An Overview of Radiotherapy Dosimetry at the NPL. Hugo Palmans National Physical Laboratory, UK

Transcription

1 An Overview of Radiotherapy Dosimetry at the NPL Hugo Palmans National Physical Laboratory, UK Radiotherapy users group meeting, NPL, 5 June 2007

2 Overview High-energy photon and electron beams + 60 Co: Absorbed dose to water standards Low- and medium-energy x-rays + 137Cs + 60Co: Air kerma standards Brachytherapy: 192 Ir: Air kerma standard + recent developments towards absorbed dose to water Opthalmic beta applicators: Absorbed dose to water standard Radiotherapy users group meeting, NPL, 5 June /33

3 Absorbed dose standards: definition of absorbed dose D med = dε dm med Mean energy imparted to matter per unit of mass. Energy imparted = energy incident minus energy leaving + energy released in transformations Example hν 1 dm T T hν 2 hν 3 e- hν 4 ε =hν 1 (hν 2 + hν 3 + T ) Radiotherapy users group meeting, NPL, 5 June /33

4 Absorbed dose standards: calorimetry radiation energy turns into heat heat is tiny even though biological effect is lethal in radiotherapy tiny, but measurable / our primary standards for absorbed dose are calorimeters Radiotherapy users group meeting, NPL, 5 June /33

5 Absorbed dose standards: principles of calorimetry D=c T T c. = c. α. 2 T t D + t c (J kg -1 K -1 ) T/D (mk Gy -1 ) α (m 2 s -1 ) water graphite Radiotherapy users group meeting, NPL, 5 June /33

6 Absorbed dose standards: principles of calorimetry thermistors to measure temperature differences ultimate sensitivity is around one µk signal is around one mk signal to noise is of the order 1000:1 R e Radiotherapy users group meeting, NPL, 5 June /33 R β 1 T 1 T = 0 0

7 Absorbed dose standards: the most direct method is water calorimetry (e.g. PTB, NRC, METAS,NMi, LSDG) Beam Air 4ºC Cooling fluid Radiotherapy users group meeting, NPL, 5 June /33

8 Absorbed dose standards: graphite calorimetry Beam D w D C Radiotherapy users group meeting, NPL, 5 June /33

9 Absorbed dose standards at NPL: primary standard for high-energy x- rays Radiotherapy users group meeting, NPL, 5 June /33

10 Radiotherapy users group meeting, NPL, 5 June /33 Absorbed dose standards at NPL: conversion procedure for high-energy photons (1) w C en C w C w C w C w ρ µ β β = d d Q Q D D (2) w C en C w C w C w 2 ρ µ β β = K f f D D

11 Absorbed dose standards at NPL: electron graphite calorimeter (electron calibration service) Radiotherapy users group meeting, NPL, 5 June /33

12 Absorbed dose standards at NPL: conversion procedure for high-energy electrons Reference depth in water z w = 0.6 R, w cm Range scaling to get depth in graphite z g = z w R R 50, g 50, w Calibrate chamber against graphite calorimeter, in graphite N D, ref, g = M D g ref, g Theoretical conversion of dose to graphite to dose to water N ref, w D, ref, w = ND, ref, g pref, g w, g, Radiotherapy users group meeting, NPL, 5 June /33 p Compare user and reference chambers at z w in water N D, userw, = ND, ref, w M M s s ref, w user, g (McEwen et al 1998, Phys Med Biol 43: )

13 Absorbed dose standards at NPL: portable graphite calorimeter (off site measurements) Radiotherapy users group meeting, NPL, 5 June /33

14 Absorbed dose standards at NPL: Dose determination with ion chamber D w = D s w, p E.g. extrapolation chamber: D = Q m W e = Q ρ V W e D = ρ 1 Q V W e 1 dq W = ρ dv e 1 dq W = ρ A dx e HV x guard c.e. A Radiotherapy users group meeting, NPL, 5 June /33

15 Absorbed dose standards at NPL: alanine dosimetry Radiotherapy users group meeting, NPL, 5 June /33

16 Absorbed dose standards at NPL: alanine dosimetry Not a primary standard but plays a significant role: Auditing and quality assurance Tomotherapy and potentially IMRT dosimetry Ophtalmic beta applicators See presentation Simon Duane Radiotherapy users group meeting, NPL, 5 June /33

17 Absorbed dose standards at NPL new directions: proton and ion beams 30 mm CCO experiment (Palmans et al 2004, Phys Med Biol 49: ) T (ºC) Radiotherapy users group meeting, NPL, 5 June /33 time (s)

18 Absorbed dose standards at NPL new directions: proton and ion beams Importance: new recommendation on proton dosimetry by ICRU/IAEA (Jones 2006, Rad Phys Chem 75:541-50) New ICRU/IAEA 37,0 36,0 (w ) p (J/C) 35,0 34,0 IAEA TRS398 New ICRU/IAEA Medin et al 2006 (Phys Med Biol 51: ) 33, E (MeV) Radiotherapy users group meeting, NPL, 5 June /33

