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DOI:10.2214/AJR.06.0171
AJR 2006; 187:1124-1128
© American Roentgen Ray Society

Clinical Observations

PET/CT Characterization of Fibroosseous Defects in Children: 18F-FDG Uptake Can Mimic Metastatic Disease

Geoffrey S. Goodin1,2, Barry L. Shulkin1, Robert A. Kaufman1,2,3 and M. Beth McCarville1,2

1 Division of Diagnostic Imaging, Department of Radiological Sciences, MS 210, St. Jude Children's Research Hospital, 332 N Lauderdale St., Memphis, TN 38105-2794.
2 Department of Radiology, University of Tennessee Health Science Center, Memphis, TN 38163.
3 Department of Pediatrics, University of Tennessee Health Science Center, Memphis, TN 38163.

Received February 1, 2006; accepted after revision March 30, 2006.

 
Supported in part by the American Lebanese Syrian Associated Charities (ALSAC).

Address correspondence to M. B. McCarville (beth.mccarville{at}stjude.org).


Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
OBJECTIVE. The purpose of this study was to characterize the anatomic appearance and metabolic activity of nonossifying fibromas, fibrous cortical defects, and cortical desmoids on PET/CT images.

CONCLUSION. Over a 14-month period, we identified eight nonossifying fibromas, four fibrous cortical defects, and two cortical desmoids in 330 children who underwent PET/CT for the evaluation of a known or suspected malignancy. CT, conventional radiography, MRI, or clinical follow-up was used to confirm the diagnoses of these fibroosseous lesions. Eleven of the 14 children underwent multiple PET/CT examinations; thus, 34 studies were included. The lesions showed variable metabolic activity as indicated by 18F-FDG uptake: 19 PET/CT examinations showed lesions with mild 18F-FDG uptake, eight showed moderate 18F-FDG uptake, and seven showed intense uptake. When PET reveals metabolically active osseous abnormalities in children who are at risk for bone metastases, benign fibroosseous lesions should be considered in the differential diagnosis before additional diagnostic procedures are undertaken. Benign fibroosseous lesions may be metabolically active and thus mimic metastatic osseous disease in children with underlying malignancies. Correlative CT or other anatomic imaging can confirm the benign nature of these lesions.

Keywords: cortical desmoid • fibroosseous defects • fibrous cortical defect • nonossifying fibroma • nuclear imaging • pediatric imaging • PET/CT


Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Benign fibroosseous lesions such as nonossifying fibromas, fibrous cortical defects, and cortical desmoids are relatively common skeletal lesions that are often discovered incidentally on radiographs of children and young adults [1-6]. Nonossifying fibromas and fibrous cortical defects are most often located in the metaphysis or diametaphyseal junction of the distal femur or proximal tibia and are thought to be variants of the same pathologic process. These lesions are classified as fibroxanthomas because of their histopathologic features [4].

Nonossifying fibromas and fibrous cortical defects generally undergo spontaneous regression over time and are rarely seen radiographically after the second decade of life [2, 3]. During the involutional phase, osteoblastic activity increases as the lesion is replaced by new bone. The lesion initially appears sclerotic, and as healing occurs it eventually disappears. Nonossifying fibromas can exhibit periods of substantial growth and tend to take longer to regress than do fibrous cortical defects.

Cortical desmoids are small (1-3 cm) fibrous or fibroosseous defects located on the posteromedial surface of the distal femur at the site of attachment of the extensor tendinous fibers of the adductor magnus muscle [4, 7, 8]. Also known as periosteal desmoids and cortical avulsive injuries, these lesions occur in growing children as a result of repetitive traction microavulsions with subsequent fibroblastic response.

The conventional radiographic, CT, and MRI characteristics of nonossifying fibromas, fibrous cortical defects, and cortical desmoids have been extensively studied [1-5]. Several studies have also described the appearance of nonossifying fibromas and fibrous cortical defects on 99mTc methylene diphosphonate (99mTc MDP) bone scintigraphic images [6, 9, 10], but to our knowledge the appearance of nonossifying fibromas, fibrous cortical defects, and cortical desmoids on PET/CT images has not been reported.

This study was prompted by the observation of 18F-FDG-avid bone lesions in children with underlying malignancy. These lesions were discovered on PET/CT images and shown by the corresponding CT images or conventional radiographs to be nonossifying fibromas, fibrous cortical defects, or cortical desmoids. In this article, we describe the appearance of these benign bone lesions on 34 PET/CT scans obtained in 14 children.


Figure 1
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  		Fig. 1A —6-year-old girl with 2.1-cm nonossifying fibroma of right proximal tibial diametaphysis. Axial PET image shows mild 18F-FDG uptake at location of nonossifying fibroma (arrow).

