Thermal Ablation of Osteoid Osteoma: Overview and Step

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EDUCATION EXHIBIT
2127
Thermal Ablation of
Osteoid Osteoma:
Overview and Stepby-Step Guide1
Online-Only
CME
See www.rsna
.org/education
/rg_cme.html
Daria Motamedi, MD • Thomas J. Learch, MD • David N. Ishimitsu, MD
Kambiz Motamedi, MD • Michael D. Katz, MD • Earl W. Brien, MD
Lawrence Menendez, MD
the
indications and contraindications for
RF ablation in the
treatment of osteoid
osteoma.
Osteoid osteoma is a small, benign but painful lesion with specific
clinical and imaging characteristics. Computed tomography is the
imaging modality of choice for visualization of the nidus and for treatment planning. Complete surgical excision of the nidus is curative,
providing symptomatic relief, and is the traditionally preferred treatment. However, surgery has disadvantages, including the difficulty of
locating the lesion intraoperatively, the need for prolonged hospitalization, and the possibility of postoperative complications ranging from
an unsatisfactory cosmetic result to a fracture. Percutaneous radiofrequency (RF) ablation, which involves the use of thermal coagulation
to induce necrosis in the lesion, is a minimally invasive alternative to
surgical treatment of osteoid osteoma. With reported success rates approaching 90%, RF ablation should be considered among the primary
options available for treating this condition.
■■Describe
©
LEARNING
OBJECTIVES
After reading this
article and taking
the test, the reader
will be able to:
■■Identify
the characteristic clinical
and imaging manifestations of osteoid
osteoma.
■■Recognize
the differences between
surgical treatment
and RF ablation
with regard to postprocedural care and
potential complications.
RSNA, 2009 • radiographics.rsna.org
TEACHING
POINTS
See last page
Abbreviation: RF = radiofrequency
RadioGraphics 2009; 29:2127–2141 • Published online 10.1148/rg.297095081 • Content Codes:
1
From the Departments of Imaging (D.M., T.J.L., D.N.I.) and Orthopedic Surgery (E.W.B.), Cedars-Sinai Medical Center, 8700 Beverly Blvd, Los
Angeles, CA 90048; Department of Radiology, University of California at Los Angeles, Los Angeles, Calif (K.M.); and Departments of Radiology
(M.D.K.) and Orthopedic Surgery (L.M.), University of Southern California, Los Angeles, Calif. Presented as an education exhibit at the 2008
RSNA Annual Meeting. Received April 3, 2009; revision requested May 6 and received July 6; accepted July 10. All authors have no financial relationships to disclose. Address correspondence to D.N.I. (e-mail: [email protected]).
©
RSNA, 2009
2128 November-December 2009
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Figure 1. Histologic features of osteoid osteoma. Medium-power photomicrograph (original magnification,
×80; hematoxylin-eosin stain) of the nidus of an osteoid
osteoma demonstrates irregular masses of eosinophilic
osteoid matrix (white arrow) and intensely stained
bone trabeculae (black arrow) rimmed by osteoblasts
(arrowhead).
Introduction
Teaching
Point
Osteoid osteoma is a relatively common entity.
In a Mayo Clinic review of 11,087 primary bone
tumors that were subjected to either biopsy or
complete surgical resection, osteoid osteoma
accounted for 13.5% of all benign tumors (1).
There is a male predominance, with a reported
male-to-female ratio of 4:1 in one large patient
series (2). Most of those affected are young; approximately one-half are in the 2nd decade of life
at presentation. The most common symptom is
bone pain, which often worsens at night and is
usually dramatically relieved by aspirin or other
nonsteroidal anti-inflammatory drugs. Pain initially may be described as a dull ache but may
progress to severe localized pain over the site
of the tumor. It may cause arousal from sleep,
with resultant sleep deprivation (3). The pain is
thought to be mediated by release of prostaglandins, which helps to explain the relief experienced
after the ingestion of prostaglandin inhibitors
such as nonsteroidal anti-inflammatory drugs (4).
