A treatment planning platform for interstitial microwave hyperthermia was developed for practical, free-hand clinical implants. Such implants, consisting of non-parallel, moderately curved antennas with varying insertion depths, are used in HDR brachytherapy for treating locally advanced cancer.

Numerical models for commercially available MA251 antennas (915 MHz, BSD Medical) were developed in COMSOL Multiphysics, a finite element analysis software package. To expedite treatment planning, electric fields, power deposition and temperature rises were computed for a single straight antenna in 2D axisymmetric geometry. A precomputed library of electric field and temperature solutions was created for a range of insertion depths (5-12 cm) and blood perfusion rates (0.5-5 kg/m³/s). 3D models of multiple antennas and benchtop phantoms experiments using temperature-sensitive liquid crystal paper to monitor heating by curved antennas were performed for comparative evaluation of the treatment planning platform.

A patient-customizable hyperthermia treatment planning software package was developed in MATLAB with capabilities to interface with a commercial radiation therapy planning platform (Oncentra, Nucleotron), import patient and multicatheter implant geometries, calculate insertion depths, and perform hyperthermia planning with antennas operating in asynchronous or synchronous mode. During asynchronous operation, the net power deposition and temperature rises were approximated as a superposition sum of the respective quantities for one single antenna. During synchronous excitation, a superposition of complex electrical fields was performed with appropriate phasing to compute power deposition. Electric fields and temperatures from the pre-computed single-antenna library were utilized following appropriate non-rigid coordinate transformations. Comparison to 3D models indicated that superposition of electric fields around parallel antennas is valid when they are at least 15 mm apart. Phantom experiments with curved antennas produced temperature profiles quite similar to those created using the planning system.

The hyperthermia planning software allowed users to select power and phasing, assess the corresponding 3D contours of energy and temperature, and optimize treatment parameters through gradient search techniques. The system produces fairly accurate temperature distributions in cases when the antennas are at least 15 mm apart.

Date Published: 12 March 2015
PDF: 9 pages
Proc. SPIE 9326, Energy-based Treatment of Tissue and Assessment VIII, 93260X (12 March 2015); doi: 10.1117/12.2079701

Published in SPIE Proceedings Vol. 9326:
Energy-based Treatment of Tissue and Assessment VIII
Thomas P. Ryan, Editor(s)