 


Figure 2
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  		Fig. 1B —6-year-old girl with 2.1-cm nonossifying fibroma of right proximal tibial diametaphysis. Corresponding axial CT image shows nonossifying fibroma (arrow).

 

Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Patient Selection
From September 2004 until November 2005, we performed 675 PET/CT studies on 330 children with known or suspected malignancies. The PET/CT reports were reviewed, and 32 patients were identified who had metabolically active lesions that were suspected to be nonossifying fibromas, fibrous cortical defects, or cortical desmoids, or that were of unclear cause. After institutional review board approval, the PET/CT studies of these 32 patients were reviewed by one pediatric radiologist.

PET/CT Scanning Parameters and Image Review
Whole-body PET/CT was performed on a Discovery LightSpeed PET/CT scanner (GE Healthcare). Patients were instructed to fast for 4 or more hours before receiving an injection of 18F-FDG (0.15 mCi/kg [55 MBq/kg] of body weight) approximately 60 minutes before imaging. The CT portion of the scanning was performed at a maximum of 90 mAs (adjusted for body weight), 120 kVp, and a slice thickness of 5 mm. No IV or oral contrast material was given. PET images were obtained in two acquisitions: first from the pelvis cranially to the skull vertex and then from the pelvis caudally to the toes. Images were reconstructed in axial, coronal, and sagittal planes and were reviewed at an Xeleris workstation (GE Healthcare).

From the PET images, we determined the maximum standardized uptake value (SUV) of the lesion by drawing a region of interest around the area of 18F-FDG uptake and locating the region of maximum uptake. We qualitatively assessed the 18F-FDG uptake of each lesion and subjectively categorized it as follows: mild if the 18F-FDG uptake of the lesion was judged to be ≤ 1.5 times that of the surrounding soft tissue (Fig. 1A), moderate if it was judged to be > 1.5 and < 3 times that of the surrounding soft tissue (Fig. 2A), or intense if it was judged to be 3 3 times that of the surrounding soft tissue (Fig. 3A). For patients who underwent multiple PET/CT scans over time, we recorded the qualitative 18F-FDG uptake and SUV measurements of the lesions on each scan.


Figure 3
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  		Fig. 2A —13-year-old boy with 3.2-cm nonossifying fibroma of left distal femoral diaphysis. Axial PET image shows moderate 18F-FDG uptake at location of nonossifying fibroma (arrow).

 

Figure 7
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  		Fig. 3A —14-year-old boy with 2.9-cm nonossifying fibroma in left femoral metaphysis. Axial PET image shows intense 18F-FDG uptake at location of nonossifying fibroma (arrow).

 
From the CT images, we recorded the anatomic site of the lesion (i.e., metaphyseal or diaphyseal), cortical or medullary involvement, and margin distinctness. Reconstructed axial, coronal, or sagittal CT images or conventional radiographs were analyzed using previously described characteristic radiographic features [2-4, 6]. Specifically, a fibrous cortical defect was defined as a radiolucent, cortically based round to ovoid lesion measuring less than 2.0 cm in greatest diameter, with sclerotic margins and no associated soft-tissue abnormality. A nonossifying fibroma was defined as a lesion with similar features but measuring 2.0 cm or larger in greatest diameter with possible extension into the medullary cavity. For patients who underwent multiple PET/CT examinations, we compared the measures of greatest diameter from each scan and used the largest value to define the lesion as a nonossifying fibroma or a fibrous cortical defect. Cortical desmoids were defined as radiolucent, 1- to 3-cm cortical irregularities located on the posteromedial surface of the distal femur with no associated soft-tissue mass [2]. Conventional radiographs, MR images, and 99mTc MDP bone scans obtained within 5 weeks of the PET/CT examination were also reviewed by a study radiologist.


Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Fourteen of the 32 patients whose PET/CT images were reviewed had lesions that met our inclusion criteria. This cohort included 10 boys and four girls whose mean age was 13.9 years (range, 6-19 years). Their primary diagnoses included Ewing's sarcoma (n = 3); rhabdomyosarcoma (n = 2); Hodgkin's lymphoma (n = 2); and one each of ganglioneuroblastoma, nonrhabdomyosarcoma soft-tissue sarcoma, paraganglioma, osteosarcoma, non-Hodgkin's lymphoma, epithelioid sarcoma, and aggressive fibromatosis.

