FDA Alert on MR-Guided Laser Interstitial Thermal Therapy Devices

Megan Brooks

April 25, 2018

There is a risk of tissue overheating owing to inaccurate magnetic resonance (MR) thermometry with MR-guided laser interstitial thermal therapy (MRgLITT) devices, the US Food and Drug Administration (FDA) said today in a safety communication.

The agency reports that these devices are used in neurosurgical procedures to ablate brain tumors, epileptic foci, or radiation necrosis. During the procedures, the laser probe tip is placed within the tissue to be ablated, and controlled thermal energy is delivered to the target tissue. The clinician monitors temperature changes at the treatment site using MR thermometry.

The FDA said it is currently reviewing data that suggest that potentially inaccurate MR thermometry information can be displayed during treatment.

"For example, MR parameters such as voxel size (measurement of the image resolution or detail) and MR image acquisition time (eg, up to 8 seconds) may contribute to inaccurate MR thermometry readings and potential errors in the ablation assessment," the agency said.

"In addition, MRgLITT devices may not account for the continued thermal spread of energy to the surrounding tissue (as the target ablation area returns to its baseline temperature), which may result in an underestimation of thermal damage," they point out.

The FDA said medical device reports and literature reports describe adverse events such as neurologic deficits (eg, focal motor deficits, aphasia, cognitive changes), increased intracerebral edema or pressure, intracranial bleeding, and/or visual changes (eg, visual field deficits, blurry vision) when the devices were used to treat intracranial lesions.

"Several of these reports note events which required urgent medical and/or surgical intervention, and may have been associated with patient deaths. However, it is unclear at this time whether an inaccuracy of MR thermometry directly caused or contributed to these events," the FDA said.

The agency recommends that providers consider, and discuss with patients, the benefits and risks of these devices, as well as any alternative treatment modalities on a case-by-case basis.

If an MRgLITT device is determined to be the best available treatment option, they recommend that physicians consider heating the target tissue slowly to reduce the potential for inaccurate MR thermometry readings. Additionally, heating the tissue slowly may lessen unanticipated thermal spread.

For example, heating tissue at a rate of 4° Celsius per second in conjunction with an MR acquisition time of 8 seconds may result in uncertainties of up to 32° Celsius during MR thermometry readings. In contrast, heating tissue at a rate of 1° Celsius per second, with an MR acquisition time of 8 seconds, may result in uncertainties of up to 8° Celsius during MR thermometry readings, the FDA notes in a letter to healthcare providers.

While heating the target tissue slowly, physicians should also consider using one of the following settings to mitigate the risk of unintended damage to surrounding tissue or nearby critical structures:

  • Set the low temperature targets on nearby critical structures to 43° Celsius or less.

  • Follow the outer perimeter or isothermal contour line at the cumulative equivalent minutes of 43° Celsius (CEM43) = 2 minutes or less so that no critical structures are within the contour line.

  • Contact the applicable manufacturer for further information regarding recommended heating parameters.

The FDA encourages device manufacturers and users to comply with the medical device reporting regulations and promptly report adverse events related to these devices.

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