AJR 2002; 178:717-720
© American Roentgen Ray Society
Fibrous Dysplasia Involving the Base of the Skull
Vincent F. H. Chong1,
James B. K. Khoo2 and
Yoke-Fun Fan1
1
Department of Diagnostic Radiology, Singapore General Hospital, Outram Rd.,
Singapore 169608, Republic of Singapore.
2
Department of Diagnostic Radiology, National Cancer Center Singapore, 11
Hospital Dr., Singapore 169610, Republic of Singapore.
Received June 29, 2001;
accepted after revision August 23, 2001.
Address correspondence to V.F.H. Chong.
Introduction
Fibrous dysplasia is a developmental anomaly that can affect any bone in
the body. The skull and facial bones are the affected sites in 10-25% of
patients with monostotic fibrous dysplasia and in 50% of patients with
polyostotic fibrous dysplasia. Conventional radiographic findings reveal
characteristics of fibrous dysplasia. CT findings also show characteristics of
fibrous dysplasia and consist of the following three varieties: the
ground-glass pattern (56%), the homogeneously dense pattern (23%), and the
cystic variety (21%) [1].
Unfortunately, the MR imaging characteristics of fibrous dysplasia do not
share the distinctive features seen on radiography and CT. In fact, the MR
imaging appearances of fibrous dysplasia often resemble that of tumors. The
intent of this pictorial essay is to highlight the MR imaging features of
fibrous dysplasia involving the base of the skull. An awareness of this
potential diagnostic pitfall can help to reduce the possibility of
misdiagnosing fibrous dysplasia of the skull base for neoplastic disease.
Radiologic Findings
The most common appearance of fibrous dysplasia on CT is an expanded bone
showing a ground-glass appearance (Fig.
1A,1B,1C,1D,1E).
The diagnosis of fibrous dysplasia on CT is usually straightforward
[2,
3]. However, localized fibrous
dysplasia on MR imaging often mimics a tumor because fibrous tissue can
enhance brilliantly after the injection of contrast material. The signal
intensity of fibrous dysplasia has been reported to be low on T1-weighted
images [4]. However, the signal
intensity of fibrous dysplasia on T1-weighted images may be intermediate, thus
resembling that of a soft-tissue tumor (Figs.
2A,2B,2C
and
3A,3B,3C,3D).
The patient illustrated in Figure
2A,2B,2C
was diagnosed with a soft-tissue tumor involving the left ethmoid sinus and
anterior skull base, and he subsequently underwent a biopsy that showed
fibrous dysplasia. This pathologically proven case of fibrous dysplasia showed
strong contrast enhancement, but the T2-weighted images exhibited relatively
low signal intensities.
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Fig. 1A. —36-year-old man with fibrous dysplasia involving floor of
anterior cranial fossa. Coronal CT scan shows expanding lesion in floor of
left anterior cranial fossa (arrow). Note typical ground-glass
appearance.
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Fig. 1C. —36-year-old man with fibrous dysplasia involving floor of
anterior cranial fossa. Axial unenhanced T1-weighted MR image shows
low-signal-intensity lesion protruding into left anterior cranial fossa
(arrow).
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Fig. 2A. —64-year-old man with biopsy proven fibrous dysplasia of
ethmoid region and anterior skull base. Axial unenhanced T1-weighted MR image
shows intermediate-signal-intensity (similar to brain) lesion in left ethmoid
sinus (arrow). Note early encroachment of left orbit and orbital apex
(curved arrow).
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Fig. 2B. —64-year-old man with biopsy proven fibrous dysplasia of
ethmoid region and anterior skull base. Axial contrast-enhanced T1-weighted MR
image shows intense uniform enhancement mimicking soft-tissue tumor.
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Fig. 3A. —23-year-old man with incidental frontoethmoidal lesion found
during MR imaging of pituitary gland. Sagittal unenhanced T1-weighted MR image
shows intermediate-signal-intensity lesion in frontoethmoidal region
(arrows). Note anterior high-signal-intensity proteinaceous
secretions (curved arrow) resulting from frontal sinus
obstruction.
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Fig. 3B. —23-year-old man with incidental frontoethmoidal lesion found
during MR imaging of pituitary gland. Sagittal T1-weighted contrast-enhanced
MR image shows enhancement in lesion. Anterior high-signal-intensity fluid
seen on T1-weighted image is relatively lower in signals because of
windowing.
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Fig. 3C. —23-year-old man with incidental frontoethmoidal lesion found
during MR imaging of pituitary gland. Axial T2-weighted MR image shows
low-signal-intensity fibrous dysplasia (arrows) and
high-signal-intensity fluid in frontal sinus (curved arrow)
responsible for high signals seen in A and B.
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Fig. 3D. —23-year-old man with incidental frontoethmoidal lesion found
during MR imaging of pituitary gland. Axial CT scan shows expansion of floor
of anterior cranial fossa and typical ground-glass appearance. Note fluid in
frontal sinus and enlarged crista galli (arrow) due to fibrous
dysplasia.
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The signal intensity of fibrous dysplasia on T2-weighted images is often
variable, ranging from low to high signals in some patients
[5,
6] (Fig.
4A,4B,4C,4D,4E).
These high signal intensities on T2-weighted images correspond to
nonmineralized areas and regions of cystic changes seen on CT. In some areas
within the bone affected by fibrous dysplasia, there may be collections of
bone marrow to produce high signal intensities on T1-weighted images. These
high-signal-intensity regions show corresponding low signals in fat-suppressed
contrast-enhanced images, thus verifying the presence of fatty marrow.
