Spinal laser interstitial thermal therapy (LITT) appears to be a promising novel modality for treatment of epidural metastatic spine disease in patients who are poor candidates for larger-scale oncologic spinal surgery and can act synergetically with spinal stereotactic radiosurgery to maximize local control and palliate pain. This technique is ideally suited for the intraoperative MRI suite to monitor the extent of the ablation in the epidural space. As percutaneous navigation, imaging, and LITT technology improve, broader applicability of this minimally invasive technique in spinal oncology is foreseen.
Key points
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Spinal laser interstitial thermal therapy (LITT) appears to be a promising novel modality for treatment of epidural metastatic spine disease in patients who are poor candidates for larger-scale oncologic spinal surgery and can act synergetically with spinal stereotactic radiosurgery to maximize local control and palliate pain.
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This technique is ideally suited for the intraoperative MRI suite to monitor the extent of the ablation in the epidural space.
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As percutaneous navigation, imaging, and LITT technology improve, broader applicability of this minimally invasive technique in spinal oncology is foreseen.
Metastatic epidural spinal cord compression (ESCC) is a major clinical problem, negatively impacting the quality of life of a significant subset of patients with cancer. As systemic treatments for advanced malignancies have improved survival, more patients are living longer with metastatic disease. Spinal metastases occur in 20% to 60% of all patients with malignant tumors and up to 10% develop spinal cord compression. Treatment strategies for metastatic spine disease must emphasize durable local control and preserving neurologic function while minimizing treatment-related morbidity.
Conventional external beam radiation therapy (cEBRT) has been the most common method for delivering radiation to the spine. This modality allows treatment of a wide target field in which all tissue receive a prescribed dose of radiation, requiring fractionation and adjustments to respect radiation-sensitive tissue and organs near the tumor being treated. Given this limitation, there is wide variability in response rates of different tumor histology, in which some abnormalities are known to be highly radiosensitive (such as lymphoma and multiple myeloma) whereas others are known to be radioresistant (such as renal cell carcinoma).
Historically, the role for surgery was unclear. Simple posterior decompression led to poor patient outcomes, which in turn limited the application of surgical treatment. However, in the last 2 decades, improvements in techniques for circumferential decompression and advancements in spinal stabilization have promoted better outcomes in patients with epidural metastatic spine disease. Patchell and colleagues demonstrated the value of thorough surgical decompression coupled with stabilization. The study established that cEBRT alone was inferior to circumferential decompression and stabilization followed by cEBRT in maintaining ambulation, preserving continence, improving pain, and promoting survival in patients with solitary spinal metastases and ESCC.
Spinal stereotactic radiosurgery (SSRS) has emerged as an alternative, noninvasive, and highly precise treatment modality for spinal metastasis. SSRS can deliver higher doses of radiation focused on a specific target with a sharp falloff, decreasing toxicity to adjacent tissues, and most importantly, overcoming the traditional “radioresistance” to fractionated cEBRT. Several series that have demonstrated good local control in cases of radioresistant histologies helped establish SSRS as an effective treatment modality for the management of spinal metastatic disease.
Despite the reliable and favorable results of SSRS for spinal metastasis, the presence of epidural tumor compressing the spinal cord requires adjustment in the dose prescribed to the epidural space in order to respect the radiation constraints of the spinal cord, which can negatively affect the local control rates. In these cases, a surgical strategy named “separation surgery” followed by SSRS has been used to minimize surgical morbidity and maximize local control. This surgery entails resection of the epidural disease to allow reconstitution of the thecal sac and stabilization of the spine. Bilsky and Smith proposed an algorithm in which neurologic, oncologic, mechanical, and systemic factors are analyzed, and the utilization of cEBRT, SSRS, and/or surgery is recommended based on tumor sensitivity to radiation and the extent of epidural involvement. In this context, local control is achieved with radiation (either cEBRT or SSRS), and surgery is used to remove the epidural tumor and stabilize the spine (separation surgery). The extent of resection loses importance in terms of local control, provided an adequate coverage with tumoricidal doses of stereotactic radiosurgery is delivered to the residual disease. Laufer and colleagues reported a cumulative incidence of local progression of 4.1% and 9%, respectively, using hypofractionated (24–30 Gy in 3 fractions) or a high single-dose (24 Gy) SSRS after separation surgery. Unfortunately, patients with metastatic disease are often deconditioned and have multiple comorbidities; thus, the morbidity of open circumferential decompression and fusion must not be underestimated.
