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,¹ direct extension or invasion,² and seeding via the cerebrospinal fluid.³ 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.⁷ 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.⁸

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 cerebrospinal fluid (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, followed by the paravertebral regions and epidural space. Intramedullary and intradural metastases have a low incidence and are usually the result of CSF seeding. With regard to region, the thoracic spine is the most common site for tumor localization (45% to 70%), followed by the lumbar spine (20% to 35%) and finally by the cervical (10% to 20%) and sacral (10% to 20%) regions.⁹

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 the aggressive resections of spinal lesions arising from indolent, well-controlled systemic disease (e.g., renal cell carcinoma, breast adenocarcinoma) might provide oncologic control and improve overall prognosis. All such goals can be successfully obtained via surgical intervention; however, 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.¹⁰ 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 colleagues¹¹ 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 colleagues¹² 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, the Tokuhashi and Tomita scoring systems do not definitively indicate the exact appropriate patient treatment, particularly because of recent advances such as stereotactic surgery. However, the general framework of these systems with respect to patient prognosis is fundamentally important 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 destabilizing the posterior elements, 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 2005.¹³ Their data suggest that surgery with radiation is statistically superior to radiation alone as demonstrated by the greater post-treatment 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.

Determining the Surgical Approach

Numerous factors must be considered when determining a surgical approach for resection and reconstruction in the case of spinal metastases. First, the spinal segment involved must be accurately identified. Next, the exact location of the tumor within the vertebral column must be clarified. It may also be helpful to obtain a histologic profile on the tumor if there is ambiguity regarding the primary cancer. Last, in combining and analyzing all available information, one should determine the type of spine reconstruction necessary.

Determining the spinal segment involved is crucial for determining the surgical approach because not all segments are equally accessible. For example, most spinal metastatic disease localizes to the vertebral body and most spine surgeons are familiar with anterior exposures of the cervical spine, making lesions in this area readily accessible. However, the upper thoracic segment of the spine (T1–4) may be difficult to expose anteriorly (Fig. 192-1). In such an instance, an anterolateral cervical approach combined with sternotomy or thoracotomy could be required. Some cases require access to spinal segments that are occluded by the presence of great vessels, and in such instances a posterolateral or transpedicular approach is often used (Fig. 192-2). For the lower thoracic region (T5–12), a right-sided thoracotomy allows access to the spine while still avoiding the great vessels and aortic arch. However, if the majority of the tumor is located on the left, a left-sided thoracotomy is appropriate. Access for a thoracolumbar junction (T11–L1) decompression and resection often requires a combined thoracotomy and retroperitoneal approach. It is possible to approach the lumbar spine (L1–5) anteriorly; however, a posterior approach allowing posterior excision and stabilization is often indicated for managing metastatic spine disease (Fig. 192-3).

FIGURE 192-1 Images demonstrating metastatic epidural cord compression (MESCC) at T1. A and B, T2-weighted images showing spinal cord compression in the axial (A) and sagittal (B) planes. Although this is the upper thoracic region, the area could be approached anteriorly without the need for sternotomy. C, A vertebrecomty was performed at T1 with an anteroposterior reconstruction as viewed on a sagittal CT scan.

FIGURE 192-2

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