High dose-rate induced temperature artifacts: Thermometry considerations for simultaneous interstitial thermoradiotherapy

Purpose : The goal of the present study was to investigate the effect of high dose-rate radiation on a flouroptic thermometry system commonly used during microwave hyperthermia. Methods and Materials : Measurements were performed by placing the flouroptic thermometry sensors at distances of ≤ 1.5, 5, 10, and 15 mm from a remote afterloading high dose-rate 192Ir source in a water bath (at two different temperatures) and in a tissue equivalent radiation bolus medium. A simulated volumetric clinical setup using a radiation bolus medium was performed with thermometry sensors placed at 1.5, 7.5, 8.4, and 10.6 mm from a scanning high dose-rate source. Results : It was found that high dose-rate radiation caused thermometry artifacts greater than 1.5°C within 2 min for flouroptic thermometers placed 1.5 mm from a 5 Ci activity high dose-rate source. Simple calculations showed that artifacts of this magnitude could not be due to any heating caused by the energy deposited by the high dose-rate source. The artifact decayed, but was still evident 24 h after the exposure. The effect strongly depended on distance with a 0.7°C artifactual increase in temperature seen for the probe 5 mm from the high dose-rate source. Moreover, experiments performed under conditions that represented a clinical setup with a 7 Ci high dose-rate source showed that for exposure times of 10 s, at distances of 1.5 mm, significant artifacts (> 0.5°C) are produced. Conclusions : These findings indicate that high dose-rate-induced temperature artifacts should be taken into account in the quality assurance procedures for the treatment of patients with simultaneous interstitial thermoradiotherapy.