Kanakavel Kandasamya, Dr. James Jebaseelan Samuelb
a – Research Scholar; b – Research Guide
Department of Physics, School of Advanced Science, Vellore Institute of Technology, Vellore, Tamilnadu, India.
Modern cancer radiotherapy treatments use a high-energy X-ray beam without a flattening filter (FF) in a linear accelerator as a flattening filter-free (FFF) beam to increase the therapeutic ratio, thereby increasing tumor control probability and decreasing normal tissue complication probability. The removal of the flattening filter (FF) results in significant changes in the beam’s characteristics, making it difficult to measure. Profile analysis using the conventional method is unsuitable for FFF beams because the intensity changes dramatically along the beam’s lateral axis. So, measuring the dosimetric parameters of the FFF beam with high-dose rate beams is very challenging and requires extreme care in choosing the right detector for the right measurement. We want to find out if the different ion chambers and silicon detectors work well with the FFF beam by looking at their linearity, sensitivity, reproducibility, energy dependence, dose rate dependence, percentage depth dose (PDD), profiles, and output factors. This included research on the feasibility of using different two-dimensional array systems for profile measurement, as well as their impact on results. In addition to the above, we also did some novelty work with a new flat ion chamber for surface dose measurement for both 6FF and 6FFF beams. The findings reveal that the detector’s type and sensitive volume, measurement resolution during PDDs and profiles, and analysis software all have an impact on the measured field size, penumbra, output factor, and degree of unflatness. A larger volume results in a higher volume averaging effect, which in turn leads to a higher penumbra. Similarly, the higher the measurement resolution, the more accurate the profile analysis was possible. The flat chamber needed to have an overresponse correction for the buildup region of the given beam, especially for determining the surface dose. Overall, a measuring chamber must have a minimal volume and minimal perturbation effect for routine beam measurements. 2D arrays must have optimal detector resolution for routine profile measurements.
Keywords: FFF Beam, Detector volume, Measurement resolution, Flat chamber, 2D array, Volume averaging effect.