Table 1 summarizes the metabolic activity and sizes of the fibroosseous defects. Nine patients with nonossifying fibromas or fibrous cortical defects underwent multiple PET/CT examinations. Of those, six had lesions that showed no subjective change in 18F-FDG uptake but slightly varying SUVs, and three had lesions that showed subjective increases in 18F-FDG uptake with little or no change in the corresponding SUVs. Both patients who had cortical desmoids underwent multiple PET/CT examinations: one lesion showed no change in the subjective assessment of 18F-FDG uptake and little change in the associated SUV, and the other showed decreased subjective 18F-FDG uptake with no substantial change in the associated SUV. Considerable overlap occurred in the SUVs for lesions placed in the three subjective 18F-FDG uptake categories and for lesions of varying size.


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  		TABLE 1: Characteristics of Lesions of 14 Pediatric Patients on PET Images

 

The greatest diameter measured in the group of 12 nonossifying fibromas and fibrous cortical defects ranged from 1.3 to 5.4 cm (mean greatest diameter, 2.6 cm). Anatomic sites of the lesions included the distal femur (n = 7), proximal tibia (n = 3), and distal tibia (n = 2). Seven lesions occurred near the diametaphyseal junction, three in the diaphysis, and two in the metaphysis. All 12 lesions showed the characteristic radiographic appearance previously described for nonossifying fibromas (Figs. 1B, 2B, and 3B) or fibrous cortical defects.


Figure 4
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  		Fig. 2B —13-year-old boy with 3.2-cm nonossifying fibroma of left distal femoral diaphysis. Corresponding axial CT image shows nonossifying fibroma (arrow).

 
In five of the 12 patients, radiographs confirmed the benign radiographic features consistent with nonossifying fibroma and fibrous cortical defect. Five of the 12 patients with a nonossifying fibroma or fibrous cortical defect had 99mTc MDP bone scanning performed within 1 month of the PET/CT study, in four of whom the scanning showed no abnormal tracer uptake at the site of the lesion (Figs. 2C and 2D), and in one of whom minimally elevated 99mTc MDP uptake was seen.


Figure 5
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  		Fig. 2C —13-year-old boy with 3.2-cm nonossifying fibroma of left distal femoral diaphysis. Anterior (C) and posterior (D) images from technetium-99m methylene diphosphonate bone scintigraphy, which was performed 1 week before PET/CT, show no activity at location of nonossifying fibroma.

 

Figure 6
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  		Fig. 2D —13-year-old boy with 3.2-cm nonossifying fibroma of left distal femoral diaphysis. Anterior (C) and posterior (D) images from technetium-99m methylene diphosphonate bone scintigraphy, which was performed 1 week before PET/CT, show no activity at location of nonossifying fibroma.

 


Figure 8
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  		Fig. 4B —14-year-old boy with 2.9-cm nonossifying fibroma in left femoral metaphysis. Corresponding axial CT scan shows nonossifying fibroma (arrow).

 
MR images of the lesion site were available for one patient with nonossifying fibroma. MRI showed the lesion to be centrally hypointense on T1-weighted images, with a mixed internal signal on T2-weighted images, and a hypointense rim on both T1 and T2 images. This lesion showed moderate heterogeneous enhancement with gadolinium.

Both cortical desmoids showed characteristic radiographic features of these lesions—that is, saucer-shaped radiolucent cortical irregularity, the lack of an outer margin, and a location at the posteromedial aspect of the distal femoral metaphysis at the attachment of the adductor magnus tendon (Fig. 4A, 4B). A corresponding conventional radiograph, 99mTc MDP bone scan, and MR image were available for one patient. The radiograph showed a subtle cortical radiolucency at the distal posterior femoral metaphysis. The 99mTc MDP bone scan showed no increased activity associated with the lesion. The MR image revealed a cortically based lesion without soft-tissue involvement that had a bright center and a dark peripheral margin on T2-weighted images and that showed central enhancement in a dark peripheral margin on contrast-enhanced T1-weighted images.


Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Our results indicate that nonossifying fibromas, fibrous cortical defects, and cortical desmoids may appear metabolically active on PET. In addition, 18F-FDG PET may be a more sensitive method for detecting and characterizing the metabolic activity of benign fibroosseous lesions than 99mTc MDP bone scintigraphy. In a small series (n = 10) reported by Greyson and Pang [6], the 99mTc MDP bone scintigraphy appearance of nonossifying fibromas and fibrous cortical defects varied with the developmental stage of the lesions. Inactive or healed lesions showed no uptake of 99mTc MDP on three-phase bone scans, while those in the healing or involutional stage showed faint to moderate uptake on delayed imaging.

In our study, five patients had corresponding 99mTc MDP bone scans. Of those, four did not show increased uptake of 99mTc MDP. These included two lesions that showed moderate 18F-FDG uptake and two that showed mild 18F-FDG uptake on PET/CT. In the remaining patient, the nonossifying fibroma with mild uptake of 99mTc MDP showed moderate 18F-FDG uptake.


