Metastatic Disease

      Drop metastases generally originate from either primary, or less commonly, metastatic disease of the brain with subsequent seeding into the subarachnoid space. Because of either the dynamics of cerebrospinal fluid flow or gravitation, neoplastic cells implant on the spinal arachnoid or pia, resulting in a variety of appearances attributable to intradural metastases. Primary brain neoplasms that may lead to drop metastases include medulloblastoma, ependymoma, germinoma, choroid plexus carcinoma, teratoma, glioblastoma, and pineoblastoma. Secondary neoplasms metastatic to brain with a predilection for drop metastases include lymphoma, melanoma, and breast, lung and renal cell carcinoma. Intradural metastases may also occur from direct spread of extradural tumor or from lymphatic or hematogenous dissemination.

      Several appearances of intradural tumor seeding have been noted. Most common are focal nodular masses that may vary substantially in size, ranging from only a few millimeters to greater than 1 cm. They are frequently spread throughout the subarachnoid space, including laterally in the recesses and root sleeves.

      A second variety of intradural metastases is diffuse coating of the spinal cord with tumor, resulting in either smooth or nodular pseudo-expansion of the cord. This appearance may actually simulate an intramedullary lesion on routine non-contrast MR. The third expression of intradural metastatic disease is a homogeneous increase in signal within the subarachnoid space. This effect is seen predominantly in the lumbar region and is probably due to a combination of increased protein content within the CSF, malignant cells, and damping of CSF pulsations.

      Gadolinium should be used in all cases of suspected intradural or "drop" metastases. It significantly increases the sensitivity for detection of the smaller nodules and readily distinguishes the pseudoexpansion described above from an intramedullary process. 

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