Characteristics and performance evaluation of the capacitive Contact Flexible Microstrip Applicator operating at 70 MHz for external hyperthermia

Purpose: To characterise and evaluate the capacitive Contact Flexible Microstrip Applicator operating at 70 MHz, CFMA-70. This applicator is introduced for the treatment of superficial tumours with extension in depth beyond the range of regular superficial applicators. Methods: E-field measurements were performed in an elliptical phantom filled with a saline solution using an E-field vector probe. E-field distributions and SAR patterns are compared to those of the CFMA-434 and of 70 MHz waveguides. The applicator has been used for the treatment of 6 patients with breast cancer with a tumour depth exceeding 4 cm. Results: The effective heating depth of the CFMA-70 is 50% larger than for the CFMA-434. Its effective field size is 26 x 20 cm (aperture 29 x 20 cm), larger than for an equally sized CFMA-434. In contrast to the CFMA-434 the SAR pattern of this applicator is insensitive to the bolus thickness. Comparison to 70 MHz waveguides shows that the E-field component normal to the applicator is 100% larger for the CFMA-70. During clinical applications acceptable temperatures were realised for individual sessions (also at depth), but in many cases treatment limiting hot spots occurred close to superficial bony structures near the applicator edge. Both surface irregularities and the normal field component may be responsible. Conclusions: The CFMA-70 has adequate penetration depth for the treatment of superficial tumours exceeding a depth of 4 cm. However, the relatively large normal E-field component may induce treatment-limiting hot spots at tissue interfaces in the direction normal to the applicator's surface