New Dosimetry Measurements- New Developments
J. Seuntjens J. Gibbons
I. Das
WA-A-211A Continuing Education Session
Much of what we will say…
Clinical Dosimetry Measurements in Radiotherapy
(AAPM 2009 Summer School)
D. W. O. Rogers & J. Cygler Editors ISBN: 9781888340846 Published: 2009
1,128 pp Hardcover
Price: US $140.00
AAPM, July 29, 2009 2
Session Outline
• Part I: Measurement dosimetry principles and application to nonstandard beams - J. Seuntjens
• Part II: Practical aspects of measurement dosimetry - J. Gibbons
• Part III: Small field dosimetry - I. Das
3
Outline - Part I
• Background• Definitions and nomenclature• Fundamental issues in nonstandard beam
dosimetry• New formalism for nonstandard beam
reference dosimetry
Background
5
Background
• Developments in RT techniques have substantially increased the use of small fields and larger uniform or non-uniform fields, composed of small fields
• Beam modifiers shaping small and composed fields such as standard-, mini- and micro- MLCs have become common equipment on conventional linacs
• Radiation therapy procedures are shifting from traditional 3D conformal deliveries to:
– novel, dedicated treatment units specifically designed for stereotactic (GammaKnife, CyberKnife) or IMRT treatments (TomoTherapy)
– traditional units delivering dynamically composed fields (RapidArc, VMAT, IMRT, etc)
AAPM, July 29, 2009
6AAPM, July 29, 2009 7
In practice…Modality Typical
calibration field size (static)
S&S or dynamic capabilities?
IMRT, SRS 10 x 10 cm2 Yes
TomoTherapy 5 x 20 cm2 Yes
Cyberknife 6 cm diameter Yes
GammaKnife 1.6 cm / 1.8 cm diameter
Yes
8
Issues with nonstandard beams
• There are no primary standards that measure dose directly in nonstandard beams
• Delivery protocols in nonstandard beams are not standardized (and not easily “standardizable”)
• The path between a static open 10x10 cm2
field calibration and delivery conditions introduces significant uncertainties
9
Large differences in Output Factorsamong users/machines
Statistics of 45 Output Factors for 6 mm and 18 mm square fields Novalis, SSD = 100 cm, depth = 5 cm, various detectors)
From Wolfgang Ullrich, BrainLab
factor of 2 in dose determination!
Definitions and nomenclature
11
Small photon fields
broad photon field
volume volume
narrow photon field
A small field is defined as a field with a size smaller than the “lateral range” of charged particles
12
Nonstandard field: small and composite
• Non-standard fields are defined as:– Single small fields
• CPE or TCPE is distorted
– Composition of small fields either in step-and-shoot, arcs, or dynamic multiple fields
• CPE or TCPE may be (but is not necessarily and not intentionally) distorted
• Examples: TomoTherapy, Cyberknife, etc.
• Why are we worried about CPE or TCPE in measurement dosimetry?
Fundamental issues in nonstandard beam dosimetry
14
Classification of dosimeters and mechanisms
Dosimeter Mechanism
Gas-filled ionization chamber Ionization in gasses
Liquid ionization chamber Ionization in liquids
Semiconductors (diodes, diamond detectors, MOSFET)
Ionization in solids
TLD Luminescence
Scintillation counters Fluorescence
Film, gel, Fricke Chemical reactions
Calorimetry Heat
Measurement dosimetry in medium
where:
dose to medium at the point
raw signal, corrected for environmentalconditions as given by detector
detector cavity dose calibration coefficient(coupling constant)
dose conversion coefficient converts averagedetector dose into dose to medium at point
Dmed (r ) = cdetM(r )fmed (r )
Dmed (r )
M(r )
fmed (r )
cdet
Dosimeter dependent coefficients & coupling constants
Factor orcoefficient
Dosimetry techniqueCalorimetry Fricke
dosimetryIon chamberdosimetry
M T∆ ODLρ
∆ Q
cdet C 13( )Fe Gε +
1 Wgasm egas
fmed Unity ( )medD Fricke
smed,gaspQ
17
Why do we worry about CPE or TCPE?
unperturbed medium fluence (SA → CPE)
restricted collision s.p.
