C l l ti BIR th d Calculations - BIR method and NCRP 147

Calculations l - BIR methodand NCRP 147David Sutton PhD

1. Cath Lab• Indicative of all C-arm applications• Only secondary radiation– Primary Beam is stopped by the imageintensifier

Geometry BIR9.5 mPreparation2.6 mMonitorsComputer room6 mRecovery63m 6.3

Geometry NCRP 14714.6 mPreparation4 mMonitorsComputer room9.2 mRecovery97m 9.7

Workload BIR calculation• Sources:– ‘Doses to patients from radiographic andfluoroscopic x-ray imaging procedures inthe UK - 2005 Review’ : HPA– Potential Irish dose reference levels forcardiac interventional procedures’ : BJR2009– BIR report (2000)

Workload BIR calculation• Basis for calculations– Chose 75th percentile of dose distributionin both surveys– Conservative– Basis for national dose reference levels.

UK DataExam N Sites DAP/kVexam (mean)CA 34236 38 29 79PTCA (1 6111 9 50 71stent)PPI 2682 17 11 75Cathlab - - 26 80BIR 1Cathlab - - 50 80BIR 2(100)

Irish DataExam N Sites DAP/kVexam (mean)CA 967 14 42 -PCI 463 14 84 -CA-PCI 134 14 107 -PPI 240 14 21 -

Variation with complexity(PCTA)Exam N Room DAP/ kVs exam (mean)PCTA 1 6111 28 50 71PCTA 2 779 9 74 -PCTA 3 289 6 130 --

BIR Calculation• Evaluate barrier thickness required– 25 patients per week– Assume exams performed at 100 kV forconservatism– Use 2.6m and 4.0 m as critical distance– Dose constraint• 0.3 mSv / y• 1.0 mSv / y

Results (300 uGy)Code 3Exam KAP Smax (100) mm @2.6m mm @4mUKCA 29 162 1.02 0.72Cath (BIR) 50 280 1.22 0.90PPI 11 62 068 0.68 042 0.42PTCA 50 280 1.22 0.90IRELAND BJR 2009CA 42 235 1.16 0.84PCI 84 470 1.42 1.09CAPCI 107 599 1.51 1.18PPI 21 118 0.90 0.61

Results (1000 uGy)Code 3Exam KAP Smax (100) mm @2.6m mm @4mUKCA 29 162 0.61 0.36Cath (BIR) 50 280 0.78 0.51PPI 11 62 0.34 0.16PTCA 50 280 0.78 0.51IRELAND BJR 2009CA 42 235 0.73 0.46PCI 84 470 0.97 0.67CAPCI 107 599 1.06 0.75PPI 21 118 0.51 0.29

Effect of increasing workload• In reality use a mixed workload.• Here just use PCTA• Increase the number of procedures perweek

Effect of increasing workloadCode 42.001.501.002.6m4m0.500.000 20 40 60 80 100Patients/wk

Effect of changing doseconstraint PTCA 25/wk2.00mm Pb150 1.501.00050 0.502.6 m4 m0.000 1000 2000Dose Constraint

NCRP Methodology• Secondary dose of 3800 uGy at 1 metrefrom each patient in a cath lab.• No breakdown by procedure• 176 patients surveyed in 4 institutions in1996• Slightly different fitting coefficients

NCRP 1472.6m 4mDC300 2.17 1.821000 1.68 1.33

NCRP 1472.6m 4mDC300 2.17 (code5) 1.82 (code 4)1000 1.68 1.33BIR300 122 1.22 09 0.91000 0.78 0.51

Different Outcomes3.002.502.002.6m4m2.6 m NCRP4m NCRP1.501.000.500.000 20 40 60 80 100Patients/wk

Differences• Scatter model• Field area assumption• Patient workload data• Dose constraintst• Transmission data

Differences - Scatter125 kVp100 kVp85 kVp70 kVp50 kVp

Differences - ScatterPCTA KAP 80 100 1201 stent 50 247 280 3112 stent 74 366 414 4603 stent 130 643 728 809CAPCI 107 530 599 666

Differences - ScatterPCTA KAP 80 100 1201 stent 50 247 280 3112 stent 74 366 414 4603 stent 130 643 728 809CAPCI 107 530 599 666CLAB 680 3365 3808 4230

NCRP scatter determination• Symmetrical, not S shaped• Expressed as fractional scatter per cm 2• Assume 12” (38cm) II• At 140 degrees= 7x10 -6• At 117 degrees = 5.75 x 10 -6• 3800 uGy at 1 m predicts KAP of 439 - 535Gy cm 2

Scatter fractionNCRP 147BIRESR 100 8.64x10 -6 56x10 5.6x10 -6

Coefficients for transmissionequation200 2.001.501.00BIRNCRP0.500.000 50 100 150Patients / wk

Which only leaves workload• Weekly workload of 25 patients ataverage of 160mA min per patient.• Equates to a KAP per patient ofbetween 440 and 680 Gy cm2• Much higher than UK & Irish (Europeannumbers)•Why?

