IRRADIATORS. September 11, :00 am-5:00 pm Hock Plaza Auditorium Duke University Medical Center, Erwin Road

Transcription

1 IRRADIATORS Terry Yoshizumi,, PhD September 11, :00 am-5:00 pm Hock Plaza Auditorium Duke University Medical Center, Erwin Road Radiation Countermeasures Center of Research Excellence 1

2 Acknowledgements Sam Brady Garrett Muramoto John Chute Giao Nguyen Greta Toncheva Lauren Daigle Dave Jorgensen Greg Egan Netiti Moori Brad Thrasher Beverly Steffey Oana Craciunescu 2

3 X-ray irradiator Production of x-rays x (bremsstrahlung) How the x-ray x machine works Bremsstrahlung X-ray spectrum Factors affecting dosimetry Case study (John Chute) 7

4 Production of x-rays x (bremsstrahlung( bremsstrahlung) Charged particle (e.g., electron) passing near a nucleus may be deflected by the strong electrical forces exerted on it by the nucleus As the projectile electron passes by the nucleus, it slows down, changes its course, and leaves with reduced kinetic energy. This loss in kinetic energy reappears as an x-ray x (called bremsstrahlung x-rays) 8

5 How the x-ray x machine works X-ray tube Removable filter AGFA X-RAD 320 9

6 ANATOMY OF ANODE TUBE Melting point 1083 deg C Z=74, High melting point 3380 deg C 10

7 TUBE VOLTAGE AND TUBE CURRENT Tube voltage (kvp) Tube current 11

8 CATHODE ASSEMBLY Small filament for high-resolution imaging Larger filament for higher intensities (large ma) 12

9 TARGET ANGLE AND DIRECTION OF BREMSSTRAHLUNG Target angle Target angle degrees Direction of bremsstrahlung radiations (approx. perpendicular to the direction of electrons) See next slide 13

10 RELATIVE INTENSITY OF BREMSSTRAHLUNG For low energy electrons, radiated predominantly at right angle to the motion of the particles The probability of bremsstrahlung production varies with Z 2 of the absorbing materials 14

11 Bremsstrahlung x-ray spectrum Max. kvp 15

12 FILTERS, AND BEAM RESTRICTORS # types of filters # 1 # 4 # 8 #1: 1.65 mm Aluminum #4: 0.1 mm Cu mm Al #8: 0.8 mm Tin mm Cu +1.5 mm Al 16

13 Key factors affecting dosimetry X-ray energy (kvp( kvp) Tube current (ma( milliamperes) Beam filtration (filters) Distance Attenuation in mouse Backscatter as a function of field size 17

14 Factors affecting dosimetry X-ray energy (kvp( kvp) Rule of Thumb (dose increases to kvp 2 ) Dose Dose kvp ( ) kvp Dose 2 Dose 1 ( ) kvp1 2 kvp Example: 120 kvp to 140 kvp Dose 140 kvp ~Dose 120kVp * (140/120) 2 = 1.36* Dose 120kVp i.e., Dose increases by 36%. 18

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16 Factors affecting dosimetry Tube current (ma( milliamperes) A change in ma results in a directly proportional change in the amplitude of the x-ray x emission spectrum at all energies. Example: if you double the ma from 200-mA to 400-mA, the area under the curve (x-ray quantity) doubles. 20

17 Factors affecting dosimetry Effects of beam filtration (filters) Available filters #1: 1.65 mm Aluminum #4: 0.1 mm Cu mm Al #8: 0.8 mm Tin mm Cu +1.5 mm Al 21

18 Factors affecting dosimetry Effects of beam filtration (filters) The overall result of added filtration is an increase in the effective energy of the x-ray x beam with an accompanying reduction in x-ray x quantity. Peak energy shift 22

19 #4 filter being used in Sands Building #4 filter #1 filter #8 filter #1: 1.65 mm Aluminum #4: 0.1 mm Cu mm Al #8: 0.8 mm Tin mm Cu +1.5 mm Al 23

20 Factors affecting dosimetry Effect of distance (source-to to-surface distance, SSD, or target (focal spot)-to to surface distance, TSD) Inverse-square square law (dose decreases inversely to the square of the distance) Example: 4Gy *(50cm/100cm) 2 = 4Gy*(1/4)=1 Gy 24

21 X-ray geometry- mouse Hummm I feel more heat! Dose depends on the source-to-mouse (target) distance. The closer to the tube, 25 the higher the dose.