Figure 9
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  		Fig. 5A —8-year-old boy with 1-cm cortical desmoid located at posteromedial surface of distal femur. Axial PET image shows moderate 18F-FDG uptake at location of cortical desmoid (arrow).

 


Figure 10
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  		Fig. 5B —8-year-old boy with 1-cm cortical desmoid located at posteromedial surface of distal femur. Corresponding axial CT scan of cortical desmoid (arrow).

 
We found no association between the size of lesions and the subjective assessment of 18F-FDG uptake or the SUV. Among the 11 patients who had serial PET/CT examinations, no significant change was seen in SUVs in those lesions that showed subjective changes in 18F-FDG uptake over time. Furthermore, considerable overlap occurred in SUVs for lesions in the three subjective 18F-FDG uptake categories. Although our sample size is small, the variability in SUV measurements among lesions that were determined qualitatively to have similar or very different 18F-FDG uptake suggests a poor correlation between the subjective assessment of metabolic activity in the lesions and this quantitative parameter.

The reported incidence of nonossifying fibromas and fibrous cortical defects is 20-40%, and that of cortical desmoids is approximately 11% in boys and 3.6% in girls [1-3]. Although we found a much lower incidence of these lesions (3.6% for nonossifying fibromas and fibrous cortical defects and 0.6% for cortical desmoids) in our cohort, our assessment was based on metabolic activity. Therefore, only lesions that showed 18F-FDG uptake were included. In addition, the intense physiologic uptake of 18F-FDG typically observed around the physes of children's long bones may have obscured small lesions and could also, at least partially, explain why we identified mostly diametaphyseal (n = 5) and diaphyseal (n = 3) lesions and few (n = 2) metaphyseal lesions.

The correlative CT performed during PET/CT accurately located and characterized these lesions, thereby dispelling concerns of osseous metastatic disease and obviating additional diagnostic procedures. Although CT is not typically indicated for evaluation, it can provide additional detailed information about the integrity of the cortex of the lesion and whether an associated soft-tissue component is present. CT can also help determine whether a pathologic fracture has occurred [2-4]. Therefore, information gained from the CT assessment of the lesion can guide further management and help determine whether additional characterization is needed.

We have shown that the metabolic activity of nonossifying fibromas, fibrous cortical defects, and cortical desmoids is probably independent of lesion size and varies among patients and over time, as indicated by their 18F-FDG uptake on PET. This finding is consistent with the natural history of these fibroosseous lesions. Our findings also suggest that PET/CT is more sensitive than conventional 99mTc MDP bone scintigraphy in the detection of these benign lesions. This result is important to consider when PET or PET/CT is performed on children with an underlying malignancy because the PET appearance of these lesions can mimic bone metastatic disease. These fibroosseous lesions have classic radiographic features; therefore, the information obtained either from the CT component of the PET/CT examination or from conventional radiography is invaluable for characterizing these lesions and determining whether further imaging or biopsy is necessary.


References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

  1. Caffey J. On fibrous defects in cortical walls of growing tubular bones: their radiologic appearance, structure, prevalence, natural course, and diagnostic significance. Adv Pediatr1955; 7:13 -51[Medline]
  2. Brant WE, Helms CA. Fundamentals of diagnostic radiology, 2nd ed. Philadelphia, PA: Lippincott Williams & Wilkins, 1999: 969-971
  3. Betsy M, Kupersmith LM, Springfield DS. Metaphyseal fibrous defects. J Am Acad Orthop Surg 2004;12 : 89-95[Abstract/Free Full Text]
  4. Smith SE, Kransdorf MJ. Primary musculoskeletal tumors of fibrous origin. Semin Musculoskelet Radiol 2000;4 : 73-88[CrossRef][Medline]
  5. 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]
  6. Greyson ND, Pang S. The variable bone scan appearances of nonosteogenic fibroma of bone. Clin Nucl Med1981; 6:242 -245[CrossRef][Medline]
  7. Burrows PE, Greenberg ID, Reed MH. The distal femoral defect: technetium-99m pyrophosphate bone scan results. J Can Assoc Radiol 1982; 33:91 -93[Medline]
  8. Jung C, Choi YY, Cho S, Park KC. Symptomatic cortical desmoids detected on knee SPECT. Clin Nucl Med2002; 27:437 -438[CrossRef][Medline]
  9. Brenner RJ, Hattner RS, Lillien DL. Scintigraphic features of nonosteogenic fibroma. Radiology 1979;131 : 727-730[Abstract]
  10. Gilday DL, Ash JM. Benign bone tumors. Semin Nucl Med 1976; 6:33 -46[Medline]

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