Spinal metastases are the most common tumors of the spine, compromising approximately 90% of masses encountered with spinal imaging. Spinal metastases are more commonly found as bone metastasis, although they are not limited to bone metastasis, and approximately 20% present with symptoms of spinal canal invasion and cord compression. Within the spinal column, metastasis is more commonly found in the thoracic region, followed by the lumbar region, while the cervical region is the least likely place professionals find metastasis.
When evaluating spinal metastasis on MRI imaging, a defining feature of these lesions is the sparing of intervertebral disc space. This disc space is almost always involved during an infection. Metastatic diseases to the spine spread through several different routes which include venous hematogenous spread versus the arterial spread, direct tumor extension, and lastly lymphatic spread. Among the routes mentioned above, hematogenous spread through Batson’s plexus system is the most common pathway for tumor embolization and spinal invasion. The following summary emphasizes the essential knowledge necessary to have while treating patients with spinal metastasis.
Most common spine metastasis involving primary metastasis originate from the following tumors in descending order: breast (21%), lung (19%), prostate (7.5%), renal (5%), gastrointestinal (4.5%), and thyroid (2.5%). While all tumor can seed to the spine, the cancers mentioned above metastasize to the spinal column early in the disease process. While the exact mechanism and gene expression necessary for tumor invasion of the spine is a focus of continuous active research, the presence of certain factors like RANK and RANKL (receptor activator of nuclear factor kappa beta (NFkB ligand)), which interact with receptors to activate osteoclastic cells, appear to play a central role in setting an island of invasion.
Spinal metastasis is common feature practitioners encounter while treating oncologic patients. In a study of post-mortem corpses examination of cancer patients with known primary spinal metastasis was as high as 70% to 90% for breast and prostate primary types of cancer. Morbidity related to spinal metastasis is related to excruciating back pain requiring high doses of narcotic medications for management, pathologic bone fractures, hypercalcemia, and spinal cord compression due to the invasion of epidural space. Rarely spinal metastasis will seed inside the spinal cord itself with no apparent bone involvement. In these cases, when there is no known history of primary cancer, the diagnosis of metastasis, diagnosis becomes difficult, and often the correct diagnosis is determined after pathology confirms the type of tumor.
The general spread of metastases involves hematogenous spreading to the center of the vertebral body. Through an initial interaction between tumors associated factors and intrinsic bone cells like osteoclasts, a nidus of invasion is established. More commonly the tumor spreads backward often involving pedicles, which is an important point of understanding for surgical management of spinal metastasis that would require spinal fusion. Due to this fact, screw fixation through the involved pedicles often is nonoptimal and requires extending fusion of several segments above and below the lesion. Commonly, blood supply to the metastatic nidus comes from pedicular arteries making selective embolization through interventional procedures a viable option. Further understanding of the interaction between invading metastasis and surrounding bone cells and factors produced like IL-6, IL-1, TGF-beta and RANK/RANKL have offered chemical therapy in the treatment of painful bone metastasis, especially breast and prostate as known primaries. A complete description of the mechanisms and factors involved is outside the scope of this short clinically focused review.
Pain is the most common presenting complaint in the patients with spinal metastasis. High clinical suspicion should be present with any oncologic patient that presents with neck or back pain. While pain is not the most fearsome or morbid symptom of spinal metastasis, it is the most common initial symptom requiring evaluation from the treating physician. In addition to being an initial presenting symptom, neck and back pain in metastasis is quite often localized requiring further diagnostic imaging on a patient that cannot undergo complete imaging of the spine. The pain is aching and deep, often occurs at night, and wakes the patient from sleep. Also, if nerve involvement has occurred which would suggest more extension of tumor, the pain is sharp and shooting in specific dermatome distribution. When the tumor has extended inside the spinal canal motor, and sensory weakness becomes permanent and more worrisome. The defining concept for treatment of spinal metastasis includes the degree of weakness and extension of the deficit. The worst the deficit is upon presentation, the worse chances are for recovery. Also, time lapsed from deficit progression to the physician identifying cause determines the possibility and chances for recovery.
The simplest test often available to evaluate an oncologic patient presenting with neck or back pain is an x-ray of the spinal column. Plain anterior and posterior (AP) and lateral images are often non-sensitive or specific and require at least 50% bone erosion before an abnormality can be observed. Spine MRI is the gold standard for evaluating these lesions. MRI would show extension, levels involved, and canal compromise, as well as provided clues to etiology of the metastasis. However, this is not always possible, for example, when a patient has an implanted pacemaker or internal defibrillator. Myelography associated with or without CT imaging should be considered in patients that are not candidates for MRI. Myelography provides the advantage of sending cerebrospinal fluid for pathology evaluation, but it has significant limitations when a lesion completely occludes the canal. In these cases, multiple spinal injections of contrast might be required to bypass the level of obstruction. PET scan is another modality which is sensitive but offers poor anatomical localization.
If metastasis involves multiple bony structures with no canal compromise or associated bone fracture, these patients can be managed without surgery. However, it is important to involved spine surgical oncologist in order to get advice regarding spinal instability related to multiple metastases to spine. However, in most patients with multiple spine metastases and normal neurological examinations, these patients will benefit from primarily radiation therapy and chemotherapy. If tissue for pathologic diagnosis is necessary and there is no primary or easily reachable metastasis with any other route, a patient could benefit from an image-guided biopsy of the bone lesion. Open diagnosis is very rarely necessary after several attempts at obtaining a sample through a needle biopsy.
When tumor involves spinal canal treatment, strategy changes significantly, and surgical consultation should be immediate as these patients can progress to becoming bed bound within days. Studies have shown that paralysis due to metastatic spine disease significantly shortens the life expectancy of cancer patients. On the other hands, surgical intervention of patients with acute paralysis due to spinal cord compression from the metastatic disease will significantly reduce mortality and morbidity associated with acute paralysis.
Dependent on rapidity and seriousness of neurologic compromise these patient treatments should involve the addition of steroids. Dexamethasone has been shown in clinical trial to decrease pain and improve symptoms. However, the exact dose that would benefit patient the most is not known. In one study comparing bolus dose of 100 mg dexamethasone with 10 mg of dexamethasone bolus initial injection, no significant clinical benefit was observed with the higher dose. A good starting dose would be bolus dose of 10 mg IV followed by maintenance dose of 4 mg every six hours tapered over two weeks as clinical scenario allows. Immediate imaging should be obtained to evaluate extension and possible surgical intervention. If the suspicion is low for very radiosensitive tumor, if there is complete paralysis that has been present for more than 24 hours or the expected survival of the patient is less than three or four months, surgical intervention is warranted. Further treatment after surgery should involve a multidisciplinary approach of chemotherapy and focused radiation. Radiation therapy usually involves 30 GY to 40 Gy in a ten-treatment session. Wound closure becomes a concern post radiation and chemotherapy, and the patient should be followed closely to identify wound breakdown and repair promptly.