Introduction
The sacrum is composed of bone, cartilage, marrow elements as well as notochord remnants.[1] Sacral tumors can arise from any one of these components. Since the sacrum contains
hematopoietic bone marrow, it is a common site of metastases, lymphoma, multiple myeloma,
or plasmacytoma. In fact, metastases are the most common tumors of the sacrum ([Fig. 1]). Primary malignant sacral tumors are less common and include chordoma, chondrosarcoma,
osteosarcoma, and Ewing sarcoma. Magnetic resonance imaging (MRI) and computed tomography
(CT) are the preferred imaging modalities for evaluating sacral masses. In this pictorial
review, we will briefly describe the anatomy, imaging, and pathological features of
common and unique sacral tumors and techniques to safely obtain a diagnostic biopsy
specimen.
Fig. 1 Pie chart showing the relative incidence of various types of sacral masses.
Benign Tumors
Giant Cell Tumor
Age and Sex
Between the ages of 20 and 40 years. Giant cell tumors (GCTs) are twice as common
in women.[2]
Incidence
About 3 to 7% of all GCTs occur in the spine and most GCTs of the spine occur in the
sacrum.[3] GCTs make up approximately 2 to 5% of primary sacral tumor.[4]
Location in the Sacrum
Sacral GCTs are frequently eccentric and abut or extend across the SI joint ([Fig. 3A]).[5]
Fig. 3 (A–F) Illustration (A) showing the characteristic location for sacral giant cell tumor
(orange shaded areas), most commonly arising eccentrically and frequently traversing
the sacroiliac joint. Radiograph of the pelvis (B) demonstrating a lytic lucent lesion
in the left hemisacrum (solid black arrow). Axial CT of the pelvis (C) demonstrates
a lytic mass (dashed black arrow) with associated cortical thinning and pathologic
fracture. Axial T1 nonenhanced image (D) shows a hypointense mass in the left sacrum
with moderate postcontrast enhancement (E) (dashed white arrow). The photomicrograph
(F) demonstrates a sheet of benign neoplastic mononuclear cells with indistinct cell
border, scant eosinophilic cytoplasm, round to ovoid nuclei, admixed with numerous
multinucleated osteoclast-like giant cells (white arrowhead). CT, computed tomography.
Presentation
Nonspecific, pain, and neurologic deficits. Malignant transformation is reported in
less than 2% of patients.[3]
[6]
Imaging
GCTs are purely lytic destructive lesions without dystrophic calcification or matrix
mineralization. CT often demonstrates a soft-tissue attenuation mass with a narrow
zone of transition and nonsclerotic margin which may remodel bone or extend through
the cortex into the soft tissue. On MRI, GCTs are frequently heterogeneous because
of the presence of necrosis (T1 hypointense signal), hemorrhage (hyperintense signal
on T1- and T2-weighted sequences or fluid–fluid levels), fibrosis (T1 and T2 hypointense),
or cystic spaces (T2 hyperintense). Contrast-enhanced (CE) MRI is often non-specific
([Fig. 3B–E]).
Additional Features
GCTs are generally very aggressive locally in the sacrum, despite their benign pathology.
