RF Interstitial Hypertheapy--Geometry Factors Influence SAR Distribution

IntroductionHyperthermia, when applied properly, has been shown to be an effective adjuvant treatment for malignant tumors which are refractory to convention therapy with surgery, radiation or themotherapy. Interstitial hyperthermia (IHT) is often combined with brachytherapy to treatment malignant tumors and classified radiofrequency ( RF) , micro wave ( MW) and hot source ( HS). In the procedure of brachytherapy, the implant needles interval is often 1. 0 ~ 1. 5cm and implementation of IHT. This study will analysis the effect of geometry factors to SAR distribution.Material and MethodsThe apparatus was NRL - II interstitial hyperthermia machine with 1MHz, output power 300W, reflect power 10W. A muscle -e-quivalent agar phantom of 8 x8 x83cm in size was used, which composed of water (95. 66) , sodium chloride (NaCl: 0. 34) and agar powder (4.0). The musle - equivalent agar phantom had permitivity of 81 and a conductivity of 0.62S/m.The internal electrodes were put by different length, different interval and slope. The temperatures were measured before heating andimmediately after switching the power off ( within 60s). the specific absorption rate SAR(WAg) distribution was calculated based on increasing temperatures by subtracting the temperature before heating from that after heating . SAR = 4816C ( AT/At). C=4.2 x 103 J/kg ℃ and heating time was set as At =60s. The values of temperature at every 5mm interval was recorded by computer.ResultsThe same length of eletrode, different interval. The interval 1. Ocm, SAR distribution curve is smooth and even and the maximal value is 160, the minimum value 20. 95. If the interval 1. 5cm, SAR distribution has wave. The maximum value is 174 and the minimum value is 13.95. The SAR distribution of the same interval and different length of electrode is similar.If the electrode be slope, the minimum interval is 0. 5cm and the maximum interval is 1. 5cm. The SAR distribution will deviate the center of the volume and close to the minimum interval.ConclusionThe SAR distribution of 1. Ocm interval is rational and even, which can form a model for clinical uses. Unparallel of electrodes can not be taken.