Sci—Wed PM: Delivery—02: Characterization and Clinical Evaluation of RADPOS 4D <i>in vivo</i> Dosimetry System

A novel 4D in vivo dosimetry system, RADPOS, has recently been developed. The system combines a MOSFET dosimeter with an electromagnetic positioning sensor and is currently used in a clinical trial to determine its potential for application in external beam treatments. The probe is small (1.3 mm diameter) and can be used either on patient skin or inside natural body cavities 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 the last one is used alone. The system can measure displacements with an accuracy ranging from (1.1–1.3) mm, within a volume defined by the coordinates: , , and , related to the transmitter. The stability of the RADPOS position readout is within 0.21 ± 0.07 mm and is not affected by the linac beam. The clinical trial involves lung cancer patients treated with external beam radiotherapy. Four RADPOS detectors are positioned at marked points on the patients' abdomen and chest during an initial 4DCT and subsequent treatment fractions. Position coordinates of the sensors are read in real time and compared to determine deviation from initial breathing patterns measured during the 4DCT. Total dose is read at the end of each fraction. Initial clinical results show that the RADPOS system can measure the amplitude and period of the patients breathing pattern and can provide a means to identify corresponding changes in dose.