NPL’s progress towards absorbed dose standards for proton beams
H. Palmans1, R. Thomas1, D. Shipley1, A. Kacperek2
1 National Physical Laboratory, Teddington, United Kingdom2 Clatterbridge Centre of Oncology, Wirral, United Kingdom
Presented at the Workshop on Absorbed Dose and Air Kerma Standards, Paris, 9-11 May 2007
Overview
• Proton therapy and to a lesser extent ion therapy are treatment modalities of increasing importance
• Dosimetry has not been as well established as in high-energy x-ray beams
• NPL’s activities in improving proton and ion dosimetry:SR project 2002-2004Graphite calorimetryInteraction dataAlanine dosimetryMonte Carlo simulation of perturbation correction factors
Topics discussed in this talk
• Calorimetry- Graphite calorimetry in CCO beam- Development of a primary standard level graphite calorimeter for light-
ions• Interaction/basic data
- (wair)p value- Stopping powers- Non-elastic nuclear interaction cross sections
• Correction factors for ionization chambers related to:- Recombination- Dose gradients- Secondary electrons- Non-elastic nuclear interactions
Why protons?
30 mm
Graphite calorimetry for protons CCO(Palmans et al 2004, Phys Med Biol 49:3737-49)
27.396
27.398
27.400
27.402
27.404
27.406
0 500 1000 1500 2000 2500 3000 3500 4000 4500
time (s)
T (º
C)
Graphite calorimetry for protons CCO(Palmans et al 2004, Phys Med Biol 49:3737-49)
Heat transfer: FE (Comsol)
depth (cm graphite)
0.950
0.970
0.990
1.010
1.030
1.050
0.0 0.5 1.0 1.5 2.0 2.5
k vol
,unm
odul
ated
0.995
0.996
0.997
0.998
0.999
1.000
1.001
1.002
1.003
1.004
1.005
kvol,modulated
Volume/gap effects: MC (McPTRAN.RZ)
-0.0010
0.0010
0.0030
0.0050
0.0070
0.0090
0.0110
0.0130
0.0150
0.0170
100.0 150.0 200.0 250.0 300.0 350.0 400.0
t (s)
ΔT
(K)
measurement
simulation
Graphite to water conversion 1: interaction cross sections(ICRU-49 and ICRU-63)
0.75
0.80
0.85
0.90
0.95
1.00
0 100 200 300
Energy (MeV)
[ σnu
cl/A
] w,g
(b)
1.10
1.11
1.12
1.13
1.14
1.15
1.E+00 1.E+01 1.E+02 1.E+03
Energy (MeV)
s w,g
(a)
Graphite to water conversion 2: dose conversion(ICRU-49 and ICRU-63)
0.985
0.990
0.995
1.000
1.005
1.010
1.015
1.020
1.025
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Water equivalent depth (cm)
Dw =
Dg x
con
vers
ion
stopping power ratio only
stopping power ratio and nuclearinteractions
0.5%
1%
Graphite to water conversion 3: fluence correction
0.990
1.000
1.010
1.020
1.030
1.040
1.050
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Water equivalent depth (cm)
Flue
nce
corr
ectio
n fa
ctor
GEANT4
MCNPX
McPTRAN.MEDIA
Experiment pp inphantom (prelimin)Experiment FC(prelimin)
Graphite calorimetry results CCO
0.96
0.97
0.98
0.99
1.00
1.01
1.02D
cal/D
ion
0.98
0.99
1.00
1.01
1.02
1.03
1.04
NE2561 (Co-60)NACP02 (Co-60)Markus (Co-60)NACP02 (e-19)Markus (e-19)
modulated beam
Jun-03
Jun-03
Jun-03
Jun-03
Jun-03
Jun-03
Jun-03
non-modulated beam
Uncertainty
