**5. Evolving role of radiation**

Both cEBRT (conventional external beam radiation therapy) and SBRT (stereotactic body radiation therapy) and SRS (stereotactic radiosurgery) have been heavily involved in the treatment of MESCC (metastatic epidural spinal cord compression). Historically, palliative radiation in the form of cEBRT was used and has been shown to have stronger outcomes in pain relief, neurologic status, and local control in patients with radiosensitive tumors compared with radioresistant tumors [16, 17]. Additionally, the rate of local control was found to be inversely proportional to tumor size for patients undergoing cEBRT [18]. The advent of SRS and SBRT has significantly improved the treatment of MESCC by its ability to deliver high doses of radiation to smaller targets, minimizing damage to adjacent neurologic structures (**Figure 1**). Compared with cEBRT, SRS is able to provide local control independent of tumor histology [19]. Tumor recurrence in patients that have undergone SRS has been shown to be dependent on the amount of radiation delivered rather than radiosensitivity of the tumor [5]. Furthermore, for patients undergoing epidural spine decompression followed by SBRT, the majority of tumor

### **Figure 1.**

*(A) Axial, (B) coronal, (C) sagittal images showing a highly conformal stereotactic radiosurgery treatment plan to a C2 metastatic lesion secondary to thyroid cancer. The isodose lines can be seen around C2 and the structures at risk are also outlined (the oropharynx and upper esophagus in green and the spinal canal in purple). (D) Shows the relative isodose lines in graphic form, with the tumor dose curve on the far right and the overlapping green and purple dose curves in the middle of the graph representing the dose to the oropharynx and spinal canal, respectively.*

recurrence arose from the portion of the tumor in the epidural space adjacent to the spinal cord that invariably receives an inadequate dose, due to the need to protect the neural structures from the potential damage of radiation [18, 20, 21]. Clearly, the benefits of radiation therapy must be balanced against the risks of damaging normal tissue.

### **6. Advent and benefits to separation surgery**

It has been well established that radiation therapy is highly effective for local tumor control. A landmark study by Patchell et al. showed that direct decompressive surgery followed by conventional radiation for symptomatic epidural cord compression was superior to radiation alone [22]. This finding established the essential role of surgery in the management of MESCC. At the same time, advances in stereotactic radiosurgery made radiation alone an attractive alternative to surgery once again. However, cord tolerance always constrains the dose of radiation delivered to the tumor close to the spinal cord, in order to avoid irreversible neurological damage (radiation myelopathy). Radiation failure and tumor recurrence of epidural metastatic disease most often occur adjacent to the spinal cord and dura mater, given this is where the radiation dose is limited to prevent injury to important neurological structures. Continued advancements in microsurgical and radiosurgery techniques have led to the advent of separation surgery, which has decreased the need for aggressive approaches for gross total resection [3, 8, 12, 21].

The goal of separation surgery is to create space between the neural elements and the tumor, so an adequate radiation dose can be delivered to the tumor. The surgical technique involves circumferential dissection around the dura to create an ablative target for SRS while preserving or restoring neurologic function and

*Minimally Invasive Treatment of Spinal Metastasis DOI: http://dx.doi.org/10.5772/intechopen.102485*

providing local tumor control [8, 21]. This strategy is most beneficial to radioresistant tumors such as metastatic renal cell carcinoma, melanoma, thyroid carcinoma, colorectal carcinoma as well as previously radiated tumors and may decrease surgical-related complications of gross total resection or en bloc resection [11, 23].

The shift toward separation surgery has allowed for the introduction of subtotal resection with tubular or expandable retractors through a minimally invasive approach [8, 12]. Furthermore, the small incisions associated with MIS approaches may allow for earlier radiation therapy [11]. The most common approach for separation surgery is the posterolateral approach, which allows for posterior instrumentation and stabilization as well as circumferential decompression [11, 24]. The use of tubular retractors with ventral decompression via a transpedicular approach is growing in popularity [8]. Surgical access from a tubular retractor has the ability to create enough space ventral to the dura to allow for delivery of an adequate dose of radiation without harming the neural structures. This less invasive technique is also associated with a relatively low rate of hardware failure. Amankulor cited 2.8% incidence of hardware failure that may be associated with inadequate reconstruction of the anterior column following minimally invasive tumor debulking [25].

## **7. Laser interstitial therapy**

Laser interstitial thermal therapy is an alternative method for treatment of epidural cord compression that may be performed via a percutaneous minimally invasive approach. This technique may achieve both epidural decompression and local control when combined with radiosurgery with less morbidity than surgery [26]. However, the time it takes for the tumor to respond to the treatment and shrink away may preclude widespread adoption of this technique. Compared with open decompression, there may be shorter interval to resume systemic treatment averaging 7.8 days [26]. Small, early studies suggest noninferiority of laser interstitial thermal therapy plus XRT compared with open decompression plus XRT in select patients [26].

### **8. MISS techniques for treating mechanical instability**

Metastatic disease to the vertebral column requires assessment for mechanical instability via the SINS criteria. SINS score of 7–12 signifies potential instability and may require bracing, kyphoplasty, percutaneous stabilization, or a combination of the three. Higher SINS scores involve more serious deformity including translation, significant vertebral body collapse, and bilateral pedicle involvement, which require more extensive approaches including vertebrectomy with instrumentation.

