A silicon strip detector for a novel 2D
dosimetric method for complex
radiotherapy treatment verification
Alessio Bocci
DITANET Experienced Researcher
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
1
Outline
 Complex radiation therapy treatments - IMRT
 Detectors for dosimetric applications
 RADIA2 project : experimental set-up
 Measurements and results
 Conclusions
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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Motivation and Objective
 Motivation:
Cancer is the second most frequent cause of death in developed countries. At
present, although surgery is the most effective way to remove the malignant tissue,
when it is combined with radiation therapy improves the cure rate by
40% approximately.
 Objective
Characterization of a silicon strip detector dedicated to 2D dose measurements in the
axial plane of a phantom for the verification of complex radiation therapy treatment
plans (i.e. IMRT)
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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Intensity Modulated Radiation Therapy
Many beam directions and entrance points
for conformal doses distributions
modulating in space the fluence of each radiation field
DITANET Workshop 08 November 2011
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Dosimetry for radiotherapy verification
2D Detectors dedicated to radiotherapy
verification plans
 Ionization chambers – “gold standard” detectors
 Radiographic and radiochromic films
 Semiconductor dosimeters: Silicon diodes
DITANET Workshop 08 November 2011
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Dose distribution verification
Film dosimeters: traditional detectors for verifying treatment plans dose distribution
Film dosimetry used as 2D dosimeters :
Advantages: high spatial resolution (sub-mm), good
uniformity, axial plane
Disadvantages: Unusable as on-line detectors
and are unreliable and time consuming detectors
DITANET Workshop 08 November 2011
Coronal plane
(ventral and dorsal)
Axial plane
(inferior and
superior)
Sagittal plane
Alessio Bocci, CNA
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2D Digital Detectors in radiotherapy
treatments
Single silicon detectors and ionization chambers assembled for 2D detectors
IMRTLog (ONCOlog Medical) 2D array silicon diodes
spatial resolution
0.5 to 1.0 cm
I’MRT MatriXX (Scanditronix/Wellhöfer)
array of ionizations chambers
MapCHECK (Sun Nuclear) silicon diodes
Spatial resolution is still poor with respect to film dosimeters
These devices are useful only for one irradiation angle and they can measure dose distribution
only in the coronal plane of the patient
DITANET Workshop 08 November 2011
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New detection systems
Traditional
detectors
New detection
systems
Radiographic films
Film dosimeters
2D commercial
digital
detectors
New detection
systems
no


Spatial
resolution

poor

2D detectors

 yes
Not monolithic!

