Enhanced accuracy of the permanent surveillance of IMRT deliveries by iterative deconvolution of DAVID chamber signal profiles.

In vivo dosimetry systems, capable of permanently monitoring IMRT treatment deliveries throughout all fractions, are increasingly used in clinical practice. The first of these solutions is the DAVID system, a translucent multiwire ionization chamber placed in the accessory holder of the treatment head below the MLC. Each wire is exactly adjusted along the midline of its associated leaf pair, thereby generating a signal correlated with the aperture of this leaf pair. However, there is some blurring of the profile of the wire signals across the beam due to the lateral transport of scattered secondary electrons in the air gap of the DAVID chamber. This paper deals with a numerical correction by which this effect is eliminated. The true photon fluence profile is calculated from the measured signal profile by an iterative deconvolution algorithm, based upon the convolution kernel formed by the lateral wire signal profile when only one leaf pair is opened. Lateral fluence profiles are thereby obtained with increased resolution, and errors in MLC positioning are revealed with enhanced sensitivity. The needed computational time of less than 1 s has made it feasible to implement the deconvolution algorithm into the daily routine for the accurate surveillance of IMRT deliveries.