Scintillation dosimetry:
Review, new innovations and
applications
Sam Beddar, Ph.D.,
Professor
Department of Radiation Physics, UT MD Anderson Cancer Center
& UT Graduate School of Biomedical Sciences
&
Luc Beaulieu, Ph.D.,
Associate Professor
Department of Physics, Université
Université Laval & Department of Radiation
Oncology, CHU de Quebec
OUTLINE
• Introduction
• Properties of plastic
scintillation detectors
• Applications
– Daily QA
– Radiosurgery
– Clinical prototype
– Proton therapy
– Works in progress
• Conclusions
INTRODUCTION
•
•
•
•
Introduction
Properties
Applications
Conclusion
• In the last 20 years, significant advances have been made
in scintillation dosimetry
• They have a unique set of advantages
• With the increasing complexity of radiotherapy treatments,
scintillation detectors could be used for quick and accurate
dose measurements even in complex geometries
The purpose of this presentation is to show the advantages
of scintillaton dosimetry and to explain how it can be used
in modern radiotherapy
INTRODUCTION
•
•
•
•
Introduction
Properties
Applications
Conclusion
• In scintillation detectors:
• Impinging particles or photons will excite atoms or
molecules of the scintillating medium.
• Electron beams: Interacting electrons are not the same as
the ionization electrons
• The decay of these excited states will produce
photons in the visible part of the spectrum.
• Photon beams: Interacting photons are not the same as the
detected photons
• These photons will be guided to a photodetector and
then converted in an electric signal.
INTRODUCTION
•
•
•
•
Introduction
Properties
Applications
Conclusion
• Two types of scintillators:
A. Inorganic materials
- NaI,
NaI, NaI(Tl),
NaI(Tl), CsI,
CsI, CsI(Tl),
CsI(Tl), CsI(Na),
CsI(Na), CsF,
CsF, BaF2,
CdWO4, …
- Advantages for stopping particles: high Z and
high density (up to 88-9 g/cm3)
• Disadvantage for dosimetry
- Need an activator:
e.g. Tl (Thalium)
Thalium)
• NaI:
NaI: 330 nm
• NaI(Tl):
NaI(Tl): 410 nm
INTRODUCTION
•
•
•
•
Introduction
Properties
Applications
Conclusion
• Two types of scintillators:
B. Organic materials (e.g. plastics)
- Lower density … and!!! nearly equivalent to water
• Density on the order of 1.031.03-1.06 g/cm3
• (Generally) No high Z materials content.
- Excitation and emission spectra are similar in
solid, liquid or vapor states
PLASTIC SCINTILLATING MATERIALS ••
Introduction
Properties
Applications
Conclusion
•
•
• Core (bulk solvent)
– Polyvinyltoluene (plastic scintillators)
scintillators)
– Polystyrene (plastic
(plastic scintillating fibers)
fibers)
• Cladding (scintillating fibers)
– Polymethylacrylate (PMMA)
– Improves transmission of light to optical fiber
SCINTILLATION PROCESS
•
•
•
•
Introduction
Properties
Applications
Conclusion
Dose deposition and scintillation
C.P. Interactions
(keV to MeV)
Energy Deposition
Energy Absorption
Ionization - Excitation
De-excitation Processes
Light Emission
(a few eV)
SCINTILLATION PROCESS
•
•
•
•
Introduction
Properties
Applications
Conclusion
• Prompt fluorescence following excitation (10-9 – 10-7 s)
important for radiation detection
SCINTILLATION PROCESS
•
•
•
•
Introduction
Properties
Applications
Conclusion
• At room temperature,
e- almost all in S00
– ΔE S1-S0 about 3-4 eV
>> 0.025 eV
• S1 band (about 0.15
eV apart) decay
radiation-less to the
S1 base state
– Photon emitted only to
S0 states
• T1 band: lower energy
and longer process
SCINTILLATION PROCESS
•
•
•
•
Introduction
Properties
Applications
Conclusion
• Aromatic molecules
– Large number of “shared”
electrons (π-electrons)
» 4n+2
» Anthracene has 14 of
such electrons.
– Excitation of π-electrons in S
states = fast fluorescence.
– Ionization of π-electrons =
mostly T states.
