Leptomeningeal carcinomatosis (LC) refers to diffuse seeding of the leptomeninges by tumor metastasis.

 Pathophysiology: Primary tumors can spread to the leptomeninges in a variety of ways. Direct extension may occur from an intraparenchymal or periventricular primary brain tumor that forms in tissue near the CSF, and this is commonly found in medulloblastomas and other PNETs, ependymoma, and occasionally in glioblastoma multiforme. Arterial metastases can invade the CSF by pial rupture, ependymal invasion, or by extension along Virchow-Robin spaces.

Tumors also can extend in a perineural fashion along cranial nerves to eventually enter the subarachnoid space, and this pathway is particularly associated with squamous cell tumors of the head and neck. A similar method of spread along perineural spaces of the spinal nerves can occur with vertebral body or lymph node metastases.

Venous hematogenous access to the subarachnoid space can occur by a number of pathways, such as Batson plexus (internal vertebral venous plexus), the choroid plexus, or through the vessels of the arachnoid. Leukemia classically spreads hematogenously and has been shown to gain access to the CSF by invading the walls of arachnoid veins as well as through microinfarcts that break down the blood-brain barrier.

Headache is the most common symptom at presentation, affecting approximately 50% of patients, while pain, nausea and/or vomiting, weakness, and sensory disturbances are each seen in approximately one third

Preferred Examination: Contrast-enhanced MR. of the brain and spine is the imaging modality of choice because of its safety, excellent contrast resolution, and multiplanar abilities.

Recent studies have tended to show higher sensitivity rates due to advancements in MR. technology, particularly better T1-weighted images as well as the advent of 3-dimensional T1-weighted sequences and postcontrast fluid attenuated inversion recovery (FLAIR). Collie et al recently reported a 100% sensitivity for intracranial LC in 25 patients evaluated with gadolinium-enhanced MR.

When the patient has a contraindication to MR, CT myelography is the next best test to evaluate the spine and has the added benefit of allowing CSF sampling at the same time as the diagnostic test is performed. The physician must exclude obstructive hydrocephalus prior to beginning the procedure, as removal of CSF below the obstruction may result in downward herniation and death.

Plain myelography demonstrates the thickened nerve roots, subarachnoid masses, and blockage of the subarachnoid space, but has not been used as a primary diagnostic tool since the increased availability of good quality CT and MR. Plain myelography is still used to provide additional imaging during a CT myelogram.

Limitations of Techniques: CT continues to be used as a screening tool in the metastatic workup of many cancer patients but is relatively insensitive compared to MR., particularly in the detection of LC. Contrast-enhanced CT may miss tumor in up to one third of cases and mischaracterize Leptomeningeal tumor deposits as intraparenchymal in another third. CT may be necessary when a contraindication to MRI exists, such as a pacemaker or certain aneurysm clips, or when the patient is unable to hold still long enough for an MR.

MR. depicts leptomeningeal tumor well, particularly when magnetization transfer or postcontrast FLAIR techniques are used. MR is much better at depicting metastases from solid tumors than those from hematologic malignancies.

Findings: Contrast-enhanced CT (CECT) scans of the brain in LC are not sensitive in depicting the disease, with a false-negative rate of more than 50%. Common findings include noncommunicating hydrocephalus, intraparenchymal volume loss, and various patterns of meningeal enhancement This enhancement can appear as multiple nodules, diffuse leptomeningeal enhancement, ependymal or subependymal enhancement, dural enhancement, or a combination. In the nodular form, pial enhancement is difficult to distinguish from intraparenchymal enhancement, although recognizing that the nodules follow the course of sulci assists in the diagnosis.

Cranial nerve enhancement is poorly visualized on CECT because of the proximity to osseous structures. Dural enhancement often is missed for the same reason.

In the spine, CECT also has low sensitivity, although CT myelography is approximately equal in sensitivity to MR in the detection of nerve root thickening and nodularity. The nerve roots appear thickened and beaded, and this is best visualized in the cauda equina. Tumor deposits along the surface of the cord lend the cord an irregular border, and the cord may be thickened. In extreme cases, the entire spinal canal can be filled with tumor, causing a complete CSF block.