Verification of Absorbed Doses Determined With Thimble and Parallel-plate Ionization Chambers in Clinical Electron-beams Using Ferrous Sulfate 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 Co-60 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 x 10(-6) m(-2) kg(-1) Gy(-1) for epsilon(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.