DOI:10.2214/AJR.05.0264
AJR 2006; 187:830-832
© American Roentgen Ray Society
PET/CT Follow-Up in Nonossifying Fibroma
Andrei Iagaru1,2 and
Robert Henderson1
1 Keck School of Medicine, University of Southern California, PET Imaging
Science Center, Los Angeles, CA.
2 Present address: Division of Nuclear Medicine, Stanford University Medical
Center, 300 Pasteur Dr., Rm. H-0101, Stanford, CA 94305.
Received February 15, 2005;
accepted after revision April 6, 2005.
Address correspondence to A. Iagaru
(aiagaru{at}stanford.edu).
Keywords: bone • CT • lymphoma • nonossifying fibroma • PET
Introduction
Nonossifying fibroma is a common benign finding encountered in the practice
of radiology. Nonossifying fibromas are well-circumscribed, solitary fibrous
proliferations. The lesions are found mostly in children, with 75% occurring
in the second decade. Lesions are more common in males than in females, and
may occur in as many as 35% of all children. The process is nonneoplastic and
occurs in the juxtaepiphyseal region of the long bones. The most common site
is the femur, followed by the tibia
[1]. Clinically, nonossifying
fibromas are asymptomatic and are usually discovered as an incidental finding
on a radiograph. Occasionally, a larger lesion presents as a pathologic
fracture. Nonossifying fibromas normally regress spontaneously. The only
definite indication to treat nonossifying fibromas is a pathologic fracture
[2].
On unenhanced radiographs, a nonossifying fibroma appears as an eccentric
radiolucent lesion with thinned cortex, which can have a multilocular
appearance and, often, a sclerotic margin. With time, radiographs will show
increasing marginal sclerosis followed by progressive ossification of the
lesion extending from its diaphyseal aspect. If a CT or MRI is obtained for
nonossifying fibroma, the cortex will often appear interrupted, which can be
interpreted as cortical destruction
[3]. However, this is secondary
to cortical replacement by benign fibrous tissue. The CT will show an
eccentric lesion with central radiolucency. The MRI may show variable signal
intensity and septations, depending on the lesion's stage of healing
[4]. On a technetium 99m
(99mTc) methylene diphosphonate bone scan, the nonossifying fibroma
will show minimal or no increased uptake of the radiopharmaceutical, unless
traumatized [5].
To our knowledge, the appearance of nonossifying fibroma on 18
F-FDG PET and the possibility of following up regression of nonossifying
fibroma with PET have not been previously reported.
Case Report
A 12-year-old boy diagnosed with large-cell anaplastic lymphoma was
referred to the University of Southern California PET Imaging Science Center
for restaging of the disease. Prior diagnostic tests included a whole-body
bone survey, a 99mTc bone scan, and a gallium-67 (67Ga)
scan (Fig. 1A), which revealed
a lesion in the distal right tibia. The diagnosis of nonossifying fibroma was
considered; however, follow-up of this lesion with a whole-body PET/CT was
recommended. The patient had two PET/CT examinations at 6-month intervals.
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Fig. 1A —12-year-old boy with large-cell anaplastic lymphoma.
Whole-body gallium-67 (67Ga) scan demonstrating lymphoma
involvement above and below diaphragm (arrowheads) and mild uptake in
nonossifying fibroma (arrows).
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Both studies were acquired with a Biograph LSO PET/CT scanner (Siemens
Medical Solutions/CPS Innovations). The system consists of a dual-slice,
helical CT (Somatom Emotion, Siemens Medical Solutions) in tandem with an
Accel PET scanner (Siemens Medical Solutions) and is optimized for use in
whole-body oncology. Data were acquired in 3D mode, with attenuation
correction calculated from coregistered CT images. Images were acquired 60
minutes after IV injection of 444 MBq on the first study and 500 MBq on the
follow-up scan. The patient fasted for 6 hours before imaging. The images were
interpreted on a Microsoft Windows NT-based computer system with a Siemens
Medical Solutions/Syngo user interface.
