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Lesions of the sacrum. Differential diagnosis and radiological evaluation.

PRIMARY lesions of the sacrum are relatively uncommon. They frequently are accompanied by nonspecific symptoms such as low back pain and pelvic pressure. Large lesions may involve the sacral nerves and are often palpable on rectal examination. The diagnosis of an early lesion may prove difficult when vague symptoms are accompanied by subtle radiological findings. The curvilinear shape of the sacrum makes it difficult to evaluate on standard anteroposterior projections. A helpful adjunct is the distorted view of the sacrum, which is obtained with the tube angled 15 degrees cephalad and with the patient's hips flexed. This stretches or straightens out the sacrum, enhancing visualization. On frontal projections, the symmetry and osseous integrity of the sacral foramina should be carefully observed. Fecal material and bowel gas frequently overlie and obscure the sacrum, particularly when the sacral nerves are involved1(Fig 1, top left). The lateral view demonstrates the anterior border