Sandra Hopkins Final Report.pdf - University of Surrey

I H G F E D C B A1 12.8 21.2 32.7 59.5 445.7 54 29.8 16.7 11.52 18.9 30.4 47.4 234.3 411.9 73.6 37.8 24.8 16.83 31.1 44.8 76.2 102.4 107.6 87 50.9 28.8 19.64 55.5 549.6 583 351.8 399 281.2 66.8 36.1 21.15 64.7 572.3 640.6 350.0 449.6 268.9 84.5 44.1 25.56 46.7 77.7 53.0 113.2 149.8 201.6 110.5 55.7 28.87 25.4 36.5 98.4 65.6 120.6 929.5 1067.8 63.5 27.78 15.2 21.0 34.2 24.9 68.5 964.3 1154.3 56.2 26.29 12.0 14.4 20.4 25.9 43.8 64.3 64.4 39.4 20.2Table 7 : Thermoluminescent dosimeter doses in mGy and their location on the Gafchromic filmTaking those TLDs that were closest to the centre of each field to determine the skin dose thefollowing skin doses for each orientation were determined. These doses were compared with theindicated doses given by the X-ray system (entire dose report available in appendix C).Orientation TLD dose(mGy)+/10%System indicateddose (mGy) +/- 5%Difference betweenmeasured and indicatedAP 450 298 -33%RAO 30 641 474 -26%CAUD 40 446 310 -30%LAO 20 CRAN 40 1154 785 -32%The skin dose measured by the TLD is 26% to 33% higher than the skin dose indicated by thesystem. This is significantly different to comparisons made using free in air and scatter conditionsusing an ionisation chamber. In this case the indicated dose under reads whereas for previousmeasurements it was shown to over read. These differences need to be considered when deciding onthe accuracy of the skin dose indicator. It is considered that the TLD dose is likely to be a moreaccurate indicator of skin dose than the ionisation chamber as it includes contributions from scatterfrom other acquisition runs in the neighbouring region.4.2 Measuring patient doses and comparison between old and new cath labsAppendix D and E give the detailed dose data for the old and new cath laboratories respectively.Both systems are from the same manufacturer and have the same options available so that they canbe programmed in the same way.24