Verification of absorbed doses determined with thimble and parallel‐plate ionization chambers in clinical electron beams using ferrous sulphate dosimetry

Absorbed dose values determined with the commonly applied NACP and PTW/Markus parallel‐plate chambers and the cylindrical NE2571 Farmer chamber were compared to values obtained with ferrous sulphate dosimetry in a number of electron beams. For the ionometry with the parallel‐plate chambers the dose‐to‐air chamber factor N D (or N gas ) was derived from a 60 Co beam calibration free in air with an additional buildup layer of 0.54 g cm −2 graphite as proposed by the protocol for electron dosimetry published by the Netherlands Commission on Radiation Dosimetry. For the product k att k m in this calibration geometry values of 0.980±0.003 [1 standard deviation (s.d.)] and 0.993±0.004 (1 s.d.) were obtained for the parallel‐plate NACP and PTW/Markus chambers, respectively. The behavior of the fluence perturbation correction factor p f versus the mean electron energy at depth was deduced for the flat PTW/Markus and cylindrical NE2571 chamber by comparison with the NACP chamber, for which p f was assumed unity. Our results show a small but significant energy dependence of p f for the PTW/Markus chamber. The absorbed dose values, determined ionometrically with the different chambers considered in the study using the experimentally determined k att k m and p f values, are systematically 0.5% higher than those obtained with ferrous sulphate dosimetry adopting 352×10 −6 m −2 kg −1 Gy −1 for ε m G . The performed comparative study confirms also that for the NACP chamber p f is unity independent of the electron energy down to a mean energy at depth of 2 MeV.