Patient Dosimetry in Mammography and Tomosynthesis:

Transcription

1 2013 ICTP/IAEA Training Course on Radiation Protection of Patients Trieste Patient Dosimetry in Mammography and Tomosynthesis: What to measure, why and how John M. Boone, Ph.D., FAAPM, FSBI, FACR Professor and Vice Chair (Research) of Radiology Professor of Biomedical Engineering University of California Davis Medical Center Sacramento, California 1

2 Mammography and Tomosynthesis Dosimetry Mammography Why measure breast dose? Basic Concepts of Breast Dosimetry (how) Mean Glandular Dose (MGD) (what) DgN coefficients Skin Thickness Issues Breast Density Issues Tomosynthesis Differences between tomo and mammo Summary 2

3 U.S. Breast Cancer Statistics (2006) 212,290 new cases 40,970 deaths 1 / 8 women will get breast cancer (12.5%) Breast Cancer Lung Cancer Incidence * 31% 12% Mortality * 15% 26% Ravdin, et al., NEJM * of all cancers

4 death rate per 100,000 U.S. Cancer Mortality ( ) Jemal A, et al., Cancer Statistics

5 Mammography: Standard of Care CC MLO CC MLO 5

6 Annual screening dose accumulation average size breast 6

7 Mammography and Tomosynthesis Dosimetry Mammography Why measure breast dose? Basic Concepts of Breast Dosimetry (how) Mean Glandular Dose (MGD) (what) DgN coefficients Skin Thickness Issues Breast Density Issues Tomosynthesis Differences between tomo and mammo Summary 7

8 Digital mammography unit at UC Davis 8

9 Breast Dose is highly dependent upon the x-ray spectrum used. Different Anode/Filter combinations are used. ANODE Mo Mo Rh W W W FILTER Mo Rh Rh Rh Ag Al S/F Digital Tomo 9

10 In addition to the Anode / Filter combo, the kv and HVL need to be well characterized 10 compression paddle 10

11 X-ray Tube Voltage Evaluation 11 non-invasive kv meter 11

12 In addition to the Anode / Filter combo, the kv and HVL need to be well characterized mm thick Al filters (99.999%) ion chamber calibrated to mammo energies air gap 12

13 HVL as a function of tube voltage 13

14 14 Establishing x-ray tube output mgy air kerma per 100 mas at 50 cm ion chamber 14

15 X-ray output cm) versus tube voltage 15

16 anode / filter combo glandular fraction kv breast thickness HVL s 16

17 Validation of Breast Thickness Accuracy 4.1 cm breast phantoms (slabs) 17

18 Estimating Entrance Air Kerma From Bloomquist, et.al. Med Phys 33: ,

19 Estimating Entrance Air Kerma Recorded in patient s record (digital) anode filter kv ma exposure time compressed breast thickness 19

20 Estimating Entrance Air Kerma dgn values normalized glandular dose coefficients air kerma 20

21 Estimating Entrance Air Anode/Filter kv: air kerma per cm = k ESK = k (mas/100) ISL ISL 50 SID T 2 50 cm SID - T usually ~65 cm breast thickness 21

22 Mammography and Tomosynthesis Dosimetry Mammography Why measure breast dose? Basic Concepts of Breast Dosimetry (how) Mean Glandular Dose (MGD) (what) DgN coefficients Skin Thickness Issues Breast Density Issues Tomosynthesis Differences between tomo and mammo Summary 22

23 Mean Glandular Dose 23

24 Mean Glandular Dose skin glandular tissue adipose tissue 24

25 Mean Glandular Dose (Monte Carlo Calculations) skin homogeneous mixture glandular tissue? glandular fraction adipose tissue? 25

26 Mammography and Tomosynthesis Dosimetry Mammography Why measure breast dose? Basic Concepts of Breast Dosimetry (how) Mean Glandular Dose (MGD) (what) DgN coefficients Skin Thickness Issues Breast Density Issues Tomosynthesis Differences between tomo and mammo Summary 26

27 DgN values are based on MC Studies assumed 4 mm skin thickness 27

28 DgN values are based on MC Studies assumed 4 mm skin thickness 28

29 Monoenergetic MC eval of E deposition 29

30 Converting Energy imparted to dose. MGD = m en r m en r glandular breast comp Energy Imparted Mass f-factor (SI) 30

31 Mean Glandular Dose (Monte Carlo Calculations) ESK DgN = MGD ESK MGD 31

32 Dose Calc with Normalization by ESE 32

33 To convert the monoenergetic DgN values to realistic polyenergetic values, spectral models are used to weight the monoenergetic values 33

