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Main partners of the research
Scientific collaboration involving:
Department of Materials ScienceProf. Anna Vedda and colleagues
Department of Physics Department of Medical Physics
Background
Dosimetry needs in the modern X-rays radiation therapy
New irradiation technologies with some common feautures: • beam modulation (i.e. Volumetric Modulated Arc Therapy, VMAT)• small fields (Stereotactic Body Radiation Therapy – SBRT)• dose conformation ! dose escalation + hypofractionation
New needs in dosimetry for:• Commissioning of the innovative machines• QC of the radiation beams• QA of the individual treatment plan in phantoms (check of the dose distribution)• In-vivo dosimetry
Background
Scintillating optical fiber dosimeter
SCINTILLATOROPTICAL DETECTOR (PMT, photodiode, CCD…)
OPTICAL FIBER (PASSIVE)
Advantages: • small dimensions (point measurement, small field dosimetry)• real time measurement of the dose/dose rate • do not need HV (in-vivo dosimetry)• unaffected by magnetic fields (ideal for the new Hibrid MRI-LINAC systems)
Major Challenge:• Stem effect (Cerenkov light and scintillation of the passive fiber)
1992 …~20 years later
The Stem Effect
• Second (reference) fibre
(Beddar, Radiat. Meas. 41, 2007)
• Temporal gating(Andersen et al. Radiat. Meas. 46, 2011)
• Spectral method(Guillot et al. Med. Phys. 38, 2011)
Research of a scintillator free from any spectral superposition with the stem effect
Rare earth-doped silica optical fibers
300 400 500 600 700 800 900 1000 11000
25
50
75
100
300 400 500 600 700 800 900 1000 11000
25
50
75
100
300 400 500 600 700 800 900 1000 11000
25
50
75
100
SiO2:Yb
SiO2:Eu
SiO2:Ce
RL
(re
lati
ve i
nte
ns
ity)
Wavelength (nm)
SOL-GEL + «rod in tube» method
Doped fibre
µ -ionizationchamber
Diode
Stereotactic collimator
Ce-doped silica optical fibers
• Fast: mainscintillating time ~ 55 ns (5d !4f Ce3+)
Applications:• Dosimetry/QA in diagnostic examinations,
brachytherapy and pre-clinical RT studies! stem effect not relevant
• Ion/Proton beams monitoring (dosimetryimpaired by quenching of the RL)
• High RL efficiency! minimum detectable
dose rate << 10-2 mGy/s
Radiat. Meas, 45, 2010
Appl
. Ph
ys.
Lett
. 85
, 20
04
Advances in Optics, 2014
• J Phys D 46, 2013• Radiat Meas 56, 2013
Eu-doped silica optical fibers
Wavelength (nm)
300 350 400 450 500 550 600 650 700 750
RL
(re
lati
ve in
ten
sit
y)
0
10
20
30
40
50
60
70
80
90
100
110
Ce-doped fibreEu-doped fibre
• Narrow emission at ~ 620 nm relatedto the 5D0 – 7F2 transition of Eu3+
• Spectral region still interested by the stem effect in unlucky irradiationconditions
CHGNNBGAA ⋅−=
An effective method for removingthe stem effect was implemented, but of difficult implementation in the clinical practice
Yb-doped silica optical fibers
Appl
. Ph
ys.