Questions• ? Age of data and / or case mix• American practice ??• ? Effect of copper filtration– mA min will increase dramatically withcopper, but air kerma will decrease.H d th l th d• How do you use the general methodhere (cf copper filtration)?

WARNING!• UK workload data apply only in UK• UK workload drivers– Legislation• Justification/ Optimisation– Public Healthcare provisionDerive your own workload data

2. CT• 2 different methodologies.• NCRP analagous to the KAP versusscatter approach.• BIR method reliant on manufacturersupplied isodose data.

120 kVp; 250 mAs

CT Calculation lation (G)Distance:(4.0/cos 36) = 4.9 mWeekly workload:3340 x 250 mAs slicesDistance to 2 µGy isodose:1.7 mDose per week:2 x 3340 x (1.7/4.9) 2= 800 µGyTransmission:7.2 x 10 -3Concrete thickness:100 mmThickness at 36°:100 x (1 + cos36)/2= 90 mm

JRP December 2008• Three manufacturer’s scanners.• Scattered radiation measured with tldmaterial.• Measurements compared with kermapredicted either from CTDI / DLP /Critical direction from isodose plots.• JRP 28 (2008) 511-523

Geometry

Scanner 1 (German)d mSv BIR DLP CTDI1 45 4.5 14 1.4 10.9 44 4.4 30 3.02 2.65 7.0 35.3 12.6 8.6Gantry 2.15 0.5 1.9 19.1 13.14 24 2.4 87 8.7 23.4 15.3 10.55 2.25 4.8 40.2 17.4 12.0

Scanner 2 (British)d mSv BIR DLP CTDI1 42 4.2 14 1.4 34 3.4 33 3.3 18 1.82 4.0 4.2 6.6 3.6 2.0Gantry 2.15 1.5 0 12.6 7.04 262 2.62 63 6.3 13.11 84 8.4 47 4.75 2.75 11.5 17.4 7.7 4.3

Scanner 2 (British)d mSv BIR DLP CTDI1 42 4.2 14 1.4 34 3.4 33 3.3 18 1.82 4.0 4.2 6.6 3.6 2.0Gantry 2.15 1.5 0 12.6 7.04 262 2.62 63 6.3 13.11 84 8.4 47 4.75 2.75 11.5 17.4 7.7 4.3

Scanner 3 (Dutch)d mSv BIR DLP CTDI1 412 4.12 05 0.5 22 2.2 16 1.6 06 0.62 2.58 3.3 5.8 4.0 1.4Gantry 1.74 0.2 0.6 8.7 3.14 288 2.88 20 2.0 14 1.4 32 3.2 11 1.15 2.25 2.25 9.3 5.2 1.9

IPEM Annual Scientific MeetingBarrier 32006AHBarrier 2Barrier 4AHIsocentre-wallBarrier 1

Results 1NCRP MethodBIR MethodDirection Distance (m) Annual Dose Distance (m) Annual DosemGymGyBarrier 1 2.26 334 2.76 341Barrier 2 2.32 317 2.34 491Barrier 3 262 2.62 248 283 2.83 320Barrier 4 4.14 99 4.3 138Ceiling 226 2.26 334 29 2.9 366

Results 2NCRP MethodBIR MethodDirection Dose per Slice uGy Dose per Slice uGyHead Body Head BodyBarrier 1 1.06 4.46 3.25 2.85Barrier 2 1 4.24 4.8 4Barrier 3 079 0.79 332 3.32 29 2.9 28 2.8Barrier 4 0.32 1.33 1.28 1.18Ceiling 106 1.06 446 4.46 327 3.27 325 3.25

Observations• NCRP methods get it wrong at the gantry (does it matter)• The BIR method is complicated, requiring i a number ofmeasurements from scale drawings, interpolationsand conversion of workload into standard slices.• The NCRP method is much simpler and quicker requiring onlylimited measurements from scale drawings.• The BIR produces most overestimates of actual dose and hencemost ‘conseravtive’ shielding.• CTDI based technique produces underestimates and should beapproached with caution• Manufacturer’s isodose plots should be approached with caution• Apart from gantry, DLP method produces closest match, but stilloverestimates in general.

Mammography• NCRP 147 : 36 uGy per patient.• BIR : 7.6 uGy per film, 3 films perpatient = 23 uGy per patient• No difference in overall conclusion.

Dental• NCRP 147 doesn’t consider dentalradiography.• BIR : Max kerma at 1 m = 1uGy/film forintra oral radiography.• Table showing number of films perweek versus barrier thickness anddistance from the x-ray set.

Dental Intra-oralBarrier distance (m)Films/ week 1 1.5 2 2.5 310 0.58 - - - -20 0.29 0.65 - - -50 0.12 0.26 0.46 0.72 -100 0.06 0.13 0.23 0.36 0.71200 003 0.03 006 0.06 012 0.12 018 0.18 035 0.35

DEXA• Handwaving on all sides.• Usually, no need to shield.

THAT’S IT –THANK YOU