Author: Jason Ellsworth, Mark Mishra, Jason K Molitoris, Lei Ren, Dario B. Rodrigues, Paul Turner, Graeme Woodworth 👨🔬
Affiliation: University of Maryland School of Medicine, Department of Neurosurgery, University of Maryland School of Medicine, University of Maryland, Department of Radiation Oncology, University of Maryland School of Medicine, Pyrexar Medical 🌍
Purpose: Hyperthermia therapy (HT) involves increasing tumor temperatures to 40-44°C and is a potent radio- and chemosensitizer for treating solid tumors. Current clinical strategies to heat deep-seated targets consist of noninvasive phased-array applicators, but none of these devices have the appropriate frequency, geometry, and number of sources to focus at depth in brain. This study presents a novel annular phased-array applicator designed to target brain tumors using focused microwave energy for HT delivery.
Methods: The brain applicator was designed at 915MHz with 72 dipole antennas enclosed in a cylindrical frame: diameter=26cm and length=13cm. Numerical simulations were implemented using a multiphysics software that couples electromagnetic and thermal physics to simulate SAR (W/kg) and temperature patterns. Heating patterns were simulated in an anatomical head model with spherical, 2-5cm diameter tumor targets and variable blood perfusion. Experimental validation included MR thermometry measurements and targeting accuracy on a cylindrical head phantom with 14.2cm diameter and 25.1cm length, which included a gel mixture that mimics brain electric properties.
Results: Simulations predicted a therapeutic focus (≥40°C) of 3×2×2 cm3 with an ellipsoid shape that can be centered anywhere within the skull. Heating of realistic tumor models to a minimum of 40°C was demonstrated for practical tumor perfusion cases, while sparing healthy tissue. MR thermal imaging confirmed reliable and predictable focus steering in the experimental head phantom, while showing a good correlation with temperature simulations within a 10% margin of error.
Conclusion: The feasibility of heating small targets in brain was demonstrated with experiments and numerical simulations. By providing a novel HT brain applicator, this form of adjuvant HT offers the potential to provide non-invasive HT for a spectrum of brain lesions, including tumors, epileptic foci, and radiation necrosis.