GCTs may have fluid–fluid levels as they can coexist with secondary aneurysmal bone
cysts.[7]
Pathology
Microscopic features include numerous multinucleated osteoclast-like giant cells scattered
among several mononuclear cells that have round to ovoid nuclei, scant eosinophilic
cytoplasm, and indistinct cell borders ([Fig. 3F]). The nuclei of the mononuclear cells and the multinucleated giant cells are morphologically
similar.[7]
Nerve Sheath Tumor (Schwannoma and Neurofibroma)
Age and Sex
Most commonly diagnosed between the ages of 20 and 40 years. No sex predilection.[8]
Incidence
They commonly occur in the spine, accounting for 6 to 30% of spinal lesions. In the
spine, nerve sheath tumors may be intradural extramedullary, extradural, or both.[9]
[10] Neurogenic tumors account for 8% of primary sacral tumors.[4]
Location in the Sacrum
Nerve sheath tumors do not arise from the osseous sacrum, but instead, arise from
the neurogenic tissue within the sacral canal or neural foramina ([Fig. 4A]).[11]
[12]
Fig. 4 (A–F) Illustration (A) showing the characteristic location of nerve sheath tumors. Intradural/extramedullary
and/or extradural mass can arise from the lumbar nerve roots and extend along the
course of the nerve through the neural foramina. Axial CT image (B) shows bilateral
symmetric cystic masses (solid black arrow) extending through the enlarged neural
foramen. Coronal T2 MRI (D) showing dumbbell configuration of nerve roots as they
traverse the neural foramen (white arrowhead). The multiseptated T1 hypointense mass
(D) demonstrates area of solid enhancement (E) (dashed white arrow). The photomicrograph
(F) shows a benign neurofibroma featuring a diffuse proliferation of loosely arranged
bland spindle cells with wavy elongated nuclei (dashed black arrow) and associated
with shredded-carrot-like collagen fibers. CT, computed tomography; MRI, magnetic
resonance imaging.
Presentation
The most common signs and symptoms are pain and radiculopathy due to compression of
the affected nerve root.
Imaging
The slow growth of these tumors causes remodeling and widening of the sacral neural
foramina by exerting mass effect on the foramina, best seen on CT ([Fig. 4B]). The tumors are generally solitary lesions with intradural and extradural components.
The presence of more than one nerve sheath tumor or tumor extension along the length
of a nerve (a plexiform neurofibroma) should prompt investigation for syndromic association
including neurofibromatosis or schwannomatosis. On MRI, the masses are most commonly
intradural and extramedullary, T2 hyperintense, and often contain enhancing solid
components ([Fig. 4C–E]). A peripheral nerve sheath tumor containing both intra- and extradural components
is often recognized by a “dumbbell sign” as it courses through the neural foramina,
with relative expansion of the mass on both sides of the neural foramina. The “target
sign” (central T2 intermediate or hypodensity surrounded by T2 hyperintensity) is
commonly seen with peripheral nerve sheath tumors, due to a combination of fibrous
and collagenous tissues (T2 intermediate/hypointense) surrounded by myxoid tissue
(T2 hyperintense). On positron emission tomography (PET)/CT, nerve sheath tumors may
demonstrate fluorodeoxyglucose (FDG) avidity, with significantly more uptake seen
in nerve sheath tumors that have undergone malignant degeneration.[13]
Pathology
Microscopic features include diffuse proliferation of loosely arranged bland spindle
cells with elongated wavy nuclei ([Fig. 4F]) associated with shredded-carrot-like collagen bundles.[3]
[7]
Malignant Tumors
Chordoma
Age and Sex
Usually diagnosed in the 5th decade with men being twice as commonly affected as women.[14]
[15]
Incidence
Chordomas are relatively rare tumors that account for 2 to 4% of all primary malignant
bone tumors.[7] Chordomas make up approximately 44% of primary sacral tumors.[4] Approximately 50 to 60% of chordomas occur in the sacrococcygeal region and it is
the most common primary malignant sacral tumor.[7]
[14]
Location in the Sacrum
This tumor arises from intraosseous notochordal remnants. Most of the chordomas present
as a midline mass involving the lower sacral segment ([Fig. 5A]).
Fig. 5 (A–F) Illustration (A) showing the characteristic locations of chordomas. Chordomas arise
in the lower sacral segments in the midline (orange shaded areas). Axial CT image
(B) demonstrates lytic erosion through the sacrum (solid black arrow). Sagittal T2
images (C) demonstrate a heterogeneous hyperintense soft tissue mass arising from
the S3 sacral body in midline (solid white arrows). Numerous characteristic T2 hypointense
septa are seen with characteristic erosion of the lower sacrum and coccyx. Unenhanced
coronal T1 image (D) shows a hypointense mass in the lower sacrum (solid black arrows)
with avid postcontrast (E) enhancement (dashed black arrow). The photomicrograph (F)
depicts large epithelioid cells some featuring physaliphorous cells with intracytoplasmic
bubbly vacuoles (black arrowhead) embedded in a myxoid stroma. CT, computed tomography.
