Referensdosimetri. Crister Ceberg Medical Radiation Physics Lund University Sweden

Transcription

1 Referensdosimetri Crister Ceberg Medical Radiation Physics Lund University Sweden

2 Reference dosimetry Determination of absorbed dose to water under reference conditions Not accounting for uncertainties related to non-reference conditions (for instance the patient...) High accuracy is needed (~ 1.5%) Requires a rigorous dosimetry protocol

3 Reference dosimetry protocols AAPM TG-51 (1999), North America DIN (1997, revised 2008), Germany NCS (1997), The Netherlands IPEMB (1996), UK IAEA TRS-398 (2000)

4 IAEA TRS-398 Published in 2000 Replace previous protocol TRS-277 Based on calibration of the instruments in absorbed dose to water

5 IAEA TRS-398

6 International measurement system

7 Traceability Bureau International des Poids et Mesures (BIPM) 1875 International Laboratory for SI units Primary Standard Dosimetry Laboratory (PSDL) Widely acknowledged Highest metrological qualities Accepted without reference to other standards About 20 worldwide Secondary Standard Dosimetry Laboratory (SSDL) Depends on calibration at a PSDL Clinical user at the hospital Depends on calibration at SSDL or PDSL

8 Primary standard laboratory (PSDL) ISO definition of primary standard a standard that is designated or widely acknowledged as having the highest metrological qualities and whose values is accepted without reference to other standards of the same quantity This requires an absolute dosimeter, that can be assembled and used to measure the absorbed dose deposited in its own sensitive volume without calibration in a known field of radiation (Attix)

9 Absolute dosimeters Ionization chamber (BIPM) Fricke dosimeter (PTB until 2006) Water calorimeter (NIST, NRC, PTB since 2006) Graphite calorimeter (NPL)

10 Ionization chamber Graphite ion chamber with well defined volume Designed to fulfill the Bragg-Gray conditions with very little perturbation

11 Ionization chamber Boutillion and Perroche, PMB 38 (1993)

12 Ionization chamber M/m W Boutillion and Perroche, PMB 38 (1993)

13 Fricke dosimeter Ferrous ions (Fe 2+ ) in water Interaction with ionizing radiation leads radiolysis of water Iron ions are oxidized to ferric ions (Fe 3+ ) The UV transmission spectrum is affected Response is determined by total absorption of electrons

14 Fricke dosimeter Feist, PMB 27 (1982)

15 Calorimeter Measurable temperature increase in medium Water Graphite Assumes that All radiation energy is transfered to heat All heat comes from radiation energy

16 Calorimeter Measures directly the absorbed energy per unit mass D=DT i c i Increased temperature [K] Specific heat capacity [J kg -1 K -1 ]

17 Calorimeter Ross and Klassen, PMB 41 (1996)

18 Calorimeter Medin, Lund University

19 Calorimeter AC R B A Variable R DU = U A -U B Variable C R Thermistors Medin, Lund University

20 Calorimeter 6 Bridge output voltage V) ( DV post-irradiation drift -8 pre-irradiation drift Time (s) Medin, Lund University

21 Calorimeter Requires several correction factors D=DT i c i k c k p k dd (1-k HD ) -1 k c : heat conductivity k p : perturbation due to glass container k dd : radiation field inhomogeneity k HD : heat defect

22 Degree of equivalence

23 Uncertainty in D w (1SD) at PSDL BIPM (ion chamber) 0.30% NIST (water calorimeter) 0.35% NRC (water calorimeter) 0.41% METAS (water calorimeter) 0.41% PTB (water calorimeter) 0.20% LSDG (water calorimeter) 0.66% NMi (graphite calorimeter) 0.40% ARPANSA (graphite calorimeter) 0.20% BEV (graphite calorimeter) 0.37% ENEA (graphite calorimeter) 0.44% LNHB (graphite calorimeter) 0.47%

24 N D,W -based formalism D w,q0 =M Q0 N D,w,Q0 Q 0 denotes the reference beam quality at the standard laboratory D W,Q0 is known at the standard laboratory M Q0 is the dosimeter reading under reference conditions N D,W,Q0 is the dosimeter s absorbed-dose-to-water calibration coefficient

