Ablation and Combination Treatments of Musculoskeletal Lesions

Clinical Relevance

Primary or metastatic bone disease can become very painful, especially in cases of lytic lesions. Possible treatments include surgery, embolization, chemotherapy, radiotherapy, and palliative analgesics.

Hypoxic cells with limited blood flow can be resistant to chemotherapy and external beam radiation therapy. These cells may be more sensitive to ablation because of increased cell sensitivity to heat in hypoxic state and decreased heat dissipation due to poor tumor perfusion.

External beam radiation, although used frequently, provides pain relief in only 70% of patients treated. Tumor destruction with ablation treatments, although fairly recent in its application within the musculoskeletal system, has evolved and seems effective for treatment of painful skeletal lesions. Such lesions can be treated by ablation, providing local tumor control either as a single-modality treatment or as an adjunct to surgical resection or other percutaneous techniques.

Ablation can be curative for small lesions (up to 3–5 cm). In bigger lesions, its main purpose is palliative treatment and to provide local control of the disease via a percutaneous approach. It can also help diminish local spread into muscle and can be used in conjunction with other techniques (vertebroplasty, surgery, radiotherapy), especially when the size of the lesion undermines bone stability.

Ablation can be very effective for lesions spreading in soft tissues, diminishing tumor burden and mass effect on other organs.

Indications

Ablation inside bone, especially when the cortex is intact, can have an oven effect and completely destroy small lesions. The most common benign lesion treated with radiofrequency ablation (RFA) is osteoid osteoma ( Fig. 108.1 ). Aneurysmal bone cysts can also be treated by RFA before filling, thus replacing the curettage technique ( Fig. 108.2 ).

In the treatment of malignant painful lytic lesions spreading into bone and muscle, ablation can help provide local tumor control. When the lytic lesion is in a weight-bearing structure (involving >40% of healthy bone), ablation must be accompanied by bone-supporting techniques (vertebroplasty, osteoplasty, intraosseous stent or surgery) to avoid the risk of bone collapse under the induced osteonecrosis. Ablation can also help reduce the possible mass effect from the soft tissue component of the lesion ( Fig. 108.3 ).

Patients must be able to withstand the percutaneous approach as well as the ablation. When the lesion is deep in the bone structures, irritation of the periosteum is minimal and so is the pain sensation. On the contrary, when the lesion involves muscle structures or affects the periosteum, sedation is mandatory. Usually the treatment can be performed under monitored assisted conscious sedation ( Fig. 108.4 ).

Contraindications

Absolute contraindications to ablation are the same as in any image-guided percutaneous procedure and include coagulopathy disorders, skin infection, immunosuppression, and absence of a safe path to the lesion without harming vital organs or structures.

Relative contraindications depend on regional anatomy and the relationship of the lesion to the skin surface and vital structures. If the lesion is too close to the skin, adequate precautions to avoid skin burn should be observed, especially with monopolar electrodes.

As noted earlier, bone radiofrequency can be very effective when cortical bone is intact, producing an oven effect. When there is a severe lytic lesion of the cortex, electrical impulses can spread out easily, thus affecting surrounding vital structures. Especially in the spine, when the metastatic lesion is affecting the posterior wall, monopolar ablation can affect the nerves in the spinal canal. In these cases, thermal monitoring of the epidural space can help prevent irreversible damage ( Fig. 108.5 ). Injection of mediating solutions can help avoid damage of vital organs by either insulation or displacement of the structure one wishes to avoid.

Equipment

There are multiple systems for RFA. Major categories include monopolar devices, bipolar devices, and coblation devices.

Monopolar systems require a grounding pad placed on the patient to “close” the electric circuit. Their main disadvantage is the formation of aberrant currents, which do not always allow for uniform energy deposition inside the lesion. They can be divided into single electrode and multitined electrodes and are the most frequently used devices. Single electrodes have the advantage of small caliber but have a smaller ablation radius. Different kinds of single monopolar devices exist to amplify treatment size. They can feature hot, cool-tip, and water-perfused electrodes and work independently or in cluster mode. Depending on the type of electrode, the ablation shape changes. Multitined electrodes increase energy deposition by creating larger zones of coagulation and produce better lesion destruction. There are three RFA systems available with expandable needle electrodes. The three systems differ in needle electrodes, generators, and the algorithms used to maximize coagulation volumes.

Bipolar devices do not require a grounding pad, because the current passes through the same or neighboring needles, thus diminishing the risk of aberrant currents. The distance between the electrodes can vary, depending on the type of needle used.

Coblation is a controlled non–heat-driven process. It uses radiofrequency energy to excite the electrolytes in a conductive medium (e.g., saline solution), creating precisely focused plasma. The plasma’s energized particles have sufficient energy to break molecular bonds within tissue, causing it to dissolve at relatively low temperatures (typically 40°C–70°C). The result is volumetric removal of target tissue.

