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 cham ber were compared to values obtained with ferrous sulphate dosimetry i n a number of electron beams. For the ionometry with the parallel-plat e chambers the dose-to-air chamber factor N-D (or N-gas) was derived f rom a Co-60 beam calibration free in air with an additional buildup la yer of 0.54 g cm(-2) graphite as proposed by the protocol for electron dosimetry published by the Netherlands Commission on Radiation Dosime try. 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 fac tor 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 sm all but significant energy dependence of p(f) for the PTW/Markus chamb er. The absorbed dose values, determined ionometrically with the diffe rent chambers considered in the study using the experimentally determi ned k(att)k(m) and p(f) values, are systematically 0.5% higher than th ose obtained with ferrous sulphate dosimetry adopting 352 x 10(-6) m(- 2) kg(-1) Gy(-1) for epsilon(m)G. The performed comparative study conf irms also that for the NACP chamber p(f) is unity independent of the e lectron energy down to a mean energy at depth of 2 MeV.