basic principles of dosimetry - forsiden principles of dosimetry eirik malinen • radiation field...
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Basic principles of dosimetry
Eirik Malinen
• Radiation field striking a small sphere:
• Fluence:
• Energy Fluence:
Ionizing radiation field
dN
daΦ = (da is the great circle area)
dA
dE=Ψ
• A photon field with total energy Rin,u enters a volume, while
Rout,u-rl is the energy leaving the volume:
• Energy transferred:
• KERMA (kinetic energy release per mass):
KERMA
Rin,u Rout,u-rl
V
QRR rlu,outu,intr Σ+−=ε −
ρ
µΨ=
ε= trtr
dm
dK
• Kerma includes all kinetic energy given to secondary
electrons, and this energy may be lost by:
– Collisions
– Radiative losses
• Kerma may be divided into two components:
• K=Kc+Kr
• Kc: collision Kerma; provides a measure of the energy loss per
unit mass from photons resulting in collisional losses for
secondary electrons!
Components of KERMA
• Is defined by:
• Net energy transfer : total kinetic energy of secondary
electrons which is not lost as brehmsstrahlung
• May take radiative losses into account by defining the qantity
g; the fraction of kinetic energi lost as brehmsstrahlung
• Definition:
• µen/ρ: mass energy absorption coefficient
Collision Kerma
n
trc
dK
dm
ε=
trcK K(1 g) (1 g)
µ= − = Ψ −
ρ
en tr (1 g)µ µ
≡ −ρ ρ
n
trε
• Look at all energy transport (both charged and uncharged
particles) through the volume of interest:
• Absorbed dose:
Absorbed dose
Rin,u+Rin,c
in ,u in ,c out ,u out ,cR R R R Qε = + − − + Σ
Rout,u+Rout,c
dD
dm
ε= unit: [Gy] = [J/kg]
• If charged particle equilibrium (CPE) is present, Rin,c = Rout,c
• Energy imparted:
• In this case, absorbed dose equals collision Kerma:
Dose from photons, CPE
v
CPE:
n
in,u in ,c out ,u out,c in ,u out,u trR R R R R Rε = + − − = − = ε
nCPEtr en
cD Km m
ε µε= = = = Ψ
ρ
• Transient Charged Particle Equilibrium: electrons originating
from upstream contributes to the dose, while the photon
contribution (Rin,u-Rout,u) is given by the collision Kerma
• Assumption: absorbed dose propotional to Kc
TCPE
TCPE
c TCPE
TCPE
D K (1 f )
f 0
= +
≥
• If no photon interactions, equilibrium of secondary electrons,
constant stopping power over point of interest:
Dose from charged particles
ρΦ= colS
D
• Exposure, X : number of charges Q (either positive or
negative) prodused in a gass of mass m:
• Number of charges per mass proprtional to dose:
X ∝ Dair
• The quantity relating X to Dair is the mean energy per ion
pair:
Exposure
dQX
dm=
Energy lost by ionizing particle
Number of ion pairs producedN
E e/W =
• For air, is 33.97 J/C
• The dose to air:
• Thus, by measuring the number of charges produced per mass
unit of air, Dair may be determined – indepedent of the
radiation quality ( is close to being constant for all
electron- and photon energies)
Dose to air, Dair
W / e
W / e
airair
aire
WX
e
W
m
Q D
=
=
Bragg-Gray cavity theory
B-G conditions:
1. Charged particle fluence is not perturbed by cavity
2. Absorbed dose entirely due to charged particles
wall
cav
e-,p, ....
wall
wall
cav
cav
S D
S D
=
=
ρΦ
ρΦ
cav
wallwall
cav S
D
D
=⇒
ρ
• Estimate dose to medium from measurements of
ionizations in air:
Applied BG theory
med
air
air
med
airairmed Se
WXSD D
==
Stopping power ratios
• Ionometry: the art of measuring ionizations
• Number of ionizations proportional to dose
• Air filled ionization chamber (”thimble”):
Ionization chamber
~ 300 V
Parallel-plate ionization chamber
• Problems with ion chambers is e.g. inacuracies in determining
air volume
• In practice, the ion chamber is calibrated at a point where the
dose is known – performed at a primary standards laboratory
(PSDL)
Dosimeter calibration (ion chambers)
H2O
γ, e- …A given dose gives reading M
electrometer
Ion chamber
• For a given dose to water, Dw, an ion chamber reading M is
obtained. Thus:
• The calibration factor is:
• Thus, the dose may be determined without using W/e, µen/ρetc
Dosimeter calibration
w w D,wD M D MN∝ ⇔ =
wD,w
DN
M=
• Calibration factor is dependent on radiation type and energy
• Usually, the chamber is calibrated in a well defined field, e.g. 60Co γ-rays (average energy 1.25 MeV)
• Corrections of the calibration factor, kQ, is thus introduced for
other energies (radiation ”qualities”, e.g. 15 MV X-rays)
Dosimeter calibration Film dosimeters
• Radiographic film: Ionization of AgBr in the grains forms the
latent image in the film
• Light transmission is a function of the film opacity and can be
measured in terms of optical density (OD) with densitometers
Film dosimeters
• Radiochromic film: special dye gets polymerized upon
exposure to radiation. The polymer absorbs light and the
transmission of light through the film can be measured with a
suitable densitometer
Thermoluminescence dosimetry
• Thermoluminescence (TL): thermally activated luminescence
• Measures the amount of visible light emitted from a crystal
when heated
Thermoluminescence dosimetry
• Glow curve
Thermoluminescence dosimetry
• Supralinear dose response
Thermoluminescence dosimetry
• Energy dependence
Diode dosimetry
• Radiation produces electron-hole (e-h) pairs. The charges
(minority carriers) produced in the dosimeter are swept across
the depletion region under the action of the electric field. In
this way a current is generated in the reverse direction in the
diode.
spherical
droplet
Diode dosimetry
Detector temperature
after placing on patientSensitivity dependence
Diode dosimetry
• Dependence on accumulated dose
Diode dosimetry
• Field size dependence• In vivo: In the living
• Verification of delivered dose
to individual patients
• Radiotherapy requires
accurate dose deliveryerror
Prescribed dose
Pro
bal
ilit
y
In vivo dosimetry
• Patient contour / planning basis (CT images)
• Patient motion
• Organ motion
• Dose calculations (inhomogeneities, scatter)
• Patient positioning
• Transfer of treatment data from simulator to linac
• Linac settings (energy, monitor units, field size) and
calibration
• Beam modifiers (blocks, wedges)
In vivo dosimetry – sources of error Dose characteristics
Patient curvature
beam
wedge
Output, SSD
Wedge, curvature
Thickness, density
Entrance dose:
Exit dose:
Point detector
2D detector array
Measurement issues
• Accurate and precise
• Multiple readouts
• Reusability
• No cables
• Non-destructive readout
High accuracy
Low precision
Low accuracy
High precision
Desired dosimeter properties
• Absorbed dose, D:
R: dosimeter reading
ND: calibration factor
Ci: correction factor
ii
D CRND ∏=
Dosimeter reading →→→→ absorbed dose
Rcal
Dcal
beam
dmax
water phantom
ion chamber
dosimeter
cal
calD
R
DN =
• Under reference conditions:
Calibration
Clinical example
Action level: 2.5%
measured dose
dose after correction
%2.1
008.1r
=
=
σ
Measured dose / prescribed dose
Distribution of measurements