Determination of contributions of scatter and distance error to the source strength of ¹⁹²Ir HDR brachytherapy source

High dose rate (HDR) brachytherapy commonly employs a ¹⁹²Ir encapsulated source to deliver high dose to the malignant tissues. Calibrations of brachytherapy sources are performed by the manufacturer using a well-type chamber or by in-air measurement using a cylindrical ionization chamber. Calibration using the latter involves measurements to be carried out at several distances and room scatter can also be determined. The aim of the present study is to estimate the scatter contribution from the walls, floor and various materials in the room in order to determine the reference air kerma rate of an ¹⁹²Ir HDR brachytherapy source by in-air measurements and also to evaluate the error in the setup distance between the source centre and chamber centre. Air kerma measurements were performed at multiple distances from 10 cm to 40 cm between the source and chamber. The room scatter correction factor was determined using the iterative technique. The distance error of −0.094 cm and −0.112 cm was observed for chamber with and without buildup cap respectively. The scatter component ranges from 0.3% to 5.4% for the chamber with buildup cap and 0.3% to 4.6% without buildup cap for distances between 10 to 40 cm respectively. Since the average of the results at multiple distances is considered to obtain the actual air kerma rate of the HDR source, the seven distance method and iterative technique are very effective in determining the scatter contribution and the error in the distance measurements.