Diapositiva 1

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Fricke gel dosimeters for the measurement of the
anisotropy function of a HDR Ir-192 brachytherapy
source Mauro Carrara1, Stefano Tomatis1,
Giancarlo Zonca1, Grazia Gambarini2,3, Giacomo
Bartesaghi2,3, Chiara Tenconi2, Annamaria
Cerrotta1, Carlo Fallai1,
1 Fondazione IRCCS Istituto Nazionale dei Tumori
di Milano
2 Dipartimento di Fisica, Università degli Studi
di Milano
3 Istituto Nazionale di Fisica Nucleare, Milano
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Brachytherapy
Brachytherapy (from the Greek brachios, meaning
short) is a form of radiotherapy where one or
more sealed radioactive sources are placed inside
or next to the area requiring treatment.
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HDR-Brachytherapy
In high dose rate (HDR) brachytherapy a single
sealed source (usually Ir-192) is adopted to
deliver radiation to the target with a dose-rate
of at least 12 Gy/h.
Ir-192 source (initial activity 10Ci)
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The anisotropy function
Treatment planning systems (TPS) are adopted in
clinical practice to optimize source dwell times
and positions inside the catheters, with the aim
of conforming the prescribed dose to the target
volume.
TPS dose calculation algorithm
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The anisotropy function

• Radiation propagation is anisotropic due to
• source self-absorbance
• absorbance by the capsule

The anisotropy function F(r,?) has been
recommended to account for the dose distribution
anisotropy that results adopting sealed sources.
This function is implemented in the AAPM dose
calculation formalism and is widely adopted by
the currently available treatment planning
systems.
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Purpose of the work
To develop and verify a method based on Fricke
gel layer dosimetry for the characterization of
the anisotropy function F(r,?) of an Ir-192
source.
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Fricke gel layer dosimetry (FGLD)
Fricke gels are radiochromic tissue-equivalent
dosimeters that can be established in form of
layers. Fricke gel layer dosimeters FGLDs are
prepared in our laboratory by infusing a ferrous
sulphate solution and the metal-ion indicator
Xylenol Orange (XO) in a tissue-equivalent gel
matrix. Exposure to ionizing radiation of a FGLD
produces a conversion of ferrous ions Fe2 into
ferric ions Fe3 and the complex XO-Fe3 causes
visible light absorption around 585nm, with yield
proportional to the absorbed dose.
Fe2 Fe3
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FGLD image acquisition system
Optical imaging is performed by means of
visible-light transmittance analysis.
CCD Camera
Computer
Transmitted light images are detected with a CCD
camera.
Gel-layer
Plane uniform light source
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FGLD analysis
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FGLD CHARACTERISATION
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FGLD characterisation
Irradiation set-up
Source
Catheter
FGLD
tissue-equivalent phantom
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FGLD characterisation
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FGLD characterisation
Response saturation for dose-rates higher than
400 cGy/min
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FGLD characterisation

• At doses lower than 400 cGy, the D(OD) response
  to the dose is not linear
• At doses higher than 2800 cGy and at dose-rates
  higher than 400 cGy/min, the D(OD) response to
  the dose saturates
• The dose response is independent on the energy
  over most of the adopted energy range
• Each single dosimeter may require a specific
  calibration

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FGLD calibration procedure
A double 400cGy pre-irradiation of each FGLD
resulted to be the optimal procedure for
calibration. Applying this procedure, the
obtained calibration curves were straight lines
crossing the origin.
I. irradiation elimination of the non-linear
behaviour at low doses
II. irradiation determination of the
sensitivity factor k
k
D cGy
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ANISOTROPY FUNCTION MEASUREMENT
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Anisotropy function measurement
Irradiation set-up
Tissue-equivalent phantom
Ir-192 source
FGLD
A series of measurements of the same irradiation
set-up composed of a tissue-equivalent phantom
and a FGLD with a built-in plastic catheter were
performed. The plastic catheter permitted us to
convey the source directly inside the dosimeter.
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Anisotropy function measurement
The delivered dose was of 1500cGy at 10mm
distance from the source. Images of the
irradiated dosimeters were acquired and
elaborated with a dedicated software developed in
Matlab.
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Anisotropy function measurement
The measured anisotropy function at radial
distances of 15mm and 20mm at angles between 15
and 165
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Anisotropy function measurement
The measured anisotropy function at radial
distances of 25mm and 30mm at angles between 15
and 165
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Anisotropy function measurement
The measured anisotropy function at radial
distances of 15mm and 20mm at angles between 15
and 165
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Anisotropy function measurement
The measured anisotropy function at radial
distances of 25mm and 30mm at angles between 15
and 165
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Anisotropy function measurement
The measured anisotropy function at radial
distances of 35mm, 40mm, 45mm and 50mm at angles
between 15 and 165
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Discussion and Conclusions

• At short distances from the source, percentage
  differences between tabulated and measured data
  are almost always smaller than 3.
• At increasing distances, data become more
  scattered due to the low doses delivered to the
  dosimeter.
• Measurements with a higher irradiation time
  will be performed to achiever better accuracy at
  higher distances from the source
• A FGLD results to be an accurate tool for the
  Ir-192 anisotropy function measurement. This
  instrument could be easily adopted in QA
  protocols to verify the anisotropy function for
  each newly installed Ir-192 source.

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Thank you for your attention
Vancouver Island 2009