Image-guided Therapy of Skeletal Metastases
Image-guided Therapy of Skeletal Metastases
Percutaneous ablation for the palliation of painful bone metastasis has developed into an effective, minimally invasive alternative to the more established practices of EBRT and systemic therapy.
Radiofrequency (RF) ablation is the most widely adopted method of percutaneous ablation for the treatment of soft-tissue tumors. The first application of its use in bone was described by Rosenthal et al in the treatment of an osteoid osteoma. RF ablation is performed by delivering RF energy with one or more needle electrodes placed directly into the treatment site under sonographic or CT guidance (Fig 6). High-frequency alternating current passes into the tissue, causing frictional heat that extends further into the surrounding tissue as conductive heat, inducing cell death through coagulation necrosis.
(Enlarge Image)
Figure 6.
Percutaneous radiofrequency ablation of an osteolytic bone metastasis for pain palliation. Axial CT scan of the pelvis demonstrates tines from a radiofrequency needle probe (arrow) extending into the lytic bone tumor.
Several clinical trials have demonstrated the effectiveness of RF ablation for palliation of pain from bone metastases. In a recent multicenter American College of Radiology Imaging Network trial of 55 patients treated with RF ablation to palliate pain from bone metastases, a decrease in pain severity was seen at 1 month and 3 months following treatment. Based on a 100-point pain scale, the average decrease in pain intensity was 26.9 at 1 month and 14.2 at 3 months. Patients also had a statistically significant improvement in mood. Grade 3 toxicity occurred in 3 patients (5%). In another multicenter trial of 43 patients with painful osteolytic bone metastases, the mean score for worst pain prior to treatment was 7.9 on a 10-point pain scale. This decreased to 4.5, 3.0, and 1.4 at 4, 12, and 24 weeks, respectively, following RF ablation. Opioid analgesic usage decreased significantly at 8 and 12 weeks after treatment. Adverse events were seen in 3 patients.
Percutaneous cryoablation is performed in a fashion similar to that of RF ablation. Insulated cryoprobes of 1.2 to 2.4 mm in diameter are utilized. These applicators use high-pressure argon gas that expands within the distal noninsulated segment of the cryoprobe, causing rapid cooling (the Joule-Thomson effect) to temperatures as low as –100° C (Fig 7). Following the freeze cycle, helium is introduced, which generates heat with expansion within the cryoprobe and allows thawing of the ice ball for probe removal. Cryoablation has certain advantages over RF ablation. The ablation margin is readily visible on CT imaging during treatment, due to the low attenuation of the ice ball, allowing for more precise targeting of the treatment field. Patients experience reduced pain during treatment and immediately thereafter. Compared with RF energy, deeper penetration of ice into bone is seen, which may allow more effective treatment of osteoblastic bone lesions. Interim results from single-center prospective clinical trials have been encouraging. In their study involving 14 patients, Callstrom et al reported a decrease in the mean score for worst pain in a 24-hour period, from 6.7 before treatment to 3.8 (P = .003) at 4 weeks after treatment, based on a 10-point pain scale. All 8 patients who were taking narcotic medications prior to treatment reported reduced medication use after cryoablation. Pain control appeared durable, with 4 of 5 patients in the 24-week follow-up interview reporting excellent pain relief. No serious complications were reported.
(Enlarge Image)
Figure 7.
A cryoprobe being tested with sterile water prior to the cryoablation procedure. A small ice ball has formed around the noninsulated tip of the cryoprobe following introduction of argon gas through the probe while it is submersed in water.
Microwave ablation utilizes a thin antenna (14.5 gauge) placed, under imaging guidance, in the treatment site. A microwave generator emits an electromagnetic wave through the noninsulated portion of the antenna, causing agitation of water molecules in the surrounding tissue. The agitation produces heat, resulting in cell death via coagulation necrosis. Microwaves travel through all types of tissue, including those with high impedance, such as lung or bone. Microwaves also have less susceptibility to the heat sink effect in adjacent larger vessels, thereby reducing possible distortion of the ablation zone. The heat sink effect is more problematic with other thermal ablative techniques, leading to regions of incomplete ablation. The benefits of microwave ablation also include higher intratumoral temperature, larger and more uniform ablation volume, and faster ablation time. The use of microwave ablation for treating patients with bone metastases has been limited, but early results are promising. Other ablative techniques are available, such as laser ablation and a new, nonthermal, ablative modality known as irreversible electroporation (IRE), which uses high-voltage direct current to irreversibly open nanopores of cell membranes to induce cell death. However, little data are available, especially for IRE, regarding treatment of patients with bone metastases. Clinical trials are necessary to determine the efficacy of these procedures for the palliation of pain from metastatic bone lesions.
All of the percutaneous ablative techniques described here have some limitations or disadvantages. Although minimally invasive, they require skilled interventionalists to place needle probes or applicators into the accessible target. Probe placement into osteoblastic metastases with preserved cortical bone may be difficult. The proximity of critical normal structures, including nerves, major blood vessels, bowel, and bladder, must be considered to prevent injury, and this influences patient selection. Treatment also requires regional anesthesia, moderate sedation, or general anesthesia for adequate intraprocedural pain control.
