Update: Radiography in Spinal epidural metastases

Most of the spinal epidural metastases are osteolytic, but at least 50 % of the bone must be eroded before plain x ray abnormality 1).

Not very specific.
see Winking owl sign
Plain x-rays are quite good at evaluating bony metastases, but not good at evaluating the spinal cord and surrounding soft tissues. Metastatic epidural spinal cord compression most commonly occurs at the site of vertebral involvement on plain x-ray, especially where there is evidence of vertebral collapse. Most common findings on x-rays include pedicle erosion, paravertebral soft shadow, vertebral collapse, and pathologic fracture or dislocation 2).
In the past, if there was back pain or a localizing sign and spinal x-ray was abnormal, the probability of epidural disease was 0.9, but if the x-ray was normal, the probability was only 0.1 3) 4).
In 1990, x-rays were found to have a 10% to 17% false negative rate 5).
The rate of missed metastatic epidural spinal cord compression is unacceptable.
1) Gabriel K, Schiff D. Metastatic spinal cord compression by solid tumors. Semin Neurol. 2004 Dec;24(4):375-83. Review. PubMed PMID: 15637649.
2) Perrin RG. Metastatic tumors of the axial spine. Curr Opin Oncol 1992;4(3):525-32.
3) Rodichok LD, Harper GR, Ruckdeschel JC, et al. Early diagnosis of spinal epidural metastases. Am J Med 1981;70(6):1181-8.
4) Portenoy RK, Galer BS, Salamon O, et al. Identification of epidural neoplasms. Radiography and bone scintigraphy in the symptomatic and asymptomatic spine. Cancer 1989;64(11):2207-13.
5) Bach F, Larsen BH, Rohde K, Børgesen SE, Gjerris F, Bøge-Rasmussen T, Agerlin N, Rasmusson B, Stjernholm P, Sørensen PS. Metastatic spinal cord compression. Occurrence, symptoms, clinical presentations and prognosis in 398 patients with spinal cord compression. Acta Neurochir (Wien). 1990;107(1-2):37-43. PubMed PMID: 2096606.

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