Findings
The patient characteristics of the various sacral lesions Fig. 1, their typical location Fig. 2, pattern of local spread/extension Fig. 3, bone scan and specific features Fig. 4 are described.
A. Benign sacral tumours
Giant Cell tumour (GCT)
GCT of the spine, represent 3-8% of GCT, most affecting the sacrum. Most are benign.
Typically expansile, osteolytic lesions with a thin cortical rim; without sclerotic margin or matrix calcification Fig. 5. CT and MRI often demonstrate a large soft-tissue mass, frequently with cystic foci, and sometimes with haemorrhage Fig. 6 and fluid-fluid levels. The solid component is low-to-intermediate T2 signal on MRI, a useful distinguishing feature as most sacral tumours display high T2 signal (Figure 1).
Vertebral haemangioma (VH)
VH is the commonest benign spinal neoplasm. Sacral involvement is uncommon. Most are asymptomatic.
Typically high T1- and T2-signal on MRI Fig. 7. Most have coarse vertical trabeculae featuring as polka dot sign (axial CT/MRI) Fig. 8 and corduroy stripe sign (radiographs, longitudinal CT/MRI).
Atypical VH contain less fatty and more vascular components. MRI signal is non-specific. Polka dot sign on CT is diagnostic. Maintained vertebral height, a sharp margin, intact bone cortex and enlarged paraspinal vessels also aid diagnosis.
Occasionally, VH may be aggressive causing epidural mass that encroaches upon the spinal canal Fig. 9 Fig. 10 or neural foramina causing symptoms. Pathological fracture may occur.
B. Malignant sacral tumours
Metastasis
Metastases are the most common sacral neoplasm, with lung, breast, kidney, and prostate carcinoma the most frequent causes.
Their imaging appearance is dependent on the primary tumour. Most are osteolytic with non-specific features Fig. 11. Rarely, a T2 hyperintense rim is present on MRI; a highly specific finding.
Multiple Myeloma (MM) and Plasmacytoma
MM, a monoclonal proliferation of malignant plasma cells of the bone marrow, is common in the spine (45% of primary vertebral tumours). Plasmacytoma (unifocal form), often precedes multicentric disease.
Radiographs and CT often demonstrates a predominantly osteolytic pattern Fig. 12 Fig. 13 with/without peripheral sclerosis. On MRI, appearance of plasmacytomas and myeloma lesions are non-specific Fig. 14 .
Lymphoma
Sacral lymphoma may be either primary bone lymphoma (extremely rare) or secondary to disseminated disease.
Frequently, radiographic findings are absent; when present, a permeative lytic lesion is most often seen. SL causes marrow replacement; more easily detected on MRI than CT. It may cause aggressive bone destruction or extends into soft tissues leaving the cortex intact. MRI signal characteristics are non-specific Fig. 15 Fig. 16 .
Sacral chordoma (SC) (Figure 8)
Chordoma, a tumour which arises from notochordal remnants, is the commonest primary sacral malignancy (20-40%).
SCs typically manifest as large osteolytic lesions on radiographs and CT Fig. 17 Fig. 18 . 30-70% have peripheral amorphous or punctate calcifications, best detected by CT. MRI signal is non-specific. Variable enhancement is seen on CT/MRI Fig. 20 Fig. 21. High T1 signal foci when present indicate either haemorrhage/proteinaceous content, while low T2 signal foci may be due to previous haemorrhage/fibrosis/calcification. Fig. 22 .
Chondrosarcoma
Only approximately 2% of chondrosarcoma occur in the sacrum; Primary and secondary forms may occur.
On radiographs and CT, chondrosarcoma typically feature as large aggressive osteolytic lesions with soft-tissue masses and characteristic ‘‘rings and arcs’’ chondroid matrix mineralization, best seen on CT. Unmineralized chondroid matrix displays intermediate T1 signal and high signal T2 signal on MRI Fig. 23 Fig. 24 and calcific foci as low signal. Enhanced MRI typically shows peripheral and septal enhancement.
