AJR 2000; 174:417-424
© American Roentgen Ray Society
Primary Tumors of the Sacrum
Diagnostic Imaging
J. Llauger1,
J. Palmer1,
S. Amores1,
S. Bague2 and
A. Camins1
1
Servei de Radiodiagnòstic, Hospital de la
Santa Creu i Sant Pau, Sant Antoni M. Claret, 167, 08025 Barcelona,
Spain
2
Servei d'Anatomia Patològica, Hospital de la
Santa Creu i Sant Pau, 08025 Barcelona, Spain
Received May 24, 1999;
accepted after revision August 4, 1999.
Address correspondence to J. Llauger.
Presented at the annual meeting of the American Roentgen Ray Society, New
Orleans, May 1999.
Introduction
Primary benign and malignant tumors of the sacrum are rare lesions that
account for fewer than 7% of all intraspinal primary tumors
[1,
2]. Metastatic lesions,
multiple myeloma, and lymphoma are far more common than primary sacral tumors.
Patients with sacral tumors present with nonspecific symptoms, including pain,
palpable mass, and neurologic deficits. The purpose of CT and MR imaging is to
define the anatomic origin, extent, and radiologic features of a given lesion.
Although the differential diagnosis for a sacral tumor is extensive, various
primary neoplasms have characteristic features on CT scans and MR imaging that
may aid in making a diagnosis. In this article we discuss and illustrate the
most common primary tumors that affect the sacrum. Our purpose is to interpret
the imaging features of these lesions and to emphasize the correlation between
the radiologic presentation and the pathologic findings.
Benign Tumors
Giant Cell Tumor
Giant cell tumors of the spine, representing only 3-7% of all giant cell
tumors, are uncommon. Most giant cell tumors of the spine occur in the sacrum.
Compared with chordomas, which are central lesions, sacral giant cell tumors
are frequently eccentric and abut or extend across the sacroiliac joint.
Tumors in women predominate (2:1). Patients are usually affected between the
ages of 15 and 40 years. [3].
Pain and neurologic deficits are the most common presenting symptoms. A giant
cell tumor is composed of osteoclastic giant cells within a spindle cell
stroma. Hemorrhagic and fibrotic areas are commonly found. Spontaneous
malignant transformation was reported in fewer than 2% of patients
[4], but it often occurs after
radiation therapy. Giant cell tumors are purely lytic destructive lesions.
Matrix calcifications and septations are absent (Fig.
1A,
1B). On CT scans and MR images,
giant cell tumors are frequently heterogeneous because of the presence of
necrosis (low-attenuation areas), hemorrhage (high-signal-intensity areas on
T1- and T2-weighted sequences or fluid-fluid levels), or cystic spaces. Low
signal intensity is frequently noted on T2-weighted images and is related to
the high hemorrhagic and fibrotic content of this tumor.
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Fig. 1B. —21-year-old woman with sacral giant cell tumor. CT scan shows
expanding mass in upper sacrum with extension into spinal canal and posterior
soft tissues. Small fluid-fluid levels (arrowheads) are seen within
tumor.
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Aneurysmal Bone Cyst
Aneurysmal bone cyst represents fewer than 1% of all primary bone tumors.
Approximately 20% of all aneurysmal bone cysts are located in the spine,
particularly in the cervical and thoracic regions, where the posterior
elements are typically involved; sacral involvement represents fewer than 20%
of all spinal aneurysmal bone cysts
[5]. The peak incidence is in
the second decade of life with a slight female predominance. The presenting
symptoms include pain, neurologic deficits, and mass. A typical aneurysmal
bone cyst is composed of multiloculated blood-filled spaces separated by
septations that contain the solid components of the tumor. The most common
radiographic appearance of an aneurysmal bone cyst is that of an osteolytic
expansile lesion surrounded by a thin shell of bone. CT and MR imaging are
useful methods of delineating the extent of sacral aneurysmal bone cysts (Fig.
2A,
2B,
2C). Both techniques may reveal
multiple fluid-fluid levels reflecting hemorrhage with sedimentation, a
characteristic feature of this tumor.
