Verification of absorbed doses determined with thimble and parallel-plate ionization chambers in clinical electron...

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 ND (or Ngas) was derived from a 60Co 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 kattkm 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 pf 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 pf was assumed unity. Our results show a small but significant energy dependence of pf for the PTW/Markus chamber. The absorbed dose values, determined ionometrically with the different chambers considered in the study using the experimentally determined kattkm and pf 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 mG. The performed comparative study confirms also that for the NACP chamber pf is unity independent of the electron energy down to a mean energy at depth of 2 MeV.