Based on the concept of separation surgery, the authors proposed a strategy to treat epidural metastatic disease with a less invasive percutaneous procedure associated with a faster recovery avoiding interruption in systemic treatment. This strategy uses laser interstitial thermal therapy (LITT) to ablate the tumor in the epidural space before the delivery of SSRS. In the procedure, a 1.3-mm laser catheter is percutaneously introduced into the epidural tumor. The technique requires use of the intraoperative MRI suite (iMRI suite, BrainLab Inc, Feldkirchen, Germany) as real-time MR thermography is used to monitor the temperature as the epidural tumor is ablated by heating the soft tissue with laser energy. Patients with spinal instability can be treated with an accompanying percutaneous stabilization using pedicle screw and rod constructs. Combined with postablation SSRS, the authors have been able to achieve satisfying rates of local control in these patients who generally present with worse functional status.
Rationale for laser interstitial thermal therapy
A substantial subset of patients suffering from ESCC has significant comorbidities in addition to rapidly progressive systemic disease. In such a scenario, conventional surgical intervention requires interruption of the oncologic treatment until the patient has recovered from surgery and the operative wound is completely healed. Preoperative optimization of comorbidities is not always possible, and surgical risk can be significant, leading to postoperative complications, which can result in additional delay to the resumption of oncologic treatment. In such cases, the authors have used spinal LITT (sLITT) as an alternative to surgery. Akin to treatment of metastasis in the brain with LITT, this palliative treatment is meant to be coupled with postablation SSRS in order to maximize local control with the ultimate goal of preventing neurologic decline, facilitating faster recovery, and minimizing interference with other required oncologic therapies. Performance and safety of sLITT are dependent on the use of MR thermography, which allows real-time visualization of tumor ablation and preservation of the integrity of the neural elements.
Techniques for less invasive ablations of spinal metastatic disease, including radiofrequency and cryoablation, have been sought after for decades, because many patients with cancer would be good candidates for this technique. However, because of the little room to maneuver in the spinal canal and the proximity to the spinal cord, there is significant risk of neurologic damage even with image-guided techniques. Computed tomography (CT)-guided radiofrequency ablation of vertebral and paraspinal tumors have reported thermal injuries to the spinal cord and nerve roots. In addition, in an animal study, ablation of the vertebral body in pigs resulted in neurologic deterioration when the radiofrequency electrode was placed immediately adjacent to the posterior wall or in the pedicle near the spinal canal and neural components. For this reason, these techniques have been limited to the treatment of lesions confined to the vertebral body without spinal cord compression, for which modern SSRS is now more commonly used.
The major advantage of sLITT over other percutaneous methods is its compatibility with intraoperative magnetic resonance thermal imaging (MRTI), which allows for real-time monitoring of heat generation and creation of a thermal map in the area of interest. Gradient echo sequence imaging allows measurement of changes in proton resonance frequency that occurs with changes in temperature. Using a reference gradient recalled phase image taken at body temperature and temperature changes recorded with MRTI, temperature information can be incorporated into an Arrhenius model to allow for a quantitative estimate of thermal damage. Thus, although it is a percutaneous procedure, there is full control to ensure satisfactory ablative coverage of the target and avoidance of thermal injury to the spinal cord and roots ( Fig. 1 ). Moreover, for epidural compression with preserved cerebrospinal fluid (CSF) around the cord, the fluid may theoretically act as a protective “heat sink,” which can help with contouring of the ablation of epidural disease and sparing of the spinal cord.
Rationale for laser interstitial thermal therapy
A substantial subset of patients suffering from ESCC has significant comorbidities in addition to rapidly progressive systemic disease. In such a scenario, conventional surgical intervention requires interruption of the oncologic treatment until the patient has recovered from surgery and the operative wound is completely healed. Preoperative optimization of comorbidities is not always possible, and surgical risk can be significant, leading to postoperative complications, which can result in additional delay to the resumption of oncologic treatment. In such cases, the authors have used spinal LITT (sLITT) as an alternative to surgery. Akin to treatment of metastasis in the brain with LITT, this palliative treatment is meant to be coupled with postablation SSRS in order to maximize local control with the ultimate goal of preventing neurologic decline, facilitating faster recovery, and minimizing interference with other required oncologic therapies. Performance and safety of sLITT are dependent on the use of MR thermography, which allows real-time visualization of tumor ablation and preservation of the integrity of the neural elements.