Presentation
Chordomas are slow-growing tumors that are commonly discovered from pain associated
with large masses.
Imaging
CT images often show large destructive lesions centered in the midline with an associated
soft-tissue mass ([Fig. 5B]). Calcification is present in 50 to 70% of patients.[7] On MRI, the mass appears heterogeneously hyperintense on T2-weighted images ([Fig. 5C]) due to the high water content of the myxoid matrix with numerous characteristic
hypointense septa. Hemosiderin from previous hemorrhage may be seen as T2 hypointensity.[16] Enhancement is variable and is typically moderate to intense ([Figs. 5D ]and [E ]). 18F-FDG uptake in chordomas on PET/CT is either low or intermediate.[17]
[18]
Pathology
Microscopic features of a conventional chordoma include proliferation of large epithelioid
cells, some with clear intracytoplasmic bubbly vacuoles known as physaliphorous cells
([Fig. 5F]). The stroma is frequently mucinous to myxoid.[3]
Chondrosarcoma
Age and Sex
The mean age of patients with chondrosarcoma is 45 years.[7]
[19]
[20] Males are affected two to four times more frequently than females.[7]
Incidence
Chondrosarcomas are more common than osteosarcomas in the spine. Chondrosarcomas account
for 7 to 12% of malignant primary tumors of the sacrum[7]; approximately 10% of all chondrosarcomas are found in the spine.[7]
Location in the Sacrum
Often eccentric in location and occurs in the upper segments ([Fig. 6] A).
Fig. 6 (A–F) Illustration (A) showing the characteristic location for chondrosarcoma (orange
shaded areas). Sacral chondrosarcoma is typically located eccentrically in the upper
portion of the sacrum. Axial CT image (B) with an eccentrically located lytic lesion
in the right hemisacrum. There is a subtle calcified chondroid matrix (solid black
arrow). Coronal T2 MRI (C) showing a hyperintense mass invading through the neural
foramina and extending into the epidural space (solid white arrow). The axial nonenhanced
T1 image (D) demonstrates an iso/hypointense mass with heterogeneous enhancement (dashed
white arrow) on the postcontrast images (E). The photomicrograph (F) shows an infiltrative
proliferation of malignant chondrocytes featuring enlarged hyperchromatic nuclei with
prominent nucleoli (dashed black arrow). CT, computed tomography; MRI, magnetic resonance
imaging.
Presentation
Pain.
Imaging
Radiographs and CT images reveal large lytic lesions with characteristic chondroid
matrix-ring and arc calcification.[7]
[21] Calcifications are manifested as areas of signal void on all pulse sequences at
MRI. On MRI the noncalcified portion is hyperintense on T2-weighted images and hypo-
to isointense to skeletal muscle on T1-weighted images. CE MRI typically shows peripheral
and septal enhancement (rings and arches) corresponding to vascular septations between
cartilaginous lobules ([Fig. 6B–E]).[3]
[22]
Pathology
Microscopically, chondrosarcoma features an infiltrative growth pattern, replacing
the existing bone marrow with large malignant chondrocytes demonstrating increased
cellularity, enlarged hyperchromatic nuclei, and large nucleoli ([Fig. 6F]).[7]
Ewing’s Sarcoma
Age and Sex
Most lesions occur in patients between 10 and 30 years old, more commonly in males
than females.[19] In addition, 95% of all Ewing’s sarcoma cases are diagnosed before the age of 30.