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Fig. 2C. —27-year-old woman with aneurysmal bone cyst. Photomicrograph of
histologic specimen shows tumor composed of blood-filled spaces that lack
healthy endothelium. Solid portions contain benign reactive osteoid, spindle
cells, and multinucleated cells in collagenous stroma. (H and E,
x200)
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Osteoid Osteoma
Osteoid osteoma of the spine accounts for 10% of all osteoid osteomas. Only
2% of spinal osteoid osteomas are found in the sacrum
[6]. Men, usually between the
ages of 10 and 20 years, are affected two to three times more frequently than
women. Patients present with pain that is often worse at night and is relieved
by nonsteroidal drugs. The tumors most typically arise from the articular
process of S1. Osteoid osteoma appears as a radiolucent lesion less than 2 cm
in diameter surrounded by marked perifocal sclerosis. Central calcification
may be observed within the osteolytic nidus. CT is particularly useful for
detecting and identifying spinal osteoid osteomas. The usefulness of MR
imaging in detecting the nidus is unclear; when an osteoma is detected, signal
intensity is generally low on T1-weighted images and intermediate to high on
T2-weighted images. In some patients, the nidus is undetectable as a result of
marrow edema, soft-tissue edema, or surrounding sclerosis (Fig.
3A,
3B).
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Fig. 3B. —19-year-old man with osteoid osteoma of sacrum. Sagittal T2-weighted
MR image shows lesion to be of low signal intensity reflecting dense sclerosis
surrounding small nidus (arrowheads).
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Osteoblastoma
Osteoblastomas are rare lesions constituting 1-2% of all primary bone
tumors. Approximately 40% of all osteoblastomas are located in the spine; 17%
of spinal osteoblastomas are found in the sacrum. The mean age of patients at
presentation is 20 years, with a slight male preponderance. Pain, scoliosis,
and neurologic deficits are the most common presenting symptoms. Spinal
osteoblastomas most frequently involve the posterior vertebral elements.
Osteoid osteoma and osteoblastoma having similar clinical and histologic
manifestations may be considered variants of the same process. The typical
osteoblastoma shows a lytic defect larger than 1.5 cm in diameter surrounded
by a sclerotic ring. Calcifications, when present, are usually multiple. In
some patients, osteoblastomas showing cortical destruction and extension into
adjacent soft tissue have an aggressive appearance. On MR imaging, signal
intensity patterns are usually nonspecific. Peritumoral edema in marrow and in
soft tissue (flare phenomenon), representing an inflammatory response to the
lesion, is a characteristic finding (Fig.
4A,
4B).
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Fig. 4A. —21-year-old man with sacral osteoblastoma. Sagittal T1-weighted MR
image shows large lobulated expanding mass (arrow) involving S1 and
S2. Mass extends into spinal canal. Note peritumoral marrow and soft-tissue
edema.
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Hemangioma
Cavernous hemangioma is by far the most common benign spinal neoplasm,
occurring in 11% of spines at autopsy. Vertebral hemangiomas constitute 75% of
all osseous hemangiomas. Multiple lesions are seen in 25-30% of patients. They
most commonly occur in the lower thoracic and lumbar regions. Vertebral
hemangiomas usually involve the vertebral body. Sacral involvement is
uncommon. Vertebral hemangiomas usually present in the fourth to sixth
decades, with a female preponderance (2:1). Most patients are asymptomatic,
but some lesions can expand and cause pain, pathologic fractures, and cord
compression. In patients with angiomatosis, multiple osseous lesions occur,
particularly in the pelvis, femur, ribs, and calvaria. On radiographs, most
osseous hemangiomas show a coarse vertical or radiating trabecular thickening.
Other lesions present a characteristic honeycomb appearance (Fig.
5A,
5B,
5C,
5D).
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Fig. 5A. —42-year-old-woman with osseous angiomatosis. Frontal radiograph
shows lesions composed of lytic areas and pattern of irregular and radiating
trabecular thickening in sacrum, left iliac wing, and right acetabulum.
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Fig. 5B. —42-year-old-woman with osseous angiomatosis. CT scan of sacrum
reveals diffuse involvement and typical honeycomb appearance reflecting
multiple osteolytic lesions with sclerotic margins. Note left iliac
lesions.
