AJR 2005; 184:S169-S174
© American Roentgen Ray Society
Radiological Reasoning: A Benign-Appearing Bone Mass
Felix S. Chew, MD and
Michael L. Richardson, MD
Department of Radiology, University of Washington, Box 354755, 4245
Roosevelt Way NE, Seattle, WA 98105.
Received March 14, 2005;
accepted after revision March 18, 2005.
Address correspondence to F. S. Chew
(fchew{at}u.washington.edu).
Abstract
Objective
We discuss the case of a 20-year-old woman who presented with a bone mass
in the anterior aspect of the distal femur. Radiographs and CT showed a
predominantly mature bony mass on the surface of the cortex with a smaller,
less mineralized soft-tissue component. MRI showed hyperintensity in the
soft-tissue component on T2-weighted images and contrast enhancement following
gadolinium infusion. The final pathologic diagnosis was parosteal
osteosarcoma.
Conclusion
A wide variety of unusual, benign bone lesions that often have little
clinical significance have been described in the literature. It is important
to distinguish lesions that require specific therapy from those that do not;
it is much less important to try to obtain a specific imaging diagnosis for
lesions that do not require therapy.
Case History
A 20-year-old woman presented with aching knee pain of several months
duration. She denied significant trauma or previous surgery. Radiographs were
obtained, followed by CT and MRI. The patient then was referred to a specialty
clinic for diagnosis and definitive care.
Radiographs
Anteroposterior (AP) and lateral radiographs of the knee were obtained
(Figs.
1-2).
On the lateral view, there is an extensive region of heaped-up,
mature-appearing bone forming a mass along the anteromedial aspect of the
distal femoral shaft. The lesion does not appear to encroach on the marrow
space. A focal sclerotic lesion is present more distally within the
metaphyseal trabecular bone. The prefemoral fat plane is displaced anteriorly
over the bony mass, but it remains sharply defined. On the AP view, there is
mild expansion of the cortex in the medial direction. There is no
fracture.
Expert Discussion (Dr. Richardson)
Morphologically, the bulk of this lesion looks like mature bone. In my
experience, this would turn out to be heterotopic ossification more than 95%
of the time, and in the remainder of cases, something from the osteoid series
of tumors. Here are my differential diagnoses, in order of likelihood: (1)
heterotopic ossification, (2) osteochondroma, (3) osteoma, (4) osteoid
osteoma, (5) osteoblastoma, (6) melorheostosis, and (7) osteosarcoma.
Osteochondroma is a possibility, but these tumors often are associated with
some accompanying dysplasia in their immediate neighborhood. This lesion looks
as if it was acquired after skeletal maturity because the shape and
proportions of the femur are normal. It is not a great location for osteoma or
osteoblastoma, and the lack of the characteristic pain syndrome (night pain
relieved by aspirin) or a nidus makes osteoid osteoma less likely.
Nonetheless, I need some cross-sectional imaging to look for a nidus.
Melorheostosis is one of those weird entities that I see once in a blue moon.
For this reason, I usually do not mention it in most of my differentials. It
can, however, mimic heterotopic ossification, osteoma, and even parosteal
osteosarcoma. Therefore, for unusual cases that do not quite fit, I
reluctantly would add it to the list. Most types of osteosarcoma generally
present as a much more aggressive-looking process. The only one I would
seriously consider in this case would be a very well-differentiated parosteal
osteosarcoma. Radiographs are notoriously poor at demonstrating soft-tissue
extension, something that I would expect in an osteosarcoma; this gives us
another reason to obtain a CT or MRI. The small, distal lesion is a bone
island.
Why not just call it heterotopic ossification and follow the patient with
serial examinations? This probably is great advice for the majority of
patients who come in with a similar radiographic appearance. Such an approach
would be bolstered greatly by any history of focal trauma or the presence of
another cause for heterotopic ossification. However, in this patient's case,
the presence of pain and the lack of any such history would warrant further
imaging and even biopsy.
Computed Tomography
CT was performed without contrast enhancement. At the superior portion of
the lesion (Fig. 3), thickening
and sclerosis of the cortex are present. Peripheral to the cortex, there is an
amorphously mineralized soft-tissue component with a lobulated shape. The soft
tissues of the quadriceps mechanism are displaced away from the soft-tissue
component, with an intervening fat plane. One is able to see a thin line
anteromedially that represents the endosteal surface of the original cortex.
The marrow space is intact. At the inferior portion of the lesion
(Fig. 4), the anterior cortex
also is thickened and sclerotic, with mature bone formation and a sparse
soft-tissue component. A nidus is not demonstrated on these or other images.
Inferior to the main lesion (Fig.
5), CT shows that the sclerotic focus in the metaphysis has the
characteristics of a bone island.
Expert Discussion (Dr. Richardson)
The soft-tissue component was unexpected, but that is why cross-sectional
imaging was necessary. The presence of a soft-tissue component pretty much
rules out all of the benign lesions, and puts osteosarcoma at the top of the
revised differential diagnosis. The presence of mineralization within the
soft-tissue component is strong evidence for osteosarcoma. At this point, we
could proceed with a biopsy.
Magnetic Resonance Imaging
MRI was performed without and with gadolinium infusion. The sagittal
T1-weighted MRI in the plane of the intercondylar notch of the femur
(Fig. 6) shows the low signal
in the thickened anterior cortex, typical for a sclerotic lesion. There is no
involvement of the marrow space. The sagittal T2-weighted fat-suppressed MRI
in the plane of the lateral femoral condyle
(Fig. 7) shows the thickened
anterior cortex with high signal in the soft-tissue component at the superior
end of the lesion. The surrounding soft tissues do not have reactive edema.
