Calculation of the dose distribution around 137Cs tubes used in clinical brachytherapy.

The dose distribution in water around a CDC.J 137Cs tube is calculated with a computer. In the transverse direction the calculated dose rates showed to be almost unsusceptible for the value of the platinum absorption coefficient. In the longitudinal direction, however, the value of the linear absorption coefficient plays a more prominent role. An effective absorption coefficient for the filter material was determined by measurement, using a photographic film. This was done by relating the filmdensity along the source axis with the filmdensity along the transverse axis, where the dose rate is well known. In this way an effective linear absorption coefficient for platinum was obtained which gives a good agreement between the experimental and theoretical results. For use in a computer program for clinical use, the dose distribution around a cesium tube has been calculated in a specific spherical coordinate system using 101 radii.