Axial plane

no

On-line
DITANET Workshop 08 November 2011
It is necessary to develop new
detection systems that enhance the
traditional ones, and that are able to
verify in a simple and accurate way
complex treatment planning
inexpensive
radiation hard
easy to use
Alessio Bocci, CNA
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From Nuclear Physics to Medical Applications
Medical applications can benefit from the developments on nuclear and high energy physics
detector technology
Silicon strip detectors mounted
@ CNA in a telescope configuration
Silicon strip detectors mounted @
Virgen Macarena Hospital in Seville
silicon tracking detectors
silicon detectors for medical
applications
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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RADIA2 Project Si-DETECTOR
• Commercial silicon detector
•Low cost
Normally used on particle detection
(W1 type from Micron Semiconductor Ltd)
• Single sided 16 strips (3.1 mm pitch)
• Active area 50 x 50 mm² & 500 µm thick
DITANET Workshop 08 November 2011
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Why Silicon detectors ?
Advantages:
Linear relationship between photon energy and e/h pairs ;
High sensitivity (Si - 3.6 eV/pair – sensitivity per unit volume: 640 nC Gy-1/mm3 )
(about 18000 times the sensitivity per unit volume of an ionization chamber) ;
Small active volume that allows to obtain high spatial resolution ;
Well developed technology for the production of segmented monolithic planar
detectors ;
Radiation hard material ;
C. Talamonti et al, NIM-A 658 (2011) 84–89
DITANET Workshop 08 November 2011
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Single-sided-Si-strip detectors (SSSSD)
γ photons
Water-equivalent phantom
Radiotherapy energies
photon interactions :
Compton Effect
Secondary electrons from the
phantom interacts centemeters
away
Signals from each strip are
converted to the
absorbed dose
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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RADIA2 Project – Collaboration
Z. Abou-Haidar1, M. A. G. Alvarez1 ,R. Arrans3, A. Bocci1,
M. A. Cortes-Giraldo2 , J. M. Espino2 ,M. I. Gallardo2, A. Perez Vega-Leal4
F. J. Perez Nieto5, J. M. Quesada2
1. DITANET group @ National Accelerator Centre (CNA)
2. Department of Atomic, Molecular and Nuclear Physics (FAMN),
University of Seville
3. Virgen Macarena University Hospital, Seville
4. School of Engineering, University of Seville
5. Instalaciones Inabensa S.A.
A. Bocci et. al., Empirical characterization of a silicon strip detector for a novel 2d mapped method for dosimetric
verification of radiotherapy treatments, Radiotherapy and Oncology, Volume 99, Supplement 1, May 2011, Page S172
A. Bocci et. al., A silicon strip detector for a novel 2D dosimetric method
for radiotherapy treatment verification
submitted to NIM-a (October 2011)
M. A. Cortes-Giraldo et al. , "Geant4 Simulation to Study the Sensitivity of a
MICRON Silicon Strip Detector Irradiated by a SIEMENS PRIMUS Linac",
Progress in Nuclear Science and Technology, in press (2011)
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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LINAC accelerator
University Hospital Virgen Macarena
(Seville, Spain)
Siemens PRIMUS linac dual energy machine
operating at 6 MV photon
mode
A treatment planning system TPS (Philips Pinnacle) was used to calculate dose
distributions. Calculations were compared to experimental data
DITANET Workshop 08 November 2011
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Experimental Set-up
Two phantoms prototypes were designed and built
Polyethylene slab material
1. A slab phantom for:
detector characterization
(sensitive area perpendicular
to the beam direction)
DITANET Workshop 08 November 2011
Cylindrical phantom
2. A cylindrical phantom for:
angular response measurements &
2D treatment plans verification in the
axial plane
Alessio Bocci, CNA
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Detector in the axial plane
Set-up 2:
The active area of the
detector is parallel
to the beam direction (axial plane)
Since common ways to present dose
distributions is a dose map in the
patient axial plane, as an innovation the
detector was placed in the cylindrical
phantom in the axial plane, parallel to
the beam axis
This work is part of a more ambitious
project aiming to:
Employ this prototype for obtaining
2D dose maps by an in-house developed
algorithm dedicated to verify IMRT
treatment plans.
This system is patent pending
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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Electronics
discrete electronics developed in-house
School of Engineering, University of Seville
School of Engineering, University of Seville
Charge integrators for each strip (electrometers) digitized (12 bits)
and analyzed by a Digital signal processor (DSP)
Virgen Macarena
University Hospital,
Seville