22 Factors affecting dosimetry Attenuation in mouse For 135 kvp expect some attenuation in the mouse Backscatter as a function of field size 26

23 John Chute, PI Objectives CASE STUDY Compare absorbed dose between clinical protocol and direct TLD method Clinical protocol used by Garrette: : Parameters: 135kVP, 100cGy/min, FS (collimated beam): 20 cm x 20 cm 27

24 Mouse Anatomy % Bone Marrow Skull:17.6% Mandible:2.0% Clavicle and Scapula: 1.0% Spine: 33.7% Cervical: 4.2% Thoracic: 7% Lumbar: 9.9% Sacral:8.1% Coccygeal: 2.3% Humerus:2.4% Forearm:0.7% Sternum:3.8% All Ribs:5.0% Femur: 5.8% Tibia: 3.0% Pelvis:11.9% Mus musculus (Common species of Lab mouse): -Avg overall length: 16.9cm (head to tail) -body length: 6-10cm (head to base of tail) -hind foot: 1.8cm -Avg weight adult mouse: 17-25g -Average height: 3-5cm 28

25 TISSUE-EQUIVALENT EQUIVALENT MOUSE PHANTOM A B C TLD 15, 16 TLD 13, 14 TLD 11, 12 1 cm 0.3 cm 0.6 cm 2.0 cm 3.8 cm 2.7 cm 3.8 cm 2.4 cm 3 cm 12 cm A B C TLD chips (3 mm x 3 mm x 1 mm thick) 29

26 Results: x-ray x irradiator Dose rate suspicious LOCATION A (Head) 1 cm deep CLINICAL SETTING 100 cgy/min 500 cgy TLD MEASURED DOSE cgy % DIFF -23% B (Middle) 0.3 cm deep 500 cgy cgy -17% C 0.6 cm deep 500 cgy cgy -13% 30

27 Clinical Setting 500 cgy X-ray Mouse Dose (cgy) Dose (cgy) A (1.0 cm) B (0.3 cm) C (0.6 cm) TLD locations 31

28 New Calibration factors for Garrett geometry Location A Dose Rate (cgy/min) 81.1 SD Dose Rate (cgy/min) 3.1 B C AVERAGE

29 Why mice did not die? Time (min) Clinical DR used (100 cgy/min) New DR (85.0 +/- 2.7 cgy/min) % Diff / % Target 700 cgy was actually 595 +/- 19 cgy. 33

30 50 cm cgy d cm min Desired Dose (cgy) 50 cm cgy d cm min 2 Desired Dose (cgy)=( ) x DR ( ) x T(min) T(min) = 2 ( ) x DR ( ) STD=50 CM d cm B A Dose Rate (DR) in cgy/min 34

31 GEOMETRY STD=50 cm B: 44.2 cm 46.5 cm A: STD=45.5 cm 4.8 cm 1 cm below 2 cm 3 cm 3.5 cm 0.5 cm deep STD=45.5 cm 35

32 Results of mice dosimetry x-ray irradiator LOCATION B: Assume source- to-target target distance =44.2 cm A: Assume source-to to- Target distance =45.5 cm Dose rate being used clinically by Chute group Dose rate 95.0 cgy/min cgy/min 100 cgy/min Dose for 5 min exposure cgy 504 cgy 500 cgy 36