Microwave ablation (MWA) is a relatively recent ablation mode that delivers cytotoxic effects and cellular death by means of coagulation necrosis. Microwave antennae are inserted within the tumor and transmit high-frequency energy that facilitates molecular movement and frictional forces, resulting in temperature elevation and subsequent necrosis. With MWA, a larger ablation zone is created within a shorter period of time. The applied energy extends approximately 2 cm around the antenna and not throughout the whole body as in RFA. The ablation zone is not governed by the heat-sink effect produced by nearby vessels, and in contrast to RFA, no grounding pads are necessary. All the aforementioned factors make MWA the treatment of choice for large tumors in close proximity to a vessel. Technical parameters of the ablation session are determined by the manufacturer of the antenna. In most protocols, 5 to 10 minutes of ablation is performed at 40 to 60 watts. When choosing appropriate therapeutic protocol, one must balance killing all malignant cells against minimizing surrounding tissue damage. A safety margin of 5 to 10 mm is mandatory for MWA. In selected cases, multiple antennae can be applied simultaneously, depositing energy in a synergistic fashion, which will result in even larger ablation zones.

Cryoablation is the application of extreme cold to destroy tumor cells by means of both direct cellular and vascular injury. In contrast to other ablation types, up to 25 probes can be used within the same lesion (depending on lesion size). In cases of multiple placements, probes should be more or less parallel and about 2 cm apart. During the cryoablation session, two 10-minute cycles of freezing and one 8-minute intervening cycle of thawing are performed per position. As in all cases of ablation, a safety margin of roughly 5 mm is necessary to ensure complete tumor ablation. The size and shape of the iceball is governed by the expansion space at the tip of the probe. In cases where tumor location makes some kind of insulation necessary, fluid must be avoided (air via antimicrobial filter or CO ₂ are more appropriate insulation agents). When the size of the lesion undermines bone stability and a combination of cryoablation and cement augmentation is necessary, one should wait until the tumor temperature rises back to normal levels to avoid cement leakages. In comparison with RFA, cryoablation seems to be less painful. Another advantage is the visibility of the iceball under computed tomography (CT) imaging during freezing, which allows visualizing the ablation margins. The major factors governing a successful cryoablation session include rapid freezing to lethal temperature (around −40°C), slow thawing, and repetition of the same process. As in any image-guided percutaneous procedure, excellent monitoring is a requisite.

High-intensity focused ultrasound (HIFU) is application of locally concentrated acoustic waves (focused by an acoustic lens) that results in focal energy deposition. Tissue absorption of this energy deposition results within seconds in frictional heating, causing focal temperature increase (sonication). Because a single sonication’s size is similar to a rice grain, multiple sonications arranged in such a way as to cover the whole tumor volume are necessary for ablation of a lesion. Bone readily absorbs this kind of acoustic energy, so in osseous lesions, application of lower energy levels results in higher temperature elevation. Because osseous lesions cause pain by means of their periosteal innervation, the heating of bone at temperatures above 60°C will effectively destroy the innervation of the periosteum, resulting in pain reduction. The typical sonication energy for osseous lesions is 1000 to 1500 joules, using a wide-beam approach and requiring shorter session duration (≈60 minutes). Treatment monitoring is performed by real-time ultrasound or magnetic resonance imaging (MRI). Because ultrasound cannot penetrate bone, intraosseous lesions are treated under MRI. MRI-guided HIFU offers increased image quality with additional direct imaging of temperature changes (by means of thermal-sensitive sequences). The technique totally lacks any invasive character (no needles are inserted inside the patient), but anesthesiologic control is required (ranging from conscious sedation to general anesthesia, with the decision varying among different centers). Owing to concerns for heat conduction through the bone to the spinal cord, HIFU is still not applied on vertebral bodies (an exception might include application of the technique on lesions of the spinous process). Similarly, care must be exercised when the target lesion is close to sensitive anatomic structures (e.g., blood vessels, nerves).

Since any kind of ablation apart from tumor necrosis results in bone weakening as well, combined therapies (ablation and cement augmentation) are necessary to achieve necrosis and at the same time support the bone to avoid postablation pathologic fractures (especially in the spine and other weight-bearing areas). To further enhance local tumor control, ablation can be combined with transarterial chemoembolization (especially for hypervascular lesions).

Vertebral body support (VBS) is a recently developed percutaneous technique that in many ways resembles kyphoplasty. A significant difference is that in VBS, before cement injection, an endovertebral prosthesis (composed of polyether ether ketone polymer, Nitinol, or titanium) is implanted inside the vertebral body, aiming at long-lasting height restoration. VBS technique includes the use of trocars through which the implant is inserted inside the vertebral body. Similar to bone augmentation, a liquid polymer, poly(methylmethacrylate), is then injected, creating a complex of implant and cement. The additional step in VBS is deployment of the implant to augment structural support.