### **8.1 Vertebral augmentation**

Compression fractures of the anterior column and combination of anterior and middle columns with preservation of the posterior elements are amenable to percutaneous kyphoplasty. High-level evidence supports kyphoplasty and vertebroplasty as highly effective for stabilizing symptomatic pathologic compression fractures [12, 27, 44] and may be done via an extrapedicular or transpedicular approach. Kyphoplasty may be combined with radiofrequency ablation and biopsy, which allows for diagnosis as well as oncological treatment (**Figure 2**). Minimal incision provides the ability for expedited recovery without interruption of radiation and chemotherapy. Patients often experience improvement in pain and functional status after these minimally invasive outpatient procedures [14].

### **Figure 2.**

*(A) MRI shows multiple painful metastatic lesions at T10, T11, T12 despite treatment with fractionated radiation. (B) Lateral and (C) AP intraoperative images showing pedicle cannulation at all three levels and radiofrequency ablation probe in position of the T11 level.*

### **8.2 Percutaneous stabilization**

Indications for percutaneous stabilization include mechanical instability or as an adjunct to a decompressive surgery for neurologic deficit [10]. Instability is an indication for surgical stabilization regardless of radiosensitivity of the tumor [10, 23]. Percutaneous instrumentation can be performed via MISS or mini open approach over the levels of interest. MISS and mini-open approaches share the advantages of quicker healing time, decreased pain, and the potential for expedited time to administration of chemotherapy and radiation [28].

When considering components of the SINS criteria, compression fractures in junctional areas, as well as fractures with more than 50% loss in height, are subject to additional mechanical stress that may exacerbate fracture, deformity, and mechanical pain. These lesions may benefit from kyphoplasty with additional percutaneous stabilization. Furthermore, compression fractures with involvement of the posterior elements benefit from percutaneous stabilization and kyphoplasty. Burst fractures with significant retropulsion may require decompression with percutaneous stabilization.

Many cancer patients are predisposed to instrument-related complications given the metastatic nature of vertebral bodies combined with osteoporosis from systemic steroids and prior radiation. Combining fenestrated screws and cement augmentation with shorter constructs may lessen the cantilever effect on the spine and reduce incidence of screw pullout or pedicle fracture and proximal junctional kyphosis [12, 29, 30].

Patients with spinal instability and limited life expectancy may undergo percutaneous fixation without fusion. Silva and colleagues conducted a multicenter retrospective study that observed low implant failure rate in short and medium term without fusion [31]. Percutaneous screws may at times be placed with chemotherapeutic agents in attempt to reduce tumor size prior to resection. A case report describes the use of percutaneous screw stabilization with denosumab 6 months prior to en bloc spondylectomy for a spinal giant cell tumor associated with instability [32]. The tumor shrunk during this period, allowing for easier resection. This may be a consideration for a primary bone tumor, which requires aggressive total resection.

### **8.3 MISS decompression**

Primary indications for surgical decompression of spinal metastasis are cord compression from radioresistant tumors as well as mechanical radiculopathy that can be localized to nerve root compression on imaging studies [6].

*Minimally Invasive Treatment of Spinal Metastasis DOI: http://dx.doi.org/10.5772/intechopen.102485*

Tubular retractors may be used primarily for decompression of the posterior elements, but may also be used for ventral decompression as well as lateral decompression.

### **Figure 3.**

*Patient presenting with worsening back pain lower extremity paresthesias. (A) CT axial (B) CT showing lytic lesion with three column involvement, including unilateral pedicle involvement. (C) T1 MRI axial precontrast and T1 MRI sagittal post contrast shows epidural compression. Patient underwent thoracic laminectomy with unilateral transpedicular corpectomy with percutaneous stabilization two levels above and below the affected vertebrae.*

### **Figure 5.**

*Some expandable retractor systems used in minimally invasive unilateral decompression may be attached to the spinal fixation system used for percutaneous pedicle screw placement. Pictured is the space-D© retractor system by Medtronic.*

### **8.4 Corpectomy with stabilization**

Higher-grade SINS criteria involve significant vertebral body and posterior element compromise, which may require corpectomy with stabilization.

Mini-open and MIS approaches have been described for corpectomy with vertebral reconstruction. Such approaches are not as commonly utilized compared with the open approach. The open approach is often met with high morbidity, which has the potential to be especially detrimental to a cancer patient. A retrospective analysis of cohort of 49 adult patients with thoracic metastasis conducted by Lau et al. showed miniopen approach for thoracic transpedicular corpectomy with instrumentation had significantly less blood loss and hospital stay with no significant difference in complications or ASIA grade compared with the traditional open approach [33].

Extension of metastatic disease into pedicle or facet can cause mechanical radiculopathy as well as further destabilization of the spinal column [34]. If the lesion involves the anterior and middle columns and one pedicle, then unilateral approach tubular or expandable retractor may be used (**Figures 3**–**5**). If more extensive disease involves both pedicles, then bilateral tubular or expandable retractors can be used.