Less common manifestations of osteoid osteoma include growth disturbance, bone deformity, and painful scoliosis. If the lesion is located
within a joint capsule, it may cause joint swelling,
synovitis, and restricted mobility. Physical examination may disclose focal tenderness; however,
signs of inflammatory disease, including erythema
and warmth, are almost always absent. The results
Figure 2. Radiographic appearance of osteoid
osteoma. Lateral radiograph shows a faint tibial
diaphyseal circular lucent defect with a diameter of less than 1.5 cm (arrow) and surrounding sclerosis.
of laboratory analyses are typically normal. Studies
of the natural history of the lesion have demonstrated cases of spontaneous regression, but treatment is usually required to obtain relief (2).
The standard treatment traditionally has been
surgical resection. However, the potentially serious complications of surgery have made percutaneous radiofrequency (RF) ablation an attractive
alternative. The article summarizes the indications and contraindications for RF ablation, with
emphasis on the histologic and radiologic appearances of osteoid osteoma and its mimics, and offers a detailed step-by-step guide for performing
successful ablation. Preprocedural preparations,
procedural technique, and postprocedural care
are described and illustrated.
Histologic and Radiologic Imaging Characteristics
Osteoid osteoma is composed of a nidus of woven bone and osteoid rimmed with osteoblasts,
with a surrounding reactive zone of thickened
cortical or trabecular bone and loose fibrovascular tissue (Fig 1). Lesions are classified according
to the location of the nidus at radiologic imaging.
Those with a cortical location are the most com-
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Figure 3. Double-density sign produced by
radionuclide uptake in osteoid osteoma. Anterior
planar view of the knees obtained with technetium medronate scintigraphy in a 20-year-old
woman demonstrates intense activity in the nidus of a lesion in the intercondylar region of the
distal left femur (arrow) with surrounding mild
activity. This combination of findings, known as
the double-density sign, is pathognomonic of
osteoid osteoma.
Teaching
Point
mon and are characterized by reactive sclerotic
cortical thickening surrounding a central radiolucent nidus. Intramedullary and subperiosteal lesions are less common, are usually intra- or juxtaarticular in location, and usually demonstrate
less osteosclerosis, which may appear at some
distance from the nidus.
Most lesions are found in the long bones of
the lower extremity, particularly the metadiaphyseal regions of the femur and tibia. Other
common sites include the spine, hands, and feet.
However, the tumor may occur in any bone.
Radiographic Findings
Radiographs characteristically show a circular
or ovoid cortical lucency representing the nidus
(usually less than 1.5 cm in diameter) with a variable degree of surrounding sclerosis (Fig 2). If
sclerosis is extensive, it may interfere with visualization of the radiolucent nidus. Intramedullary
and subperiosteal lesions may not demonstrate
significant osteosclerosis, and the cortex overlying
the site may appear normal, making intraoperative surgical localization difficult.
Scintigraphic Findings
Radionuclide skeletal scintigraphy characteristically reveals intense activity at the site of the nidus
and relatively decreased activity in the surrounding
reactive zone, a pattern referred to as the double-
Motamedi et al 2129
Figure 4. CT appearance of osteoid osteoma. Coronal reformatted CT image of
the proximal right femur in a 17-year-old
boy reveals a radiolucent nidus (arrow)
with faint internal mineralization and mild
surrounding reactive sclerosis.
density sign (Fig 3). Scintigraphy may be useful
for lesion localization, particularly in cases with
normal or nearly normal radiographic findings.
Computed Tomographic Findings
Computed tomography (CT) remains the modality of choice for detecting osteoid osteoma and
generally provides the best characterization of both
the nidus and the surrounding cortical sclerosis
(2). The nidus appears as a well-defined radioluTeaching
cent region and demonstrates varying degrees of
Point
central mineralization in approximately 50% of
cases. Although the use of intravenous contrast
material is not necessary to obtain images of diagnostic quality, the nidus enhances at contrastenhanced CT. Marked reactive sclerosis around
the nidus is common; however, some lesions may
have little to no reactive sclerosis (Fig 4).
