Abstract
The purpose of this work was to investigate the influence of a new transmission detector on 6 MV x-ray beam properties. The device, COMPASS (IBA Dosimetry, Germany), contains 1600 plane parallel ionization chambers with a detector spacing of 6.5 mm and an active volume of 0.02 cm3. Butson et al (1996 Australas. Phys. Eng. Sci. Med 19 74–82) studied the causes for the surface dose and dose in the build-up region and concluded that the surface dose at 6 MV is mostly due to electron contamination. We used PTW Markus parallel plane chamber for measurements at the surface and in the build-up region and corrected the over-response using the Mellenberg method (Mellenberg 1990 Med. Phys. 17 1041–4) and we found that for moderately narrow beam geometric conditions, the increase in surface dose was small. For the largest field size investigated (20 × 20 cm2) at 90 cm SSD, the surface dose with the detector was 34.9% versus 26.8% in the open field. It was found that beyond dmax, the difference in relative dose (profiles and PDDs) between open and COMPASS fields was insignificant. In summary, the transmission detector was found to increase the relative dose in the buildup region, but had a negligible effect on the beam parameters beyond dmax.
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