A thirty-five-year-old female presented with progressively increasing, lower back pain radiating to both legs since three months. It was associated with tingling and numbness. She had a recent history of constipation; however there was no urinary incontinence. She had no spinal surgery or trauma in the past.
The patient underwent a plain and contrast MRI of the lumbosacral spine (3 Tesla Siemens MRI scanner). A well defined, expansile, destructive mass lesion measuring 7.3x6.5x7.6 cm was seen involving the sacrum. The lesion was seen to extend up to the subarticular portion of the right sacroiliac joint and superior endplate of the S1 vertebral body. Significant narrowing of the sacral canal with soft tissue extension was present. Anteriorly, the mass was extending into the presacral space with no involvement of the rectum. Posteriorly, it was infiltrating the adjacent paraspinal and bilateral pyriformis muscles. It was isointense to the muscle on T1W images, isointense to mildly hyperintense on T2W and STIR images. Few small T2 & STIR hyperintense cystic areas were seen within it. There were no fluid-fluid levels. On post-contrast MR images, it showed heterogeneous enhancement with non-enhancing necrotic areas. Surrounding soft tissue oedema was present. Screening of the rest of the spine revealed no other lesions. CT, in addition, revealed a pathological fracture of right paramedian portion of the sacrum extending into the S5 vertebral body.
CT-guided biopsy was performed; histopathology revealed multiple giant cells with aneurysmal changes. There were no features of atypia. The patient was referred to another centre for further management.
Various pathologies can involve the sacrum, which may include infective, inflammatory, post-traumatic and neoplastic lesions. These can extend into the presacral space.  Clinical history such as the patient’s age and gender, and imaging findings such as location, calcifications or fluid-fluid levels can aid the diagnosis.  MRI has superior soft-tissue resolution compared to CT. It is thus, better at showing the anatomic relations of any lesion in this space, as well as for neural involvement.  It is the preferred modality; however, CT may be used to evaluate the matrix of the lesion in certain cases, for detection of pathological fracture, guided biopsy and in those patients in whom MRI is contraindicated.
Giant cell tumour is the second most common primary sacral tumour. It occurs in 2nd-4th decades of life and is more common in females. It is lytic, expansile, and eccentric. A soft tissue mass with thin sclerotic rim may be seen on CT. On MRI, it is hypointense on T1W images and isointense on T2W images. Fluid-fluid levels may be present. There is avid post contrast enhancement on both CT and MR imaging; areas of haemorrhage or necrosis may be present. A list of differential diagnoses is following.
Another benign tumour in young patients includes aneurysmal bone cyst which presents as a multiloculated, expansile lesion with multiple fluid-fluid levels. Neurogenic tumours include neurofibromas, schwannomas, paragangliomas and chordomas. Target appearance on T2W images is seen in case of neurofibromas while paragangliomas are very vascular and may show haemorrhage. Neurogenic tumours can expand the sacral canal and neural foramina. [1, 2] Chordoma is the most common primary malignant sacral tumour and occurs more commonly in middle-aged to elderly men. It occurs in the midline due to its origin from notochordal remnants. They are typically lytic and expansile, appearing hyperintense on T2 –weighted images due to the presence of mucin. Haemorrhage is not uncommon, and heterogeneously enhancing soft tissue component is usually present. They are locally aggressive. [1, 2, 3, 4] Chondrosarcomas present as destructive, lytic lesions with endosteal scalloping and extraosseous extension. ‘Ring and arc’ calcifications representing chondroid matrix may be seen. Ewing’s sarcoma is a lytic lesion with large extraosseous component invading the spinal canal.  Plasmacytoma occurs in older age. Metastasis in sacrum may occur from primary pelvic malignancy. Increased FDG uptake on PET/CT may be useful in the evaluation of tumours such as lymphoma.
Surgical resection is the treatment of choice for GCT, most commonly, curettage and bone grafting. Radiotherapy is another option for large tumours. A pre-operative CT and MRI is very important for surgical planning. 
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