A PC allows to control and to retrieve data via an RS-232 serial
bus based on a LabVIEW software
DITANET Workshop 08 November 2011
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Measurements
Detector
Set-up 1:
characterization
• Linearity
• Uniformity
• Calibration
• Percent Depth Dose
(PDD)
• Penumbra
Angular response
Set-up 2:
• TPS and Geant4
Simulations
• Angular measurements
• Final calibration
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
18
Measurements
Detector
Set-up 1:
characterization
 Linearity
• Uniformity
• Calibration
• Percent Depth Dose
(PDD)
• Penumbra
Angular response
Set-up 2:
• TPS and Geant4
simulations
• Angular measurements
• Final calibration
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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Linearity
Set-up 1:
SSSSD perpendicular
to the beam direction
Linearity with dose better than 0.1 %
for all channels
DITANET Workshop 08 November 2011
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Measurements
Detector
Set-up 1:
characterization
• Linearity
 Uniformity
• Calibration
• Percent Depth Dose
(PDD)
• Penumbra
Angular response
Set-up 2:
• TPS and Geant4
simulations
• Angular measurements
• Final calibration
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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Uniformity
Set-up 1:
SSSSD perpendicular
to the beam direction
Non-uniformities depend by the different strip efficiency and gain of the electronics
Uniformity better than 0.5 %
for all channels
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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Measurements
Detector
Set-up 1:
characterization
• Linearity
• Uniformity
 Calibration
• Percent Depth Dose
(PDD)
• Penumbra
Angular response
Set-up 2:
• TPS and Geant4
simulations
• Angular measurements
• Final calibration
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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Calibration
Set-up 1:
SSSSD perpendicular
to the beam direction
Monitor Units –> Voltage -> cGy
Calibration in standard condition
radiation field 10 × 10 cm2
source-to-surface distance (SSD) = 100 cm
1.5 cm of water-equivalent solid slabs
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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Measurements
Detector
Set-up 1:
characterization
• Linearity
• Uniformity
• Calibration
 Percent Depth Dose
(PDD)
• Penumbra
Angular response
Set-up 2:
• TPS and Geant4
simulations
• Angular measurements
• Final calibration
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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Percent Depth Dose
Set-up 1:
SSSSD perpendicular
to the beam direction
Dose at different depth measured using different water-equivalent solid slabs
The difference between SSSSD and ionization chamber is:
0.68 % at 10 cm and 0.73 % at 15 cm
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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Measurements
Detector
Set-up 1:
characterization
• Linearity
• Uniformity
• Calibration
• Percent Depth Dose
(PDD)
 Penumbra
Angular response
Set-up 2:
• TPS and Geant4
simulations
• Angular measurements
• Final calibration
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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Penumbra
Region between 20 % and 80 % of the maximum dose levels at 1.5 cm water depth
SSSSD: 6.17 ± 0.56 mm - single silicon diode: 3.92 ± 0.20 mm
Set-up 1:
SSSSD perpendicular
to the beam direction
SSSSD penumbra value larger than the one obtained when using a single silicon detector
This was mainly due to the SSSSD strips pitch of 3.1 mm
Geant4 simulations gave compatible results
Improvements with a future prototype (next project)
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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Measurements
Detector
Set-up 1:
characterization
• Linearity
• Uniformity
• Calibration
• Percent Depth Dose
(PDD)
• Penumbra
Angular response
Set-up 2:
 Geant4 simulations
and TPS calculations
• Angular measurements
• Final calibration
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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Geant4 simulations and TPS calculations
Treatment head
Geometry Model
Phantom
SSSSD Detector
The geometry of the Siemens
treatment head at 6 MV nominal
energy photons, was reproduced
in detail
The geometric model of the
phantoms and of the SSSSD was
also reproduced
The absorbed dose in each strip
was calculated
Geant4 Simulations were
performed for calculating
dose-to-water
case
comparisons
with
calculations
M. A. Cortés Giraldo, Ph. D. Thesis, 2011
DITANET Workshop 08 November 2011
also
the
for
TPS
M. A. Cortes-Giraldo et al. , "Geant4 Simulation to Study the Sensitivity of a
MICRON Silicon Strip Detector Irradiated by a SIEMENS PRIMUS Linac",
Progress in Nuclear Science and Technology, in press (2011)
Alessio Bocci, CNA
30
Measurements
Detector
Set-up 1:
characterization
