Based on phase 3 clinical study findings subsequently reported at this week’s 54th Annual Meeting of the American Society for Radiation Oncology (ASTRO) in Boston, MA, the FDA has announced that InsighTec’s ExAblate magnetic resonance imaging-guided focused ultrasound (MRgFUS) thermoablation system will be the first such device approved in the United States for pain palliation in patients with bone metastasis, who have not benefited from (or who are not candidates for) traditional radiotherapeutic palliation. 

The FDA’s announcement follows similar approvals in Europe and Asia. It expands the set of clinical tools available for patients suffering a difficult-to-treat form of pain, whose hopes until now have been limited largely to palliative radiation therapy and narcotic pain medications.

InsighTec’s combined-modality ExAblate system allows targeted thermal ablation of nerve cells near bone tumors, disrupting the signaling of pain to the brain.

While the hardware and software engineering is quite complex, the underlying principles of MRgFUS are elegantly simple.

“When we think about diagnostic ultrasound, you have an ultrasound transducer that propagates energy outward, and it is reflected back,” said Mark Hurwitz, MD, Director of Thermal Oncology and Vice Chair of the Radiation Oncology Department at Thomas Jefferson University in Philadelphia.

“With focused ultrasound, as opposed to one transducer, you have hundreds of — and sometimes now over 1,000 — tiny transducers focused on a single point or very small area,” he told The Advisor Blog. “All of this energy heats and kills that tissue.”

Acoustic energy causes rapid molecular vibration at targeted tissues, which generates thermal energy, destroying cellular proteins and triggering nerve death.

But unlike traditional high-intensity focused ultrasound (HIFU), ExAblate uses MRI in place of ultrasound imaging to guide the procedure.

“That has a number of important advantages,” Dr. Hurwitz said. “One is, you have higher-quality imaging, so you can visualize the target better than with ultrasound. You also have a complete picture of the beam path – the tissues the energy passes through to get to the target.”

That allows treatment planning to avoid critical nerves, blood vessels, and organs, he said.

“You can also use the MRI to monitor temperatures in target tissues, and to record what we call the ‘thermal dose’,” Dr. Hurwitz said. “Thermal dose imagery allows us to ensure we’re achieving the temperatures desired, that are sufficient to provide the desired effect. We can monitor target temperatures and also temperatures around the target tissue.”

ExAblate uses proton resonance frequency shift (PRF) to measure temperature, which can be sensitive to patient-motion artifacts, others have noted. MR thermometer measures change in temperature rather than absolute temperature; absolute temperatures are calculated using a user-defined background value or a default temperature value of 37⁰C, according to a recent paper in the Journal of Magnetic Resonance Imaging.