Findings at Magnetic Resonance Imaging
At magnetic resonance (MR) imaging, the signal in the nidus typically is isointense to that of
muscle on T1-weighted images and is variable on
T2-weighted images. Signal hyperintensity is seen
in the surrounding reactive zone on T2-weighted
or short inversion time inversion-recovery images (Fig 5). However, the imaging findings may
be nonspecific and may mimic those of a stress
fracture or osteomyelitis if extensive surrounding
edema obscures the nidus. Dynamic MR imaging
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Figure 5. MR imaging appearance of osteoid osteoma. Coronal T1-weighted (a) and T2-weighted (b) pelvic MR
images obtained in the same patient as in Figure 4 show the nidus in the right femoral neck (arrow) with surrounding
marrow edema.
Figure 6. Brodie abscess. Axial CT
image, obtained in a previously healthy
21-year-old woman with a 2-month
history of nighttime pain in the right
thigh, shows a focal cortical lucency
(arrow) with central calcification and
surrounding sclerosis, features resembling those of osteoid osteoma, in the
proximal right femur. Coronal reformatted images showed a 5-cm-long
craniocaudal extension of the lesion, a
finding atypical for an osteoid osteoma
nidus. Laboratory analyses of a biopsy
specimen helped confirm the diagnosis
of Brodie abscess with a positive culture
of Staphylococcus aureus.
with the use of a gadolinium-based contrast material may provide increased conspicuity of the nidus
and help improve overall diagnostic accuracy in
cases with indeterminate findings at CT or unenhanced MR imaging (5).
Differential Diagnosis
A Brodie abscess may resemble an osteoid osteoma at radiography, CT, and MR imaging (Fig
6). A bony sequestrum within an abscess may be
confused with calcification in an osteoid osteoma
nidus. The presence of a linear or serpentine
tract leading away from the abscess cavity may be
helpful for differentiation (6). Bone scintigraphy
typically reveals decreased radionuclide activity
in the abscess cavity, in contrast to the doubledensity sign seen in osteoid osteoma (2).
A stress fracture also may simulate osteoid
osteoma, with findings of osteosclerosis at radiography and CT and marrow edema at MR
imaging; however, no nidus should be seen (2).
In the presence of a stress fracture, a linear radiolucency at CT or a low-signal-intensity line at
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Figure 7. Intracortical chondroma. Coronal short inversion time inversion-recovery
MR image, obtained in a healthy 3-year-old
boy with leg pain after minor trauma, shows
a focus of high signal intensity in the anterolateral left tibial cortex (arrow), a finding that
corresponds to a 6-mm circular lucent lesion
seen at initial radiography. The lesion is surrounded by a region of reactive bone marrow
edema. The differential diagnosis included
osteoid osteoma. Pathologic analysis of a
surgical specimen showed intracortical chondroma, a rare benign lesion that may mimic
osteoid osteoma clinically and radiologically.
MR imaging may be depicted with an orientation
perpendicular to the cortex. The nidus of an osteoid osteoma, by contrast, is round or ovoid and
usually parallels the cortex.
Chondroblastoma may elicit a surrounding
tissue reaction similar to that in osteoid osteoma,
with extensive marrow edema. Chondroblastoma
has a characteristic predilection for the epiphyseal
centers of growing bones, but osteoid osteoma
may occur in a similar location and age group.
The lytic focus in chondroblastoma tends to be
larger and more lobular in contour than the nidus
of osteoid osteoma, and the presence of a calcified chondroid matrix may be suggestive of the
diagnosis (7).
Osteoblastoma may be indistinguishable from
osteoid osteoma at radiologic imaging. The lesion
size and natural history are its main differentiating features: Osteoblastoma tends to be larger
(nidus diameter, >2 cm) and exhibits growth
progression (6).
