121P CLINICAL EVALUATION OF A NOVEL 4D IN-VIVO DOSIMETRY SYSTEM FOR LUNG CANCER RADIOTHERAPY

Abstract

Background: A novel 4D dosimetry system called RADPOS has recently been developed and is in use at The Ottawa Hospital Cancer Centre as part of a clinical trial to determine its potential for use in external beam treatment for lung cancer patients. The RADPOS combines a MOSFET dosimeter with an electromagnetic positioning sensor to allow for simultaneous measurement of dose and spatial coordinates at a specific location. The probe is small (1.3mm diameter) and it can be used either on patient skin or inside natural body cavities. The RADPOS system can be used on patients as a real time treatment verification tool providing immediate feedback about patient motion and the accuracy of the delivered dose. Preliminary tests have shown that the RADPOS has dosimetric characteristics similar to microMOSFET detectors, when used alone. The system can measure displacement with an accuracy of (0.75±0.07)mm for total displacements up to 200mm. The stability of the RADPOS position readout is within 0.21±0.07mm and is not affected by the linac beam. Aluminum, brass, steel and lead were the only materials of those tested that caused significant interference with this accuracy. Methods: The clinical trial involves lung cancer patients treated with external beam radiotherapy. Four RADPOS detectors are positioned at marked points on the patients’ abdomen during an initial 4DCT and subsequent treatment fractions. Position coordinates of the sensors are read in real time at a rate of 20 25Hz and total dose is read at the end of each fraction. Amplitude and period of the breathing motion are analyzed and compared to determine deviation from initial breathing patterns measured during the 4DCT. Results: Results show that the RADPOS system can be set-up quickly, requiring minimal additional time for each scheduled treatment fraction. The RADPOS measured breathing pattern agrees very well with the simultaneous measurement done with the Philips Bellows System and Varian RPM systems. When used to measure simulated breathing motion on a 4D Quasar phantom, the RADPOS-measured amplitude was within 0.2mm of measurements made by the Philips Bellows System. Conclusions: In conclusion, the RADPOS provides sufficient information to identify changes in the patients breathing pattern and other patient motion.