Sandra Hopkins Final Report.pdf - University of Surrey

3. Method3.1 Verification of Patient Dose Indicators.3.1.1 Dose area productSince the Dose Area Product data is used to assess the dose to the patient and the values are used foraudit it is necessary to ensure that the meter is correctly calibrated. The manufacturer’s work to atolerance of +/- 30% but ideally better accuracy is preferred.The C-arm is positioned in a Lateral positionas shown in figure 11. An MDH 9020 meterand 6cc ionisation chamber is placed in thecentre of the image field at a known distancefrom the x-ray tube focus. For different setkV values the dose area product is recordedfrom the system and the total output from theionisation chamber. In order to calculate thedose area product it is necessary to obtain thefield area at the ionisation chamber distance.Figure 11 : Standard set up for measuring skin dose removing table attenuation and backscatter.This is done by replacing the chamber with an imaging plate at exactly the same distance. This isexposed and the field area is measured. Typically this is carried out during annual performancetesting of the equipment with all automatic Copper filters removed from the beam. In order toincrease exposure factors during testing it is necessary to place large area copper filters in front ofthe detector. The increased attenuation drives up the exposure factors and allows the calibration tobe checked across the full clinical kilovoltage range. Accuracy of the MDH chamber is ensured bysending it annually to a national calibration test house and applying any correction factors provided.3.1.2 Skin Dose IndicatorMethod 1 – Ion chamber free in airSince it is known that the skin dose indicator is either calculated from the Dose Area Product dataor directly from exposure factors the set up above in figure 11 can be used since this eliminatedboth table attenuation and back scatter. This is a free in air exposure. Some manufacturers calibratetheir dose meters free in air whilst others include the table within their measurement. This is usuallycarried out with automatic filters removed from the beam. However, on this occasion somemeasurements were also made with Copper attenuation added to confirm that the calibration took15