Other rare mimics of osteoid osteoma include
primary benign and malignant neoplasms such as
intracortical chondroma (Fig 7) and intracortical
osteosarcoma (6,7).
Treatment Options
Medical Management
Osteoid osteoma may be self-limiting, and the
regression of some lesions has been documented.
However, the onset of regression is generally de-
layed (it may not occur until 7 years after symptom onset), and patients invariably present before
it occurs (8). The mechanism of involution is
unknown, but the leading theory involves tumor
infarction. Aspirin or other nonsteroidal anti-inflammatory medications frequently provide effective pain control, but long-term therapy may be
unacceptable because of refractory pain, recurrent nighttime pain with resultant sleep deprivation, or gastrointestinal complications. Articular
or periarticular osteoid osteomas are particularly
resistant to conservative therapy, and more aggressive intervention is often necessary.
Surgical Management
Complete surgical resection has historically been
the treatment of choice for osteoid osteoma,
with success rates of 88%–97% for en bloc open
resection (9,10). Lesion resection leaves a bone
defect that may be vulnerable to fracture and, in
some cases, may necessitate internal fixation and
bone grafting. To minimize the amount of excised
bone, precise intraoperative localization of the
lesion is important. Yet localization is difficult in
some cases, even with the use of various specialized methods such as needle- or wire-based localization of the nidus, tetracycline labeling, and
intraoperative scintigraphy. Even in cases of successful localization, the surgically created bone
defect may lead to a fracture (Fig 8). Surgical
Teaching
Point
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Figure 8. Postsurgical fracture.
(a) Lateral radiograph obtained
after an excisional biopsy (same
patient as in Fig 7) reveals a tibial
curettage defect. (b) Anteroposterior
radiograph obtained 5 months after
surgery shows a fracture at the site
of tibial curettage.
resection of a lesion may be incomplete, necessitating a second surgery (Fig 9). The location of
some lesions may preclude surgical excision or
increase the risk of injury to adjacent structures
(Fig 10). The excision of articular and epiphyseal
lesions may require arthrotomy, with resultant
impairment of bone growth, joint mobility, or
both. Other postsurgical complications include
hematoma and infection. The average postoperative hospital stay is 3–5 days (9). Weight-bearing
activity is limited for 1–6 months after surgery,
and the use of crutches may be necessary in cases
involving the lower limb.
CT-guided percutaneous resection is a lessinvasive alternative method of treatment that may
allow a reduced hospital stay and earlier resumption of weight-bearing activity. However, it is associated with postoperative complications similar
to those of open surgical excision, including he-
matoma, osteomyelitis, and fracture. The failure
rate with this method of treatment in one patient
series was 16% (11).
RF Ablation
The use of RF ablation to treat osteoid osteoma
was first described in 1989 (12), with initial
results published in 1992 (13). The procedure
is safe and effective (13–17), widely available,
and should be considered the current method of
choice for treatment. RF ablation is performed
with the use of CT for guidance in lesion localization and treatment and with general, spinal,
or propofol-induced anesthesia. Local anesthesia
alone usually results in insufficient pain control,
particularly during entry into the nidus of the
lesion (4,18). The average procedure can be
performed in approximately 90 minutes, and the
duration of postprocedural hospitalization for
observation is 3–24 hours (19). All daily activities may be resumed immediately without the
use of a cast, splint, or other external supportive
apparatus.
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Figure 9. Incomplete resection. (a) CT image obtained in a 24-year-old man shows an osteoid osteoma in
the right ilium (arrow). The lesion was subsequently resected. (b–d) Postoperative CT images show surgically
created bone defects at levels superior (b) and inferior (c) to the nidus, which is still visible in d.
Figure 10. Surgically inaccessible lesion. Axial (a) and coronal (b) CT images obtained
in a 15-year-old girl demonstrate an osteoid osteoma in the left sacrum (arrow) with adjacent sclerosis and with narrowing of the left S2 neural foramen secondary to hyperostosis.
