Study of scattered radiation for in-air calibration by a multiple-distance method using ionization chambers and an HDR 192Ir brachytherapy source.

The aim of this study is to estimate the room-scatter correction when measuring air kerma rate of an HDR 192Ir brachytherapy source by in-air calibration. The variation in scattered radiation due to the specially designed jig and from the room walls was also studied. Two therapy ion chambers of volume 0.1 cm3 and 0.6 cm3 were used in the present study. Air kerma was measured by placing the source at several distances between 10 cm and 20 cm from the chamber. The scatter radiation was determined by superimposing the theoretically derived model curve of known scatter (based on the inverse square law) over the plot of measured air kerma strength values. The scatter radiation was estimated in terms of percentage of the primary radiation at 10 cm measurement distance. The scatter estimated by the 0.6 cm3 chamber at two positions was 0.33% and 0.59%, respectively. Similarly the scatter estimated at two other positions by the 0.1 cm3 chamber was 0.58% and 1.11%. This variation in scatter with position as well as with the chamber was due to the varying scatter contribution from components of the measurement set-up. The scatter radiation becomes constant at a distance greater than 100 cm from the walls of the room. We conclude that a fixed chamber with changing source positions should be used in multiple-distance measurement of air kerma rate when using a measurement jig.