Imaging of Sacral Tumors

Betty Jean Manaster, MD, PhD, Travis Graham, MD

Disclosures

Neurosurg Focus. 2003;15(2) 

In This Article

Imaging Workup of Sacral Lesions

Because lesions of the sacrum are often large by the time they are diagnosed and because their primary treatment is usually wide resection, careful imaging is a crucial part of the workup. Following radiographic identification of a sacral lesion, radionuclide bone scanning should be conducted. The bone scan is not obtained to evaluate the extent or aggressiveness of the lesion; rather, it is used to determine whether the lesion is polyostotic. If there are multiple lesions, the differential diagnosis (presuming an aggressive appearance) is usually limited to metastases, multiple myeloma, Paget disease, and vascular tumor. Once the bone scan demonstrates that the lesion is monostotic, cross-sectional imaging should be undertaken. Occasionally there is a specific reason to perform CT scanning; this generally relates to a need to evaluate the character of any matrix that may be present. Computerized tomography scanning may also be performed if MR imaging is contraindicated in a particular patient. If CT scanning is substituted for MR imaging, and there is a presacral soft-tissue mass, both rectal and intravenous contrast should be administered to evaluate involvement of the pelvic structures. When possible, however, MR imaging is the cross-sectional imaging modality of choice.

Magnetic resonance imaging is performed both to help specify the diagnosis and for site evaluation. As previously described, the combination of T1- and T2-weighted imaging or its equivalent may provide specific enough information based on signal intensity, location, morphology, and prevalence of a disease to make a very strong presumptive diagnosis. At other times, these characteristics will be nonspecific and only yield a differential diagnosis. In either case, MR imaging is conducted for site evaluation, which in turn helps guide biopsy sampling and resection, as well as give substantial information regarding prognosis for postoperative functional status. Magnetic resonance imaging should not be performed as a standard examination. The radiologist must work with the surgeon to determine exactly what information must be gleaned from the examination. Generally, this includes exact localization and extent of both bone and soft-tissue mass, as well as involvement of any adjacent soft tissues and neurovascular bundles. In the sacrum, the multiplanar capabilities of MR imaging can be utilized to obtain "true" coronal and axial images of the sacrum, which in fact are oblique coronal and axial images relative to the tabletop (see Fig. 14 I and J). In this way, involvement of nerve roots and the presacral visceral structures and neurovascular bundles can best be evaluated. Sagittal imaging is best for evaluating fluid levels within a lesion. Please note that axial images are required to evaluate for involvement of muscles, neurovascular bundles, bowel, bladder, or other visceral structures. These structures are not adequately evaluated using coronal or sagittal imaging.

The radiologist should bear in mind, when considering site evaluation, that sacral lesions sparing the majority of S-1 and the sacroiliac joints may be amenable to complete excision[9] and remain mechanically stable. Functionality considerations show that if the lesion can be removed with the upper three sacral nerves on one side and two on the other side preserved, the patient will maintain close to normal function. On the other hand, loss of all but the first nerve root bilaterally will result in rectal and urinary incontinence and impaired sexual function.[10] Invasion of the bowel requires resection and colostomy. Studies have shown that it is difficult to distinguish adhesions from rectal wall invasion by cross-sectional imaging, even with insufflation of air in the rectum during MR imaging examination.[10]

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