Surgical access to a lesion in such close proximity to the nerve roots is a challenge.
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Figure 11. Spinal osteoid osteoma. Axial CT
image of the cervical spine in a 32-year-old
woman demonstrates a 1.4-cm osteoid osteoma
in the region of the left inferior articular process
with mild narrowing of the left lateral recess. The
lesion is too close to nerve roots and the cervical
spinal cord to be treated with RF ablation.
Figure 13. Coronal reformatted CT image
shows a cortically based sclerotic lesion (arrow),
a finding consistent with a diagnosis of osteoid
osteoma. (Figs 13–22 were obtained in the same
patient as Fig 12.)
Relative contraindications to RF ablation
include the location of a lesion in the hand or
the spine (<1 cm from vital structures such as
nerves) (20), pregnancy, cellulitis, sepsis, and coagulopathy (Fig 11). Lesions with a nidus larger
than 1 cm generally require multiple applications
of the electrode in various positions (21).
All patients should undergo preprocedural
screening, during which a medical history is
obtained and a physical examination and basic
laboratory analyses are performed. The skin
overlying the lesion must be intact at physical
examination. Patients should be counseled that
the development of any changes such as rash or
Figure 12. Lesion localization in a 17-year-old
boy. Axial thin-section CT image obtained with
the leg in external rotation to facilitate needle
placement clearly depicts an osteoid osteoma with
a radiolucent nidus (arrow) in the femoral neck.
Figure 14. Approach planning. Axial CT image of the proximal femur demonstrates the preferred angle of approach, nearly perpendicular
to the cortical surface (arrow). This approach
was planned to avoid an adjacent neurovascular
bundle (arrowhead).
infection in the skin overlying the planned entry
site will result in postponement or cancellation
of the procedure.
Step-by-Step Guide
for Performing Ablation
Lesion Localization
Localization of the lesion is performed by acquiring multiple thin-section CT images at the level
of the osteoid osteoma, within a region of interest of approximately 4 cm (Fig 12). Multiplanar
reformatted images may be helpful for preproce-
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Figure 15. Placement of grounding
pads. Grounding pads placed on the ipsilateral thigh and contralateral extremity help minimize the transmission of
RF energy through the body and help
prevent excessive heating.
Figure 16. Bone biopsy system. (a) Photograph shows a 14-gauge penetration cannula (large
green cap) with inner stylet (smaller green cap), coaxial drill tip (white cap), and coaxial biopsy
needle (blue cap). (b) Photograph provides a magnified view of the tips of the instruments in a.
dural planning (Fig 13). A single skin entry point
is planned for lesions less than 1 cm in diameter.
For lesions larger than 1 cm, multiple skin entry
points are required to obtain the 1-cm-wide ablated margin required for treatment success (21).
Approach Planning
Radiopaque markers are placed on the skin
overlying the lesion. The preferred approach is
at an angle perpendicular to the cortical surface
of the involved bone. The route is planned so as
to avoid any adjacent neurovascular structures
(Fig 14). In some cases, entry through the opposite normal cortex may be necessary to avoid
overlying critical structures.
Time-Out and Grounding Pad
After the entry site is marked, a time-out should
be taken to confirm the patient’s identity and
verify the side and site of the planned procedure. Grounding pads are then put in place to
inhibit the transmission of current through the
patient (Fig 15) (18). The targeted extremity
should be secured to prevent movement during
biopsy and ablation. The skin overlying the lesion is then prepared and draped in accordance
with sterile technique. One gram of cefazolin
sodium is administered intravenously before the
start of the procedure.