Percutaneous Ablation
Percutaneous ablation for the palliation of painful bone metastasis has developed into an effective, minimally invasive alternative to the more established practices of EBRT and systemic therapy.
Radiofrequency Ablation
Radiofrequency (RF) ablation is the most widely adopted method of percutaneous ablation for the treatment of soft-tissue tumors. The first application of its use in bone was described by Rosenthal et al in the treatment of an osteoid osteoma. RF ablation is performed by delivering RF energy with one or more needle electrodes placed directly into the treatment site under sonographic or CT guidance (Fig 6). High-frequency alternating current passes into the tissue, causing frictional heat that extends further into the surrounding tissue as conductive heat, inducing cell death through coagulation necrosis.
(Enlarge Image)
Figure 6.
Percutaneous radiofrequency ablation of an osteolytic bone metastasis for pain palliation. Axial CT scan of the pelvis demonstrates tines from a radiofrequency needle probe (arrow) extending into the lytic bone tumor.
Several clinical trials have demonstrated the effectiveness of RF ablation for palliation of pain from bone metastases. In a recent multicenter American College of Radiology Imaging Network trial of 55 patients treated with RF ablation to palliate pain from bone metastases, a decrease in pain severity was seen at 1 month and 3 months following treatment. Based on a 100-point pain scale, the average decrease in pain intensity was 26.9 at 1 month and 14.2 at 3 months. Patients also had a statistically significant improvement in mood. Grade 3 toxicity occurred in 3 patients (5%). In another multicenter trial of 43 patients with painful osteolytic bone metastases, the mean score for worst pain prior to treatment was 7.9 on a 10-point pain scale. This decreased to 4.5, 3.0, and 1.4 at 4, 12, and 24 weeks, respectively, following RF ablation. Opioid analgesic usage decreased significantly at 8 and 12 weeks after treatment. Adverse events were seen in 3 patients.
Cryoablation
Percutaneous cryoablation is performed in a fashion similar to that of RF ablation. Insulated cryoprobes of 1.2 to 2.4 mm in diameter are utilized. These applicators use high-pressure argon gas that expands within the distal noninsulated segment of the cryoprobe, causing rapid cooling (the Joule-Thomson effect) to temperatures as low as –100° C (Fig 7). Following the freeze cycle, helium is introduced, which generates heat with expansion within the cryoprobe and allows thawing of the ice ball for probe removal. Cryoablation has certain advantages over RF ablation. The ablation margin is readily visible on CT imaging during treatment, due to the low attenuation of the ice ball, allowing for more precise targeting of the treatment field. Patients experience reduced pain during treatment and immediately thereafter. Compared with RF energy, deeper penetration of ice into bone is seen, which may allow more effective treatment of osteoblastic bone lesions. Interim results from single-center prospective clinical trials have been encouraging. In their study involving 14 patients, Callstrom et al reported a decrease in the mean score for worst pain in a 24-hour period, from 6.7 before treatment to 3.8 (P = .003) at 4 weeks after treatment, based on a 10-point pain scale. All 8 patients who were taking narcotic medications prior to treatment reported reduced medication use after cryoablation. Pain control appeared durable, with 4 of 5 patients in the 24-week follow-up interview reporting excellent pain relief. No serious complications were reported.
(Enlarge Image)
Figure 7.
A cryoprobe being tested with sterile water prior to the cryoablation procedure. A small ice ball has formed around the noninsulated tip of the cryoprobe following introduction of argon gas through the probe while it is submersed in water.
Microwave Ablation
Microwave ablation utilizes a thin antenna (14.5 gauge) placed, under imaging guidance, in the treatment site. A microwave generator emits an electromagnetic wave through the noninsulated portion of the antenna, causing agitation of water molecules in the surrounding tissue. The agitation produces heat, resulting in cell death via coagulation necrosis. Microwaves travel through all types of tissue, including those with high impedance, such as lung or bone. Microwaves also have less susceptibility to the heat sink effect in adjacent larger vessels, thereby reducing possible distortion of the ablation zone. The heat sink effect is more problematic with other thermal ablative techniques, leading to regions of incomplete ablation. The benefits of microwave ablation also include higher intratumoral temperature, larger and more uniform ablation volume, and faster ablation time. The use of microwave ablation for treating patients with bone metastases has been limited, but early results are promising. Other ablative techniques are available, such as laser ablation and a new, nonthermal, ablative modality known as irreversible electroporation (IRE), which uses high-voltage direct current to irreversibly open nanopores of cell membranes to induce cell death. However, little data are available, especially for IRE, regarding treatment of patients with bone metastases. Clinical trials are necessary to determine the efficacy of these procedures for the palliation of pain from metastatic bone lesions.
Summary
All of the percutaneous ablative techniques described here have some limitations or disadvantages. Although minimally invasive, they require skilled interventionalists to place needle probes or applicators into the accessible target. Probe placement into osteoblastic metastases with preserved cortical bone may be difficult. The proximity of critical normal structures, including nerves, major blood vessels, bowel, and bladder, must be considered to prevent injury, and this influences patient selection. Treatment also requires regional anesthesia, moderate sedation, or general anesthesia for adequate intraprocedural pain control.
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