The position of the opposite flat applicator changes the SAR and thermal distributions of the RF capacitive intracavitary hyperthermia

The variations of the specific absorption rate (SAR) and thermal distribution in the JSHO QA phantoms were investigated by radiofrequency (RF) capacitive intracavitary hyperthermia (ICHT) applicator (AP-T01, Omron Electric Co., Kyoto, Japan) with the changing position of the opposite flat applicator (15-cm in diameter). The thermal distribution was observed with the thermographic camera and the normalized SAR distribution was calculated with the thermal data that were measured with the thermocouple thermometers. The SAR and thermal distributions of AP-T01 significantly varied with the position of the opposite flat applicator. The slope of the normalized SAR became gradual towards the side of the flat applicator. During the operating of a high flow rate (1500ml/min) cooling system, the region between AP-T01 and the flat applicator was widely and rather homogeneously heated, except the hot spot around the end of AP-T01. This hot spot may be due to the imbalance of cooling of AP-T01 and the warming-up of the electrode. These results suggest that the RF capacitive ICHT using AP-T01 may be clinically effective on the deep-seated tumours in the direction of either the end wall, such as cervical cancers, or the upper wall, such as prostatic cancers and the mediastinal metastatic lymph nodes, if an improvement of the cooling system is achieved.