BY Jack W. Jennings, MD, PhD
Assistant Professor in Radiology
Director of Musculoskeletal and Spine Intervention
Mallinckrodt Institute of Radiology
Washington University in St. Louis School of Medicine
Over the last decade, percutaneous image-guided thermal ablation has emerged into an effective treatment option in the management of benign and malignant bone tumors. Increasingly, interventional radiologists are asked to provide minimally invasive therapies for the palliation of bone lesions at the same time biopsy is performed. More and more biopsies are being requested by oncologists for tissue that will be sent for genomics and molecular profiling.
The ability to perform both diagnostic and therapeutic image-guided procedures in one encounter, often using the same access needle, is convenient for the patient and cost-effective.
Osseous metastatic disease is very common in patients with breast, prostate, and lung cancers, with nearly 85 percent having bone metastases at the time of death. Complications from skeletal metastasis include severe, intractable pain and pathologic fractures, which can lead to decreased mobility with resultant reduction in performance status and quality of life.
Palliation of painful bone metastases is problematic and often exhausts traditional local and systemic therapies, which include opioid and non-steroidal analgesics, radiotherapy, surgery, and chemotherapy. While radiation therapy remains the gold standard, many meta-analyses have shown that one month after radiation less than 30 percent of patients experience complete pain relief and there is a recurrence of pain in nearly 60 percent at a median of 15 weeks following radiation therapy.
With the advancement of minimally invasive, thermal ablative techniques, the interventional radiologist has become an integral component of the patient’s multidisciplinary team of medical, orthopedic, and surgical oncologists and can offer a palliative, focal treatment alternative to conventional therapies.
Radiofrequency (RF) ablation is a form of high-temperature thermal ablation that results from transforming radiofrequency energy into heat using an RF generator (60 to 250 W) to deliver a high-frequency (375-480 kHz) alternating current through an active exposed tip of the probe, creating a voltage between the probe and one or more grounding pads placed on the patient’s skin. The alternating current agitates ions in the cells, leading to frictional heating. At tissue temperatures of 60 to 100°C, irreversible cellular damage occurs.
Osteoid osteoma was the first bone tumor treated with RF ablation, which is now the standard of care for osteoid osteoma due to higher rates of technical success, decreased morbidity, and lower costs than those obtained with surgery. In addition to its role in treating benign bone tumors, image-guided RF ablation is effective for palliation of painful bone metastases in patients who have recurrent symptoms or inadequate pain relief after radiotherapy, which is the longstanding standard of care.
A recent multicenter trial of 55 patients demonstrated the efficacy and safety of RF ablation as well as its benefits to those patients who had not responded to conventional treatment, had a contraindication to initial or repeat radiation, or had limited disease. The treatment goal is to ablate the bone-soft tissue tumor interface which is thought to be the source of pain, and not necessarily the entire tumor, especially in large tumors that are over 5 centimeters in size.
The most recent innovation in RF ablation is the development of targeted, bipolar RF ablation systems for the treatment of malignant spinal lesions. Temperature monitoring by the use of two thermocouples on the articulating electrode can determine ablation size and morphology. The articulating electrode and navigational ability allow for easy access to posterior vertebral body lesions, which was previously difficult to access with other ablation devices.
Cryoablation, a form of low-temperature thermal ablation, has a long history in the treatment of primary and metastatic neoplasms of the kidney, liver, lung, and prostate. Cryoablation is a relatively new technique in the palliative treatment of bone metastases, with the first prospective trial reported in 2006 demonstrating its safety and efficacy.
Argon gas flowing through a sealed probe creates an ice ball due to rapid expansion of the pressurized gas in the sealed probe tip, resulting in a temperature drop to -100°C within seconds (Joules-Thomson effect). Helium gas is then used to attain active thawing of the ice ball. The size of the ablation depends on the diameter of the cryoprobe, the length of the uninsulated tip, and the time of freezing.
A single freeze-passive thaw-freeze cycle is performed for 10-5-10 minutes, respectively. Unlike that for RF ablation, the ablation zone in cryoablation can be monitored directly with computed tomography (CT) as a low attenuation elliptical region originating from the distal probe shaft. The margin of the visible ice ball represents 0°C ice, and cell death reliably occurs within 3 to 5 mm inside the edge.
The mechanisms of cell death occurring between -20 and -40°C include the formation of intracellular and extracellular ice crystals and the creation of osmotic differences across the cell membranes.
Many advantages of cryoablation have been reported. The ability to use multiple synchronous cryoprobes allows treatment of larger lesions in a single session, thus likely decreasing procedure time and greatly reducing the chances of leaving residual tumor behind, which is possible when doing overlapping ablations with RF ablation on large tumors.
A recent study evaluating the immediate pain response comparing RF ablation and cryoablation found that cryoablation had a greater reduction in analgesic dose and a shorter hospital stay after the procedure as compared with those for patients who had RF ablation.
Cementoplasty refers to the percutaneous placement of methylmethacrylate cement into bone; the term vertebroplasty is used when this is performed in the spine.
Cementoplasty is performed in weight-bearing bones for prophylaxis and treatment of pathologic fractures, most commonly in the supra-acetabular ilium. Stabilization of microfractures with the metastasis is the postulated mechanism of pain relief. Cementoplasty is often performed in conjunction with thermal ablation for stabilization of the ablated lesion.
Real-time monitoring with fluoroscopy or CT fluoroscopy during cement instillation and the use of high-viscosity cement are essential to avoid cement leakage, particularly within the hip joint on supra-acetabular lesions.
Image-guided percutaneous ablation and cement augmentation have proven to be useful in the treatment of osseous metastatic disease for pain palliation and pathologic fracture stabilization. Thermal ablation can be repeated at the same site with recurrent metastasis or with new bone metastases without adverse consequences, and does not prohibit the use of concurrent chemotherapy, radiation, or other adjuvant therapies.
The low morbidity of this procedure makes it attractive for the sick patient and those for whom radiation therapy has been exhausted and surgical treatment is not an option. More recently, our institution and others have shown that metastatic lesions can be treated definitively with this localized therapy and that local control can be achieved in patients with oligometastatic disease with no clinical or imaging evidence of residual or recurrent disease.