Introduction
Bone is the third most common site of tumor metastasis after lung and liver. Of all bones, the spine is the most frequent site of metastasis ( Table 8.1 ). Osseous metastatic lesions can be seen in up to 80% of cancer patients at the time of their death, with up to half of these metastases located in the spine. This is thought to be related to the vascularity and hematopoietic role of the vertebrae. The cancer causes pain and erosion of the bone, leading to fracture ( Table 8.2 ). The pain from these metastases can be debilitating for patients, impairing patient mobility and leading to depression and a decrease in quality of life. These lesions are traditionally managed with a combination of radiation therapy (mainstay), chemotherapy, medical therapy with bisphosphonates or targeted bone agents, and occasionally surgical intervention should the patient be a candidate.
|
| Osteolytic | 70% |
| Osteoblastic | 8% |
| Mixed | 21% |
These interventions do have their limitations, however. Incomplete or lack of pain relief in patients receiving radiotherapy for painful bone metastasis has been documented in a large meta-analysis study in 60% and 23% of patients respectively. Pain relief may also take as long as 4 to 6 weeks to be fully effective. Complications from these treatments are not to be ignored as patients undergoing radiotherapy have documented fracture rates as high as 39%. In patients with longer life expectancy, recurrence of symptoms after initial response has been observed as well. Additional radiotherapy is often avoided in these patients due to risk of radiation myelopathy. These limitations have allowed radiofrequency ablation (RFA) with vertebral augmentation to demonstrate its ability to provide accelerated pain control faster with fewer complications when compared with the current standard of care.
Multiple studies have demonstrated the role of RFA of metastatic bone lesions followed by vertebral augmentation in reducing pain scores. The benefits include improvement in mood, improved quality of life, and a reduction in opioid use. Safety and efficacy of RFA of metastatic bone lesions has also been demonstrated. One study examined 87 patients experiencing pain from metastatic disease in the spine and/or sacrum who underwent RFA with subsequent cementoplasty. The outcome was a rapid and statistically significant improvement in pain within 3 days of the procedure with sustained relief at 6 months in the majority of patients.
The analgesic effect of RFA is attributed to destruction of tumor cells and nociceptive nerves, lower tumor-mediated cytokine release, and delays in tumor progression to the periosteum. Subsequent cementoplasty allows for stabilization of fractured trabeculae and may prevent vertebral body collapse as a result of ablation or tumor invasion. Patient selection, workup, contraindications, and the procedure itself are described in the following sections.
Background
The biological effect of radiofrequency (RF) waves on tissue was described by the French physiologist Jaques D’Arsonval (1851–1940) in the late 19th century (1891). However, use of RF energy in medicine came with the invention of the Bovie knife. Rossi et al. independently described using RF thermal therapy to treat a tumor nodule in the liver in 1992. Since then, this mode of therapy has grown in popularity and has been used for tumor ablation worldwide.
Physics of radiofrequency
RF waves are part of the electromagnetic spectrum. Electromagnetic waves are massless vibrating photons that carry varying amounts of energy depending on their frequency. Waves with frequency ranges of 30 Hz to 300 GHz are called radio waves . RF ablative devices work in the 450- to 500-kHz range. RFA causes coagulative necrosis by inducing thermal heating of tissue. An RF machine emits RF waves through an electrode, the probe in this case. The alternating current fed to the generator causes the generator to create a high-frequency alternating electromagnetic field. The dipoles in the electromagnetic field are forced to align within the field. This alternating field causes the adjacent dipoles—in this case, the water molecules surrounding the tissues—to vibrate at the frequency of the RF waves, resulting in agitation. This agitation generates heat within the tissue. The heat generated by the friction, when high enough, leads to protein denaturation and cell death. The electrode itself never gets hot; all of the heat is generated within the tissue. The tissue biopsy examined under the microscope will reveal coagulative necrosis. The size of the thermal burn depends on tissue ability to handle the heat and the power of the device to deliver it. In general, the size is proportional to the net energy deposited. Temperature above 45° C is lethal to mammalian cells in 15 minutes or more. At temperatures above 50° C, this time is reduced to a few minutes. At temperatures above 100° C, tissue vaporizes instantly, creating air bubbles, which then restrict the conduction of heat and, thus, limit burn size ( Table 8.3 ). Therefore slow delivery of heat at 50° C to 90° C to the surrounding tissue creates optimal size burn.
