No abstract

Dose is attenuated in the 'shadow' of the tissue expander port in patients receiving PMRT. This is likely to be clinically insignificant for most, but centres should undertake appropriate measurements before utilising TPS predictions.

Relative output factors are used in radiation therapy for treatment planning purposes including treatments using kilovoltage X-ray beams. The output factor is the relative dose output for a particular applicator relative to a reference applicator. Due to the differences in the scatter contribution from the inside of an applicator, it is more accurate if output factors are measured for all combinations of X-ray beam energy and applicator. Previously published papers and various kilovoltage X-ray beam dosimetry protocols have discussed the validity of using various cylindrical and parallel plate ionization chambers for relative output factor measurements. In this work, we evaluated the suitability of Gafchromic™ EBT3 film for the determination of output factors for kilovoltage X-ray beams. Output factors were measured with Gafchromic™ EBT3 film for beam qualities of 50, 75, 100 and 125 kVp and applicator sizes of 2, 3, 4 cm diameter, 8 × 8 and 12 × 12 cm(2) square applicators. The film read out was performed with a flatbed EPSON Expression 10000XL scanner. The measured data was compared with benchmark data from measurements using an Advanced Markus ionisation chamber as well as comparing with ratios of published backscatter factor values. The agreement between output factors measured with EBT3 film and the ionisation chamber was generally better than 2 %, with the largest difference of 3.3 % occurring for the 2 cm diameter field with the 50 kVp X-ray beam. These differences were consistent with the estimated total uncertainty in the measurements as calculated by the ISO GUM. The agreement between the output factors measured with film and the published BSFs was generally better than 5 % but differences of up to 12 % occurred for the smallest field size. The results demonstrate that Gafchromic™ EBT3 film is a suitable dosimeter for output factor measurements of kilovoltage X-ray beams.

No abstract

A novel method is described for the measurement and calculation of surface and skin dose during megavoltage x-ray therapy utilising an extrapolation technique with optically stimulated Dosimeters. The technique utilises three configurations of an Optically Stimulated Luminescence (OSL) detector which provide different effective depth of measurement points. This allows the user to assess percentage build up dose at effective depths of 0.35, 0.55 and 0.85 mm. Performing an extrapolation technique, percentage dose at other effective depths such as the surface (0 mm) or the Basal Cell layer (70 μm) depth can be estimated. Results have shown that extrapolated percentage dose of Dmax at a depth of 0.048 mm are 17.3% ± 2.3%, 28.2% ± 3.5% and 37.8% ± 3.8% for field sizes of 10 cm × 10 cm, 20 cm × 20 cm and 30 cm × 30 cm respectively. Comparative results measured with the Attix chamber which has an estimated effective depth of 0.048 mm were 16.5%, 28% and 36.8% respectively, thus showing close agreement (within 1%) with the extrapolation technique. The OSL extrapolation technique will allow users of OSL detectors to accurately assess surface and skin dose using this technique.