Techniques for less invasive ablations of spinal metastatic disease, including radiofrequency and cryoablation, have been sought after for decades, because many patients with cancer would be good candidates for this technique. However, because of the little room to maneuver in the spinal canal and the proximity to the spinal cord, there is significant risk of neurologic damage even with image-guided techniques. Computed tomography (CT)-guided radiofrequency ablation of vertebral and paraspinal tumors have reported thermal injuries to the spinal cord and nerve roots. In addition, in an animal study, ablation of the vertebral body in pigs resulted in neurologic deterioration when the radiofrequency electrode was placed immediately adjacent to the posterior wall or in the pedicle near the spinal canal and neural components. For this reason, these techniques have been limited to the treatment of lesions confined to the vertebral body without spinal cord compression, for which modern SSRS is now more commonly used.
The major advantage of sLITT over other percutaneous methods is its compatibility with intraoperative magnetic resonance thermal imaging (MRTI), which allows for real-time monitoring of heat generation and creation of a thermal map in the area of interest. Gradient echo sequence imaging allows measurement of changes in proton resonance frequency that occurs with changes in temperature. Using a reference gradient recalled phase image taken at body temperature and temperature changes recorded with MRTI, temperature information can be incorporated into an Arrhenius model to allow for a quantitative estimate of thermal damage. Thus, although it is a percutaneous procedure, there is full control to ensure satisfactory ablative coverage of the target and avoidance of thermal injury to the spinal cord and roots ( Fig. 1 ). Moreover, for epidural compression with preserved cerebrospinal fluid (CSF) around the cord, the fluid may theoretically act as a protective “heat sink,” which can help with contouring of the ablation of epidural disease and sparing of the spinal cord.
Patient selection
Patients with spinal metastasis of radioresistant histology or who have previously undergone cEBRT are discussed in an interdisciplinary conference with radiation oncology, radiology, and neurosurgery. To evaluate the extension of epidural disease, the authors use Bilsky’s ESCC grading system. If the tumor encroaches upon the thecal sac enough to compromise SSRS dosing to tumor (generally Bilsky grade Ic or greater), then separation surgery is considered ( Fig. 2 ). Among these patients, if the patient is a poor surgical candidate for an open circumferential decompression and stabilization or if the patient’s systemic disease requires prompt resumption of chemotherapy, sLITT is considered. Patients with unstable pathologic fractures as defined by Spinal Instability Neoplastic Score (SINS) criteria can be treated with accompanying percutaneous posterior spinal stabilization. If progressive or severe incomplete neurologic deficits related to cord compression are present, urgent open decompression is preferred. Patients who cannot undergo iMRI or have spinal instrumentation at the level of interest that can interfere with MR thermography are not considered candidates. Patients with thoracic radiculopathy as a result of epidural and foraminal disease below T1 are often good candidates for sLITT because ablation of the tumor and the associated nerve root often relieves their pain. If the epidural disease involves functional nerve roots (C3-T1 roots, lumbar and sacral roots), open surgery is favored to allow dissection of the nerve root from the tumor.
Technique
The procedure is performed inside an iMRI suite (BrainLab Inc, Feldkirchen, Germany) under general anesthesia. The patient is positioned prone, with the arms parallel to the body in the iMRI transfer table. A fluoroscope (Siemens, Munich, Germany) is positioned inside the iMRI room, at a safe distance to obtain confirmatory anteroposterior and lateral fluoroscopic images. Initially, in the authors’ experience with sLITT, they used a preoperative CT scan and localized using intraoperative C-arm fluoroscopy to perform percutaneous placement of the laser catheter cannulas. However, currently they use an iMRI registration scan using surface fiducials for navigation ( Fig. 3 ). The authors have found this MR-based image-guidance method can be performed with submillimeter accuracy on average (Tatsui C, et al, unpublished data, 2016). Fiducial markers (Izi Medical Products, Owing Mills, MD, USA) are placed on the back overlying the area of interest and marked with a surgical pen (see Fig. 3 A). A plastic cradle is used to support a Siemens body matrix coil, avoiding contact with the skin and the fiducials (see Fig. 3 B). The patient is transferred in the operative position to the iMRI, and T2 sequence registration scans are obtained. These scans are uploaded to the navigation system (Brainlab, Inc, Feldkirchen, Germany) and prepared for surface matching registration (see Fig. 3 C, D). The patient is removed from the iMRI and placed at a safe distance away from the magnetic field. A reference array is secured to the skin of the patient with stitches and adhesive, and the fiducial markers are registered. Accuracy is verified by applying the navigation tool to the fiducials and over the midline using the spinous processes as landmarks (see Fig. 3 E, F).
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