Calculati tions - BIR meth thod and NCRP 147

Transcription

1 Calculations l - BIR method and NCRP 147 David Sutton PhD

2 1. Cath Lab Indicative of all C-arm applications Only secondary radiation Primary Beam is stopped by the image intensifier

3 Geometry BIR 9.5 m Preparation 2.6 m Monitors Computer room 6 m Recovery 63m 6.3

4 Geometry NCRP m Preparation 4 m Monitors Computer room 9.2 m Recovery 97m 9.7

5 Workload BIR calculation Sources: Doses to patients from radiographic and fluoroscopic x-ray imaging procedures in the UK Review : HPA Potential Irish dose reference levels for cardiac interventional procedures : BJR 2009 BIR report (2000)

6 Workload BIR calculation Basis for calculations Chose 75th percentile of dose distribution in both surveys Conservative Basis for national dose reference levels.

7 UK Data Exam N Sites DAP/ kv exam (mean) CA PTCA ( stent) PPI Cathlab BIR 1 Cathlab BIR 2 (100)

8 Irish Data Exam N Sites DAP/ kv exam (mean) CA PCI CA-PCI PPI

9 Variation with complexity (PCTA) Exam N Room DAP/ kv s exam (mean) PCTA PCTA PCTA

10 BIR Calculation Evaluate barrier thickness required 25 patients per week Assume exams performed at 100 kv for conservatism Use 2.6m and 4.0 m as critical distance Dose constraint 0.3 msv / y 1.0 msv / y

11 Results (300 ugy) Code 3 Exam KAP Smax (100) UK CA Cath (BIR) PPI PTCA IRELAND BJR 2009 CA PCI CAPCI PPI

12 Results (1000 ugy) Code 3 Exam KAP Smax (100) UK CA Cath (BIR) PPI PTCA IRELAND BJR 2009 CA PCI CAPCI PPI

13 Effect of increasing workload In reality use a mixed workload. Here just use PCTA Increase the number of procedures per week

14 Effect of increasing workload Code m 4m Patients/wk

15 Effect of changing dose constraint PTCA 25/wk 2.00 mm Pb m 4 m Dose Constraint

16 NCRP Methodology Secondary dose of 3800 ugy at 1 metre from each patient in a cath lab. No breakdown by procedure 176 patients surveyed in 4 institutions in 1996 Slightly different fitting coefficients

17 NCRP m 4m DC

18 NCRP m 4m DC (code5) 1.82 (code 4) BIR

19 Different Outcomes m 4m 2.6 m NCRP 4m NCRP Patients/wk

20 Differences Scatter model Field area assumption Patient workload data Dose constraints t Transmission data

21 Differences - Scatter 125 kvp 100 kvp 85 kvp 70 kvp 50 kvp

22 Differences - Scatter PCTA KAP stent stent stent CAPCI

23 Differences - Scatter PCTA KAP stent stent stent CAPCI CLAB

24 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 ugy at 1 m predicts KAP of Gy cm 2

25 Scatter fraction NCRP 147 BIR ESR x x10 5.6x10-6

26 Coefficients for transmission equation BIR NCRP Patients / wk

27 Which only leaves workload Weekly workload of 25 patients at average of 160mA min per patient. Equates to a KAP per patient of between 440 and 680 Gy cm 2 Much higher than UK & Irish (European numbers) Why?

28 Questions? Age of data and / or case mix American practice??? Effect of copper filtration ma min will increase dramatically with copper, but air kerma will decrease. H d th l th d How do you use the general method here (cf copper filtration)?

29 WARNING! UK workload data apply only in UK UK workload drivers Legislation Justification/ Optimisation Public Healthcare provision Derive your own workload data

30 2. CT 2 different methodologies. NCRP analagous to the KAP versus scatter approach. BIR method reliant on manufacturer supplied isodose data.

31 120 kvp; 250 mas

32 CT Calculation lation (G) Distance: (4.0/cos 36) = 4.9 m Weekly workload: 3340 x 250 mas slices Distance to 2 µgy isodose: 1.7 m Dose per week: 2 x 3340 x (1.7/4.9) 2 = 800 µgy Transmission: 7.2 x 10-3 Concrete thickness: 100 mm Thickness at 36 : 100 x (1 + cos36)/2 = 90 mm

33 JRP December 2008 Three manufacturer s scanners. Scattered radiation measured with tld material. Measurements compared with kerma predicted either from CTDI / DLP / Critical direction from isodose plots. JRP 28 (2008)

34 Geometry

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36 Scanner 1 (German) d msv BIR DLP CTDI Gantry

37 Scanner 2 (British) d msv BIR DLP CTDI Gantry

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39 Scanner 2 (British) d msv BIR DLP CTDI Gantry

40 Scanner 3 (Dutch) d msv BIR DLP CTDI Gantry

41 IPEM Annual Scientific Meeting Barrier A H Barrier 2 Barrier 4 A H Isocentre -wall Barrier 1

42 Results 1 NCRP Method BIR Method Direction Distance (m) Annual Dose Distance (m) Annual Dose mgy mgy Barrier Barrier Barrier Barrier Ceiling

43 Results 2 NCRP Method BIR Method Direction Dose per Slice ugy Dose per Slice ugy Head Body Head Body Barrier Barrier Barrier Barrier Ceiling

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

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

46 Dental NCRP 147 doesn t consider dental radiography. BIR : Max kerma at 1 m = 1uGy/film for intra oral radiography. Table showing number of films per week versus barrier thickness and distance from the x-ray set.

47 Dental Intra-oral Barrier distance (m) Films/ week

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

49 THAT S IT THANK YOU