Surgery for Metastatic Spine Disease

Mechanism of Spread to the Spine

Metastatic tumors often exhibit characteristic biological behavior of the primary tumor, and thus the mechanism of spread is correlated to the type of primary tumor. Understanding the mechanism is crucial when surgically treating such patients because it can predict recurrence (local versus distant) and can change the goals of resection (debulking versus extensive local control). These mechanisms include hematogenous seeding, direct extension or invasion, and seeding via the cerebrospinal fluid (CSF). Hematogenous seeding occurs via arterial or venous pathways and is the most usual route of metastatic spread to the spine, presumably because of the highly vascular nature of the vertebral bodies. Venous spread occurs mainly via Batson’s plexus, which connects to multiple venous networks including renal, pulmonary, intercostal, azygous, portal, and caval systems. Owing to the valveless nature of these veins, tumor cells may deposit both antegrade and retrograde in the spine as physiologic pressure changes occur within the major body cavities. Multiple studies in animal models have supported this hypothesis for prostate metastases, finding that occlusion of the caval system leads to backfilling of the Batson system and routing of the tumor cells into these veins.

With regard to direct extension, primary tumors of the pelvis, abdomen, or thorax can directly invade the spine, resulting in symptomatic spinal metastases. For instance, lung tumors can extend superiorly to invade the cervicothoracic junction as Pancoast tumors, or they can extend posteriorly to invade the thoracic spine. The sacrum may be invaded by multiple pelvic primaries, including colorectal, bladder, and prostate cancers. Finally, seeding of tumor cells into the CSF can rarely occur, usually following surgical manipulation of cerebral primary or metastatic lesions, leading to metastases within the subarachnoid space or the spinal cord itself.

Classification of Spine Tumors

Classification is based on the anatomic location of the lesion: intramedullary, intradural-extramedullary, or extradural. The main reason for this is that once the anatomic location is determined, the differential diagnosis of the lesion can be quickly narrowed. Of these locations, the extradural compartment is the site at which spinal metastasis most commonly occurs. Extradurally, the most common site to which spinal metastases localize is the vertebral body (70% to 85%), followed by the paravertebral regions and epidural space. Intramedullary and intradural metastases have a low incidence, occurring in only 0.1% to 0.4% ^, and 4% to 15% of all cancer patients, respectively, and are usually the result of CSF seeding. With regard to region, the thoracic spine is the most common site for tumor localization (46% to 87%), followed by the lumbosacral spine (25% to 39%) and finally by the cervical (8% to 26%) regions.

Management

Proper treatment of metastatic spine disease requires a multidisciplinary approach necessitating the involvement of various types of therapy and medical specialties such as neurosurgery, orthopedic surgery, surgical oncology, medical oncology, radiation oncology, interventional radiology, pain specialists, and rehabilitation medicine. Classically, treatment goals have generally been considered to be palliative, centered on mechanical stabilization, pain relief, and preservation of neurologic function. However, preliminary data now suggest that aggressive resections of spinal lesions arising from indolent, well-controlled systemic disease (e.g., renal cell carcinoma, breast adenocarcinoma, thyroid carcinoma) might provide oncologic control and improve overall prognosis. This benefit is not seen in patients with more advanced systemic disease, or in those with more aggressive tumor types, so both factors must be considered when debating surgery. Additionally, before choosing surgical intervention as a treatment, patient variables such as age, life expectancy, functional status, and tumor burden must be carefully considered.

Selecting Patients for Surgical Intervention

Increased patient survival time due to advancements in systemic chemotherapy and radiation therapy has paralleled more aggressive surgical decompression and fixation for patients with metastatic spine disease. For example, since the 1990s, the combination of improvements in implant and instrumentation technology combined with reduction in approach-related morbidity have resulted in improved outcomes following tumor resection and spinal reconstruction. Selection of patients for surgical treatment remains challenging; however, in general most clinicians agree that a patient’s survival must be in excess of at least 3 months to be considered for surgical treatment. ^{, , , , , ,}

To facilitate the selection process for surgical candidates, scoring systems have been created to evaluate the preoperative status and predict the postsurgical outcome of patients following large decompression and fixation procedures. Tokuhashi and colleagues created a scoring system that considers primary tumor histology, number of vertebral metastases, presence of extraspinal or visceral metastases, overall functional status, and neurologic function. They were able to correlate such factors with survival following tumor resection. Not surprisingly, presence of less-aggressive tumors, single vertebral metastases, lack of other metastases, good overall functional status, and normal neurologic function were associated with a higher point total and longer survival. As a result, they recommended that patients with a score greater than 9 of 12 should be considered for aggressive excisional surgery with reconstruction. Patients with scores lower than 5 of 12 had a poor prognosis, and they recommended either palliative surgery (limited decompression) or no surgery at all.

Tomita and colleagues also constructed a scoring system to assess surgical candidates. In Tomita’s system, unlike Tokuhashi’s, lower scores indicate a positive prognosis and vice versa. In addition, Tomita’s system is based on only three main parameters: primary tumor histology, presence of visceral metastases, and solitary versus multiple vertebral metastases.

Although they are useful guides, neither the Tokuhashi and Tomita scoring systems have been shown to consistently predict patient outcome following surgery. ^{, , , , ,} Furthermore, they do not account for recent advances such as stereotactic surgery, the use of which is becoming increasingly popular, particularly in high-risk patients who are otherwise too unhealthy to undergo surgery. ^, However, the general framework of these systems with respect to patient prognosis is fundamentally important. Consequently, they are recommended as decision aids when deciding if surgery is an option. ^{, , , ,}

Moving beyond the selection of a patient for surgical intervention, planning of the surgical approach and stabilization necessitates a thorough understanding of the histopathology and anatomy of the metastatic tumor and its surrounding structures. In addition, knowledge of the functional spine biomechanics and the pathologic biomechanical alterations that follow vertebral metastases is also required for optimal planning of a surgical intervention.

More recently, significant attention has been paid to the concept of metastatic epidural spinal cord compression (MESCC). In the past, surgical interventions were restricted to decompressive laminectomies. ^{, , ,} However, decompressive laminectomy alone does not resolve anterior compression originating from the vertebral body, and it induces spinal instability by disrupting the posterior tension band, ^{, , ,} leading to increasing pain and potentially worsening neurologic function. For this reason, direct decompression and reconstruction approaches are now the standard for treating patients with a reasonably good prognosis. The first prospective randomized controlled trial of direct decompressive surgical resection with radiation therapy versus radiation therapy alone in the treatment of patients with MESCC was published by Patchell and colleagues in 2005. Their data suggest that surgery with radiation is statistically superior to radiation alone, as demonstrated by the greater posttreatment ambulatory rate, duration of ambulation, maintenance of ambulation after treatment, and return of ambulation after treatment. The study also demonstrated that patients who received both surgery and radiation had improved functional ability (Frankel scores), muscle strength (ASIA scores), continence, and survival time when compared to the group that received radiation alone. Important exclusion parameters within the selection criteria include patients with highly radiosensitive tumors such as small cell lung carcinoma, myeloma, and lymphoma. In such patients radiation alone may still be the preferred treatment for MESCC.