Image-guided ablation eases bone tumor pain

Tumor ablation is defined as the direct application of chemical or thermal therapies to achieve substantial tumor destruction. Modalities such as ethanol ablation, radiofrequency ablation, laser ablation, and cryoablation have been used widely, primarily for the management of hepatic neoplasms.1

Image-guided ablation of bone tumors represents an expanding field in interventional and musculoskeletal radiology.2 Advantages over conventional therapies include limited invasiveness, selective destruction of tumors, decreased morbidity and complication rates, and cost-effectiveness. Procedures are performed through needles and applicators that have external diameters no bigger than a few millimeters. This minimally invasive approach reduces hospital stays and recovery times. Patients with surgical contraindications can be considered as well. The decreased need for anesthesia means that cancer patients who have a short life expectancy can undergo palliative procedures under conscious sedation.

Procedures are most commonly performed under CT guidance. MRI is used less frequently, though it has superior contrast resolution and can monitor thermal modifications under treatment.3 Several factors restrict the more general use of MR for image guidance: lack of access to equipment, interference between RF systems and MRI coils, and lack of MR-compatible instruments. Fluoroscopy and ultrasound may be combined with CT scanning but are seldom sufficient alone.

Use of imaging allows guidance of instruments to the lesion site more easily and precisely than in conventional surgery. This benefit is particularly evident when dealing with deep-seated lesions or small tumors that are difficult to locate without imaging control or even with simple fluoroscopy.

Ethanol, laser, RF, and cryoablation have all been employed for the destruction of bone tumors. These procedures were originally developed for management of cancer-related pain, principally at Strasbourg University Hospital in France and at the Mayo Clinic in Rochester, Minnesota, in the U.S.4,5 These methods are now becoming more widely accepted, and the list of indications and techniques is growing.

EASING PAIN

With palliative applications, the treatment will improve symptoms but not eradicate the tumor entirely. Bone metastases represent a frequent source of pain in cancer patients and can significantly impair terminally ill patients' quality of life. Suggested mechanisms for the cause of pain include tumoral distention of the periosteum, weakening of bone and subsequent fractures, and tumoral involvement of nervous structures.6 In addition to specific treatments for the neoplastic disease (i.e., chemotherapy, hormonal therapy), the management of bone metastases relies mainly on external-beam radiation therapy, bisphosphonates, analgesics, and sometimes surgery.

Radiation therapy provides variable and often incomplete pain relief and is insufficient in up to 30% of patients. Efficacy is often delayed and transitory, and this approach offers no possibility of retreating previously irradiated bone.7 Major analgesics do not always provide sufficient pain relief. Side effects such as nausea, constipation, and obtundation can also limit the use of analgesics, requiring practitioners to seek a therapeutic adjunct.

Ethanol ablation was the first such technique to be described, though it has subsequently been superseded by others. The simple technique consists of injecting absolute alcohol through small-gauge needles positioned within the tumor to cause tissue necrosis (Figure 1). Ethanol diffusion is difficult to control, however, and cannot be predicted accurately from prior contrast injection. This difficulty raises the likelihood of incorrectly targeted or incomplete ablation and precludes the use of ethanol ablation for lesions close to critical neurovascular structures.8

RF ablation has been used to manage a wide range of tumoral lesions, most often in the liver, kidney, and lungs. Delivery of an alternating current to tissues causes ionic agitation and results in tissue heating. The temperature required to cause tumor cell death ranges from 60 degrees C to 100 degrees C. This is reached at the active tip of the electrode, producing a well-delineated spherical or ovoid-shaped necrosis. The necrotic area is less than 15 mm in diameter for a simple electrode but can be modulated by different electrode configurations. Cooled-tip, perfused, multitined, and multipolar electrodes have been designed to increase the ablated diameter.9,10 The bipolar technique is of particular interest because it restricts coagulation to the tissue area between two electrodes, so increasing safety. This allows ablation of lesions adjacent to critical structures, such as spinal tumors (Figure 2).11,12

RF ablation appears to be particularly effective in treating painful bone metastases, providing significant pain relief for most patients within a week.13 Because dramatic pain relief can be obtained without complete tumor ablation, treatment of large metastases focuses on the periosteal interface.5 Complete tumoral ablation should, however, be attempted for differentiated thyroid cancer metastases, because this will allow a reduction in therapeutic doses of radioactive iodine.14