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Fig. 4A. —49-year-old man with fibrous dysplasia of sphenoid bone.
Axial CT scan shows fibrous dysplasia involving body of sphenoid bone. Note
expanded right pterygoid process (black arrow) and inflammatory
changes in left sphenoid sinus (white arrow) and left maxillary
sinus.
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Fig. 4D. —49-year-old man with fibrous dysplasia of sphenoid bone.
Coronal T1-weighted MR image shows sphenoid sinus floor expansion
(straight arrows) and areas of fat signal intensity presumably due to
presence of marrow (curved arrow).
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Fig. 4E. —49-year-old man with fibrous dysplasia of sphenoid bone.
Coronal contrast-enhanced T1-weighted MR image with fat-saturation technique
shows enhancement in lesion. Note areas of suppressed marrow
(arrow).
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When T2-weighted images show high signal intensities, the differential
diagnosis should include an inflammatory lesion or a neoplastic process (Fig.
5A,5B,5C,5D,5E).
A destructive pattern is not a feature of uncomplicated fibrous dysplasia.
Because fibrous dysplasia is typically a painless anomaly, the presence of
pain should also alert the radiologist to a more sinister process.
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Fig. 5A. —51-year-old man with biopsy proven sarcomatous degeneration
in fibrous dysplasia. Axial CT scan shows destructive lesion involving greater
wing of left sphenoid bone (asterisk). Note fibrous dysplasia
involving right greater wing of sphenoid bone (arrow).
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Fig. 5B. —51-year-old man with biopsy proven sarcomatous degeneration
in fibrous dysplasia. Axial unenhanced T1-weighted MR image shows intermediate
signal intensity involving left greater wing of sphenoid bone (S). III-defined
foci of high signal intensity probably reflecting hemorrhage are present. Note
high signal intensities in right greater wing (straight arrow) of
sphenoid presumably due to marrow. Fibrous dysplasia in left ethmoid appears
low in signal intensity (curved arrow).
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Fig. 5C. —51-year-old man with biopsy proven sarcomatous degeneration
in fibrous dysplasia. Axial T2-weighted MR image shows heterogeneous high
signal intensity in lesion. Note low signal intensity corresponding to
nonneoplastic fibrous dysplasia on contralateral side.
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Fig. 5D. —51-year-old man with biopsy proven sarcomatous degeneration
in fibrous dysplasia. Coronal CT scan shows gross destruction involving left
greater wing (asterisk) and sphenoid sinus. Note typical features of
fibrous dysplasia involving left alveolar ridge (curved arrow) and
cranial vault (straight arrow).
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Fig. 5E. —51-year-old man with biopsy proven sarcomatous degeneration
in fibrous dysplasia. Coronal T2-weighted MR image shows high signal intensity
in area of sarcomatous degeneration. Compare low signal intensity of left
alveolar ridge (arrow) with CT features in D.
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Discussion
MR imaging is likely to be the modality of choice in patients with skull
base lesions. Alternatively, fibrous dysplasia involving the base of the skull
may be detected as an incidental finding on MR imaging performed for an
indication unrelated to the skull base. Hence, a radiologist should have a
high index of suspicion to diagnose fibrous dysplasia of the base of the skull
because this lesion often resembles a tumor on MR imaging
[7]. When fibrous dysplasia
shows bone expansion that conforms to the general shape of the bone of origin,
the diagnosis is relatively easy to recognize. However, fibrous dysplasia with
localized involvement is potentially a diagnostic pitfall.
Histologically, fibrous dysplasia consists of varying amounts of spindle
cell bundles and trabeculae of immature woven bone. On T1-weighted images, the
signal intensity is usually low to intermediate depending on the ratio of
fibrous tissue to mineralized matrix. Lesions with high fibrous tissue content
tend to have intermediate signal intensities, whereas lesions with highly
mineralized stroma tend to show lower signal intensities. On T2-weighted
images, the MR signal intensities are more variable. Some lesions with a
highly mineralized matrix show correspondingly low signal intensities, whereas
lesions with high fibrous tissue content and cystic spaces return high signal
intensities. Unlike mature scar tissues that show low signal intensities on
all imaging sequences, the fibrous tissues in fibrous dysplasia are
metabolically active, thus accounting for the high signal intensities on
T2-weighted images. The fibrous tissues in fibrous dysplasia are well
vascularized and often show numerous small vessels in the center and large
peripheral sinusoids. These histologic features explain why fibrous dysplasia
enhances intensely after the injection of contrast material
[8].
The confidence in making a correct MR imaging diagnosis of fibrous
dysplasia is high when the signal intensities on both T1- and T2-weighted
images are low in spite of enhancement after the injection of contrast
material. Confusion arises when fibrous dysplasia shows intermediate signal
intensities on T1-weighted images and high signal intensities on T2-weighted
images and enhances vividly after the injection of contrast material. Under
such circumstances, CT should be performed to resolve the problem.
In summary, as more MR imaging studies are performed for suspected skull
base lesions, it becomes increasingly important for radiologists to be
familiar with the MR imaging features of fibrous dysplasia. Fibrous dysplasia
shows characteristic radiographic and CT findings, but on MR imaging it can be
easily confused with neoplastic disease.
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Introduction
Radiologic Findings