Skin Entry
A bone biopsy system is used to obtain percutaneous access to the lesion, perform a biopsy, and
guide the RF electrode for ablation (Fig 16). Typical components include a penetration cannula
and inner stylet, a coaxial drill tip to penetrate the
outer cortex, and a coaxial biopsy needle. Local
anesthesia is attained with an infusion of 1% lidocaine solution via a superficial skin wheal along a
preselected path to the bone surface. The cannula
with stylet is then inserted and advanced through
the soft tissues to the bone surface, and a localizing scan is performed to confirm that the cannula
2136 November-December 2009
Figure 17. Skin entry and verification of cannula positioning. Spot CT image shows appropriate positioning
of the cannula for biopsy of the lesion. After the cannula and inner stylet are advanced to the bone surface,
the stylet is removed to avoid a metallic artifact, which
might obscure a small lesion at verification scanning.
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Figure 18. Bone entry. Photograph shows the cannula
containing the drill used to penetrate the outer cortex.
is correctly positioned. The inner stylet should be
removed before scanning to avoid metallic artifact,
which may obscure a small lesion (Fig 17).
Superficial Bone Entry and Drilling
Once the cannula is appropriately positioned,
upward traction should be exerted on the skin
and soft tissues around the cannula by using
the thumb and index finger to form a tentlike
structure to prevent needle tip displacement.
The stylet is then exchanged for the bone drill
(Fig 18), which is advanced through the cannula
to penetrate the outer cortex. Spot CT images
are obtained to verify the depth and direction
of the bone drill (Fig 19). When the drill tip is
positioned at the cortical edge of the nidus, the
cannula is advanced over the drill to maintain
the position. The drill is then retracted.
Biopsy
Although osteoid osteoma often has characteristic clinical and imaging findings, other neoplastic and nonneoplastic conditions may have
similar manifestations; hence, a biopsy of the
lesion should be routinely performed to help
confirm the diagnosis and direct subsequent
treatment. The cannula serves as a pathway
for the biopsy needle (Fig 20), which is ad-
Figure 19. Bone entry. Spot CT image obtained after insertion of the cannula, removal of
the stylet, and advancement of the drill through
the cannula shows proper positioning of the drill,
at the cortical edge of the lesion.
vanced into the nidus with intermittent CT for
guidance (Fig 21). A specimen is then removed,
placed in a formalin solution, and submitted for
analysis.
Cannula and Electrode Placement
The RF electrode is then inserted, with its
tip directed toward the center of the nidus,
through the fixed cannula. Spot CT images
are again obtained to confirm appropriate positioning. Prior to ablation, the cannula is partially withdrawn over the electrode to prevent
unintended heating of adjacent tissue by propagation along the metal cannula (18). The cannula
should be retracted to a point more than 1 cm
from the electrode tip (Fig 22). Spot CT images
again are obtained to confirm a satisfactory position of the electrode and cannula.
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Figure 20. Needle placement for biopsy. Photographs show the cannula and needle before (a) and after (b)
placement for biopsy.
Figure 21. Spot CT images obtained at successive intervals during needle insertion (b later than a) show
the needle extending beyond the cannula and penetrating the nidus.
Figure 22. Positioning of the electrode for ablation of osteoid osteoma. (a) Photograph shows insertion
of the electrode through the cannula and into the lesion. (b) Spot CT image shows partial retraction of the
cannula (arrow) to a position more than 1 cm proximal to the electrode tip. This step reduces the risk of heat
propagation along the metal cannula and thermal injury to soft tissues adjacent to the lesion.
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Figure 23. RF generator settings for ablation. Photograph shows the generator with cables connected to
the grounding pad (arrowhead) and electrode (arrow).
Electrode Connection
The grounding pad is connected to the dispersive electrode cord, which is then plugged into
the RF generator. The RF electrode also is connected to the generator (Fig 23). The generator
is activated with an electrical impedance value
of 200–600 Ω (18).
RF Ablation
Thermal heating is applied with the RF electrode at a targeted temperature of 90°C, with
manual adjustment of output controls during
the procedure as needed to maintain a stable lesion temperature (Fig 24). An automatic override
temperature control helps prevent excessive heating. Ablation is typically performed for a total of
4–6 minutes. Large lesions may require multiple
ablation cycles with the electrode in different
positions, and, if necessary, the cannula may be
repositioned through a separate skin incision.
