Evaluation of scatter contribution and distance error by iterative methods for strength determination of HDR 192Ir brachytherapy source

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.

Estimation of distance error by fuzzy set theory required for strength determination of HDR (192)Ir brachytherapy sources

Verification of the strength of high dose rate (HDR) (192)Ir brachytherapy sources on receipt from the vendor is an important component of institutional quality assurance program. Either reference air-kerma rate (RAKR) or air-kerma strength (AKS) is the recommended quantity to specify the strength of gamma-emitting brachytherapy sources. The use of Farmer-type cylindrical ionization chamber of sensitive volume 0.6 cm(3) is one of the recommended methods for measuring RAKR of HDR (192)Ir brachytherapy sources. While using the cylindrical chamber method, it is required to determine the positioning error of the ionization chamber with respect to the source which is called the distance error. An attempt has been made to apply the fuzzy set theory to estimate the subjective uncertainty associated with the distance error. A simplified approach of applying this fuzzy set theory has been proposed in the quantification of uncertainty associated with the distance error. In order to express the uncertainty in the framework of fuzzy sets, the uncertainty index was estimated and was found to be within 2.5%, which further indicates that the possibility of error in measuring such distance may be of this order. It is observed that the relative distance li estimated by analytical method and fuzzy set theoretic approach are consistent with each other. The crisp values of li estimated using analytical method lie within the bounds computed using fuzzy set theory. This indicates that li values estimated using analytical methods are within 2.5% uncertainty. This value of uncertainty in distance measurement should be incorporated in the uncertainty budget, while estimating the expanded uncertainty in HDR (192)Ir source strength measurement.