An experimental study of the dose distribution in water around137Cs tubes used in brachytherapy

The dose distribution in water around type CDC.G 137Cs tubes (The Radiochemical Centre, Amersham) was measured by means of a silicon surface barrier p-n junction detector (for relative values) and a small ionization chamber (for absolute values). The cylindrical semiconductor detector, of 3 mm outer diameter, had high sensitivity, good spatial resolution, and satisfactory energy dependence for measurements of a relative nature. Thirty caesium tubes were investigated; all showed dose distributions symmetrical about the longitudinal axis of the source but asymmetrical about the lateral axis, the latter effect varying in detail from source to source. The distribution for a “typical” caesium source was obtained by averaging the data of ten sources with different distribution patterns. The absolute dose rates and distributions of 137Cs and radium tubes of similar size, equal activity, and identical screenage (0·5 mm Ir–Pt) were compared. By matching the dose rates on the lateral axis (the caesium dose rates are actually about 2·8 per cent higher because of a different water/air ratio) the isodose curves were shown to be similar in shape, although some differences amounting to a few per cent in terms of dose rate were observed on the longitudinal axis and in the paraxial region. The clinical radium tubes have a screenage of 1 mm Ir–Pt and this decreases the dose rate at all depths on the lateral axis by 9·7 per cent relative to the clinical caesium tubes, with corresponding changes in the distribution as a whole. The uncertainties involved in the measurements have been assessed and the overall most probable error on the lateral axis found to be 3·7 per cent.