New standard level graphite calorimeter for light-ion beams(cfr Mark Bailey / this workshop)
• Either calibrate ionization chambers or measure kQdata
• Large enough for scatter build-up
• Light enough to be portable
• Robustness, vacuum operation, core size, perturbation, alignment and beam monitoring considerations
Derivation of (wair)p from calorimeter measurements
ccairwcair
ppairwpair
psWpsw
⋅⋅⋅⋅
≈)()()()(
,
,
cwD
ppcalw
cwD
pwDQ N
MDNN
k ==,,
,,
,,
,,
ppairwcwD
ccairwcairpcalwpair
psN
psWDw
⋅⋅
⋅⋅⋅=
)(
)()()(
,,,
,,,
pM ⋅
Importance: new recommendation on proton dosimetry by ICRU/IAEA
33,0
34,0
35,0
36,0
37,0
0 100 200 300 400
E (MeV)
(wai
r) p(J
/C)
IAEA TRS398
New ICRU/IAEA – Jones 2006 (Rad Phys Chem 75:541-50)
Medin et al 2006 (Phys Med Biol 51:1503-21)
New ICRU/IAEA
Faraday cup measurements for range and attenuation measurements
Plates
Faraday cupMonitorchamber
Protonbeam
To elec-trometer
Guard
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
0.0 1.0 2.0 3.0 4.0
Graphite thickness (cm)
Cha
rge
(nC
)
Measured data points
Attenuation fit
Tangent at 50% range
Distal edge fit
Range results
Nuclear attenuation results
• Factor 2 to 3 higher than expected from ICRU 63 tables: not as yet understood.
• Hypothesis: wide angle secondary protons:
Plates
Faraday cup
Guard
Correction factors for ionization chambers: recombination(Palmans et al 2006, Phys Med Biol 51:903-17)
BEAM
PHANTOM
IC AIR CAVITIES
IC1 IC2BEAM
PHANTOM
IC AIR CAVITIES
IC1 IC2
0.0
0.1
0.2
0.3
0.4
0.0 0.2 0.4 0.6 0.8 1.0
Time (in 1/8 revolutions)
Dos
e ra
te (G
y s-1
)
surface
z = 23 mm z = 20 mmz = 10 mm
1.000
1.010
1.020
1.030
1.040
1.050
1.060
1.070
1.080
0.0 1.0 2.0 3.0IV or IV,eff (nA)
I V/I V
/n
(d)
0.998
1.000
1.002
1.004
1.006
1.008
1.010
1.012
1.014
1.016
1.018
Equati
on (1
)
Pulsed
(Boa
g)
Markus
1Mark
us 2
p ion
Perturbation correction factors for ionization chambers
protons
⎟⎟⎠
⎞⎜⎜⎝
⎛ρ
⋅water
air
S⋅=waterD airD ⋅ pcav ⋅ pwall ⋅ pcel ⋅ pdis
SA
• For high-energy x-rays: typical corrections of level 1% applied since 1970’s
• For protons: not applied in any recommendation
pdis: Monte Carlo - McPTRAN.CAVITY(Palmans 2006, Phys Med Biol 51:3483-501)
1
2
3
Geometry interrogation region
Protonbeam
E
dϕ /dE
Secondary electrons: EGSnrc + variance reduction techniques (Verhaegen&Palmans 2001 Med. Phys. 28:2088)
• D in cavities~~~~~~~~~~~~~~~~~• Histories resumed
Chamber completeNew depth
~~~~~~~~~~~~~~~~~• Variance reduction
Lateral range rejectionHistory splitting
~~~~~~~~~~~~~~~~~• PDDIC compared
with PDD in homogeneous water
pdis: analytical modelIntegrating the depth dose curve(Palmans 2006, Phys Med Biol 51:3483-501)
z
sleevewallairc.e.
z0
waterx
Rcel
Rcav
Rwall
Rsleeve
O
Proton
P QS T
U
I
pdis: comparison with experimentPDD’s(Palmans 2006, Phys Med Biol 51:3483-501)
Mobit et al. 2000 Med. Phys. 27:2780-2787
78 MeV protons
Jäkel et al. 2000 Phys. Med. Biol. 45:599-607
3 GeV 12C
0.0
0.5
1.0
1.5
2.0
2.5
0.0 20.0 40.0 60.0
depth (mm)
norm
alis
ed d
ose
(a.u
.)
