The response of lif thermoluminescence dosemeters to photon beams in the energy range from 30 kV x rays to 60Co gamma rays.

The energy response of standard (TLD-100) and high-sensitivity (TLD-100H) LiF thermoluminescence dosemeters (TLDs) has been studied for photon beams with mean energies from about 25 keV to 1100 keV. Canadian primary standards for air kerma were used to establish the air kerma rates for each of the photon beams. TLDs were mounted in a PMMA holder and the air kerma response was measured as a function of energy. The EGSnrc Monte Carlo code was used to model the TLD holder and calculate the absorbed dose to the TLD chip per unit air kerma for each beam. The measured and calculated results were combined to obtain the intrinsic dose response of the TLD chip. Broadly, our results are consistent with existing data, which show a marked difference in the energy dependence of the two materials. However, the precision of our measurements (standard uncertainty of about 0.6%) has permitted the identification of features that have not been noted before. In particular, the energy dependence of the two materials is quite different in the important energy region delimited by 137Cs and 60Co gamma rays.

GafChromic MD-55: investigated as a precision dosimeter.

GafChromic MD-55 is a fairly new, thin film dosimeter that develops a blue color (lambda max = 676 nm) when irradiated with ionizing radiation. The increase in absorbance is roughly proportional to the absorbed dose. In this study, GafChromic MD-55 was irradiated with 60Co gamma rays. A double irradiation method was used in which a dosimeter is given an unknown dose followed by a known, calibration dose. With this method, GafChromic MD-55 was used to measure doses in the vicinity of 6 Gy with an uncertainty of less than 1%. It was found that the measured optical density of GafChromic MD-55, as presently fabricated, is affected by the polarization of the analyzing light, an important consideration when using GafChromic MD-55 as a precision dosimeter. GafChromic MD-55 was found to consist of seven layers. The response to polarized light was measured for the whole dosimeter and for the three Mylar films which form part of the dosimeter.

Prenatal diagnosis in advanced maternal age. Amniocentesis or CVS, a patient's choice or lack of information?

Ninety-six women of advanced maternal age were interviewed about the way they obtained information on prenatal diagnosis and about how the decision was made as to what procedure was to be performed (transabdominal chorionic villus sampling (TA-CVS) or amniocentesis). In the CVS group, women visited their physician or midwife earlier in pregnancy (mean 7.1 weeks) than those in the amniocentesis group (mean 10.7 weeks). The availability of prenatal diagnosis was not mentioned during the first antenatal visit in 55 per cent of women from the amniocentesis group as opposed to 25 per cent from the TA-CVS group. Approximately 40 per cent of women eligible for prenatal diagnosis did not receive any information from the referring body prior to counselling at our centre. Only 29 per cent of women who underwent amniocentesis had actually chosen this procedure; 71 per cent were too late to undergo TA-CVS at 12 weeks. It is concluded that information to the patient must be improved in order to ensure early referral for prenatal diagnosis.