Magnetic Resonance-Guided High Intensity Focused Ultrasound for Palliation of Painful Skeletal Metastases

Bone metastases are common among patients with advanced cancer and have been reported in up to 85% of cancer patients at autopsy. Complications of bone metastases include pain, functional limitation, decreased quality of life, pathological fracture, spinal cord compression and cauda equina syndrome.

External beam radiotherapy is the current standard treatment for patients with painful uncomplicated bone metastases. Unfortunately up to 30 % of patients treated with radiotherapy do not respond to therapy, and 30% of responders have their pain recur at some point after treatment. Radiotherapy re-treatment is also limited by cumulative doses delivered to sensitive structures. Ablative techniques such as cryotherapy and percutaneous radiofrequency ablation are not similarly limited by cumulative effects, however, they are invasive procedures that place patients at risk of complications.

MR-guided high intensity focused ultrasound (MR-HIFU) is a non-invasive, outpatient modality being investigated for the treatment of cancer. In MR-HIFU, a specially designed transducer is used to focus a beam of ultrasound energy into a small volume at a specific target site in the body. The focused beam produces therapeutic hyperthermia in the target field but only harmlessly warms the immediately surrounding tissue. Magnetic resonance (MR) imaging is used both to focus the ultrasound beam on the target field in the bone (the metastatic lesion and adjacent periosteum containing the nerves and vasculature for the tumor) and to perform real-time thermal mapping at and around the target. The mechanism of action of pain response is thought to be thermal periosteal denervation and / or thermal ablation of the tumor mass that diminishes pressure on the surrounding tissue.