AttixCapintec PR06Reconstructed
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
115.0 120.0 125.0 130.0
depth (mm)
rela
tive
dose
ReferencePTW-30006MarkusReconstructed
pcav,e: SA cavity theory
⎟⎟⎠
⎞⎜⎜⎝
⎛ρ
⋅=med
airairmed
SDD
⎟⎟⎠
⎞⎜⎜⎝
⎛ρ
⋅=med
airair
SD ⋅ cav,ep
med
air
med
airecav
SSp ⎟⎟⎠
⎞⎜⎜⎝
⎛ρ⎟⎟
⎠
⎞⎜⎜⎝
⎛ρ
=SA
,
protonsSA
δ-electrons
due to secondary electrons
pwall,e: SA cavity theory
δ-electrons SA
⎟⎟⎠
⎞⎜⎜⎝
⎛ρ
⋅=med
airairmed
SDD
protons
⎟⎟⎠
⎞⎜⎜⎝
⎛ρ
⋅med
wall
Swall
⎟⎟⎠
⎞⎜⎜⎝
⎛ρ
⋅=med
airair
SD ⋅ wall,epSA
med
air
wall
airwall,ep = S
⎟⎟⎠
⎞⎜⎜⎝
⎛ρ
SA
⎟⎟⎠
⎞⎜⎜⎝
⎛ρ
⋅med
wall
S S⎟⎟⎠
⎞⎜⎜⎝
⎛ρ
SA
pcav,e & pwall,e: SA cavity theory for
FWT-IC18 type chamber
0.998
1.000
1.002
1.004
1.006
1.008
1.010
0.01 0.10 1.00 10.00 100.00Rres
p cav
,e.p
wal
l,e
PCAV*PWALL_P_TEP_E_TEE50 MeV
150 MeV250 MeV
FWT-IC18
pcav,e & pwall,e: Monte Carlo versus SA cavity theory (spher r = 0.25cm, Δ = 13.2 keV)
1.000
1.002
1.004
1.006
1.008
1.010
0 50 100 150 200 250 300
Eeff (MeV)
p cav
,e.p
wal
l,e
A150CPMMAwater
pcav,e & pwall,e: comparison with experiment
0.995
1.000
1.005
1.010
1.015
1.020
0 5 10 15
Chamber #
Dw
,NE2
571
/Dw
,Ch
C-C&PTW30002
A150-Al&NE2581
PMMA-Al&PTW30001
Nylon66-Al
IC18
ExrT2
0.0E+00
1.0E-04
2.0E-04
3.0E-04
4.0E-04
5.0E-04
0 20 40 60 80 100 120 140E (MeV)
φ E( cm-
2 MeV
-1 )
0.0E+00
2.0E-06
4.0E-06
6.0E-06
8.0E-06
1.0E-05
0 20 40 60 80 100 120 140E (MeV)
φ E( cm-
2 MeV
-1 )
p: α:
pwall,n: (simplistic) analytical model for slowing down spectra secondaryp & α (NE2571, 150 MeVp)
water bulk
“water” wallgraphite wall
due to secondaries from nonelastic nuclear interactions
pwall,n: secondary p & α perturbation (NE2571)
• BUT:
• ICRU 63 data (uC ~ 30-40%)
• Crude model
• → MC study needed
0.992
0.994
0.996
0.998
1.000
1.002
0 50 100 150 200 250 300E (MeV)
pert
urba
tion
fact
or
pp+α
Summary
• Graphite calorimetryMany operation characteristics, perturbation factors and heat
transfer phenomena are similar as for photonsPrimary standard level calorimeter is being builtConversion to dose to water is a serious issue
• Interaction/basic data:Substantial contribution to (wair)p valueRange and attenuation measurements
• Ionization chambersCorrections for recombination, gradients and secondary electronsFurther work: non-elastic nuclear interactions
Acknowledgments
• Simon Duane, Thomas Russell, David Shipley, Mark Bailey, Alan DuSautoy, Andrzej Kacperek (CCO), Frank Verhaegen, Jan Seuntjens and…