Percutaneous Interventional Techniques for Treatment of Spinal Metastases

Abstract

This article details an approach for evaluation as well as minimally invasive percutaneous treatment of spinal metastases focusing on thermal ablation and most recent advances. Safe and effective management of certain subgroups of patients with spinal metastases can be achieved by minimally invasive percutaneous thermal ablation with or without vertebral augmentation. Adjunctive palliative treatment options such as epidural or neuroforaminal corticosteroid and long-acting anesthetic injections may also be performed in patients who have nerve and radicular pain including those who are not candidates for thermal ablation. Thermal protection strategies should be implemented to minimize the risk of neural thermal injury.

The vertebral column is commonly affected by osseous metastases and approximately 40% of patients with metastatic cancer will develop tumor involvement of the spine. ^{1 2} In up to 50% of these patients, spinal osseous metastases will result in intractable pain due to direct tumor involvement of bone, pathologic fracture, and/or compression of nerve roots or spinal cord. ^{3 4} Pain and neurologic compromise with or without spinal instability often negatively influence patients' functional independence and quality of life. ⁵

Over the past two decades, there have been substantial advances in minimally invasive percutaneous management of vertebral metastases including thermal ablation with or without vertebral augmentation that may be performed in conjunction with (or supplemented by) adjuvant radiation therapy, chemotherapy, or surgery. ^{6 7 8 9 10 11 12 13 14} This review details an approach for evaluation as well as minimally invasive percutaneous treatment of spinal metastases focusing on thermal ablation and most recent advances.

General Principles

A multidisciplinary approach is recommended for the evaluation and treatment of patients with vertebral metastases, typically including radiation oncologists, medical oncologists, oncologic spine surgeons, and interventional radiologists to take into consideration and implement the most recent advances in each medical discipline for improved patient care. ¹⁵

While at present external beam radiation therapy is considered the reference standard for local control and pain palliation of spinal metastases, one should consider its limitations particularly efficacy in providing timely and adequate pain palliation. Incomplete pain response to radiation therapy has been reported in up to 75% of patients along with typically a 10- to 20-week delay to achieve maximum pain reduction following completion of radiation treatment, which is substantial considering diminished life expectancy of many patients with spinal metastases. ^{16 17 18} Furthermore, radiation-resistant tumors and cumulative tolerance of the spinal cord limiting further treatment of tumor progression or recurrence lead to additional limitations. ¹⁹

The morbidity associated with vertebral surgical interventions coupled with patient's frequent poor functional status limits efficacy of these procedures which are mainly considered for patients with neurologic compromise or spinal instability. Although World Health Organization's analgesic ladder recommends opioids for the management of cancer-related bone pain, ²⁰ this approach remains suboptimal due to common side effects of such medications, particularly constipation, nausea, and dependence, as well as incomplete alleviation of pain. Also, given the current opiate crisis which includes the cancer population, nonopiate palliative alternatives and interventions are becoming more relevant and widely adopted.

Recent advances and evolution of percutaneous minimally invasive thermal ablation technologies with or without vertebral augmentation offer attractive options for certain subgroups of patients with spinal metastases (such as suboptimal response or contraindications to radiation therapy) with several distinct advantages including timely and durable pain palliation, excellent local tumor control rates, reinforcement of the treated vertebra, as well as short- and long-term improvement in functional status without compromise in adjuvant radiation or chemotherapy. ^{6 7 8 9 10 11 12 13 14}

Treatment Goals

Percutaneous thermal ablation has been established as part of the treatment paradigm for the management of vertebral metastases. ^{6 7 8 9 10 11 12 13 14 15 16} The goals of treatment are typically pain palliation and/or local tumor control (often with vertebral augmentation for pathologic fracture stabilization or prevention) for majority of the patients. In addition, osseous oligometastases (less than five lesions) may be approached for definitive cure.