34 DgN Tables (poly) The units used in DgN Tables have varied over the years and by the country of origin. These units are mrad / R, but mgy/mgy is in more common usage today 34

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36 DgN versus monoenergetic x-ray energy 2 cm 4 cm 6 cm 8 cm 36

37 Mammography and Tomosynthesis Dosimetry Mammography Why measure breast dose? Basic Concepts of Breast Dosimetry (how) Mean Glandular Dose (MGD) (what) DgN coefficients Skin Thickness Issues Breast Density Issues Tomosynthesis Differences between tomo and mammo Summary 37

38 t = 4 mm? 38

39 Developed algorithms to segment skin from breast CT images and measure thickness t 39

40 Skin Thickness measurements were relatively precise for each women (18%) Measurements from the same women 40

41 Skin Thickness measurements ranged from 0.9 mm to 2.3 mm (m = 1.45 mm) Measurements from 51 different women 41

42 Left / Right Comparison provided a consistency check, with good results 42

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44 DgN values versus skin thickness 44

45 Change (in %) in DgN values versus skin thickness, Relative to the assumption of t = 4 mm 45

46 Mammography and Tomosynthesis Dosimetry Mammography Why measure breast dose? Basic Concepts of Breast Dosimetry (how) Mean Glandular Dose (MGD) (what) DgN coefficients Skin Thickness Issues Breast Density Issues Tomosynthesis Differences between tomo and mammo Summary 46

47 Mean Glandular Fraction high medium low 100% 50% 0% 47

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49 Breast Density Analysis segmentation glandular % adipose risk assessment & dosimetry validation of 2D approaches (M. Yaffe) 49

50 including skin excluding skin UC Davis breast CT N =

51 Density analysis done on mammograms at University of Toronto Data from UC Davis Breast CT 51

52 UCD & UT data combined (N = 2831) Median 16 % 52

53 UCD & UT data combined (N = 2831) Cumulative Distribution 95% 90% 75% 50% 53

54 Breast density decreases about 5 % per decade 54

55 volume fraction (%) Areal Fraction versus Volume Fraction 100% 0% 0% 100% areal fraction (%) 55

56 DgN for 0% Glandular Breast 56

57 DgN for 100% Glandular Breast 57

58 12% 16% 50% increase in average DgN coefficient of: ( )/237 = 12% 5 cm 2 cm 4 cm 6 cm 8 cm 58

59 Mammography and Tomosynthesis Dosimetry Mammography Why measure breast dose? Basic Concepts of Breast Dosimetry (how) Mean Glandular Dose (MGD) (what) DgN coefficients Skin Thickness Issues Breast Density Issues Tomosynthesis Differences between tomo and mammo Summary 59

60 Tomosynthesis (limited angle tomography) 60

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62 Conventional versus Tomosynthesis Conventional Tomosynthesis Conventional Tomosynthesis Similar display characteristics 62

63 Tomosynthesis (limited angle tomography) ±7.5 63

64 Combo Tomo / Mammo mode output mammography mr/mas or mgy per 100 mas tomosynthesis 64

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66 Physical measurements of dose in Tomo/Mammo Polyethylene is an excellent surrogate for adipose 66

67 Output of real time exposure meter 15 tomo exposures mammo exposure filter changes & grid is inserted 67

68 Point dose measures: Tissue Air Ratios 68

69 Mammography and Tomosynthesis Dosimetry Mammography Why measure breast dose? Basic Concepts of Breast Dosimetry (how) Mean Glandular Dose (MGD) (what) DgN coefficients Skin Thickness Issues Breast Density Issues Tomosynthesis Differences between tomo and mammo Summary 69

70 Mammography and Tomosynthesis Dosimetry Summary Complete characterization of the x-ray system is necessary (kv accuracy, HVL, and air kerma / 100 anode/filter) Ionization chambers must be mammo beam compatible thin windowed and calibrated Practical Assessment requires table of DgN values specific to the conditions of the actual exam Current DgN tables may slightly under-estimate dose due to % glandular fraction and skin thickness issues Tomosynthesis is similar enough to the geometry of mammo anode/filter combo may differ; different DgN tables needed 70

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