The postgadolinium sagittal T1-weighted fat-suppressed MRI in the plane of the
lateral condyle (Fig. 8) shows
enhancement in the soft-tissue component only.
Expert Discussion (Dr. Richardson)
The MRI is helpful in a confirmatory way, and can be used for staging and
surgical planning. A bone lesion with an enhancing soft-tissue component
probably is malignant.
Clinical Management
The lesion was resected with clear margins, and the limb was reconstructed.
The final pathologic diagnosis was parosteal osteosarcoma. The patient's
prognosis for survival is excellent.
Expert Discusssion (Dr. Richardson)
Of all of the parosteal osteosarcomas in my experience, I have seen exactly
one other one that looked this benign. The anterior portion of this bone
formation is arguably slightly different in character than the more
juxtacortical portion. If one were to biopsy anywhere in this lesion, this
would be the place. However, one could also make a good argument that this is
all just a mass of heterotopic bone, with some areas more mature than others.
There is an old rule of thumb for distinguishing heterotopic ossification from
parosteal osteosarcoma: if it is denser centrally, it is more likely to be
osteosarcoma; if it is denser peripherally, it is more likely to be
heterotopic ossification. It is difficult to apply that rule with confidence
here, since the majority of the bone formation is so benign in appearance.
18F-FDG PET might have been quite useful in this patient. Even
though the bulk of the lesion appeared to be quite benign, a high standardized
uptake value (SUV) focus would have provided additional early evidence that
this was a sarcoma, rather than just another garden variety case of
heterotopic ossification. When the sum of the history and images do not quite
add up, as in this case, the safest course is to get a piece of the patient's
disease. This means, of course, an appropriate biopsy of the appropriate
portion of the lesion. PET often is helpful in this regard, pointing out an
area of high metabolic activity that may otherwise be inapparent.
Commentary (Dr. Chew)
In the evaluation of focal bone lesions, radiographs and CT generally are
considered the techniques of choice for arriving at an imaging diagnosis.
Radiography provides an overall perspective of size, shape, and location,
while generally allowing an assessment of the presence and rate of bone
destruction, the presence and quality of reactive bone formation, and the
presence and characterization of matrix mineralization
[1]. CT is valuable because it
can detect and measure soft-tissue mineralization better than can radiography
and also because it provides tomographic images, thus reducing, if not
eliminating, perceptual errors caused by superimposition of shadows on
radiographs. MRI may be helpful for identifying fluid and other avascular
regions, for identifying and characterizing soft-tissue involvement, and for
demonstrating anatomic setting and extent
[2].
At the University of Washington, the imaging protocol for focal bone
lesions includes the following pulse sequences as a minimum: T1-weighted
without fat suppression, T2-weighted with fat suppression, and postgadolinium
T1-weighted with fat suppression. We use the axial plane as our basic study
but add sequences in the long axis tailored to the anatomic site. Neoplastic
lesions generally will show mass effect, hyperintensity on T2-weighted or
other fluid-sensitive pulse sequences, and enhancement following gadolinium
infusion. However, densely mineralized lesions may not show hyperintensity on
fluid-sensitive sequences and may not show enhancement following gadolinium
infusion. A radionuclide bone scan may be helpful for determining the
osteoblastic activity in a lesion, and PET may be used to give an indication
of cellular metabolic activity
[3]. CT-guided biopsy may be
the ultimate diagnostic procedure, if one has appropriately qualified
pathologists available to examine the specimens that are obtained
[4,
5]. For the nonspecialist, it
is reasonable to obtain a CT and perhaps an MRI when confused by a focal bone
lesion with an unfamiliar appearance on radiographs. The key to appropriate
management then depends on recognizing when a lesion has an unfamiliar
appearance.
Parosteal osteosarcoma is a variant of osteosarcoma that arises on the
surface of bone; of the various types of surface osteosarcoma, parosteal
osteosarcoma is the most common, comprising approximately 10% of all
osteosarcomas [6]. These
lesions are low-grade malignancies with little potential for metastasis; the
usual and unusual appearances of parosteal osteosarcomas have been documented
in the literature
[7-15].
The parosteal osteosarcoma in this case has some slightly atypical features.
In the Mayo Clinic series [6]
of 69 parosteal sarcomas, 64% were found in females, and the peak age of
presentation was between 25 and 30 years. Forty-four percent occurred in the
distal femur, and there was a pronounced tendency to involve the posterior
aspect of the distal femoral shaft. Local surgical resection is the usual
treatment.
Besides heterotopic ossification, a common phenomenon that frequently
follows trauma, a wide variety of unusual, benign bone lesions that often have
little clinical significance have been described in the literature
[16-21].
It is important to distinguish lesions that require specific therapy from
those that do not; it is much less important to try and arrive at a specific
imaging diagnosis for lesions that do not require therapy.
Meet the Expert
Michael L. Richardson plays the fiddle, practices tai chi and aikido, and
spends a lot of time hiking and geocaching outdoors in the Pacific Northwest.
His under-graduate degree was in physics, and he has since acquired some
additional graduate training in biostatistics. He has served on several ARRS
committees, and continues to pursue a number of geeky research interests as
part of his day job as a professor of radiology at the University of
Washington.
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Abstract
Case History
