Variation of relative dose rate with distance from extended sources of 99Tcm

Determining the variation of relative dose rate with distance from a patient cannot be made directly with measurements by thermoluminescent dosemeters (TLD) since they would require an extended period of exposure. There are also considerable limitations to determining this variation in detail from a patient by measurements with a portable dose rate meter. Such an instrument may have insufficient sensitivity for measurements at large distances, an effective centre which varies with distance from the patient, and an adequate spatial resolution. Carrying out direct measurements on a patient may cause unnecessary alarm, and in previous studies with dose rate meters, measurements were limited to just a few distances from the patient. It was concluded from these studies that different shaped sources and TLDs should be suitable for a more detailed characterisation of the variation of relative dose rate with distance. This method has been used for a preliminary investigation of the variation of relative dose rate with distance, and the results have been compared with the variation predicted by theoretical methods and with some data published from patient studies. (author)