title: The focus-to-detector distance as a source of systematical errors in the measurement of Chaoul therapy units.
abstract: The skin exposure rates measured on 22 Chaoul units in two consecutive years were compared and their variance was analysed. The statistical fluctuation of the ionization method was 3.1% by a factor of about 2 to 2.5 smaller than the variations due to lack of reproducibility of the Chaoul units. The authors observed systematical errors among exposure rate measurement performed at different focus-to-detector distances. The effective source-to-detector distance is different for various ionization chambers. It is the sum of nominal focus-to-detector distance plus a geometrical constant. The geometrical constant is for a particular chamber only to a small extent dependent on the front wall thickness and on the focus-to detector distance. Sufficient standardization of both calibration procedure and construction of ionization chambers may help in avoiding this effect.
mesh_terms: D000704:Analysis of Variance; D006801:Humans; D007477:Ions; D011829:Radiation Dosage; D011878:Radiotherapy / Q000295:instrumentation*; D012867:Skin / Q000528:radiation effects
chemical_list: D007477:Ions