Ionizing Radiation Group K.B. Lee

Transcription

1 Ionizing Radiation Group K.B. Lee 1

2 Contents ha standard equipment for metrology in in nuclear medicine hwhy Monte Carlo technique in in nuclear medicine hpenelope Monte Carlo program hmonte Carlo modeling hpreliminary simulation results hfuture plan 2

3 Traveling standard A new ion chamber has been developed in KRISS, serving as a traveling standard used to calibrate the radionuclide calibrators in hospitals and clinics. Radionuclide calibrator : The de facto standard instrument for activity measurement of radiopharmaceuticals in the medical institutes 3

4 Ionization chamber hwell-type, re-entrant hgamma sensitive hwell diameter :: 2 inches hcentronics h15 h15 atm atmargon gas gas hwell liner liner hsample holders of of different heights hkeithley 6517A electrometer hcurrent mode hbuilt-in power supply 4

5 IC characterization Efficiency variation over applied voltage Efficiency variation over source position 5

6 Consistency check 6

7 Scheme of QA in radioassay hcalibration service service (KRISS) Accuracy requirements Diagnostic accuracy total total :: < % instrumental :: < 5 % Therapeutic accuracy total total :: < 5 % instrumental :: < 2 % hintercomparison and and proficiency test test (KFDA) 7

8 Calibration factor The chamber needs to be calibrated to obtain the activity from the current measured. Calibration factor depends on the measurement geometry calibration factor ( pa => MBq ) hsolution volume and and density hglass wall wall thickness of of container (( vial vial and and syringe) hmetal wall wall thickness of of chamber 8

9 Why Monte Carlo technique The chamber calibration factor depends on the volume of the source and on the type of the measurement vessel. Modification of the geometry requires to recalibrate the chamber in the new geometry. + Solution volume ml, ml, 1 ml, ml, 2ml, 2ml, ml, ml, 5ml 5ml Experimental determination of all the calibration factors cost much time and money Use the PENELOPE Monte Carlo code to calculate the calibration factor under varying conditions and complement the experimental determinations. 9

10 PENELOPE code PENetration and Energy LOss of Positrons and Electrons PENELOPE V 2003 Applicable from 100 ev to 1 GeV energy Photon, electron and positron simulation Photon : Rayleigh, photoelectric, compton, pair production Electron/positron : elastic, inelastic, Bremsstrahlung, positron annihilation Neither hadronic interactions nor bragg diffraction Atomic relaxation (characteristic X-ray, Auger electron) 10

11 Simulated model Composed of 35 elementary volumes described by 62 surfaces Radioactive solution in the glass vial PMMA liner and source holder Ionization chamber The materials Water, air, aluminum, iron, argon, pyrex glass, rubber, stainless steel Established using the specifications of the manufacturer for the pressure, the type of materials, and the dimension of the chamber 11

12 Simulation parameters E abs : cut-off energy for absorption W cc, W cr : cut-off energy loss for hard interactions C 1 : determine the mean free path between hard elastic events C 2 : maximum average fractional energy loss in a single step E abs (electron) (kev) E abs (photon) (kev) E abs (positron) (kev) C 1, C 2 W cc, W rc (kev) Argon The other materials Best compromise between speed and accuracy Each calculation begins with different random generator seeds to avoid any correlation in the simulation. No variance reduction method 12

13 Geometry IC geometry (in mm) 8R vial geometry (in cm) 13

14 IC and vial simulated 14

15 Simulation result (I) Deposited energy distribution in the gas for 100 kev photons generated inside the source volume Response depends on volume of solution (different volume => different geometry and attenuation) Source volume dependence Source volume Deposited energy difference 0.5 ml (971.7 ± 1.5) ev 2.8 % 1 ml (947.4 ± 1.5) ev 0.3 % 1.5 ml (945.0 ± 1.5) ev 0.0 % 2 ml (941.3 ± 1.5) ev -0.4 % 2.5 ml (929.9 ± 1.5) ev -1.6 % 15

16 Simulation result (II) Variation in wall thickness of glass vials is ± 0.05 mm in 1 mm Different wall thickness requires to use different calibration factors Vial type dependence Vial type 8R vial Wall thickness 1 mm Deposited energy Difference (971.7 ± 1.5) ev 0.0 % 10R vial 1 mm (961.5 ± 1.5) ev -1.0 % P6 vial 1.2 mm (877.4 ± 1.5) ev -9.1 % 16

17 Future plan Test and validate the Monte Carlo simulation by comparing between calculated and experimental calibration factors. Tune the Monte Carlo code for the absolute measurements of activity by the ion chamber. Use the Monte Carlo calculation to provide calibration services to the radiopharmaceutical industry. 17