Thermal Ablation Options

Radiofrequency Ablation

Radiofrequency ablation (RFA) systems operate by delivery of high-frequency, alternating current to target tissue through the exposed active tip of an electrode to generate frictional heat and ultimately achieve coagulation necrosis (temperature between 60 and 100°C). ²¹

There have been substantial advances in RF electrode technology including recent introduction of navigational bipolar RFA electrodes which have several distinct advantages compared with traditional unipolar straight electrodes which are particularly beneficial for the treatment of vertebral metastases. ^{6 8 12 13} These advantages include (1) bipolar electrode design obviating the need for grounding pad placement and eliminating the risk of skin thermal injury; (2) real-time accurate intraprocedural monitoring of ablation zone size made possible by built-in thermocouples along the electrode shaft; and (3) navigation of the electrode tip which can be articulated in different orientations through one skin/bone entry site, allowing treatment of challenging-to-access tumors particularly along the posterior vertebral body, and achieving larger ablation zones via a transpedicular approach. ^{6 8 12 13} Furthermore, tissue char (carbonization) is minimized by the use of internally cooled electrodes. ²¹ In addition, intact vertebral cortex minimizes undesired RF energy propagation. ¹³

Simultaneous bipedicular vertebral RFA is a novel technique that efficiently generates two confluent, coalescent, and overlapping ablation zones in close proximity decreasing convective cooling effect (heat sink), the risk of thermal injury, as well as charring and impedance-related issues ¹³ ( Fig. 1 ). More thorough ablation may be achieved by this approach, in alignment with the stereotactic spine radiosurgery paradigm to treat the entire vertebral body volume and pedicles for improved local tumor control rates and more durable pain palliation. ^{19 22}

Fig. 1.

A 56-year-old woman with metastatic breast cancer and painful L1 lesion. Sagittal T1-weighted MRI image ( a ) shows L1 vertebral body bone marrow replacing lesion with pathologic compression fracture ( a , arrow). Anteroposterior fluoroscopic image during simultaneous bipedicular radiofrequency ablation ( b ) shows inferomedial articulation of electrode tips which are 5–10 mm apart (width of the spinous process as landmark). Radiofrequency ablation was immediately followed by vertebral augmentation (not shown). Sagittal ( c ) and axial ( d ) T1-weighted fat-saturated contrast-enhanced MR images obtained 3 months following treatment show local tumor control with no evidence of tumor recurrence. Note the enhancing granulation tissue along the posterior vertebral body ( c , arrow). Note the hypointense cement within vertebral body ( c and d ).

RFA is mainly utilized for the treatment of vertebral metastases with the following characteristics: (1) primarily osteolytic tumors as the higher impedance of osteoblastic lesions renders RFA ineffective ²³ ; (2) vertebral lesions with no or small extraosseous components; and (3) tumors along the posterior vertebral body particularly centrally where access is feasible using navigational articulating electrodes.

Limitations of RFA include CT occult ablation zone, relative contraindication for use of monopolar systems in patients with metallic implants and pacemakers due to risk of skin thermal injury and pacemaker malfunction, hit-sink effect due to cerebrospinal fluid and vertebral venous plexus flow, procedure-related pain, and, frequently, increased pain during the immediate postablation period. It is important to note that the hit-sink effect along the posterior vertebral body and pedicles also decreases the possibility of neural thermal injury. ¹³

With the recent consensus recommendations by the international spine radiosurgery consortium for definition of clinical target volume (CTV) versus gross tumor volume (GTV) to account for microscopic tumor spread and marginal radiation therapy failures, there has been a paradigm shift for the treatment of spinal metastases. ²² The consensus recommendation defines CTV (to be treated by stereotactic spine radiosurgery) to include GTV plus surrounding abnormal bone marrow signal intensity on magnetic resonance imaging (MRI) to account for microscopic tumor invasion and adjacent normal osseous expansion to account for subclinical tumor spread in marrow. ²² Equipped with these recommendations and ablating the CTV, investigators have reported successful treatment of 33 vertebral metastases in 27 patients using simultaneous bipedicular RFA combined with vertebral augmentation with a local tumor control rate of 96% with no complications and no clinical evidence of metastatic spinal cord compression at the treated levels (mean follow-up, 24.2 weeks). ¹³