– Excitation or ionization of
other electrons (σ-electrons)
= no fluo or temporary /
permanent damage
SCINTILLATION PROCESS
•
•
•
•
Introduction
Properties
Applications
Conclusion
• Since all photons
come from the S10
– Shift in the emission
spectrum to lower
energy than the
excitation (absorption)
spectrum
– Mostly transparent to its
own emitted photons!
From Knoll
FLUORS
•
•
•
•
Introduction
Properties
Applications
Conclusion
• Organic fluors (scintillating materials) are used with
a bulk solvent: two components system
– BC400: >97% PVT, < 3% organic fluors
» e.g. p-TERPHENYL (C6H5 C6H4 C6H5)
– Energy deposited in the solvent is transfered to the
organic fluor molecules
» Emission is typically peaked in the violet-blue region.
• “Wavelength shifters” or three component system
– A third (organic) component can also be used to absorb
the organics fluors emitted photons and re-emit at a longer
wavelength
» POPOP [1,4-bis(5-phenyloxazol-2-yl) benzene] to get
scintillators emitting in the green or yellow region.
SCINTILLATOR SPECTRA
•
•
•
•
Introduction
Properties
Applications
Conclusion
Scintillators with
longer wavelengths
have lower light
emission
Archambault L, Arsenault J, Gingras L, Beddar A S, Roy R, Beaulieu L, "Plastic scintillation
dosimetry: Optimal selection of scintillating fibers and scintillators
scintillators““, Med. Phys 32: 22712271-2278,
2005.
SCINTILLATION EFFICIENCY
•
•
•
•
Introduction
Properties
Applications
Conclusion
• Only a small portion of the incident kinetic energy
lost is converted in fluorescent energy
– For plastic scintillating fibers like BCF-12 about 8000
photons/MeV
ƒ This means about 125 eV / scintillation photon.
ƒ The total energy of visible light produced (at 430 nm or ~ 2.9 eV)
represents an efficiency of 2.4% (97.6% goes in phonons!)
– The light output depends on the LET (i.e. CP type)
QUENCHING EFFECT
•
•
•
•
Introduction
Properties
Applications
Conclusion
• A decrease from the optimal scintillation efficiency, or
quenching, can occur under various conditions
– For organic scintillators, little thermal quenching
• Constant between -60 ºC and +20 ºC
• 5% variation max between +20 ºC and +60 ºC
– Radiation damage can decrease the efficiency
(Ionizations lead to temporary and/or permanent molecular
damage)
• Increased absorption due to defects (plastics turn yellow)
• Need > kGy accumulated doses (104 to 105 Gy)
QUENCHING EFFECT
•
•
•
•
Introduction
Properties
Applications
Conclusion
• Quenching also occurs for highly ionizing radiation
– Overlapping excitation sites and molecule damages.
– Provides alternate radiationless de-excitation processes.
– Non-linearity of Light (L) to Energy (E) response.
BIRKS’ FORMULA
•
•
•
•
Introduction
Properties
Applications
Conclusion
• The relationship between the specific energy loss
(ionization density) and the light output is given by
the Birks’ formula:
dE
S
dL
dx
=
dx 1+ kB dE
dx
S: Scintillation efficiency
B(dE/dx): Density of damaged
molecules along the particle track
k: fraction of damage contributing to
quenching
– For electrons above 125 keV: dL/dx = S dE/dx
» Thus L = SE !
– For large dE/dx, saturation can occurs along the track
» dL/dx = S / (kB) (usualy for ion particles)
Archambault L, Beddar S, Sahoo N, Poensich F, Gillin M, Mohan R, " Experimental Proof and
Feasibility of a 3D RealReal-Time Detector System for Therapeutic Proton Beams “, Med. Phys. 35:
2905, 2008. (Abstract).
SCINTILLATION PROCESS:
SUMMARY
•
•
•
•
Introduction
Properties
Applications
Conclusion
• Organic (plastic) scintillators are:
– Made of low Z materials.
– Light output is directly proportional to the exciting energy.
» Linear with deposited energy for e-,γ.
» …for electron above 100-125 keV.
– (Mostly) Transparent to its emitted photons.
– The gap is wide enough to be insensitive to a wide
range of temperatures.
– Fast time response (physics of orbital transitions).
SCINTILLATION PROCESS:
SUMMARY
•
•
•
•
Introduction
Properties
Applications
Conclusion
• Scintillation efficiency is not the collection
efficiency.