33 Discussions Sources of inaccuracies Look-up table (BSF)- probably minor Geometry changed since originakl calibration (major) Scatter geometry different in mice in the plastic holder (not uniform water phantom) 37

34 Gamma ray irradiator JL Shepherd Mark I 38

35 Gamma ray irradiator Decay scheme (Cs-137) Key differences from X-rayX Case study Quality assurance note 39

36 Decay scheme (Cs-137) Half-life= life= 30 yrs Emits β 1 particles (electrons) Gamma at 662 kev + - n p + e + γ + energy Cs Ba + β + ν + γ Cs Ba + β + ν + γ 40

37 Key differences from X-ray X irradiator Energy: Cs kev X-ray 135 kvp (ave.. energy ~45 kev) (average energy ~135 * 1/3= 45 kev) Need for annual decay correction for Cs yr -λ*t - 30 yr = e = e The dose rates must be decreased 2.3% per year. 41

38 Cs-137 Irradiator Dimension 37 cm high Cs-137 source will be raised Turntable 30 cm diam. 42

39 Cs-137 Irradiator (Chute) 4.8 cm 3 =7.6 cm rotating table 43

40 Factors affecting dosimetry Non-uniform irradiation of mice due to rotating table Dose rate point taken at central point and assume same dose distribution (not true) 44

41 CASE STUDY John Chute, PI Objectives Compare absorbed dose between clinical protocol and direct TLD method Exposure T=0.76 min Target dose =500 cgy 45

42 Geometry issues need to be addressed TLD chips Beam angle and TLD chip angle dependency The beam not always perpendicular to the chips Θ Cs-137 source Goals: Understand angle dependency Minimize angle effects in dosimetry 46

43 8/15/2006 TLD runs:cs-137 geometry TLD chip 3 x 3 x 1 mm #3 #1 Beam direction TLD perpendicular, middle #2 #2 TLD parallel to beam, middle #1 rotation #3 TLD parallel to beam, top, same as x-ray 47

44 CLINICAL PROTOCOL DOSE SET TO 500 cgy Set-up #1 Gold Standard (TLD Cs-137) A B C average A B C 48

45 Geometry #2, target dose =500 cgy #2 TLD parallel to beam, middle Case # dose (cgy) A B C average 49

46 Case #3: x-ray x geometry, Target 500 cgy A B C TLD 15, 16 TLD 13, 14 TLD 11, 12 #3 TLD parallel to beam, top, same as x-ray 1 cm 0.3 cm 0.6 cm 2.0 cm 3.8 cm 2.7 cm 3.8 cm 2.4 cm 3 cm 12 cm Case #3: x-ray geometry Dose (cgy) A B C average 50

47 New calibration factors for Garrett geometry geometry DR (cgy( cgy/min) SD (cgy( cgy/min) #1 (middle, gold standard) #2 (middle) #3 (spine curve)

48 Irradiator Dosimety Quality Assurance Note Whenever set-up changes, consult your physicists (Oana( and Beverly, Rad Onc or Radccore HP group) Geometry specific direct measurements would improve confidence and accuracy Calibration factors for specific geometries may be posted in the web-site Dose validation is important across the Radccore affiliates 52

49 Irradiator safety issues 54

50 Irradiator safety issues Don t t lose your fingers Understand radiation levels in normal operations 55

51 Radiation level under normal operations X-ray beam on Assume 10 min 1 ft, then Cs-137 Source off µ R R 1 hr hr 10 µr 60 min -6 Dose equivalent ( Rem ) = 8 * *10 min * = 1.3 x 10 R (or Rem) 6 Recall WB limit = 5 Rem per yr No radiation risks! 56

52 Cs-137 source up A location A B C D waist level 3-ft high (1 ft away from wall) ur/hr B A D At A, 5 x 10-5 Rem for 10 min exposure (Annual limit = 5 Rem) C 57

53 Safety Summary Watch your fingers! Under normal operating conditions, no radiation risks exist. 58