• Linearity
• Uniformity
• Calibration
• Percent Depth Dose
(PDD)
• Penumbra
Angular response
Set-up 2:
• Geant4 simulations and
TPS calculations
 Angular measurements
• Final calibration
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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Angular response
00
450
Set-up 2:
SSSSD parallel
to the beam direction
Two experimental measurements
gantry
detector
3150
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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Angular Response
Steps of 450
Comparison between exp.
data, TPS and Geant4
Set-up 2:
SSSSD parallel
to the beam direction
The agreement between the
tendency of experimental data
at different angles
and the TPS is notable
This implies that the new
calibration factors are
independent of the irradiation
angle
Constant calibration factors
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
33
Measurements
Detector
Set-up 1:
characterization
• Linearity
• Uniformity
• Calibration
• Percent Depth Dose
(PDD)
• Penumbra
Angular response
Set-up 2:
• Geant4 simulations and
TPS calculations
• Angular measurements
 Final calibration
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
34
Final Calibration
Constant calibration factors
Calibration factors: Experimental/TPS
Set-up 2:
SSSSD parallel
to the beam direction
gantry
detector
Calibration factors independent of
angular irradiation and of strip number
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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Final Calibration
Calibrated dose
Set-up 2:
SSSSD parallel
to the beam direction
gantry
detector
Relative difference between the calibrated dose and TPS
calculations are better than 2 %
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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Conclusions
 Main Objective: Characterize and benchmark a new detection
system based on a Si-strip detector dedicated to 2D dose
measurements in the axial plane of a cylindrical phantom
 SSSSD characterization: results shows that the prototype is
suitable for IMRT verification plans
(remarkable linearity, uniformity, PDD)
 The angular response of the detector in the axial plane was
independent of the irradiation angle and of the strip number
 Geant4 simulations gave compatible results with respect to TPS
and to experimental data
 Final calibration with respect to TPS in the axial plane gives
differences smaller than 2 % for all the strips
 This system is patent pending
OEMP PATENT number P201101009
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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Future developments
 Future: work is in progress in order to obtain 2D dose maps
from experimental data in the axial plane using an in-house
developed algorithm based on the Radon Transform
 A new SSSSD prototype and a new experimental set-up has
been designed to improve the spatial resolution, coupling
the data acquisition system with a reconstruction algorithm
and with an on-line graphical interface software
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
38
RADIA2 Project – Collaboration
Z. Abou-Haidar1, M. A. G. Alvarez1 ,R. Arrans3, A. Bocci1,
M. A. Cortes-Giraldo2 , J. M. Espino2 ,M. I. Gallardo2, A. Perez Vega-Leal4
F. J. Perez Nieto5, J. M. Quesada2
1. DITANET group @ National Accelerator Centre (CNA)
2. Department of Atomic, Molecular and Nuclear Physics (FAMN),
University of Seville
3. Virgen Macarena University Hospital, Seville
4. School of Engineering, University of Seville
5. Instalaciones Inabensa S.A.
A. Bocci et. al., Empirical characterization of a silicon strip detector for a novel 2d mapped method for dosimetric
verification of radiotherapy treatments, Radiotherapy and Oncology, Volume 99, Supplement 1, May 2011, Page S172
A. Bocci et. al., A silicon strip detector for a novel 2D dosimetric method
for radiotherapy treatment verification
submitted to NIM-a (October 2011)
M. A. Cortes-Giraldo et al. , "Geant4 Simulation to Study the Sensitivity of a
MICRON Silicon Strip Detector Irradiated by a SIEMENS PRIMUS Linac",
Progress in Nuclear Science and Technology, in press (2011)
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
39
Thank you for
your
attention!!!
Single Strip and 2D monolithic
silicon detectors
Research is directed towards silicon microstrip technology to improve
spatial resolution
1. DOSI
2. CMRP DMG
Single crystal n-Si
128 channels
32 mm x 0.2 mm
I. Redondo-Fernandez et al,
NIM-a, (2007) 141–144
128 phosphor
implanted n+
strips on a ptype silicon
wafer
J. H. D. Wong et al.,
Medical Physics 37 (2010) 427–439
3. European project MAESTRO
Pixellated monolithic silicon detectors such as
the 2D array
441 Si n+p diodes
50 µm epi layer growth on MCz p.
Active area: 6.29 x 6.29 cm2.
D. Menichelli et al., Nucl. Instr. and Meth. A, 583, 109 (2007)
DITANET Workshop 08 November 2011
Alessio Bocci, CNA
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