Spread of Neoplasms

Anatomy-Based Imaging Issues

Neoplasms affecting the spine may spread by direct extension via the lymphatic system, the hematogenous route, or along the cerebrospinal fluid (CSF) pathways. There can also be combinations of these pathways involved, with the most typical being hematogenous metastatic dissemination to vertebral bodies with subsequent direct extension into the epidural space.

Primary tumors located in soft tissues may extend into the vertebral column by direct extension. An example would include lung carcinoma extending into the chest wall and subsequently the paravertebral region and into the spinal column and epidural space. Prostate, bladder, or bowel carcinoma may extend into the presacral space and subsequently into the vertebral column and epidural space. Nasopharyngeal carcinoma extends into the clivus and skull base and may track along the cranial nerves. In rare cases, there may be direct extension of a CNS tumor along the biopsy or surgical tract. There are also rare cases of CNS tumors extending through access via indwelling shunt tubing to give systemic metastases. Findings of direct extension of neoplasm in the spine consist of a soft tissue mass with adjacent bone destruction and variable neural compromise. Direct extension of neoplasm into the epidural space is more likely from vertebral body through the posterior longitudinal ligament. The anterior longitudinal ligament and disc are relatively resistant to tumor invasion. The anterior longitudinal ligament is stronger than the posterior longitudinal ligament and has fewer perforating vessels. Once the tumor has access into the epidural space, it comes against the tough dura, which is an effective barrier to tumor penetration. These barriers result in the distinguishing features between a disc space infection with adjacent vertebral osteomyelitis (with the epicenter at the level of the disc space) and neoplastic involvement (with the epicenter involving the vertebral bodies with sparing of the disc space). Direct extension of tumor may also be seen with a primary cord tumor within the cervical spine, with extension into the infratentorial space. Rarely, brainstem or cerebellar neoplasms may extend into the upper cervical cord.

Lymphatic spread is of limited importance in spine imaging relative to the more ubiquitous hematogenous spread. Local spread of pelvic tumors within the lumbar spine without pulmonary metastases would suggest venous or lymphatic route of extension.

Hematogenous spread is the major pathway of extension of malignant tumors to the axial skeleton. Batson plexus is a longitudinal network of valveless veins running parallel to the spinal column. These veins lie outside of the thoracoabdominal cavity and communicate with multiple aspects of the venous system, including vena cava, spinal, portal, azygos, intercostal, pulmonary, and renal veins. Flow direction in Batson plexus is variable due to the variable intrathoracic and intraabdominal pressures. Tumors in multiple anatomic sites could cause metastatic lesions along the course of the venous plexus without lung or liver involvement. Prostate carcinoma cells could seed vertebral bodies via Batson plexus and not necessarily extend into the vena cava. Breast carcinoma might also seed the vertebral bodies via the azygos system into Batson plexus. There is only 5-10% of portal blood flow that might shunt to Batson plexus, explaining the relatively low frequency of spinal metastatic lesions with GI and GU primaries. For the vast majority of spinal metastases, there may be no clear answer to the precise route to the end target. Homing properties of the tumor cells and receptive properties of the implantation site may be more important than any particular vascular route.

Spread along the CSF pathways is an important route for primary intracranial tumors. Tumor emboli gain access to the CSF via fragmentation, as well as being shed during surgical manipulation. CNS tumor types showing subarachnoid spread include medulloblastoma, ependymoma, pineal tumors, astrocytoma, lymphoma, leukemia, choroid plexus carcinoma, and retinoblastoma (poor prognosis with MYCN gene amplification). Spread along the CSF pathways can also occur following initial hematogenous dissemination of tumor. For example, this pattern of spread would be present in cord and leptomeningeal metastatic disease following the initial hematogenous dissemination in lung and breast metastatic disease.

Pathologic Issues

Forty percent of patients with cancer will develop visceral or bony metastases during their illness. The spinal column is the most common site of osseous metastases. Men are more frequently affected with vertebral metastases, with a male to female ratio of 3:2. Prostate, lung, and breast carcinomas account for the vast majority of spinal metastases. Locations are primarily thoracic (70%) > lumbar (20%) > cervical.

Primary tumors are generally composed of a variety of biologically different cells in regard to ultimate metastatic potential. Cells continually shed from the primary tumor and gain access to the circulatory system. Fewer than 0.01-0.10% of tumor cells survive to reach a distant site. Successful tumor spread requires completion of a complex pathway, including tumor separation from the primary source, access to blood, CSF, or lymphatic system, survival within the transport process, attachment to the endothelium of a distant vessel, exiting that vessel into the interstitial space, and finally developing the vascular supply at a distant site. The distant host environment site is a complex milieu. Multiple anatomic pathways may be involved with varying flow patterns within the veins and arteries.