» These photons must be detected.
» Linearity must be preserved throughout the
complete detection chain
» Light must be calibrated to Dose.
SCINTILLATION PROCESS:
REFERENCES
•
•
•
•
Introduction
Properties
Applications
Conclusion
• JB Birks, The Theory and Practice of Scintillation
Counting, Pergamon Press Book, MacMillan, New
York, 1964. [Chapters 3 and 6]
• GF Knoll, Radiation Detection and Measurement,
3rd Edition, John Wiley and Sons, 2000. [Chapter 8]
• WR Leo, Techniques for Nuclear and Particle
Physics Experiments, 2nd edition, Springer-Verlag,
1992. [Chapter 7]
• FH Attix, Introduction to Radiological Physics and
Radiation Dosimetry, John Wiley and Sons, 1986.
[Chapter 15]
ADVANTAGES OF PLASTIC
SCINTILLATORS
•
•
•
•
Introduction
Properties
Applications
Conclusion
•
•
•
•
Introduction
Properties
Applications
Conclusion
• Linear response to dose
• Dose rate independence
• Energy independence
• Temperature independence
• Spatial resolution
WATER EQUIVALENCE
• W-E is achieved by:
– Media-matching (walls and sensitive volume)
– Density state of the sensitive volume (gaseous vs.
condensed)
• W-E depends on:
– Mass energy-absorption coefficients
– Mass collision stopping powers
– Size of the sensitive volume (according to Burlin
cavity theory)
Parameter
Density (g/cm3)
Electron density
(1023 e-/g)
Composition
(by weight %)
Scintillator
1.032
3.272
Polystyrene
1.060
3.238
Water
1.000
3.343
H: 8.47
C: 91.53
H: 7.74
C: 92.26
H: 11.19
O: 88.81
WATER EQUIVALENCE
•
•
•
•
Introduction
Properties
Applications
Conclusion
According to Burlin cavity theory, above 125 keV:
Dsci
= 0.980 ± 0.005
Dmed
Beddar A S, Mackie T R, Attix F H, "Water"Water-equivalent plastic scintillation detectors for highhigh-energy beam dosimetry: I.
Physical characteristics and theoretical considerations“
considerations“, Phys. Med. Biol. 37: 18831883-1900, 1992.
DETECTOR CONFIGURATION
Scintillator
εaccept
εcouple1
Fiber light guide
εtransmit
•
•
•
•
Introduction
Properties
Applications
Conclusion
Photodetector
εcouple2
Plastic scintillating fibers offer a good alternative to
regular plastic scintillators:
•
•
Increased light capture due to cladding (>internal reflection)
The cladding is also water-equivalent/ no perturbation
ORIGINAL PROTOTYPE
•
•
•
•
Introduction
Properties
Applications
Conclusion
• High sensitivity (PMT)
• Remove Cerenkov with background subtraction
Beddar, A S, Mackie, T R, and Attix, F H, "Water"Water-equivalent plastic scintillation detectors for highhigh-energy
beam dosimetry: I. Physical characteristics and theoretical considerations,"
considerations," Phys. Med. Biol. 37: 188318831900, 1992.
LINEARITY
•
•
•
•
Introduction
Properties
Applications
Conclusion
Light production is proportional to the dose deposited
DOSE RATE INDEPENDENCE
•
•
•
•
Introduction
Properties
Applications
Conclusion
•
•
•
•
Introduction
Properties
Applications
Conclusion
Not affected by dose rate variations
ENERGY DEPENDENCE
• Detecting volumes: intermediate cavities
• Best energy dependence relative to other dosimeters
used in radiotherapy
Beddar A S, Mackie T R, Attix F H, "Water"Water-equivalent plastic scintillation detectors for highhigh-energy beam dosimetry: I.
Physical characteristics and theoretical considerations“
considerations“, Phys. Med. Biol. 37: 18831883-1900, 1992.