| Temperature (Celsius) | Time to Cell Death |
|---|---|
| < 45 | No cell death, hyperemia |
| 46 | 60 minutes |
| 50–52 | 6 minutes |
| 60–100 | A few seconds |
| > 105 | Vaporization, carbonization |
Type of radiofrequency ablation
RF energy can be delivered either via unipolar electrode or bipolar electrode. In a unipolar electrode, the electromagnetic field is created between the electrode (cathode) and the grounding pad (the returning electrode or anode). This setup produces intense heat generation at the electrode tip, causing localized charring, as seen in a surgical Bovie knife. Unipolar RF does not deposit energy uniformly within the tissue, and the uneven heat deposition gets worse when the electrode diameter is increased. Hence, unipolar electrodes are not suitable for ablation of large lesions. In bipolar RF, both electrodes are present on the same needle or catheter and the electromagnetic field is created between the two electrodes. This yields a more controlled dispersion of heat and a more predictable lesion.
Bipolar RF device development has led to the safe application of this therapy to tumor lesions within the spine while avoiding any nerve or spinal cord damage.
Types of electrodes
Cooled versus non-cooled probes
Goldberg et al. first reported use of chilled saline during RF, which reduced charring and impedance, resulting in increased tumor volume ablation. The cooled RF device is commercially available and is manufactured by Medtronic (OsteoCool TM ). Using ice water (0° C), the electrode tip temperature stays around 15° C, and charring can be prevented altogether. If room temperature water is used, the tip of the probe cools down to 35° C, which prevents charring if the generator power is kept at 50 W or less. If the electrode tip is cooled to 45° C, all benefits of cooling (increased lesion size and absence of charring) are lost.
Navigational bipolar probe
The SpineSTAR Ablation system—its RF electrode has an active navigational tip that can be maneuvered into difficult spots, such as the posterior part of the vertebral body.
Perfused bipolar electrode
A saline- or dextrose-based solution is injected into tissue through the electrode during RFA. Saline works by improving heat conductivity as well as increasing ion availability within the tissue to generate more heat. Hypertonic saline increases the lesion size much more than normal saline.
Patient selection/workup
RFA is approved by the US Food and Drug Administration for patients suffering from pain due to malignancy in the bone. The National Comprehensive Cancer Network (NCCN) endorses RFA as a method for improving malignant bony pain.
A patient is a candidate for this therapy if suffering from localized back pain due to malignancy. It can be locally curative if the ablation takes out the lesion completely, preventing local recurrence. It is also useful in preventing impending fracture or collapse of a vertebral body when ablation is followed by cementoplasty.
Patient evaluation includes complete history and physical examination, with focus on determining the pathology of the lesion and whether the tumor is chemo- or radiosensitive so that the best combination therapy can be planned. Look for any sign of nerve or cord compression. Stability of the spine or possibility of cord compression should elicit surgical consultation ( Table 8.4 ).
Patient Evaluation
Treatment Options
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Imaging is imperative. Magnetic resonance imaging (MRI) with contrast is done to evaluate the size and location of the lesion in order to plan placement of the RF catheter. Look for any expansion of the lesion into the surrounding tissue, especially the epidural space and the pedicles, as this can increase the probability of nerve damage from the heat during RFA. If there is a contraindication to MRI, nuclear scintigraphic bone scan with single-photon emission computed tomography (SPECT) is the next best option. CT of the spine is useful in evaluating the integrity of the vertebrae.
It is important to screen for coagulopathy, systemic infection, and relevant metabolic abnormalities. Additional tests should be ordered when clinically warranted. All anticoagulant therapies should be held, as the procedure is considered high risk for bleeding.
Indications
RFA is considered under the following circumstances:
- 1.
Poorly controlled lesion-related pain that is nonresponsive to opioid therapy.
- 2.
The tumor is radioresistant and expanding.
- 3.
The tumor is radiosensitive but dose limits have been reached.
- 4.
The tumor is radiosensitive but pain is debilitating and urgent intervention is needed.
- 5.
When chemotherapy cannot be interrupted even to have radiation therapy or surgery.
- 6.
Patients in whom invasive surgical procedures are not warranted given short life expectancy and/or comorbidities.
- 7.
There is a risk of myelosuppression from radiation or chemotherapy.
- 8.
As a part of combination therapy (surgery + RF or radiation + RF, or chemo + RF).
Contraindications
- •
Patient refusal.
