Dosimetric applicator characterization in hyperthermia: an automated quality assurance procedure based on relative...

Ensuring reproducible treatments and therapeutic temperature rises is pivotal for wider clinical application of hyperthermia. We propose a fast automated quality assurance (QA) procedure using E‑field measurements for relative specific absorption rate (SAR) applicator characterization, with a similar workflow to radiotherapy dosimetry.

Our procedure was demonstrated for 434 MHz superficial contact flexible microstrip applicators (CFMA) with similar antennas and geometry to the ALBA ON 4000D system. Applicators were placed on a liquid saline phantom. A Cartesian robot performed E‑field measurements moving E‑field sensors through the phantom. We analysed the influence of spatial resolution on effective field size (EFS) measurements. Using volume measurements, we established effective penetration depth (EPD) variation across the aperture. We evaluated repeatability, measuring central planes on eight different days. Applicators/phantoms were characterized with flat and curved setups, using standard clinical and more excessive bolus thicknesses.

Procedures take ~40 min per setup condition. One-centimetre spatial resolution appears sufficient for QA. EPD showed errors < 5% when determined from high-SAR regions (~80-90% of the maximum). The EFS and EPD variability between different days was < 5%. Increasing EPD and decreasing EFS with increasing curvature was observed when using the clinical bolus thickness, with more homogeneous SAR distributions for curved than for flat setups. Excessive bolus thickness resulted in irregular SAR distributions and larger EFS and EPD variations.

The proposed QA procedure is characterized by fast, practical and reproducible measurements that are suitable for efficiently evaluating various setup conditions. This flexible workflow can also be used with other radiative hyperthermia systems.