Dose-Exposure Conversion Factors for Megavoltage X-ray Dosimetry

Publisher Website
Google Scholar
Abstract
It is supposed that a commercial cavity ionization chamber fitted with its Perspex build-up cap, as required for a 2 Mv X-ray exposure calibration, is centered at a specified depth in a tissue-like phantom for a dose determination. The source of radiation may be an X-ray source in the range 2 Mv to 15 Mv X-rays or a gamma-ray source Cs137 or Co60. However, the scale of the associated measuring instrument is determined by the 2 Mv X-ray exposure calibera-tion. Hence, a reading in units of 2 Mv X-ray "roentgens" is always given, irrespective of the situation of the chamber and the source of radiation used with the phantom. The problem presented is how to convert a scale reading in units of 2 Mv X-ray "roentgens" to a scale reading in rads for a particular phantom material and a particular X-or gamma-ray source. These computational problems are examined in the paper. An expression is derived for the required conversion factor. It is shown that these conversion factors are not critically dependent on the composition of the medium for low-Z materials or on small changes in the dimensions of the cavity chambers.