- •
Metastases associated with pathologic compression fracture with spinal instability.
- •
Metastases causing spinal cord compression.
- •
Highly vascular tumors (renal tumor, melanoma).
- •
Sclerotic lesions (prostate tumor, postradiation sclerosis).
- •
Patient-reported or documented history of allergy to bone cement.
- •
Bleeding disorder or current/recent use of anticoagulation that cannot be corrected.
- •
Any metal in the bone that can result in heat damage.
- •
Medically unstable, active spinal infection.
Consent
Informed consent is obtained. The benefits of procedure are effective and prompt pain control, spine stability, and potential to locally cure the tumor.
The risks come from the needle, RF therapy, and from cement injection. The risks from the needle and cement are covered separately in this book. There is a risk of unintended heat damage from RFA to the surrounding tissue, especially nerve roots and the spinal cord, which is irreversible. The risk is very low but real. Various strategies have been used to eliminate this risk, which will be discussed later. The tumor may recur after the ablation therapy.
Procedural technique
Setup
The procedure is done in a sterile fashion. It can be done with light sedation; however, depending on patient health and extent of discomfort from positioning, deeper sedation or general anesthesia may be needed.
Position
The patient is placed prone on the operating room table. If the patient is unable to tolerate prone positioning, oblique prone positioning can be considered. It is important to ensure that the intended vertebral level is clearly visible with fluoroscopic imaging. It is especially a concern in the thoracic area, where the patient’s arm can obscure the lateral view. If possible, the arm should be positioned above the head.
Antiseptic precautions
Antibiotic prophylaxis should be administered to patients using either cefazolin 1 to 2 g or clindamycin 600 mg if the patient has a penicillin allergy. The skin is cleaned with a sterile antiseptic solution. Most commonly, a combination of 2% chlorhexidine gluconate in 70% isopropyl alcohol is used, which is effective for both rapid and persistent reduction of bacterial load for a broad spectrum of organisms. Prep the skin and drape the back widely. The washed skin is covered with an antimicrobial surgical-incise-adhesive drape. The drapes have antimicrobial activity. It slowly releases iodophor onto the skin in order to provide continuous, broad-spectrum antimicrobial activity throughout the surgical procedure.
Procedure steps
- 1.
Position fluoroscopy C-arms in both anteroposterior (AP) and lateral positions. Identify the appropriate vertebral levels. Optimize the AP and lateral views of the vertebra. In the AP view, the pedicles should be clearly visualized and the end plates should be aligned with the spinous process equidistant from each pedicle. In the lateral view ( Fig. 8.1 ), the two pedicles should be superimposed, the end plates should appear as one line, and the posterior vertebral body wall should be crisp.
Fig. 8.1
Lateral view of spine.
- 2.
RFA is performed in conjunction with cementoplasty. The three main components of the procedure are as follows.
- a.
Accessing the vertebral body with one or two trocars/cannula (10–13 F)
- b.
Placing one or two RFA probes via the cannula.
- c.
Cement injection with or without kyphoplasty.
- a.
- 3.
The components (a) and (b) part of the procedure have been covered in Chapter 4 and will be reviewed only briefly here.
- 4.
Selecting the skin entry point is extremely important. The two overarching principles in planning the cannula trajectory are as follows.
- a.
Safely enter the vertebral body via the pedicle without violating the spinal canal.
- b.
The trajectory of the cannula should optimize placement of the RF probe to maximize size of the lesion.
- a.
- 5.
Two RFA systems are commercially available, OsteoCool (Medtronic, Minneapolis, MN) and SpineSTAR (Merit Medical, South Jordan, UT). The RF probe placement is different for each system.
Osteocool
- 1.
After confirmation of the appropriate level, create a small skin incision using a #15 blade.
- 2.
Insert the bone trocar(s) using a mallet. Advance into the portion of the vertebral body necessary for access to the lesion with a transpedicular or parapedicular approach and guidance with fluoroscopy. A transpedicular trajectory passes through the entire length of the pedicle into the vertebral body. This approach shields adjacent neural and vascular structures but can limit the ability to achieve a midline needle tip position. Figs. 8.2 and 8.3 demonstrate the transpedicular approach.
In the parapedicular trajectory, the lateral wall of the pedicle is penetrated, which often makes midline needle tip positioning easier to attain.
Once the trocar is positioned in the vertebral body, a bone biopsy can be done if needed for diagnostic purposes.
