Electron contamination of the photon beam of the 60Co teletherapy unit 'ROKUS'

The dose along the central axis in a tissue-equivalent phantom at various depths - from 2.3 mg/cm2 (the thickness of the chamber wall) to 570 mg/cm2 - has been measured with an ionization chamber (volume 0.03 cm3) connected to a Farmer dosemeter type 2,570. The ratio of the dose at 7 mg/cm2 depth (that of the epidermal basal layer) to the maximum dose has been calculated. Its dependence on the source to surface distance, on the field size, on the presence of additional beam shaping devices and the use of a copper filter as an electron absorber have been investigated. Under the conditions put into practice the maximum value of this ratio does not exceed 0.65 and thus demonstrating that the main advantage of the telegammatherapy - the 'skin sparing' effect - is available in case of the radiotherapy methods used. The results show that the electron contamination of the Rokus teletherapy unit is relatively low compared with that reported in the literature for other types of such units. (author)