Response of CaSo4:Dy teflon TLD discs to diagnostic X-rays for entrance and exit beam and in-phantom dosimetry

CaSO4:Dy based TLD are being widely used for entrance and exit dose measurements and collection of data on patient doses in diagnostic radiology procedures in India. These TLDs have also been used to develop conversion factors for estimating organ doses due to diagnostic X-ray practices. Responses (TL readout per mGy of air kerma on phantom measured by calibrated EIL ionization chamber) of these TLDs is found to change considerably for exit beam as compared with the response for entrance beam for the mix-D phantom measurements. The present study indicates that an error of more than 40% could result if the dosemeters are calibrated in conventional way of using X-ray beam of about 80kVp tube potential and irradiation on phantom surface or in air. However, the response for the exit dose measurements is found not to change significantly with varying phantom thicknesses of more than a few cms and X-ray beam field sizes of more than 15 x 15 cm2. Thus, by using two sets of calibrations, one for entrance beam and the other for exit beam in phantom dosimetry the superiority of CaSO4Dy; viz. high sensitivity (about 300 times that of LiF for diagnostic X-rays) and indigenous availability could still be exploited in diagnostic radiology. (author). 5 refs