Ewing's Sarcoma (ES)
ES is a high-grade small, round cell tumour. The spine is an uncommon site for primary ES (3-10%); more than half involving the sacrum. Metastases from extraspinal ES is more common.
Radiographs and CT show moth-eaten or permeative osteolysis or occasionally mixed osteolysis and sclerosis, frequently with a dominant extraosseous soft tissue component best depicted by CT or MRI Fig. 25. MRI signal characteristics are non-specific Fig. 26 .
Osteosarcoma
Sacral osteosarcoma is rare; 9% of primary malignant sacral tumours. Predisposing factors are Paget's disease or previous pelvic irradiation.
Sacral osteosarcoma typically shows an aggressive lesion causing permeative bone destruction, cortical breakthrough and an associated soft-tissue mass on imaging. The majority contain ‘‘cloudlike’’ osteoid mineralization readily detected on CT Fig. 27 which MRI appears as signal voids on all pulse sequences Fig. 28 Fig. 29. MRI helps to detect skip lesions.
C. Mimics of sacral tumours
Benign and malignant peripheral nerve sheath tumours (MPNST)
Peripheral nerve sheath tumours (PNST) can be found arising from sacral nerve roots. PNST are intradural extramedullary masses and not true primary sacral neoplasms.
Most benign PNST are purely intradural lesions without associated bony changes. Dumbbell-shaped tumours with intradural and extradural components typically cause enlargement of the neural foramina Fig. 30; a helpful diagnostic feature. MPNSTs tend to larger tumours Fig. 31 and may cause sacral destruction.
D. Tumour-like conditions
Osteomyelitis (OM)
Infection of the sacrum can result from contiguous spread of adjacent infection eg. pelvic abscess, decubitus ulcers or hematogenous spread.
OM appears as a destructive lesion associated with soft-tissue swelling which may be seen on radiographs, CT and MRI Fig. 32. Unlike sacral tumours, osteolysis associated with osteomyelitis is ill defined and poorly marginated. Other differentiating features: osseous sequestra, extensive adjacent soft-tissue inflammatory change and presacral fluid collections Fig. 34 .
Paget’s disease (PD)
PD may involve the sacrum, more commonly in polyostotic disease.
The imaging features depend upon the phase of disease. Sclerotic phase - radiographs and CT shows cortical thickening, especially at the sacral foramina and sacral side of the sacroiliac joints Fig. 35 Fig. 36 , and osseous expansion encroaching upon the sacral canal and foramina.
MRI findings are variable and diagnosis may be difficult. Lytic phase - non-specific. Sclerotic phase - low T1- and T2-signal; may mimic sclerotic metastases and tumours or chronic osteomyelitis. Fig. 37.
Insufficiency fracture (IF)
Sacral IFs are relatively common in elderly patients’ especially females with osteoporosis, and after pelvic irradiation.
Radiographs are insensitive for detecting sacral IFs. Unilateral or bilateral sclerotic bands or fracture lines parallel to the sacroiliac joints may occasionally be identified Fig. 38. “Incomplete” patterns may occur which should be distinguished from metastatic disease, which is typically patchier in distribution. CT and MRI are usually reserved for atypical cases. CT reveals a sclerotic band or discrete fracture lines, commonly with disruption of the anterior cortex of the sacral ala Fig. 41, and MRI demonstrates marrow oedema with/without discrete fracture lines Fig. 39 Fig. 40.
Osteoradionecrosis (ORN)
ORN is a delayed radiation-induced injury characterized by bone tissue necrosis and failure in healing but no extraosseous mass.
Radiographs and CT features include: osteopenia, coarsening of trabecular architecture, cortical irregularity, heterogeneous bone density, subtle fractures and dystrophic soft-tissue calcification. ORN is depicted on MRI by areas with low T1 signal, low-to-intermediate T2 and non-enhancing centre with high T1, high T2 signal and enhancing periphery Fig. 42.
Conclusion
Tumours of the sacrum are rare and the differential diagnosis includes a wide spectrum of benign and malignant conditions.
Imaging findings can aid to better characterise some of these conditions, avoiding unnecessary biopsy and surgery for non-aggressive or benign conditions, and guide appropriate further management.