A new device for the measurement of the absorbed dose to water for low energy x-ray sources used in brachytherapy

Abstract

The Physikalisch-Technische Bundesanstalt (PTB) is developing a primary standard to determine the absorbed dose to water within a water phantom for X-ray brachytherapy sources such as I-125 and Pd-103. A candidate for the standard to be newly established is a water-equivalent parallel-plate ionization chamber with continuously adjustable electrode spacing, forming an extrapolation chamber in a phantom of water-equivalent material.

A newly contructed exemplar of this chamber was put into operation. The chamber design is presented and the method of evaluation is summarized briefly. Results obtained with this chamber for ISO reference x-ray radiation qualities N20, N25, N30 were compared to those obtained by a free-air chamber, when a conversion factor to the absorbed dose to water is applied.

A good agreement within 2 % of the values for the absorbed dose to water obtained by the two methods could be stated, which demonstrate that the operability of the chamber is now given.