There is no theoretical reason why the SA evaluation cannot be performed using disequilibrium fluence.But, …, in this case we don’t expect smed,det to be a good representation of the ratio Dmed/Ddet
18
Stopping power ratios (SPRs)
• In regions of CPE and TCPE: SPR corrections accurately represent detector response (and are small for air-filled chambers in photon beams)
• In regions of non-CPE: SPR corrections DO NOT accurately reflect changes in detector response and additional, sometimes large, corrections are needed– small fields – build-up regions in any field, interface-proximal points in
heterogeneous phantoms (build-up and build-down)– sometimes: intensity modulated fields, etc
ICRU, September 2008 19
Narrow 1.5 mm field - extreme exampleRatio of avg. dose to water <Dw> to dose to cavity air
Off-axis distance (mm)
Model A14P chamberCollecting electrode diameter: 1.5 mmSeparation: 1 mm
0.80
0.90
1.00
1.10
1.20
1.30
1.40
1.50
1.60
1.70
0 2 4 6 8
<D
w>/D
air
Stopping power ratio w/air
averaged over cavity volume
Paskalev, Seuntjens, Podgorsak (2002) AAPM
Proc. Series 13, Med. Phys. Publishing,
Madison, Wi, 298 – 318.
Perturbation correction factors: traditional factorization for ionization chambers:
• Pwall: wall perturbation correction factor (pwall)• Pgr: correction for gradient effect (effective p. of meas., pdis)• Pfl: fluence perturbation correction factor (pcav)• Pcel: central electrode perturbation correction factor (pcel)
PQ = PwallPgrPflPcel
21
Perturbation correction factors
• depth, field size, and radiation quality dependent– for reference dosimetry using ionization chambers:
evaluation based on Monte Carlo calculations, measurements and relatively well documented
– for relative dosimetry: their variation relative to the reference point is traditionally ignored but can be very significant in non-CPE conditions.
ICRU, September 2008 22
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00
IMRT is far from standard reference conditions
From Art Boyer, StanfordICRU, September 2008 23
sw,air = 1.1229
sw,air = 1.1201
Data for IMRT dosimetry can be determined very accurately
(Andreo, ESTRO 2008)but what is the use, …
Bouchard et al, 2009 (Med Phys, in print)
Extreme nonstandard fields
Exradin A12Exradin A14
Bouchard et al, 2009 (Med Phys, in print)
Exradin A14
ICRU, September 2008 26
From Roberto Capote, IAEA
27
Reference dose measurements in dynamic fields
Fraser, D. et al., (2009)
PinPointIC10NE2571 =
=0.973
( ) 0.022NE
NE
x
s x
==
0.963
( ) 0.024IC10
IC10
x
s x==
0.944
( ) 0.035PP
PP
x
s x
A new proposed formalism for reference dosimetry of
nonstandard beams
29
• Two new reference fields:– Small static field dosimetry: machine-specific-
reference field (msr) for treatment machines that cannot establish a conventional reference field.
– Composite (dynamic) field dosimetry: plan-class specific reference field (pcsr). Represents a class of dynamic or step-and-shoot delivery fields, or a combination of fields, such that full charged particle equilibrium (CPE) is achieved at the position of the detector.
Reference dosimetry protocol: the IAEA/AAPM proposal
30
1. Small Static FieldsReference Calibration, ref field fmsr
0 0
,, , , , ,
msr refmsr msrmsr msr msr
f ff fw Q Q D w Q Q Q Q QD M N k k= ⋅ ⋅ ⋅
is a factor which corrects for the differences between the conditions of field size, geometry, phantom material and beam quality of the conventional reference field fref and the machine-specific reference field fmsr
,,
msr ref
msr
f f
Q Qk
refref
msr
msr
msr
msrrefmsr
msr fQ
fQw
fQ
fQwff
QQMD
MDk
,
,,, =
Andreo, ESTRO 2008
31
1. Small Static Fields Relative dosimetry, clinical field fclin
is a field factor which converts the absorbed dose to water for the machine-specific reference field, fmsr to the absorbed dose to water for the clinical field fclin. In relative dosimetry of single static fields this factor is conventionally called a field output factor, which ought to be defined as a ratio of Dw.
,,
, ,clin ms clin msr
clin ms
r
clin m rsr
f fw Q
f fQ Qw QD D= ⋅Ω
,,
clin msr
clin msr
f fQ QΩ
msrclin
msrclinmsr
msr
clin
clinmsrclin
msrclin
ffQQf
Q
fQff
QQ kM
M ,,
,, ⋅=Ω
It can be calculated directly as a ratio of Dwusing Monte Carlo alone or it can be measured as a ratio of detector readings multiplied by a Monte Carlo calculated correction factor.