Cryoablation

In cryoablation, tumor cell death is achieved by cycles of rapid freezing and gradual thawing. Using the Joule–Thomson effect, liquid argon is commonly utilized to rapidly drop the temperature at the cryoprobe tip, with the cooling effect then exchanged with the surrounding tissues resulting in a progressively enlarging ice ball. ^{7 21} The initial freezing cycle is immediately followed by a thawing phase and typically a second freezing cycle. ^{7 21} A temperature of −40°C or lower must be achieved to guarantee reliable cell death. ²⁴ Cryoablation is typically used for the treatment of spinal metastases with the following features: (1) vertebral tumors with large soft-tissue components; (2) large tumors involving the posterior vertebral elements; (3) paravertebral soft-tissue lesions ( Fig. 2 ); and (4) osteoblastic lesions.

Fig. 2.

A 74-year-old man with metastatic lung adenocarcinoma and painful T8–T10 paravertebral metastatic lesion. Axial T1-weighted fat-saturated contrast-enhanced MR image ( a ) shows infiltrative enhancing paravertebral soft-tissue metastatic lesion ( a , arrow). Due to large paravertebral soft-tissue nature of the metastasis, cryoablation was chosen as the ablation modality. Coronal unenhanced CT image during cryoablation ( b ) shows placement of three cryoprobes within the metastatic lesion with hypoattenuating ice ball encompassing the neoplastic tissue ( b , arrows). Low radiation dose CT technique results in slight compromise in visualization of the ice ball. Axial contrast-enhanced CT image obtained 10 months following cryoablation ( c ) shows nonenhancing ablated paravertebral soft tissue compatible with local tumor control ( c , arrow) with no evidence of tumor recurrence.

Cryoablation has several distinct advantages including visualization of hypoattenuating ice ball on CT, simultaneous use of several cryoprobes to achieve additive overlapping ablation zones, less intraprocedural and immediate postprocedural pain compared with heating ablation techniques, and availability of MRI-compatible cryoprobes. Disadvantages of cryoablation include often lack of distinct visualization of ice ball within osteoblastic tumors (and at times normal bone), extended procedure length in larger tumors, cost associated with use of multiple cryoprobes, and delay in cement augmentation to minimize interference with cement polymerization. It should be noted that intact cortex does not prevent expansion of ice ball which should be taken into consideration when cryoablating vertebral tumors. ^{7 21} In a single-center retrospective study, investigators reported successful cryoablation of 31 vertebral metastases in 14 patients and achieved 96.7% local tumor control and statistically significant pain palliation with no major complications. ⁷

Microwave Ablation

In microwave ablation, electromagnetic microwaves result in realignment and motion of ionic molecules, frictional heat, and eventually coagulative necrosis. Microwave ablation is less susceptible to hit-sink effect and variable tissue impedance, conceivably resulting in more uniform and larger ablation zone and more efficient ablation using a single antenna. ^{9 14} The advantages of microwave ablation include efficacy in treatment of osteoblastic lesions, lack of need for grounding pads minimizing risk of skin thermal injury, lack of contraindication in patients with metallic implants, as well as minimal risk of back-heating phenomena in latest generations of microwave antennae. ^{9 14} However, it should be recognized that, similar to cryoablation, intact cortical bone does not serve as a barrier to microwave energy propagation, and rapid deposition of high-power output (up to 100 Watts) may be a disadvantage for the treatment of vertebral lesion due to potential risk of neural thermal injury. Additionally, the microwave ablation zone is largely CT occult with less distinct ablation zone margins compared with RFA and cryoablation, resulting in a disadvantage for management of vertebral lesions. In the largest spinal microwave ablation series to date, Khan et al reported successful treatment of 102 vertebral tumors in 69 patients in a retrospective single-center study and achieved statistically significant pain palliation as well as local tumor control, with two minor complications. ¹⁴

Vertebral Augmentation and Adjunctive Techniques

Following thermal ablation of vertebral metastases, vertebral augmentation is commonly performed to achieve pathologic fracture stabilization or prevention as well as further pain palliation in patients with persistent pain or imaging tumor progression despite maximum radiation therapy, contraindication of radiation therapy, or insufficient response to systemic therapies and opioids ^{6 7 8 11 12 13 15} ( Fig. 1 ). Vertebral augmentation may not be necessary following thermal ablation of posterior vertebral elements only or lower sacral spine segments. In patients with spinal instability (detailed later) and contraindications for surgery, vertebral augmentation may be performed (without ablation) to diminish motion at the pathologic fracture site, prevent further collapse, and improve pain. However, it should be recognized that spinal stability is not completely achieved in such cases.

