 lesions. Radiology 2001;219:774–777.[Abstract/Free Full Text]
10. Aoki J, Endo K, Watanabe H, et al. FDG-PET for evaluating musculoskeletal tumors: a review. J Orthop Sci 2003;8:435–441.[CrossRef][Medline]
11. Dimitrakopoulou-Strauss A, Strauss LG, Heichel T, et al. The role of quantitative ¹⁸F-FDG PET studies for the differentiation of malignant and benign bone lesions. J Nucl Med 2002;43:510–518.[Abstract/Free Full Text]

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