19 Absorbed dose standards at NPL new directions: proton and ion beams New standard level graphite calorimeter for light-ion beams (cfr new photon electron calorimeter / Mark Bailey) Either calibrate ionization chambers or measure k Q data Large enough for scatter build-up Light enough to be portable Robustness, vacuum operation, core size, perturbation, alignment and beam monitoring considerations Radiotherapy users group meeting, NPL, 5 June /33

20 Absorbed dose standards at NPL new directions: 192 Ir Calorimeter with ring-shaped core Dimensions optimised by Monte Carlo simulations and heat transfer simulations Source self heating shielded from core by vacuum gaps Operated in isothermal mode Radiotherapy users group meeting, NPL, 5 June /33

21 Absorbed dose standards at NPL new directions: 192 Ir, schematic drawing of prototype calorimeter (RZ-geometry) Z R Z = 14 cm Absorber: Graphite ring, 2 mm x 5 mm, R = 2.5 cm R = 10 cm 1 mm vacuum gap Radiotherapy users group meeting, NPL, 5 June /33

22 Absorbed dose standards at NPL new directions: the future More sensitive thermometry (low-temperature) Non-contacting thermometry (microwave resonators) Micro-bolometers for microdosimetry Radiotherapy users group meeting, NPL, 5 June /33

23 Air kerma standards: definition of kerma K = detr dm de tr : sum of initial kinetic energies of all charged particles liberated by uncharged particles in a mass of dm Example hν 1 dm T T hν 2 hν 3 e- hν 4 E tr = T Radiotherapy users group meeting, NPL, 5 June /33

24 Air kerma standards: based on the measurement of exposure/ionization ionisation in easier to measure than heating in water limiting sensitivity is a few fa (10-15 amps) signal is hundreds of pa (10-10 amps) signal to noise is of the order 10 5 :1 free- chamber photons interact in, electrons stopped in cavity chamber photons, electrons interact in wall (e.g. graphite), not measured current corrected for : - density ρ (p,t) - humidity (NPL calibrations): 0.3% correction is included nothing else needed if RH at the time of measurement is between 20% and 70% - recombination (NPL calibrations) NOTE: kerma applies to readings without further correction absorbed dose applies to readings corrected to zero ion recombination recombination is generally significant only in pulsed beams with large instantaneous dose rates Radiotherapy users group meeting, NPL, 5 June /33

25 Air kerma standards at NPL: free- chambers primary standard of exposure (we measure the ionisation per mass of ), rather than of kerma (we don t measure the energy released) every electron track producing ionisation which will be collected should originate from a primary photon absorption or scattering event in. no electron track can reach the collecting electrodes. Radiotherapy users group meeting, NPL, 5 June /33

26 Air kerma standards at NPL: free- chambers A L A.L M K = ( ). W / e). k. k. k. k. k s h a sc V.ρ Radiotherapy users group meeting, NPL, 5 June /33 e

27 Air kerma standards at NPL: free- chambers Improved correction factors by measurement (attenuation and photon scatter but also recombination) and Monte Carlo simulations (including fluorescence generated by argon) Will lead to improved uncertainty analysis (Contact Rebecca Nutbrown) Radiotherapy users group meeting, NPL, 5 June /33

28 Air kerma standards at NPL: cavity ionisation chambers (as primary standards) at energies much higher than 300 kv, the size of a free- chamber becomes impractically large. use cavity standards instead, but ensure that volume is measurable apply corrections for wall material (graphite instead of ) the fraction going into radiation losses attenuation and scatter in the wall Radiotherapy users group meeting, NPL, 5 June /33

29 Air kerma standards at NPL: priniciple for high-energy gamma beam K K col, K col, = 1 g ( µ ρ) en K col C katt =, C, D S ρ SA C, D C k cep Q V ρ W e Radiotherapy users group meeting, NPL, 5 June /33

30 Air kerma standards at NPL: priniciple for high-energy gamma beam K Q ( ) W e S SA = C V g, ρ 1 ρ C, ( µ ρ) en ki k = k cep k att k sc k stem k hum k s k pol kion K wall relative response k att.k sc x k cep = k wall graphite thickness (mm) Radiotherapy users group meeting, NPL, 5 June /33

31 Air kerma standards at NPL: 60 Co At present based on three 1.8cc chambers A set of variable volume chambers has been constructed to replace the aging existing standards (Contact Andy Shah) Radiotherapy users group meeting, NPL, 5 June /33

32 Air kerma standards at NPL: 192 Ir (RAKR) Spherical shell (carbon) Chamber stem (carbon fibre) Central conductor and electrode (carbon) Insulator Radiotherapy users group meeting, NPL, 5 June /33

33 Air kerma standards at NPL: 192 Ir (RAKR) Comparison with LNHB (equiv. within uncertainties) Monte Carlo calculated perturbation correction factors Improved correction factors for ion recombination and polarity Improved correction factor for attenuation and scatter Improved uncertainty budget (Contact Thorsten Sander) Radiotherapy users group meeting, NPL, 5 June /33