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Nerve Sheath Tumor
Two main types of nerve sheath tumors, schwannoma and neurofibroma, can be
found arising from sacral nerve roots. Nerve sheath tumors are intradural
extramedullary masses and, consequently, are not true primary sacral
neoplasms. Most tumors are purely intradural lesions without associated bony
changes. Dumbbell-shaped tumors with intradural and extradural components
typically cause enlarging of neuroforamina and sacral destruction
(Fig. 6).
Malignant Tumors
Chordoma
Chordomas are relatively rare tumors that account for 2-4% of all primary
malignant bone tumors. However, chordoma is the most common (20-34% of
patients) primary malignant sacral neoplasm
[1]. This tumor arises from
intraosseous notochordal remnants. Nearly 50% of all chordomas originate in
the sacrococcygeal region, particularly in the fourth and fifth sacral
segments. Another 35% are in the sphenooccipital region; only 15% of chordomas
occur in the spine above the sacrum. Men are affected twice as frequently as
women; the mean age of patients is 50 years. Chordomas are slow-growing tumors
that are commonly discovered as large masses. Typical chordomas contain clear
cells with intracytoplasmic vacuoles (physaliphorous cells) and abundant
mucin, both intracellular and extracellular. In atypical or dedifferentiated
chordomas, the mucinous matrix is replaced by chondroid or osteoid elements.
On CT scans, sacral chordomas show large lytic lesions centered in the midline
and an associated soft-tissue mass. Calcification is present in 30-70% of
patients (Fig. 7A,
7B,
7C,
7D). Compared with skeletal
muscle, typical chordomas are iso- or slightly hypointense on T1-weighted
images, and typically hyperintense on T2-weighted images. These chordomas
correlate with the intratumoral accumulation of mucin.
Chondrosarcoma
In the spine, in contradistinction to the appendicular skeleton,
chondrosarcomas are more common than osteosarcomas. Chondrosarcomas account
for 7-12% of malignant primary tumors of the spine; approximately 10% of all
chondrosarcomas are found in the spine
[1]. The thoracic spine is the
most common site; sacral involvement is unusual. The mean age of patients with
chondrosarcoma is 45 years old. Primary and secondary forms (arising from
malignant degeneration of osteochondromas) may occur. Most chondrosarcomas are
central in origin and primary. Histologically, conventional chondrosarcomas
may be graded from 1 to 3 and are composed of lobules of hyaline cartilage
separated by fibrovascular septations. Radiographs and CT images reveal large
destructive lesions with characteristic chondroid matrix mineralization.
Calcifications are typically rounded or curvilinear and are also visible in
the soft-tissue component of the lesions (Fig.
8A,
8B). Calcifications are
manifested as areas of signal void on MR images. Enhanced MR imaging typically
shows peripheral and septal enhancement (rings and arches) corresponding to
vascular septations between cartilaginous lobules (Fig.
9A,
9B,
9C).
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Fig. 8A. —24-year-old man with chondrosarcoma of sacrum who presented with
pain and neurologic symptoms. Frontal radiograph shows densely mineralized
lesion extending into soft tissues without osseous destruction.
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Fig. 8B. —24-year-old man with chondrosarcoma of sacrum who presented with
pain and neurologic symptoms. CT scan displayed at soft-tissue window shows
lobulated presacral mass of soft-tissue attenuation. Irregular calcifications
within lesion were also visible in sacrum and spinal canal.
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Ewing's Sarcoma and Primitive Neuroectodermal Tumor
The spine is the principal site for only 3-10% of all primary Ewing's
sarcomas and primitive neuroectodermal tumors. However, metastatic involvement
from extraspinal primary lesions is more common. Ewing's sarcoma is the most
common nonlymphoproliferative primary malignant tumor of the spine in
children. Most lesions occur in patients between 10 and 30 years old; the age
range is more variable with primitive neuroectodermal tumors, a pathologically
distinct entity with similar features radiologically. The lumbosacral spine is
the most common site. Lesions typically occur in the vertebral bodies.
