Local hyperthermia of N2/N3 cervical lymph node metastases: Correlation of technical/thermal parameters and response

Purpose: Patients with advanced head and neck carcinomas, primarily nonresectable as well as recurrent cases, were treated in multimodality regimens with radiotherapy, chemotherapy, and local hyperthermia. Commercially available microwave and radiowave applicators were used in 50 patients with N2/N3 cervical lymph node metastases during more than 250 heat treatments. To assess technical suitability, the achieved power densities and thermal parameters were tested for correlation with anatomical and geometrical factors. To assess effectiveness, the response was compared with derived parameters of the achieved temperature distributions. Methods and Materials: The temperature measurement points (in thermometry catheters) documented by computerized tomography are labeled according to tissue depth, shielding by osseous structures, and location in relation to the external applicators. Relative and absolute specific absorption rates (SAR) and cooling coefficients are extracted from the temperature-time curves. Time-averaged temperature-position curves are evaluated to obtain index temperatures (T-90, T-50, T-20), minimum/maximum tumor temperatures, cumulative minutes T-90 greater than or equal to 43 degrees C, and 43 degrees C-equivalent min T-90. Radiation dose, treatment time, and chemotherapy regiment are also considered. A response parameter is defined using the pre- and posttherapeutic tumor volumes. A multivariate variance analysis is performed for the dependent variables power density, thermal parameters, and response. Results: A significant correlation exists between power density and absorption, presence of a fat layer, and applicator illumination. The maximum depth is 5 cm, where SAR of greater than or equal to 10 mW/g are registered. Achieved temperatures at individual measurement points are dependent on the SAR, and to a lesser extent, the perfusion-dependent cooling coefficients, but the index temperature T-90 is only significantly related to intratumorally achieved SAR. The thermal gradient (T-20-T-50) and temperature peak (T-20) are significantly influenced by the tumor volume. The response is directly related to the index temperature T-90, equivalent minute T-90 43 degrees C, and cumulative minutes T-90 greater than or equal to 40.5 degrees C, and inversely related to the tumor volume. Conclusions: Local hyperthermia using microwave and radiowave applicators in the head and neck region is a tolerable and clinically practical supplementary therapy used as part of multimodal regimens, and has already been proven to be effective. However, the analyses also demonstrated the limits of currently available technology, and confirm the need for continued methodical research.