Sacral Tumours on MRI: A Pictorial Essay

Tumours of the sacrum can be primary or secondary. Since the sacrum is rich in haematopoietic bone marrow, bone metastases are the most frequent aetiologies. However, tumours can arise from all components of the sacrum and primary bone tumours should be considered in case of a solitary lesion and absence of oncologic history. As the clinical signs are usually non-specific, magnetic resonance imaging has become an indispensable tool in narrowing the differential diagnosis and determining the therapeutic approach. This pictorial essay illustrates specific features of the most common sacral tumours on magnetic resonance (MR) imaging.


Giant cell tumour
Giant cell tumour of bone, also known as osteoclastoma, is the second most common primary tumour in the sacrum [5]. Patients aged 15-50 are most frequently affected, and women are twice as likely to develop the tumour compared to men [3,8]. Giant cell tumours are usually located on both sides of the midline and extension across the sacroiliac joint is frequent (Figure 3). The solid components show a low to intermediate signal on T1-weighted images and usually enhance intensely after administration of gadolinium. The heterogeneous signal on T2-weighted sequences is variable and non-specific. Cystic changes and fluid-fluid levels are occasionally observed [8].

Haemangioma
Although vertebral haemangiomas occur in more than 11% of the population, sacral involvement is rare. The tumour is most prevalent during the fifth decade, with a male to female ratio of 1:1.5. Typical haemangiomas are fat predominant, creating a hyperintense well-defined signal on T1-and T2-weighted images. Due to the presence of vascular elements the signal remains high on fluid-sensitive sequences. Linear areas of lower signal correspond to thickened vertical trabeculae and are referred to as the polka dot sign (Figure 4). Atypical haemangiomas are composed of less fatty and more vascular components creating a hypointense to isointense signal on T1-weighted images with variable enhancement and heterogeneous hyperintense signal on both T2-weighted images and STIR-sequences. As a result the polka dot sign is more difficult to distinguish. Maintained vertebral height, a sharp margin, intact bone cortex and enlarged paraspinal vessels are additional findings that favour the diagnosis of haemangioma [9].

Enchondroma/Chondrosarcoma
Enchondromas originate from the medullary cavity and are particularly common in long bones. Sacral location is rare. They are most often diagnosed during the fourth  and fifth decade of life with an equal gender distribution [10]. On MRI, the lobules of firm matured cartilage create a hypointense to isointense signal on T1-weighted sequences and a hyperintense signal on T2-weighted sequences. Regions of calcification and osseous content translate as a low signal on T2-weighted images with either high, intermediate or low signal on T1-weighted images. Enhancement along the fibrovascular septa surrounding the cartilaginous lobules may be observed after administration of gadolinium [11] (Figure 5). Bone or soft tissue oedema are rare, as well as soft tissue masses, cortical destruction or solid enhancement. When present, these findings are highly suggestive of chondrosarcoma and a biopsy should be performed [12].

Aneurysmal bone cyst
Aneurysmal bone cysts are primarily seen in children and adolescents with a slight female predominance (56%) [2]. The multiseptated lesion has a heterogeneous appearance on T1-and T2-weighted sequences with a low-intensity rim in the periphery, indicating a thin shell of bone. The typical fluid-fluid levels are diagnostic [13] (Figure 6). As there are usually no solid structures within the cysts, only the septations enhance on T1Gd-sequences [2]. When solid components are present, the MRI appearance is very similar to that of a giant cell tumour, albeit with a heterogeneous signal intensity on both T1-and T2-weighted images.

Ewing sarcoma
Ewing sarcoma is a high-grade malignancy composed of uniform small round blue cells. It occurs most frequently during the teenage years, with a male predominance of 62% [1]. The tumour often presents as a destructive osteolytic lesion with a soft tissue component. The lesion has a homogeneous hypo-to isointense signal on T1-weighted images and isointense signal on T2-weighted images. The extraosseous component is typically larger than the intraosseous lesions and invasion into the paraspinal area and spinal cord is common (Figure 7). After the administration of gadolinium mild to moderate enhancement is observed [1,14].

Metastases
Lung, breast, prostate, kidney, head and neck, skin (melanoma) and gastro-intestinal cancers are the most common tumours that can produce sacral metastases. Most metastases are osteolytic except for prostate cancer, where metastases are mainly osteoblastic [1]. Osteolytic lesions have a hypointense signal on T1-weighted sequences and an iso-to hyperintense signal on T2-weighted sequences compared to normal bone marrow (Figure 8). On STIR sequences the signal is usually hyperintense. Administration of gadolinium most often   leads to intense uptake although moderate, heterogeneous and low uptake are not uncommon. A peripheral hyperintense rim on T2-weighted images is a rather rare but highly specific finding. Cortical disruption is seen in 57% of all osteolytic metastases. In osteosclerotic metastases, the sclerotic areas appear hypointense on all sequences [15,16].

Conclusions
The differential diagnosis of sacral tumours is extensive, and although metastases are the most common lesions, a broad spectrum of primary bone tumours can arise from sacral components. MR imaging allows us to detect bone marrow invasion, and changes in morphology and intensity will help to determine the most probable diagnosis. In addition, imaging has an important role in the staging of the tumour and further follow-up.