TEMPERATURE
•
•
•
•
Introduction
Properties
Applications
Conclusion
No temperature dependence between 18ºC and 30ºC
CERENKOV EMISSION
•
•
•
•
Introduction
Properties
Applications
Conclusion
• In plastics like polystyrene, electrons with energies 146
keV and higher produce a blue light due to Cerenkov
emission
• This light is superposed on the scintillation signal
• If a large amount of clear optical fiber is in the radiation
field, Cerenkov emission can be significant (>15 %)
Although the light spectrum due to Cerenkov emission is different
different
from the scintillation spectrum, de Boer et al have shown that spectral
spectral
filtration is not sufficient to remove completely the Cerenkov light.
light.
de Boer S F, Beddar A S Rawlinson J F "Optical filtering and spectral
spectral measurement of radiationradiationinduced light in plastic scintillator dosimetry“
dosimetry“, Phys Med Biol 38: 945945-958, 1993.
CERENKOV EMISSION
Techniques to remove the Cerenkov effect:
Background subtraction
Dsci ∝ M − M crkv
Note: requires 2 optical fibers
•
•
•
•
Introduction
Properties
Applications
Conclusion
Simple filtering
Find the optimal S(λ)
that max. N(λ) and min.
1/λ2
Chromatic filtration
With 2 different wavelength filter S1(λ) and S2(λ),
mi =
∫
∞
0
⎛
C ⎞ −x / l( λ )
Si ( λ)dλ
⎜ kDsci N(λ ) + 2 ⎟e
⎝
λ⎠
i = 1,2
⇒ Dsci = Am1 + Bm2
Where A and B are fixed by calibration under 2
different conditions
Fontbonne J M, Iltis G, Ban G, Battala A, Vernhes J C, Tillier J, Bellaize N, le Brun C, Tamain B,
Mercier K, Motin J C, "Scintillating fiber dosimeter for radiation therapy acclerator“
acclerator“, IEEE Trans. Nucl.
Nucl.
Sci. 49: 22232223-2227, 2002.
A DAILY QA DETECTOR SYSTEM
•
•
•
•
•
•
•
•
Commercialized product
Rugged and simple to construct
Good stability and reproducibility
Independent of temperature and pressure
No high-voltage bias
Remote operation and reset
Easily used by trained technical staff
Cost effective
•
•
•
•
Introduction
Properties
Applications
Conclusion
Beddar S, “A new scintillator detector system for the quality assurance of 60Co and highhigh-energy therapy
machines”
machines”. Phys Med Biol 39: 253–
253–263, 1994.
A DAILY QA DETECTOR SYSTEM
Stability of the QA device over time
•
•
•
•
Introduction
Properties
Applications
Conclusion
Beddar A S, “A new scintillator detector system for the quality assurance of 60Co and highhigh-energy
therapy machines”
machines”, Phys Med Biol 39: 253–
253–263, 1994.
SCINTILLATION FIBER ARRAY
PROTOTYPE
•
•
•
•
Introduction
Properties
Applications
Conclusion
• PMMA
• Fixed focal
• f/# = 1.4
• Alta U2000c
• Interline
• Color
• Cooled @ -20º
• BCF12 (blue)
• Subtract Cerenkov contamination with chromatic removal
Archambault L, Beddar S, Gingras L, Roy R, Beaulieu L, "Measurement accuracy and Cerenkov
removal for high performance, high spatial resolution scintillation
scintillation dosimetry“
dosimetry“, Med. Phys. 33: 128128-135,
2006.
SCINTILLATION FIBER ARRAY
DETECTOR FOR QA
•
•
•
•
Introduction
Properties
Applications
Conclusion
Lacroix F, Archambault L, Gingras L, Beddar AS, and Beaulieu L. Clinical prototype of a plastic water
water-equivalent scintillating fiber dosimeter matrix for IMRT QA applications,
applications, Med Phys 35 (2008) 36823682-3690.
•
•
•
•
DEPTH DOSE
Introduction
Properties
Applications
Conclusion
150
Scint. 10x10
CC13 10x10
130
Relative Dose
110
1.6%
90
70
50
30
0
25
50
75
100
125
150
175
200
225
250
275
300
Depth (mm)
Lacroix F, Archambault L, Gingras L, Beddar AS, and Beaulieu L. Clinical prototype of a plastic water
water-equivalent scintillating fiber dosimeter matrix for IMRT QA applications,
applications, Med Phys 35 (2008) 36823682-3690.