The initial whole-body 18F-FDG PET examination showed no
evidence of active disease. Additional images of the lower extremities were
acquired as requested for evaluation of the right distal tibia lesion. On
concurrent CT, the lesion had the classic appearance of a nonossifying fibroma
with central radiolucency and cortical defect. The lesion corresponded on the
PET study to an 18F-FDG-avid area, with a maximum standard uptake
value (SUVmax) of 1.5 (Fig.
1B). (SUV is defined as the ratio of activity per milliliter of
tissue to the activity in the injected dose corrected by decay and per patient
body weight [SUVbw], lean body mass [SUVlbm], or body
surface area [SUVbsa]. Accuracy is greater than three significant
digits for maximum SUV value [SUVmax]
[6]). On the follow-up study
done 6 months later, the patient continued to be free of active disease. The
nonossifying fibroma showed interval increase in sclerosis of the distal right
tibial lesion on CT. On the PET scan, the lesion showed decrease in the
18F-FDG uptake, with a calculated SUVmax of 1.0
(Fig. 1C). The lower extremity
soft-tissue background SUVmax was 0.6, whereas the normal tibia had
an SUVmax of 0.4 on both studies.
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Fig. 1B —12-year-old boy with large-cell anaplastic lymphoma. CT
images show distal right tibia eccentric lesion (white and black
arrows) with central radiolucency and cortical defect, typical for
nonossifying fibroma. PET study indicates moderate 18F-FDG activity
in lesion (arrowheads), with calculated maximum standard uptake value
(SUVmax) of 1.5. Physiologic 18F-FDG uptake in growth
plates is seen.
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Fig. 1C —12-year-old boy with large-cell anaplastic lymphoma.
Ossification is noted on CT within fibroma (white and black arrows),
whereas 18F-FDG uptake is decreased (SUVmax of 1.0) from
prior study in lesion (arrowheads). Physiologic 18F-FDG
uptake in growth plates continues to be seen.
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Biopsy was not performed because the lesion was asymptomatic and the
imaging features were consistent with the diagnosis of ossifying fibroma.
Discussion
Nonossifying fibroma is a benign osseous lesion, common in children and
less common in adults, as the majority of lesions ossify during adolescence.
In those that do ossify, the nonossifying designation is a misnomer. These
lesions are inactive or minimally active on 99mTc scans in children
[5]. However, lesions that
ossify become quite active on bone scintigraphy during adolescence. An osseous
lesion active on a bone scan and a 67Ga scan in a 12-year-old boy
with anaplastic lymphoma prompted further evaluation with PET/CT. The PET scan
showed increased 18F-FDG activity in the area corresponding to the
lesion seen on the bone scan, and in the absence of anatomic localization with
CT, could have been reported as bone involvement by lymphoma. The classic
appearance on CT provided correct interpretation of the PET activity. This was
important because it maintained the patient's status as free of active disease
and saved him from unnecessary chemotherapy. The diagnosis of ossifying
fibroma was confirmed on the subsequent study, which showed the
18F-FDG activity as decreasing in the fibroma on the PET scan and
the ossification as increasing within the lesion on the CT scan. The possible
mechanism for 18F-FDG uptake in nonossifying fibroma is similar to
the one for acute fractures and consists of increased blood flow and
osteoblastic and metabolic activity.
Combined PET/CT is an essential tool in the diagnosis and evaluation of
response to therapy in various cancers, including musculoskeletal tumors
[7]. It has become widely
available to referring physicians and, with a larger referral basis, it is
presumed that more metabolically active benign lesions will be seen. It is
essential to recognize typical CT features of nonossifying fibroma to prevent
erroneous interpretations of the 18F-FDG uptake. As mentioned in
this case report, ossifying fibroma shows moderate 18F-FDG uptake
and should be identified by its CT appearance.
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Introduction
Case Report
