Sandra Hopkins Final Report.pdf - University of Surrey

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Firstly a free in air measurement with no additional copper filters in the beam. The calibration wasfound to be within 8% to 15%. The accuracy of this measurement is affected by the fact that highexposure factors are needed to get a high enough dose rate to provide sufficient accuracy. It wasalso found that the indicated dose rate was not always stable despite the exposure factors remainingstable. There was a larger difference between measured and indicated skin dose with Copper in thebeam path. However, the calibration was still inside the manufacturers tolerance and wasconsidered sufficiently accurate for the current work.Additional measurements of skin dose with Perspex in the beam to simulate the patientdemonstrated that the calibration was still satisfactory. It was expected that the radiation backscattered from the Perspex would increase the dose to the ion chamber and lead to a closercorrelation. However, this increase appears to be offset by the reduction in dose to the ion chamberdue to the table attenuation. This implies that calibration of the skin dose indicator can be checkedwith sufficient confidence with either the free in air method or with scattering media on the table.The final method for measuring skin dose was with the use of thermoluminescent dosimeters(TLDs). Whereas the skin dose indicator appeared to over read when compared to the ion chamber,the indicated skin dose indicator now appeared to under read by 26% to 33%. This is a highlysignificant difference. TLD measurements positioned outside the direct radiation path, receivedscattered radiation equal to approximately 5% of the direct dose. This value would be verydependent on position beyond the primary beam but demonstrates that for a coronary angiographyexam with a minimum of 9 runs there will be additional radiation to the skin from scatteredradiation from nearby runs.The results given here where only four runs were involved demonstrated that the actual dose couldbe higher by up to 15%. The skin dose indicator does not take this into account. Since the skin doseindicator provides a total and does not give any indication of dose distribution it is not essential.However, it demonstrates that for any skin dose distribution software the additional contributionfrom scattered radiation from adjacent runs needs to be considered.There have been a number of different studies using both film and/or TLDs to measure the skinentrance dose to the patient. One study used pre-packaged therapy verification film held in aspecially made gown with TLDs placed in specific locations on the film as a cross check betweenfilm and TLD exposure. TLDs were, on average, within 9% of the film estimate 39 . By wrappingfilm completely around a patient this would ensure that doses could be measured for lateralexaminations which was not possible for film and TLDs lying flat on the patient couch top. TLDshave also been arranged in a grid and a concentration factor determined, which is the ratio of the35
maximum skin dose to average dose 30 . In another study, results from a TLD array wrapped aroundthe patient were used to measure the maximum skin dose and action levels for DAP were proposed.A first DAP action level of 125Gy.cm 2 corresponded to the 2Gy dose threshold for optionalradiopathological follow up and a second DAP action level of 250Gy.cm 2 corresponded to the 3Gyskin dose which would require systematic follow up 40 . These values have not been compared in thisstudy but is worthy of further work. Indeed, another study demonstrated the need for both operatorand procedure specific DAP values for action levels 41 .5.2 Measuring patient DAP dosesThe old and new labs are from the same manufacturer and any differences are mainly mechanicaland software related so it is expected that with the same x-ray tube and flat plate detector and thesame staff group, patient doses should be very similar between the two labs. However, the datademonstrates that there has been an increase in patient DAP dose by 28%. This has been as a resultof a 35% increase in fluoroscopy dose and a 27% increase in acquisition dose. The mean dose nowexceeds the current Diagnostic Reference level of 29Gy.cm 2 and requires investigation and effortsto reduce this dose or justify the need for this higher dose. The mean weight of the sample grouphas reduced from 81.3kg to 80.4kg so it is clear that patient weight is not a contributory factor.There may have been a change in technique, however, the data demonstrates that the fluoroscopyscreening time has reduced from 236 seconds to 162 seconds and the mean number of acquisitionimages has reduced from 879 to 674. If there has been any change in cardiologist technique this hasresulted in improved technique resulting in reduced fluoroscopy and acquisition and should haveresulted in a patient dose reduction. This demonstrates that the increase in patient dose does notrelate to the examination technique and is related to equipment performance.Both systems have the same detector, however the manufacturer’s specification dose rates haveincreased for the newer system. The older system had field size factors based on image intensifiertechnology with the increase in dose increasing in accordance with the square of the field size. Forflat plate detectors a linear relationship has now been employed by manufacturers and the field sizefactor at 25cm and 20cm fields is higher than what had previously been recommended for imageintensifier systems. Detector dose measurements found the new system to have higher detector dosefor both fluoroscopy and acquisition. Additionally, for acquisition there were differences in theamount of copper pre-filtration that is automatically added by the system and differences inexposure factors. The significance of this to patient dose was assessed by the use of a 19cm Perspexphantom to simulate the patient. For fluoroscopy patient doses for the two systems were found to bebroadly similar. However, the new lab had less copper pre-filtration in the beam. Copper filtrationhardens the beam. The x-rays are therefore more penetrating and will result in reduced patient doseat the expense of image contrast. The improved latitude and image processing performance of36
Page 1 and 2: Calculation, verification and monit
Page 3 and 4: AcknowledgementsI would like to exp
Page 5 and 6: 4.3.2 Simulated Patient Skin Absorb
Page 7: These types of interventional proce
Page 12 and 13: 2.3.3 Equipment configurationFigure
Page 14 and 15: Skin injuries can occur when skin d
Page 16 and 17: couch height position when the cent
Page 18 and 19: This formula provides the idealised
Page 20 and 21: 3. Method3.1 Verification of Patien
Page 22 and 23: The field size was set to Mag 1 (20
Page 24 and 25: Further combined orientations were
Page 26 and 27: 70 183 67 60.7 +10.3%102 205 175 15
Page 28 and 29: Method 3 - Thermo Luminescent Dosim
Page 30: 13172125293337414549535761656973778
Page 33 and 34: 4.3.2 Simulated patient Skin Absorb
Page 35 and 36: However, a second set of orientatio
Page 37 and 38: further analysis work. An excerpt o
Page 39: Figure 29 : Intervention coronary e
Page 43 and 44: of 29% 42 . Image quality assessmen
Page 45 and 46: always resulted in a reduction in d
Page 47 and 48: References1. NRPB W4 - Radiation ex
Page 49 and 50: 30. Theodorakou C and Horrocks J A,

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