Ablation with RF energy is painful and requires conscious sedation or general anesthesia. Postoperative pain related to tumor necrosis is common. This pain usually lasts for 48 to 72 hours, but it can be reduced considerably by nonsteroidal anti-inflammatory drugs. Patients experience dramatic pain relief within a few days, and complications are uncommon. Metastases involving weight-bearing bones (e.g., spine, acetabular roof) are best treated with concomitant cementoplasty to maximize pain relief and prevent bone collapse (Figure 3).15,16

Cryoablation is another technique used to manage bone metastases. Circulation of a cryogenic gas (argon) within a cryoprobe allows an iceball to form at the probe's active extremity. Alternate cycles for freezing and thawing are then used to cause thermal damage to the tumor, including cellular dehydration, protein denaturation, and microcirculatory failure. A notable advantage of cryoablation is its ability to visualize precisely the necrosis corresponding to the iceball with unenhanced CT.17

CURATIVE ABLATION

Curative applications require that the tumor be destroyed completely.

Surgery is typically used to manage primary bone tumors. Oncological standards require obtaining a histological diagnosis before resection, extending resection margins depending on tumor stage, and obtaining histological confirmation of resected specimens. Image-guided ablation techniques are usually prohibited in such cases.

Some benign primary bone tumors that are difficult to treat surgically, however, represent the exception to this rule. These may be managed successfully with image-guided ablation.

RF and laser ablation are now recommended as the standard therapy for treating osteoid osteoma. This benign osteoid-forming tumor typically manifests with pain. Traditional surgery is difficult because of the tumor's limited size (typically less than 10 mm) and the frequent peripheral hyperostosis. Resection may be incomplete, or excessive bone may be removed, leaving patients needing osteosynthesis and bone grafts. Percutaneous trephine resection under CT guidance is effective but results in bone weakening with risk of secondary fractures.18

Targeted osteoid osteoma ablation is performed easily under CT guidance. RF and laser ablations are both extremely effective in producing thermal coagulation of the tumor. RF ablation involves placing an electrode within the tumor and heating at 90 degrees C for four to six minutes. Laser ablation requires insertion of an optical fiber connected to an infrared laser generator into the tumor through an 18-gauge spinal needle. Lesion photocoagulation is carried out in a continuous mode at low power (2 W) for five to 10 minutes (Figure 4).

Bone removal in both cases is limited to the needle track, which never exceeds the caliber of a 14-gauge perforation needle. Weight-bearing activities need not be avoided postoperatively, and most patients are able to resume their normal activities within five days. Immediate postoperative pain is common and can necessitate perfusion of nonsteroidal anti-inflammatory medication. Overnight hospitalization is consequently recommended.

Results indicate that the success rate is over 90%, and minor complications occur only in exceptional cases (e.g., skin burns with RF ablation). Recurrences due to incomplete treatment can be managed during a repeat ablation session.19,20

The only drawback of these techniques is their destructive nature. Tumor biopsy is not always feasible and conclusive because the material is frequently insufficient for histological analysis due to minimal bone removal. Only cases with an unambiguous diagnosis should be managed with such techniques. The unique clinical and imaging features of osteoid osteoma should allow practitioners to diagnose such tumors preoperatively with confidence.

Isolated reports have described other small benign tumors, such as chondroblastoma, being managed with RF ablation.21 Evidence to support the routine use of ablative treatment in these cases is still lacking. Ablation nonetheless represents an interesting alternative when conventional surgery is considered too damaging; for example, when tumors occur in deep articular and spinal locations. It is important to obtain an adequate biopsy of the tumor in these exceptional cases and to follow up with patients on a regular basis.

Image-guided ablation techniques are not intended to cure malignant bone tumors. Cases of recurrent sacrococcygeal chordoma have, however, been successfully managed with RF ablation.22

In conclusion, ablation techniques-principally RF and laser ablation at present-have become available to radiologists who are mastering different modalities for image guidance. Cryoablation will probably become more widespread in the near future. Valid indications for bone tumor ablation are the management of painful metastases and osteoid osteoma treatment. The use of ablation to treat other benign tumors that are difficult to manage surgically can be discussed on a case-by-case basis.

DR. MOSER is a clinical fellow in interventional radiology, DR. BUY is a consultant in interventional radiology, and PROF. GANGI is a professor of interventional radiology, all at the University Hospital of Strasbourg in France.

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