Adjunctive palliative treatment options such as epidural or neuroforaminal corticosteroid and long-acting anesthetic injections may also be performed in subgroup of patients with vertebral metastases, including patients with osseous central canal and neuroforaminal stenosis due to retropulsion of pathology fracture fragments, patients with contraindications for surgery or when thermal ablation is not a valid option, and immediately following thermal ablation for added pain relief and diminished postablation inflammation ( Fig. 3 ). ^{6 7 8 12 13}

Fig. 3.

A 72-year-old man with metastatic lung adenocarcinoma and painful C5 lesion. Axial T1-weighted fat-saturated contrast-enhanced MR image ( a ) shows C5 vertebral body bone marrow replacing lesion obliterating the right C5–C6 neuroforamen, extending to the epidural space, and encasing the right vertebral artery ( a , arrows). The patient was not a candidate for percutaneous thermal ablation. Pain palliation was achieved by CT-guided cervical epidural injection of steroid and long-acting anesthetic (diluted in contrast agent) at tumor level ( b ).

Patient Selection Guidelines

A multidisciplinary consensus should be achieved among medical oncologists, radiation oncologist, interventional radiologists, and oncologic spine surgeons in order for a patient to undergo percutaneous minimally invasive therapy. This approach ensures that each patient benefits from the latest advances in each medical discipline while having a clear treatment plan.

After a consensus is reached, a pretreatment consultation should be arranged with the patient to discuss the procedure details and perform a focused physical examination to reconfirm tumor origin of pain as well as a neurologic examination to identify potential neurologic symptoms and deficits.

The main factors determining a patient's eligibility to undergo percutaneous thermal ablation for the management of vertebral metastases include pain, performance status, life expectancy, status of spinal stability, the presence of metastatic epidural spinal cord compression (MESCC), and extent of visceral metastases. ^{13 15 25 26} Validated and widely accepted Karnofsky Performance Status Scale is utilized in clinical practice to evaluate patient performance status. ²⁷ Percutaneous thermal ablation is relatively contraindicated in patients with spinal instability, considering severity. Spinal instability is evaluated based on the spinal instability neoplastic score ²⁸ ( Table 1 ). Scores range from 0 to 18, and higher scores translate into greater instability. While no score cut-off exists to prompt surgical intervention, surgical evaluation for potential tumor resection and/or stabilization is recommended for scores of 7 or higher. ²⁹ Although vertebral metastases complicated by central canal stenosis are typically managed by surgical intervention, ³⁰ thermal ablation may be considered as an alternative option, for patients who are not surgical candidates, in the absence of spinal cord compression. It is important to note that in patients with central canal stenosis caused by tumor alone, thermal ablation may result in retraction or arrest of epidural tumor; however, it will not alleviate symptoms due to osseous retropulsion.

Table 1. Spine Instability Neoplastic Score.

Location
Junctional (occiput–C2, C7–T2, T11–L1, L5–S1	3
Mobile spine (C3–C6, L2–L4)	2
Semirigid (T3–T10)	1
Rigid (S2–S5)	0
Pain relief with recumbency and/or pain with movement/loading of the spine
Yes	3
No (occasional pain but not mechanical)	1
Pain-free lesion	0
Bone lesion
Lytic	2
Mixed (lytic/blastic)	1
Blastic	0
Radiographic spine alignment
Subluxation/translation present	4
De novo deformity (kyphosis, scoliosis)	2
Normal alignment	0
Vertebral body collapse
Greater than 50% collapse	3
Less than 50% collapse	2
No collapse with >50% body involved	1
None of the above	0
Involvement of posterolateral spine elements (facet, pedicle, costovertebral fracture/tumor involvement)
Bilateral	3
Unilateral	1
None of the above	0

Notes: Total score:

Stability: 0–6.

Indeterminate stability: 7–12.