Incidence
About, 3 to 10% of all primary Ewing’s sarcomas and primitive neuroectodermal tumors
occur in the spine.[3] Approximately 9% of all primary sacral tumors are Ewing’s sarcoma.[4]
[23]
Location in the Sacrum
The sacral ala is the most common site in the sacrum ([Fig. 7A]).[19]
[24]
Fig. 7 (A–F) Illustration (A) showing the characteristic location of sacral Ewing’s (orange shaded
areas). Ewing’s generally originates within the sacral ala, often with extension across
the sacroiliac joint. On the axial CT image (B), there is a mixed sclerotic/lucent
osseous lesion (dashed black arrow) in the right sacrum with aggressive sunburst periosteal
reaction (solid black arrow). Axial T2 MRI (C) demonstrates a large hyperintense soft
tissue mass that extends through both inner and outer tables of the ilium and sacrum
(solid white arrow). Coronal nonenhanced T1 MRI (D) shows a hypointense mass which
demonstrates enhancement (dashed white arrow) on the coronal postcontrast images (E).
The photomicrograph (F) demonstrates a diffuse sheet of uniform malignant small round
cells with finely dispersed chromatin and inconspicuous nucleoli, and scant cytoplasm
(black arrowhead). CT, computed tomography; MRI, magnetic resonance imaging.
Presentation
Pain and neurological changes, including loss of bowel and bladder function.
Imaging
Radiographs and CT images may demonstrate a destructive osteolytic lesion, frequently
with a dominant extraosseous soft-tissue component. An aggressive periosteal reaction
may be seen. Diffuse sclerosis is seen in 70% of cases.[12]
[25] The MRI appearance is nonspecific with a T2 hyperintense destructive soft-tissue
mass with heterogeneous contrast enhancement ([Fig. 7B–E]).
Pathology
Microscopically, the tumor consists of uniform malignant small round blue cells with
finely dispersed chromatin and inconspicuous nucleoli, and scant cytoplasm ([Fig. 7F]). The neoplastic cells typically grow in sheets, nests, or cords, sometimes with
palisading and Homer-Wright rosettes.[25]
Osteosarcoma
Age and Sex
Patients present at an older age than those with appendicular tumors with most lesions
in the fourth decade of life, compared with the second for appendicular lesions. Males
are more affected than the females.
Incidence
Osteosarcoma is the most common primary malignant bone tumor, but uncommon in the
axial skeleton with less than 3% found in the spine.[26]
[27]
Location in the Sacrum
The lumbosacral spine is the most common site (60–70% of the patients).[7] In the sacrum it is eccentrically located ([Fig. 8A]).
Fig. 8 (A–E) Illustration (A) showing the characteristic location of sacral osteosarcomas (orange
shaded areas), which nearly always involve the sacral ala and body. Axial CT (B) image
shows a large destructive lytic lesion centered within the left hemisacrum and obliterates
the left SI joint to extend into the left ilium. Amorphous osteoid matrix deposition
is seen (dashed black arrow). Coronal T2 MRI (C) shows a hyperintense mass (solid
white arrow). Postcontrast T1 image (D) shows minimal peripheral enhancement (dashed
white arrow). Photomicrograph (E) depicts irregular neoplastic woven bones lacking
osteoblastic rimming (white arrowhead), which are produced by the malignant neoplastic
polygonal, oval, and spindle osteosarcoma cells in the background. CT, computed tomography;
MRI, magnetic resonance imaging.
Presentation
Palpable mass, with 80% presenting with neurological deficits.[28] Some osteosarcomas arise from malignant degeneration of Paget’s disease and commonly
affect the pelvis and sacrum.
Imaging
Radiographs and CT images demonstrate a predominantly osteoblastic permeative lesion
with cortical breakthrough and soft-tissue extension ([Fig. 8B]). CT is useful to identify osteoid matrix mineralization. The MRI of nonmineralized
areas is nonspecific; the lesions are hypointense on T1-weighted images and hyperintense
on T2-weighted images with heterogeneous contrast enhancement. The ossified portion
is dark, isointense to cortical bone, on all sequences ([Fig. 8 B–D]).
Pathology
Microscopic features include irregular neoplastic woven bones lacking osteoblastic
rimming ([Fig. 8E]), which are produced by the malignant neoplastic polygonal, oval, and spindle osteosarcoma
cells in the background.[17]
Multiple Myeloma and Plasmacytoma
Age and Sex
Most patients are 60 years of age at presentation, with a male predominance (2:1).[19]
Incidence
Multiple myeloma accounts for 45% of vertebral tumors.[3] Plasmacytoma (solitary lesion) often precedes the development of multicentric disease.