Andreo, ESTRO 2008
32
Monte Carlo D-ratios vs experimental M-ratios. 6 MV, 5cm depth
Data from S.Dobladoet al (2007)
0 0
,, , , , ,
msr refmsr msr
msr msr msr
f ff fw Q Q D w Q Q Q Q QD M N k k= msrclin
msrclin
msr
msr
clin
clin
ffQQ
fQw
fQw DD ,
,,, Ω=Machine specific
reference field fmsr
Clinicalfclin
e.g. a GammaKnifeclinical plan
msrclin
msrclin
ffQQ
,,Ω
Tomotherapy5cm x 20cm
REFERENCE DOSIMETRY RELATIVE DOSIMETRY
GammaKnife∅∅∅∅ 1.6/1.8 cm
CyberKnife0.6 cm
1
≡ Ionizationchamber
Broad beamreference field
fref
00 ,,, QQQwD kN
Hypothetical
reference field fref
BrainLABmicro MLC10cmx10cm
refmsr
msr
ffQQk,,
Radiosurgical collimators∅∅∅∅ 18 mm
refmsr
msr
ffQQk,,
Andreo, ESTRO 2008
34
2. Composite FieldsReference Calibration, ref field fpcsr
refpcsr
pcsr
pcsr
pcsr
pcsr
pcsr
ffQQQQQwD
fQ
fQw kkNMD ,
,,,,, 00⋅⋅⋅=
A plan-class specific reference field, pcsr, is a reference field for a class of dynamic or step-and-shoot delivery fields, or a class of combinations of fields in a configuration that is as close as possible to the final clinical delivery scheme, but delivers a homogeneous absorbed dose to an extended and geometrically simple target volume
Andreo, ESTRO 2008
35
2. Composite Fields Relative dosimetry, clinical field fclin
,,
, ,pcs clinrclin
c
pcs
lin
r
clip ncsr pcsr
f f
Q
ffw Q w Q QD D ⋅Ω=
has to be established for every treatment delivery, and can be determined using the same set-up and the same phantom as for calibration. E.g. for a patient specific clinical plan, the phantom should be the same as for the dosimetry in the pcsr field using a cross-calibrated chamber.
,,
clin pcsr
clin pcsr
f f
Q QΩ
Andreo, ESTRO 2008
refpcsr
pcsr
ffQQk ,,
refpcsr
pcsr
pcsr
pcsr
pcsr
pcsr
ff
QQQQQwD
f
Q
f
Qw kkNMD,,,,,, 00
= pcsrclin
pcsrclin
pcsr
pcsr
clin
clin
ff
f
Qwf
Qw DD,,,, Ω=
Broad beamreference field
frefPlan-class
specific reference fieldfpcsr
00 ,,, QQQwD kN
Clinicalfclin
msrpcsr
msrpcsr
ffQQk,,
Hypotheticalreference field e.g. 9-field prostate
pcsre.g. 9-field prostate
clinical plan
pcsrclin
pcsrclin
ffQQ
,,Ω
(e.g. IMRT Linac)
refmsr
msr
ffQQk,,
fmsr
(e.g. Tomotherapy5cm x 20cm)
REFERENCE DOSIMETRY RELATIVE DOSIMETRY
20º 60º
100º
140º180º220º
260º
300º 340º
2
≡ Ionizationchamber
Andreo, ESTRO 2008
0.9775
0.9800
0.9825
0.9850
0.9875
0.9900
0.9925
0.9950
0.9975
1.0000
1.0025
1.0050
1.0075
Exradin A12 NE2571 Exradin A1SL Exradin A14 PinPoint 31006
Ionization chamber
k_p
csr
Fully-rotated Delivery
Farmer-type 0.6 cm3
ionization chamberssmaller ionization chambers
Ionization
Chamber
Exradin
A12NE2571
Exradin
A1SL
Exradin
A14
PinPoint®
31006
Fully-rotated Delivery
0.9918
±0.0028
0.9915
±0.0028
0.9902
±0.0028
0.9932
±0.0029
0.9921
±0.0028
Collapsed Delivery
0.9899±0.0021
0.9907±0.0021
0.9982±0.0022
1.0025±0.0022
0.9937±0.0022
Chung et al, 2009
7 beam pcsr