Instability: 13–18.

Source: Adapted from Fisher et al. ²⁸

Percutaneous thermal ablation of vertebral metastases is typically considered for the following patient population (the Metastatic Spine Disease Multidisciplinary Working Group algorithms): (1) asymptomatic spinal metastases in patients with life expectancy more than 6 months, good performance status, and few visceral metastases; (2) uncomplicated (lack of pathologic vertebral compression fracture and MESCC) painful spinal metastases; (3) stable pathologic vertebral compression fracture in patients with life expectancy more than 6 months, good performance status, and few visceral metastases. ¹⁵

The National Comprehensive Cancer Network (NCCN) has incorporated percutaneous thermal ablation for the treatment of osseous metastases in published guidelines. According to the latest NCCN guidelines for adult cancer pain (version 3.2019), thermal ablation may be considered for palliation of metastatic bone pain in the absence of oncologic emergency when chemotherapy is inadequate and radiation therapy is contraindicated or not desired by the patient. ³¹

Recently published American College of Radiology (ACR) Appropriateness Criteria indicate the following for the management of vertebral metastases ³² : (1) vertebral augmentation and thermal ablation may be appropriate for the treatment of asymptomatic pathologic spinal fracture with or without edema on MRI; (2) vertebral augmentation and thermal ablation are usually appropriate for the treatment of pathologic spinal fracture with severe and progressive pain; and (3) vertebral augmentation is usually appropriate and thermal ablation may be appropriate for the treatment of pathologic spinal fracture with spinal malalignment. The appropriateness category of “may be appropriate” is defined as follows: “The imaging procedure or treatment may be indicated in the specified clinical scenarios as an alternative to imaging procedures or treatments with a more favorable risk-benefit ratio, or the risk-benefit ratio for patients is equivocal.” And the appropriateness category of “usually appropriate” is defined as follows: “The imaging procedure or treatment is indicated in the specified clinical scenarios at a favorable risk-benefit ratio for patients.” ³²

Thermal Protection

Close proximity of the spinal cord and nerve roots to the ablation zone carries an important risk of neural injury during thermal ablation of spinal metastases. Several techniques may be implemented to minimize the risk of neural injury including active and passive thermal protection. ^{6 7 8 12 13 26} Active thermal protection is typically achieved by thermal insulation including hydrodissection with injection of warm or cool liquid in the epidural space or neuroforamina (ionic solutions should be avoided during RFA to avoid creation of plasma field and undesired energy propagation) as well as pneumodissection with epidural/neuroforaminal carbon dioxide injection. Epidural balloons may also be used for active neural thermal protection when ablating spinal lesions. ³³ Passive thermal protection strategies include patient biofeedback when ablating under conscious sedation, real-time temperature monitoring by placement of thermocouples within the epidural space and/or neuroforamina, motor and somatosensory evoked potential amplitude monitoring, as well as electrostimulation of peripheral nerves for early detection of impending nerve injury. ^{6 7 8 12 13 26} Approaches to minimize skin thermal injury include precise assessment of ablation zone size and geometry, surface application of warm saline during cryoablation, using bipolar RF electrode systems, and utilization of wider and more grounding pads with unipolar RF systems.

Complications

The most important potential complication of thermal ablation of spinal metastases is thermal injury to the spinal cord and nerve roots, most of which are transient and may be managed by transforaminal or epidural injection of steroids and long-acting anesthetics. Bone weakening and risk of ablation-related fracture are typically minimized by vertebral augmentation. Cement leakage into the central canal/epidural space or neuroforamina may result in increased pain due to central canal or neuroforaminal stenosis, spinal cord compression, or potentially spinal cord thermal injury due to exothermic effect of cement polymerization. Skin thermal injury remains a potential risk as well.

Conclusion

Safe and effective management of certain subgroups of patients with spinal metastases can be achieved by minimally invasive percutaneous thermal ablation with or without vertebral augmentation. Adjunctive palliative treatment options such as epidural or neuroforaminal corticosteroid and long-acting anesthetic injections may also be performed in patients who are not candidates for thermal ablation. Thermal protection strategies should be implemented to minimize the risk of neural thermal injury.