•
•
•
•
BEAM PROFILE
CC13 20x20
CC13 10x10
CC13 4x4
Scint. 20x20
Scint. 10x10
Scint. 4x4
100
Relative Dose
Introduction
Properties
Applications
Conclusion
80
60
40
20
0
-175
-125
-75
-25
25
Position (mm)
75
125
175
Lacroix F, Archambault L, Gingras L, Beddar AS, and Beaulieu L. Clinical prototype of a plastic water
water-equivalent scintillating fiber dosimeter matrix for IMRT QA applications,
applications, Med Phys 35 (2008) 36823682-3690.
•
•
•
•
EBRT
Accurate for electron measurements:
Introduction
Properties
Applications
Conclusion
6 MeV
Depth Dose (%)
18 MeV
Depth in Water (cm)
Depth in Water (cm)
Beddar, A. S., Mackie, T. R., and Attix, F. H., "Water"Water-equivalent plastic scintillation detectors for highhighenergy beam dosimetry: II. Properties and measurements," Phys. Med.
Med. Biol. 37, 19011901-1913 (1992) c
SPATIAL RESOLUTION
•
•
•
•
Introduction
Properties
Applications
Conclusion
Beddar, A. S., Mackie, T. R., and Attix, F. H., "Water"Water-equivalent plastic scintillation detectors for highhighenergy beam dosimetry: II. Properties and measurements," Phys. Med.
Med. Biol. 37, 19011901-1913 (1992) c
SPATIAL RESOLUTION
•
•
•
•
Scint
Diode
Introduction
Properties
Applications
Conclusion
A16
Archambault L, Beddar S, Gingras L, Lacroix F, Roy R, Beaulieu L, "Water
"Water--equivalent
dosimeter array for smallsmall-field external beam radiotherapy",
radiotherapy", Med Phys 34: 1583–
1583–1592,
2007.
A12
DOSIMETRY FOR STEREOTACTIC
RADIOSURGERY
•
•
•
•
Introduction
Properties
Applications
Conclusion
DOSIMETRY FOR STEREOTACTIC
RADIOSURGERY
•
•
•
•
Introduction
Properties
Applications
Conclusion
Beddar S, Kinsella T J, Ikhlef A, Sibata C H, “Miniature 'Scintillator'Scintillator-FiberopticFiberoptic-PMT' detector
system for the dosimetry of small fields in stereotactic radiosurgery”
radiosurgery”, IEEE Trans. Nucl. Sci. 48:
924924-928, 2001.
MONTE CARLO SIMULATIONS FOR
PROTON BEAMS
•
•
•
•
Introduction
Properties
Applications
Conclusion
• Use Geant4 for a complete simulation
– Electromagnetic and hadronic processes
– Production and tracking of visible light (scintillation and
Cerenkov)
• Understand the behavior of a plastic scintillator
irradiated with protons
• Optimize light collection efficiency
MONTE CARLO SIMULATIONS
OF PROTON BEAMS
•
•
•
•
Introduction
Properties
Applications
Conclusion
•
•
•
•
Introduction
Properties
Applications
Conclusion
Water equivalence preserved for most clinical proton
energies
QUENCHING (PROTONS ONLY)
1.0
Relative Dose
0.8
0.6
Unquenched – ion chamber
measurements & simulation
Quenched – scintillation
measurements & simulation
0.4
0.2
0.0
-10
-8
-6
-4
Depth (cm)
-2
0
2
Monte Carlo accurately predicts quenching and can
therefore be used for determining correction factors
Archambault L, Polf J C, Beaulieu L, Beddar S, “Characterizing the response of miniature
scintillation detectors when irradiated with proton beams”
beams”, Phys Med Biol 53: 18651865-1876, 2008.
SCINTILLATION SCREEN
• Rapid, but requires correction for light propagation
• Similar to a-Si dosimetry, but takes advantage of the
water equivalence of the plastic scintillator
• Not as accurate as point-like scintillation dosimeters
• See Petric et al. for more information
•
•
•
•
Introduction
Properties
Applications
Conclusion
Petric M P, Robar J L, Clark B G, “Development
and characterization of a tissue equivalent plastic
scintillator based dosimetry system,”
system,” Med Phys 33:
9696-105, 2006.
•
•
•
•
BRACHYTHERAPY
Introduction
Properties
Applications
Conclusion
• Scintillator irradiated with low energy (< 125 keV) photons
is less water equivalent than megavoltage photons
• Chemical composition of plastic scintillator may be changed
for a better response (Williamson et al., Kirov et al.)
