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Clinical Observations |
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).
Abstract
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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
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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.
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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.
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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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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.
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Both cortical desmoids showed characteristic radiographic features of these lesionsthat 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.
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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.
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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.
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