A solution for dosimetry and quality assurance in IMRT and hadrontherapy: the pixel ionization chamber.

1
A solution for dosimetry and quality assurance in
IMRT and hadrontherapy the pixel ionization
chamber.

• S. Amerioa, S. Bellettib, A. Borianoc, R. Ciriod,
  S. Codaa, M. Donettie,d,
• B. Ghedib, A. Lupariad, E. Madonf, F. Marchettod,
  U. Nastasia,
• C. Peronid,C.J. Sanz Freired,g, E. Trevisiolf, A.
  Urgesif
• a Servizio di Fisica Sanitaria, Ospedale S.
  Giovanni A.S., V. Cavour 31, I-10123 Torino
• b Servizio di Fisica Sanitaria, Spedali Civili di
  Brescia, P.le Ospedale 1, I-25123 Brescia
• c ASP, V.le Settimio Severo 65, I-10133 Torino
• d University and INFN of Torino, V. Giuria 1,
  I-10125 Torino
• e TERA Foundation, V. Puccini 11, I-28100 Novara
• f OIRM S. Anna, V. Baiardi 43, I-10126 Torino
• f partially supported by IBA, Ion Beam
  Application, Louven la-Neuve, Belgium

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Introduction
In Intensity Modulated Radiation Therapy (IMRT)
techniques like step and shoot, sliding window,
dynamic wedge and in hadrontherapy with scanning
beam the delivered dose in the treated volume is
not just function of the space but also of the
time. An instrument useful in dose measurements
has to be fast, reproducible and able to measure
in several points. We have developed, built and
tested a fast ionization chamber segmented in
pixels that measures the dose in a plane which
can help the physicist in the dose measurements
and in quality assurance. The outline of the talk
is the following detector design read out
electronics data acquisition test
performed results
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Detector design
parallel plate ionization chamber anode
segmented in 1024 square pixels pixel dimension
7.5 X 7.5 mm2 sensitive area 24 X 24
cm2 cathode made of aluminized mylar foil total
dimension 64 X 64 X 3 cm3 front end
electronics, placed around the chamber, perform
the analog to digital conversion
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Detector design
Thin version When used like monitor chamber the
detector has to be thin to minimize the effects
on the beam energy and shape. For this
configuration, anode and cathode are glued to
frames in order to assure the mechanical rigidity
and the gap thickness . Water equivalent
thickness lt 1 mm.
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Detector design
Thick version When used in plastic phantom the
detector can be thick. With photon or electron
beam air gap has to be minimized. Anode and
cathode are glued to a plastic slab, which is a
grid of 1024 holes. Each hole can be considered
as an independent sensitive volume.
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Read out electronics
The read out electronics is housed next to the
electrodes. We developed a Very Large Scale
Integration (VLSI) chip. Every chip has 64
channels that convert the collected charge in
digital output. The name of the last version is
TERA05
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Read out electronics

• TERA05 characteristics
• Very Large Scale Integration
• i?f conversion
• output ? Qint
• 100 fCltcharge quantumlt800 fC
• fmax 5 MHz ? Imax 4 ?A
• 64 channels
• 16 bit wide counters
• multiplexed digital output
• three-state output
• no dead time
• max read out 10 MHz

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Read out electronics
Recycling Integrator architecture.
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Read out electronics
The excellent linearity over a large range is due
to recycling integrator architecture.
QC 600 fC 10 pA lt I lt 2 ?A Linearity better
than 0.3
QC 100 fC 20 pA lt I lt 0.6 ?A Linearity better
than 0.7
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Read out electronics
I 49.96 nA 26 chips
Charge quantum have been measured over different
chips in order to evaluate the spread of the
value.
Results show an RMS ? 1
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Read out electronics
Charge quantum reproducibility has been measured.
I 49.96 nA Qc 600 fC 24 ºC lt T lt 27 ºC 1
month 6 measures
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Data acquisition
Slow data acquisition

• connection by twisted pair flat cables (100 m
  max)
• max rate transfer 1 MHz 2 Mb/s
• read out transfer time 1 ms
• PCI DAQ card
• LabVIEW software
• cheap solution
• easy to handle

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Data acquisition
Fast data acquisition

• connection by twisted pair flat cables (100 m
  max)
• max rate transfer 10 MHz 40 Mb/s
• read out transfer time 50 ?s
• read out cycle total time 500 ?s (FIFO
  operation data transfer)
• real time operation system
• expensive solution

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Hadron beam test
GSI - Darmstadt - Germany

• Aims
• spatial resolution
• homogeneity of response

• Beam characteristics
• raster-scan delivery system
• 270 MeV/u, C6, 8.8 mm (FWHM)
• fast data acquisition system
• data acquisition synchronized with raster-scan

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Hadron beam test
Spatial resolution
pixel dimensions 7.5 7.5 mm2 beam dimension
8.8 mm (FWHM) counts per voxel ? 103
Spatial resolution ? lt 0.2 mm
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Hadron beam test
Response homogeneity
The calibration accounts for the different gain
of each channel (gas gap variations and
electronics)
18 18 cm2 uniform field
before calibration ? 2.0 after calibration ?
1.1
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Photon beam
S. Giovanni A.S. and S. Anna hospitals Torino

• Quality Assurance
• Aims
• Measurements reproducibility
• Depth dose profile measurements and comparison to
  reference ion chamber (Farmer) results
• Profile measurements

In such measurements there arent DAQ time
constraints. Anyway the system can measure the
delivered dose in real time.
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Photon beam
Several measures have been performed to evaluate
the reproducibility. After a pre-irradiation
reproducibility is better than 1
100 MU per irradiation field bigger than
sensitive volume
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Photon beam
Depth dose profiles have been measured for
different photon energy. We use a Farmer ion
chamber in a plastic phantom to compare the
results. An excellent agreement has been found
for every photon energy.
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Photon beam
Profile comparison with a water phantom have been
performed.
Field 20 X 20 cm2 Depth 10 cm
Field 10 X 10 cm2 Depth 10 cm
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Photon beam
IMRT measurements

• Dynamic wedge case
• Moving each side of the collimator with different
  speed one obtains a wedge shape of the dose
• At the end of the treatment one has to check the
  delivered dose in different points

The pixel ion chamber can measure precisely the
2D dose in 1024 points.
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Photon beam
IMRT measurements

• Dynamic multi leaves collimator (DMLC) case
• independent leaves can be moved to shape the beam
  field
• one needs to check as a function of time the 2D
  delivered dose

The pixel ion chamber has a fast read out. Every
1ms (slow DAQ system) a complete read out can be
performed. It is possible to monitor the 2D dose
in real time.
Delivered dose in function of time
Total dose delivered
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Conclusions
We developed and tested a pixel ionization
chamber that can be used to measure and monitor
the treatment in IMRT and hadrontherapy. Its time
resolution allows to perform 2D dose measurements
in real time. Two prototypes have been built in
order to use the same technique both as monitor
chamber and to perform relative dose
measurements. Future development By stacking
several chambers 3D measurements can be obtained.