No abstract

Direct measurement is made for X-ray beams with first-half-value thicknesses in the range 0.5 mm to 8 mm Al and for field sizes ranging from 15 mm diameter to 70 mm square at 100 mm SSD. The measurements were made in a water phantom using a 0.2 cm3 thimble ionization chamber. The surface dose relative to that for full backscatter conditions decreases as the depth of water with lead beneath decreases. Further, for a given thickness of water with lead beneath, the surface dose relative to that for full backscatter conditions decreases with increasing beam energy and field size. The measured data is well described by a simple exponential function, the parameter values of which for the range of beam qualities and field sizes considered in this study are given. The measurements are compared with Klevenhagen's measurements of the build-up of backscatter with thickness of polystyrene.

The low energy (30-50 kVp) beams from an intra-operative X-ray source are modelled using a basic analytical model considering just primary beam attenuation and absorption. Spatial dosimetry at such low energies is difficult due to the rapid changes in dose-rate from the radiation source. The purpose of the model was to determine the variation with distance in water of coefficients required for beam dosimetry and to validate beam measurements performed in water of high-gradient dose distributions. The model predicts a change in mean mass-energy absorption coefficient of up to 3 % over the range of clinically-relevant distances in water. Distance-dose distributions (variation in dose with distance in water) for the X-ray source were calculated with the model and found to be in agreement with measurement (at clinically-relevant distances), to within a spatial distance comparable to the dimensions and positional accuracy of the ionization chamber used, and comparable to the expected dosimetric anisotropy of the radiation source. Measured and calculated distance-doses begin to diverge at relatively large distances from the radiation source, which is where dose-rates are so low that detector signal levels are comparable with noise.

A method is proposed to simplify the IAEA TRS398 dosimetry code of practice in respect to dose determination of high-energy electron beams. The proposed method eliminates the use of the intermediate beam quality Q(int) (and beam quality correction factor k(Q,Q(int))) applicable for cross calibration and subsequent use of the user's chamber for dose determination in water for high-energy electron beams. This method allows calculation of the dose to water calibration factor for the user's instrument at the reference beam quality N(D)(w,Q0) directly from a cross calibration in a high-energy electron beam of quality Q(cross) at the user's institute.