Lett
105
, 20
14
Spectra not corrected for the spectral response of the system
Good candidates as real time dosimeters
Spectral measurementsconfirmed the Yb RL independence of: ! the beam direction! the lenght of irradiated
passive fibre
Yb-doped silica optical fibers
Prerequisite: stability/reproducibility of the RL signal
60CoLENA
• Yb-doped optical fibers suffer of «hystereris effect»: increaseof the RL efficiency with increasing the cumulated dose
• Competitive traps are deep enough to remain filled at room temperature, enabling stability of the RL over the time
• Defects of the silica matrix acting as competitive traps
J.Phys.Chem.C. 119, 2015
Yb-doped silica optical fibers
Prerequisite: stability/reproducibility of the RL signal
60CoLENA
• Yb-doped optical fibers suffer of «hystereris effect»: increaseof the RL efficiency with increasing the cumulated dose
• Competitive traps are deep enough to remain filled at room temperature, enabling stability of the RL over the time
• Defects of the silica matrix acting as competitive traps
Appl. Phys. Lett 105, 2014
Long-term reproducibility
Yb-doped silica optical fibers: the reader
Wavelength (nm)
Q.E
. (%
)
~ 13%
975 nm
Long-pass filters (cut-on wavelength: 950 nm)
200 400 600 800 1000 1200
0
20
40
60
80
100
Inte
nsi
ty %
Wavelength (nm)
RL emission of Yb-doped fibre Filter transmission
APD (Geiger Mode)
Design and implementation of an efficient and portable optical detector for real-time measurements of the emission of Yb3+
~ 15 cm~ 90 cm
6 MV X-rays, 200 MU, 300 MU/min, 30x30 cm2
Figure adapted by Carrasco et al. Med Phys 42, 2015
Complete suppression of the stem effect by optical filtering
! No calibration procedures required
0 10 20 30 40 50 60 70
0
10
20
30
40
50
60
70
80
A Conf. B Conf. A Conf. Filtered B Conf. Filtered
RL I
nte
nsi
ty (
10
3 c
ps)
Time (s)
Yb-doped silica optical fibers: the stem effect
B A
0 1 2 3 4 5 6 7 80
10
20
30
40
50
60
Yb Weighted Linear Fit
RL Inte
nsi
ty (
10
3 c
ps)
Dose Rate (cGy/s)
r2=0.9997
Yb-doped silica optical fibers: dosimetry
Total counts (i.e. dose)independent of the dose rate
- Change of the source-detector distance
Linearity vs. dose rate:
MDDR ~0.1 mGy/s
0 100 200 300 400 500 6000
2
4
6
8
10
12
14
0
2
4
6
8
10
12
0 280 560 840 1120 1400
-4
-2
0
2
4
1400 MU/min
RL
Inte
nsi
ty (
10
3 c
ps)
Time (s)
140 MU/min
Yb Weighted Linear Fit
RL
Inte
nsi
ty (
10
3 c
ps) r
2=0.9999
∆∆ ∆∆ (
%)
Dose Rate (MU/min)
- Change of the LINAC pulse frequency
Phys Med Biol 62, 2017
Yb-doped silica optical fibers: validation
• Varian Trilogy System Linear Accelerator• 6 MV X-rays FFF • Water phantom (IBA)
• Relative dose profiles (OAR, PDD) and Output Factors* (OF)
Reference detectors:• Ion chamber (Exradin A26, Standard Imaging; CC13, IBA)• Diodes (Razor, IBA; EDGE, Sun Nuclear Corporation)• Scintillator (Exradin W1, Standard Imaging)• Gafchromics films (EBT3, ISP technologies)
*The OF of a genericfield with size ixicm2 is the ratio between the signal produced by this field and the corresponding signal produced by thestandard size 10x10 cm2 field
0.0
0.2
0.4
0.6
0.8
1.0
-16 -12 -8 -4 0 4 8 12-9
-6
-3
0
3
6
9
Re
lativ
e D
ose
Razor W1 Yb EBT3
Yb-EBT3 Razor-EBT3 W1-EBT3
∆ (
%)
Distance (mm)
10
20
30
40
50
60
70
80
90
100
110
0 40 80 120 160 200 240
-2
-1
0
1
2
Do
se (
%)
CC13 Yb
(a)
∆ (
%)
Depth (mm)
Yb-doped silica optical fibers: validation
Field size: 30x30 mm2
0 20 40 60 80 100-4
-2
0
2
4
0.7
0.8
0.9
1.0
Yb-W1 A26-W1 Edge-W1
∆ (
%)
Field Size (mm)
Yb A26 Edge W1
Outp
ut
Fact
or
Field size: 30x30 mm2
• Good agreement between the results of the fibre and of other reference detectors
• Effective and practical tool for “small field” dosimetry and promising for in-vivo dosimetry
Phys Med Biol 62, 2017
Conclusions
• Rare earth doped silica optical fibers have RL properties that can be exploited in various dosimetry applications, and more in general for ionizing radiation detection and monitoring.
• For some applications connections with industrial partners have been already established, for other applications, including medical dosimetry, contacts are currently in progress.
• Further applications are possible ! discussion and new inputs are welcome