Radiographs and CT images may reveal lytic, mixed, or sclerotic lesions.
Involvement of paraspinal soft tissues and extradural space is best depicted
on CT and MR imaging. The MR imaging appearance is nonspecific
(Fig. 10).
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Fig. 10. —Ewing's sarcoma of upper sacrum in 31-year-old man. Sagittal
T2-weighted MR image shows hyperintense tumor at S1 and S2 with extension
through disk and into spinal canal (arrowheads).
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Osteosarcoma
Osteosarcoma is the most common nonlymphoproliferative primary malignant
bone tumor but rarely affects the spine. Fewer than 3% of all osteosarcomas
are found in the spine, but these tumors account for 5% of all primary
malignant tumors of the spine
[7]. Patients present at an
older age than those with appendicular tumors. Some spinal osteosarcomas occur
in patients with Paget's disease or with previously irradiated lesions. The
lumbosacral spine is the most common site (60-70% of the patients), and the
usual location is the vertebral body. Radiographs and CT images show a purely
lytic, mixed, or predominantly osteoblastic lesion. CT allows identification
of both the matrix mineralization and the extension into the paravertebral and
extradural soft tissues. The MR imaging of nonmineralized areas is
nonspecific; the lesions have low to intermediate signal intensity on
T1-weighted images and high signal intensity on T2-weighted images. Areas of
bone formation may remain as areas of signal void on all pulse sequences (Fig.
11A,
11B).
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Fig. 11B. —53-year-old man with osteosarcoma of sacrum. Axial T1-weighted MR
image shows osseous lesion involving upper sacrum. Areas of decreased signal
intensity correspond to mineralized matrix within tumor
(arrowheads).
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Paget's Sarcoma
Paget's disease, affecting 10% of persons more than 80 years old, is
common. Neoplasms associated with Paget's disease include sarcoma, metastases,
multiple myeloma, lymphoma, and giant cell tumor
[8]. Sarcomatous degeneration,
a rare complication of the disease, is reported in fewer than 1% of patients.
Osteosarcoma, followed by malignant fibrous histiocytoma and chondrosarcoma,
is the most frequently found cause of such degeneration. The prevalence of
sarcomas in Paget's disease increases with age and in patients with
polyostotic skeletal involvement. The most common sites, the pelvis, sacrum,
and femur, reflect the skeletal distribution of the disease. The imaging
appearance indicates an aggressive tumor that has a high degree of anaplasia
usually found on pathologic examination, with permeating lysis, cortical
destruction, and large associated soft-tissue masses (Fig.
12A,
12B,
12C). In some cases,
chondrosarcomas and osteosarcomas may show, respectively, a characteristic
chondroid or osseous matrix calcification.
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Fig. 12A. —Secondary osteosarcoma in 69-year-old man with long-standing Paget's
disease. CT scan shows changes of Paget's disease involving sacrum and iliac
bones and osteolytic central lesion with soft-tissue mass
(arrows).
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Fig. 12B. —Secondary osteosarcoma in 69-year-old man with long-standing Paget's
disease. Sagittal T1-weighted MR image reveals large area of sacral
destruction. Lesion shows signal intensity similar to that of skeletal
muscle.
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Fig. 12C. —Secondary osteosarcoma in 69-year-old man with long-standing Paget's
disease. Photomicrograph shows typical pagetoid mosaic pattern on left half of
histologic specimen and osteogenic sarcoma on right half. (H and E,
x200)
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Multiple Myeloma and Plasmacytoma
Multiple myeloma is a monoclonal proliferation of malignant plasma cells of
the bone marrow. Plasmacytoma is the uniforcal tumoral form of multiple
myeloma and usually has a better prognosis than multiple myeloma. Multiple
myeloma, accounting for 45% of vertebral tumors, is common. Plasmacytoma often
precedes the development of multicentric disease. The axial skeleton is the
most common site of multiple myeloma. On MR images, plasmacytomas and myeloma
lesions are hypointense to healthy marrow on T1-weighted images and
hyperintense on T2-weighted images (Fig.
13).
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Introduction
Benign Tumors