Postprocedural imaging of the lesion site is not
routinely performed because the CT features of
the lesion are unchanged after ablation.
Figure 24. Photograph shows RF generator settings
for ablation. The numbers displayed indicate the temperature at which the automatic override is invoked
(arrowhead), planned ablation time of 6 minutes
(white arrow), and 3 minutes of ablation time remaining (black arrow). The temperature control can be
manually adjusted during ablation to ensure sufficient
heating and avoid overheating.
Physiologic Reaction
A characteristic physiologic reaction, which may
include an increase in the respiratory rate, heart
rate, and blood pressure as well as involuntary
patient motion, has been reported to occur when
the nidus of the osteoid osteoma is entered (4).
These changes typically abate during the procedure; however, deep anesthesia may be needed
during biopsy and ablation.
Postprocedural Care
After ablation, the electrode is removed and a
local anesthetic (bupivacaine hydrochloride) is
injected via the cannula for pain relief. After a
sterile dressing is applied to the skin entry site,
the patient is transferred to the postanesthesia
care unit. Pain medication may be administered
as needed, although pain often abates after the
1st day (15). Diet and activity are advanced as
tolerated, and the patient is discharged after routine discharge criteria are met, usually within 3–4
hours. Daily activities, except for driving, may be
resumed immediately after discharge (15). Excessive stressful weight bearing and prolonged strenuous activity should be avoided for 1–3 months
following the procedure if ablation is performed
in a weight-bearing bone (9,14). A follow-up visit
is scheduled for 1 month after the procedure.
Teaching
Point
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Figure 25. Intra-articular osteoid osteoma in a 31-year-old woman. (a) Axial CT scan shows an ovoid articular lesion in the femoral head (arrow). (b) Axial CT scan shows the placement of a cannula and needle
for biopsy.
Clinical Success
Clinical success of RF ablation is defined as
the absence of pain 2 years after the procedure.
Clinical success rates between 89% and 95% for
primary treatment have been widely reported
(14,17,22,23). These results compare favorably
with those of surgical treatment and other less
invasive therapies, such as CT-guided percutaneous resection (11) and laser ablation (24). Residual or recurrent pain may be due to inaccurate
needle positioning, irritation of adjacent soft tissues, or inadequate ablation of large lesions (21).
For lesions larger than 1 cm in diameter, the use
of multiple needle positions is recommended to
achieve satisfactory tumor destruction.
Follow-up CT is not routinely indicated but
may demonstrate partial or complete replacement
of the nidus with sclerotic bone within 2 months
to 2 years after ablation. After 2 years, the nidus
may be completely indistinguishable from surrounding bone. Follow-up MR imaging should
show resolution of bone marrow edema. A CT
finding of persistent radiolucency of the ablated
site, or MR imaging findings of arterial enhancement of the nidus and residual marrow edema
in patients with negative findings at CT, are suggestive of residual tumor. If residual symptoms
are present, a second application of RF ablation
is safe and is often successful (15), with reported
response rates of 80%–90% (14). However, the
outcome of repeat ablation tends to be poor in
patients who experience recurrent symptoms after a pain-free interval (14,15).
Special Cases
Large Lesions
When the nidus of an osteoid osteoma exceeds
1 cm in its greatest dimension, the use of two
or more electrode positions is often necessary
to successfully ablate the lesion. Overlap of the
treatment zones is recommended to increase the
likelihood of complete nidus ablation. Residual
or recurrent pain after ablation is more common
with large lesions than with small ones.