The axial skeleton is the most common site of multiple myeloma.
Imaging
On CT the lesions appear as punched-out lytic areas—narrow zone of transition with
a nonsclerotic margin. On MRIs, plasmacytomas and myeloma lesions are hypointense
on T1-weighted images and hyperintense on T2-weighted images. Diffuse bone marrow
infiltration may precede focal involvement in some cases of multiple myeloma ([Fig. 9A–C]).[29]
Fig. 9 (A–D) Axial CT in a patient with multiple myeloma (A) demonstrates a punched out lytic
lesion within the right sacrum (dashed black arrow) and left iliac bones. Axial T2
MRI (B) shows a hyperintense well-defined marrow replacing lesion (solid white arrow)
in the right sacrum. Axial postcontrast (C) T1 image shows a well-defined enhancing
lesion (dashed white arrow). The photomicrograph (D) depicts abnormal proliferation
of malignant plasmacytic cells with enlarged “clock-face” eccentric nuclei, and basophilic
cytoplasm as well as perinuclear hof (black arrowhead). CT, computed tomography; MRI,
magnetic resonance imaging.
Pathology
Multiple myeloma exhibits a monoclonal proliferation of malignant plasma cells of
the bone marrow. The plasma cells have eccentric nuclei with characteristic “clock-face”
chromatin without nucleoli ([Fig. 9D]).[30]
Lymphoma
Age and Sex
Primary lymphoma of the sacrum has a peak incidence during the second and third decades
of life, affecting more males than females at a ratio of 2:1.[5]
Incidence
Primary bone lymphoma involves a single bone with no evidence of disseminated disease.[31] It is extremely rare, accounting for only 1% of cases of non-Hodgkin lymphoma.[31]
Imaging
A permeative lytic lesion is most often seen, with minimal cortical destruction. On
CT, the abnormality may be very subtle. MRI demonstrates replacement of the bone marrow,
on T1- and T2-weighted imaging, which enhanced after contrast administration at the
sites of disease.
Pathology
The tumor is often composed of sheets of large cells of lymphoid lineage. There are
areas of hemorrhage and necrosis. Tumor cells have brisk mitotic activity with many
apoptotic bodies ([Fig. 10C]). Immunohistochemistry shows the tumor cells to be strongly positive for CD-45,
CD-20, and CD-19 and negative for CD-3, CD-5, CD-99, and cytokeratin.[32]
Fig. 10 (A–C) Axial CT image in a patient with lymphoma (A) shows a mixed lytic and sclerotic
lesion in the sacrum (solid black arrow) and both iliac bones. T2 MRI image (B) demonstrates
heterogeneous hyperintensity throughout the sacrum (solid white arrow), and mixed
signal within the iliac bones bilaterally. Photomicrograph (C) depicts a diffuse proliferation
of dis-cohesive malignant lymphocytes with enlarged round to lobulated nuclei with
multiple prominent nucleoli, and minimal cytoplasm. CT, computed tomography; MRI,
magnetic resonance imaging.
Metastasis
Metastases from lung, breast, prostate, kidney, head, and neck; melanoma; and gastrointestinal
cancers are the most common malignant sacral tumors.[5]
[33] Metastases are usually osteolytic ([Fig. 11A], [B]), except in prostate and breast cancers, where they are frequently osteoblastic.
Fig. 11 Coronal CT image (a) in a patient with metastasis, shows bone destruction and soft
tissue mass extending to the sacroiliac joints bilaterally (dashed black arrows).
Axial MRI T2 image (b) demonstrates an expansile T2 hyperintense mass (solid black
arrow).
Lesions usually present with pathological fracture. History of primary tumor and multiple
bone lesions raises the concern for metastatic bone disease.
Sclerotic metastases typically display low signal intensity on all MRI sequences.
Kidney and thyroid metastases may have expansile appearance, and both may be hypervascular.
