Determination of percentage depth-dose curves for electron beams using different types of detectors

According to the new AAPM TG-51 dosimetry protocol, reference dosimetry for electron beams is performed at depth of dref = 0.6R50-0.1 (cm) instead of dmax recommended in TG-21. In clinical practice most electron beams are normalized at dmax. Therefore it becomes more important to get an accurate percentage-depth-dose (%dd) curve particularly for higher-energy electron beams in which the depth dref is away from dmax. When ionization chambers are used in determining %dd curves the water-to-air stopping-power ratios and the fluence correction factors are required. The TG-51 recommends that the stopping-power ratios for realistic electron beams be used instead of the monoenergetic stopping-power ratios used in TG-21. This investigation aims to study the effects of those correction factors on the determination of %dd curves. We observed 1% deviations in the value of %dd at dref for 15 and 18 MeV beams between a plane-parallel NACP and a cylindrical IC-10 chamber without considering the fluence correction factors Pfl. We explored a method to derive the fluence correction factors at any depth by using the existing fluence correction data at dmax and tested its feasibility. We compared %dd curves measured by a diode detector and a NACP chamber with stopping-power ratios recommended by TG-51 and those recommended by TG-21. We found that for 15 and 18 MeV beams the difference in the values of %dd at dref between using those two different stopping-power ratios is about 0.5%.