Delivery and verification of intensity‐modulated x‐ray beams in radiotherapy

Abstract

This thesis deals with the implementation of intensity‐modulated radiation therapy into the clinic and a method of quality assurance which can be used on each intensity‐modulated beam prior to treatment. The first component of the thesis was to write a step‐and‐shoot leaf sequence algorithm to control the multileaf collimator (MLC) fitted to our Clinac 2300 C/D linear accelerator. Our algorithm takes into account the MLC transmission, MLC penumbra, and change in scatter conditions with field size and is also slightly more efficient than other published step‐and‐shoot type algorithms. The second (and major) component of the thesis was to investigate the use of a portal imaging device for dosimetric verification purposes. We show that an electronic portal imaging device of the scanning liquid ionization chamber type yields images which, once calibrated from a previously determined calibration curve, provide highly precise planar maps of the incident dose rate. For verification of an intensity‐modulated x‐ray beam delivered in the segmented approach with a MLC, a portal image is acquired for each subfield of the leaf sequence. Subsequent to their calibration, the images are multiplied by their respective associated monitor unit settings, and summed to produce a planar dose distribution at the measurement depth in phantom. The excellent agreement of our portal imager measurements with calculations of our treatment planning system and data from a one‐dimensional beam profiler attests to the usefulness of this method for the planar verification of intensity‐modulated fields produced in the segmented approach on a computerized linear accelerator equipped with a multileaf collimator.