• Kirov et al. have developed a 3D scintillation detector for
the characterization of Ru-106 eye plaques
Kirov A S, Piao J Z, Mathur N K, Miller T R,
Devic S, trichter S, Zaider M, Soares C G,
LoSasso T, “ThreeThree-dimensional scintillation
dosimetry method: test for a 106Ru plaque
applicator”
applicator”, Phys Med Biol 50: 30633063-3081,
2005.
see also Williamson J F, Dempsey J F, Kirov A S, Monroe J I, Binns W R, Hedtjä
Hedtjärn,
rn, “Plastic scintillator
response to lowlow-energy photons”
photons”, Phys Med Biol 44: 857857-871, 1999.
IN VIVO DOSIMETRY
FOR PROSTATE RADIOTHERAPY
•
•
•
•
Introduction
Properties
Applications
Conclusion
• Direct measurement of the dose delivered to
organs, critical structures, and within the vicinity of
a tumor is feasible and can be used to verify
treatment plan delivery.
• We believe this can be done using an in vivo
scintillation detector composed of multiple probes
arranged in an application-specific design that can
monitor true in vivo dose in real time.
NCI Grant No. 1R01CA120198-01A2
INTRAFRACTIONAL MOTION
•
•
•
•
Introduction
Properties
Applications
Conclusion
Adam Melancon (MDACC)
INTRAFRACTIONAL MOTION
GASEOUS BUILDUP
Before Tx
•
•
•
•
Introduction
Properties
Applications
Conclusion
After Tx
Adam Melancon (MDACC)
OUR GOAL:
ACCURACY & PRECISION
•
•
•
•
Introduction
Properties
Applications
Conclusion
•
•
•
•
Introduction
Properties
Applications
Conclusion
Prostate
Detector
RECTAL DETECTOR
Capillaries where
scintillation probes
can be inserted
Rectal balloons are part of
the standard treatment for
radiation therapy at MDACC
URETHRAL DETECTOR
•
•
•
•
Introduction
Properties
Applications
Conclusion
•
•
•
•
Introduction
Properties
Applications
Conclusion
Urethral detector consisting of a capillary
containing line and point probes for
measurement of dose at the urethra. The outer
diameter of the catheter is 2 mm (6F).
Foley Catheter
CLINICAL INNOVATION:
REAL-TIME HDR DOSIMETRY
A single PSD is placed in an empty
standard prostate plastic catheter.
The remaining 12 catheters are
used for planning and delivery
The PSD is read in real-time to
monitor the treatment delivery.
#4 #5 #6
#3
#1 #2
#7
#10
#8 #9
#12
#11
CLINICAL INNOVATION:
REAL-TIME HDR DOSIMETRY
•
•
•
•
Introduction
Properties
Applications
Conclusion
* Results of 5 measurements
PLATO
Scintillator*
cGy
274.5
274.54
± 13.7
SUMMARY
•
•
•
•
Introduction
Properties
Applications
Conclusion
• Scintillation dosimeters possess a unique set of
advantages
– Water equivalence, linear dose response, energy
and temperature independence, spatial resolution,
etc…
• Several types of scintillation detectors have
been developed in the last 15 years
• Main applications:
–
–
–
–
–
Conventional EBRT (photons and electrons)
Quality assurance device (daily and IMRT)
Radiosurgery
Protons
Brachytherapy
CONCLUSIONS
•
•
•
•
Introduction
Properties
Applications
Conclusion
• Scintillation dosimetry will be increasingly used in
radiotherapy.
• Their advantages make them ideal for measuring
complex dose distributions such as those produced by
IMRT.
• They can compete with other dosimeters in terms of
measurement accuracy, convenience and can be
economical.
ACKNOWLEDGEMENTS
CoCo-authors
L. Archambault
F.H. Attix
S.F. de Boer
T.M. Briere
L. Gingras
M. Guillot
A. Ikhlef
T.J. Kinsella
F. Lacroix
T.R. Mackie
J.F. Rawlinson
R. Roy
C.H. Sibata
J.V. Siebers
F. Thé
Thériaultriault-Proulx
M. Villeneuve
N. Suchowerska
S. Law
… and all others who have contributed to the field of
scintillation dosimetry