25 Correction for radiation quality D w,q =M Q N D,w,Q =M Q N D,w,Q0 k Q,Q0 Q denotes the quality at the user D W,Q is the absorbed dose in the user s beam M Q is the dosimeter reading at the user k Q,Q0 corrects for the effects of the difference between the reference beam quality and the beam quality at the user

26 The beam quality correction factor N D,w,Q k Q,Q0 = = N D,w,Q0 D w,q /M Q D w,q0 /M Q0 Most often, the reference beam quality is 60 Co, and then k Q,Q0 =k Q

27 Experimental determination of k Q,Q0 Ideally, the k Q,Q0 factor should be measured for each chamber at the desired beam quality However this requires Standard laboratories with clinical beam qualities Independent dosimetry (e.g. calorimeters) operating at these beam qualities

28 Theoretical determination of k Q,Q0 Instead, theoretically determined k Q,Q0 factors are tabulated in the IAEA TRS 398 protocol for different chamber types for different beam qualities

29 0.2%

30 Beam quality For photons TPR 20,10 TPR 20,10 =TPR(20)/TPR(10) TPR 20,10 = PDD 20, For electrons R 50 Depth where absorbed dose is 50% of maximum R 50 =1.029 R 50,ion (R 50,ion 10 cm) R 50 =1.059 R 50,ion (R 50,ion > 10 cm) 6 MeV elektoner Relativ dos Djup i vatten (mm)

31 Practical details before use Store instruments in a safe place (dry, normal room temperature, clean, etc.) Inspect the instrument before use Make an x-ray image of the instrument at the time of purchase, or if problems arise

32 Stability check Check and document long term stability (both detector and electrometer) Perform stability check before and after the detector is sent for calibration at the standards laboratory

33 Practical details during measurement Allow time to reach thermal equilibrium Turn on the electrometer at least 1-2 h before use Always collect several measurements (5-10) Pay attention to trends (can be sign of equipment failure) Leakage should be < 0.1% of M Q

34 Pre-irradiation effects Pre-irradiate the chamber to reach charge equilibrium in the exposed materials After change of voltage or polarity, be careful with restabilisation (> 20 min), consider using normalisation to an external monitor chamber

35 Pre-irradiation effects Normalized response for an NE 2571 to 60 Co irradiation McCaffrey et al. PMB 2005

36 Correction of M Q for influence quantities M Q =M k TP k elec k pol k s Uncorrected reading: M Temperature and pressure: k TP Electrometer correction factor: k elec Polarity effect: k pol Recombination effect: k s Humidity: k h (rarely used)

37 Correction of M Q for influence quantities M Q =M k TP k elec k pol k s Uncorrected reading: M Temperature and pressure: k TP Electrometer correction factor: k elec Polarity effect: k pol Given by the SSDL together with N D,w Recombination effect: k s Humidity: k h (rarely used) Not needed if calibrated at 50% humidity (20-80%)

38 Temperature and pressure The temperature and pressure correction converts the cavity air mass to the reference conditions: k TP = (273.2+T) P 0 (273.2+T 0 ) P where the reference conditions generally are T 0 =20 C P 0 =101.3 kpa

39 Polarity Polarity correction for asymmetric charge collection (mean value of readings at both polarities): k pol = M + + M - 2M where M + and M - are the readings at positive and negative polarity, respectively If not performed at the standards laboratory, a correction for the relative effect can be done

40 Recombination The recombination factor corrects for incomplete charge collection using the two-voltage method: ( M 1 ) M 2 k s =a 0 +a 1 + a 2 ( M 1 ) 2 M 2 where M 1 and M 2 are the readings at two different operating voltages V 1 and V 2, such that V 1 3 V 2, and a 0 - a 2 are constants found in the TRS 398 protocol If not performed at the standards laboratory, a correction for the relative effect can be done

41 Reference dosimetry in the user beam The absorbed dose to water is then given by D w,q =M Q N D,w,Q0 k Q,Q0 where M Q is corrected for influence quantities, and k Q,Q0 is determined based on the user s beam quality

42 Reference dosimetry in the user beam Take care that the linac is operating properly! If not, accurate reference dosimetry can be a source of errors