Intra-articular Lesions
Ablation of intra-articular lesions can be technically challenging. The hip is the joint most commonly affected by osteoid osteomas (Fig 25). If
possible, a transarticular approach should be
avoided because it increases the risks of infection,
electrode cooling, and ablation of nontargeted
tissue (20). A focal defect in the articular cartilage may be created at the ablation entry point,
and the patient should be made aware of this
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Figure 26. Spinal osteoid osteoma in a 22-year-old woman with painful scoliosis. Anteroposterior (a) and oblique (b) radiographs of the thoracolumbar spine demonstrate a mixed lytic
and sclerotic lesion of the left L3 pedicle (arrow). The diagnosis was based on histopathologic
analysis of a biopsy specimen. (Case courtesy of Deborah M. Forrester, MD, Los Angeles
County–University of Southern California Medical Center, Los Angeles, Calif.)
before providing consent for the procedure. However, articular cartilage appears to be relatively
tolerant of thermal injury induced by short-term
heating (14).
Spinal Lesions
Spinal lesions account for 10% of osteoid osteomas. The lesions involve the lumbar, cervical, and
thoracic spinal segments, in order of decreasing
frequency. Involvement of the posterior elements
is more common than that of the vertebral body,
and the spinal canal and paraspinous soft tissues
are not affected. The classic manifestation of a
spinal osteoid osteoma is painful scoliosis (Fig
26). When localizing the lesion for RF ablation,
it is important to avoid entering the facet joint
or neural foramen. The presence of intact cortex
between the nidus and spinal cord or nerve root
is also necessary to avoid thermal injury to neurovascular structures (20).
Complications
Few complications of RF ablation for osteoid
osteoma have been described. A skin burn with
thermal necrosis may occur with superficial thermocoagulation (15). Neural injury is of particular
concern in spinal and hand osteoid osteomas.
Some authors suggest electrode placement at
least 1 cm away from major nerves, precluding
the use of RF ablation in some cases (14,17,22).
In these patients, percutaneous laser ablation
may be an alternative therapy, and has been successfully employed to treat lesions less than 8 mm
from vital structures (24). Other potential complications include bleeding and infection at the
skin entry site.
RG ■ Volume 29 • Number 7
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This article meets the criteria for 1.0 AMA PRA Category 1 Credit TM. To obtain credit, see www.rsna.org/education
/rg_cme.html.
RG
Volume 29 • Number 5 • November-December 2009
Motamedi et al
Thermal Ablation of Osteoid Osteoma: Overview and
Step-by-Step Guide
Daria Motamedi, MD, et al
RadioGraphics 2009; 29:2127–2141 • Published online 10.1148/rg.297095081 • Content Codes:
Page 2128
The most common symptom is bone pain, which often worsens at night and is usually dramatically
relieved by aspirin or other nonsteroidal anti-inflammatory drugs.
Page 2129
Most lesions are found in the long bones of the lower extremity, particularly the metadiaphyseal
regions of the femur and tibia.
Page 2129
Computed tomography (CT) remains the modality of choice for detecting osteoid osteoma and
generally provides the best characterization of both the nidus and the surrounding cortical sclerosis.
The nidus appears as a well-defined radiolucent region and demonstrates varying degrees of central
mineralization in approximately 50% of cases. Although the use of intravenous contrast material is
not necessary to obtain images of diagnostic quality, the nidus enhances at contrast-enhanced CT.
Marked reactive sclerosis around the nidus is common; however, some lesions may have little to no
reactive sclerosis.
Page 2131
Aspirin or other nonsteroidal anti-inflammatory medications frequently provide effective pain control,
but long-term therapy may be unacceptable because of refractory pain, recurrent nighttime pain with
resultant sleep deprivation, or gastrointestinal complications. Articular or periarticular osteoid
osteomas are particularly resistant to conservative therapy, and more aggressive intervention is often
necessary.
Page 2138
A characteristic physiologic reaction, which may include an increase in the respiratory rate, heart rate,
and blood pressure as well as involuntary patient motion, has been reported to occur when the nidus
of the osteoid osteoma is entered. These changes typically abate during the procedure; however, deep
anesthesia may be needed during biopsy and ablation.