veronese pdf web -...

Recent progresses in scintillating optical fiber dosimeters

Milan- 29th June 2017

Ivan Veronese

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


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


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

veronese pdf web -...

Documents