Diapositiva 1

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DOSGEL 2008 5th International Conference on
Radiotherapy Gel Dosimetry Hersonissos, 30th
September 2008
A procedure to mathematically amend possible
thickness disuniformities in gel-layer
dosimetry M. Carrara1, G. Gambarini2,3, G.
Bartesaghi2,3, C. Fallai1, A. Negri2,3
1 Medical Physics Unit, National Cancer
Institute, Milan, Italy
3 National Institute of Nuclear Physics, Milan,
Italy
2 Physics Department, University of Milan, Italy
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GEL ANALYSIS
Irradiation of piled-up gel layers
Irradiation of a gel volume
Analysis of the entire volume
Analysis of each single layer



with MRI, laser or cone beam CT scanners
with CCD camera and plane light source
Elaboration of the obtained signals / matrices
3D dose distributions
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GEL-LAYER ANALYSIS
Gel-layer analysis vs. other systems

• DISADVANTAGES
• more time-consuming
• lower spatial dose resolution

• ADVANTAGES
• simplest instrumental requirements
• portable instrumentation (lt5kg)
• achieve big phantoms without problems related to
  the cooling-rate and to refractive index
  inhomogeneities

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FRICKE GEL-LAYER
Currently, we adopt Fricke gels with following
characteristics

• Composition
• Porcine skin in the amount of 3 of the final
  weight
• ferrous sulphate solution ?1mM
  Fe(NH4)2(SO4)2?6H2O
• sulphuric acid 25mM H2SO4?
• Xylenol-Orange ?0.165mM C31H27N2Na5O13S?

Dimensions 110 x 110 x 3 mm3 (length x width x
thickness) 110 x 110 x 1 mm3 60 x 60 x 2
mm3 60 x 60 x 1 mm3 160 mm 3 mm (diameter
thickness) 120 mm 3 mm (diameter thickness)
square
circular
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FRICKE GEL-LAYER ANALYSIS - 1
1) grey-level images of the transmitted light at
585nm are detected for each dosimeter before
(GL585,b) and after (GL585,a) irradiation
After irradiation (a) _at_ 585nm
Before irradiation (b) _at_ 585nm
I0
I0
Ib
Ia
(GL585,b)
(GL585,a)
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FRICKE GEL-LAYER ANALYSIS - 2
2) the difference in optical density ?(OD) is
obtained from the GL images
?(OD) log10 (GL585,b / GL585,a)
(GL585,b)
D(OD)
(GL585,a)
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FRICKE GEL-LAYER ANALYSIS - 3
3) knowing the linearity factor between D and
?(OD), superficial matrices of dose distribution
are obtained.
D k ?(OD) with k proportionality factor
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FRICKE GEL-LAYER ANALYSIS - 4
4) matrices of superficial dose distributions are
combined together to obtain 3D dose distributions

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FRICKE GEL-LAYER ANALYSIS - 5
Recently, adoption of gel layers with increased
superficial dimensions has shown that the layers
thickness might not be uniform, thus introducing
a source of inaccuracy. The thinner are the
layers, the bigger is the inaccuracy due to
thickness non-uniformity. A further step in the
gel analysis chain has been introduced to correct
this source of error.
thickness disuniformities amendment algorithm
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THICKNESS DISUNIFORMITIES AMENDMENT
1b) before irradiation, one further grey-level
image of the transmitted light is detected for
each dosimeter at 430nm (GL430,b).
Before irradiation _at_ 430nm
I0
Ib
Hypothesis Possible differences in gray level
values of these images are due only to a
thickness inhomogeneity of the layer.
Flexible thin transparent layers
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THICKNESS DISUNIFORMITIES AMENDMENT
The additional information can be adopted to
develop normalization matrices Nb (before
irradiation) and Na (after irradiation) which
correct pixel by pixel the possible thickness
disuniformities of the layers.
(Nb)
(Na)
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THICKNESS DISUNIFORMITIES AMENDMENT
2) the difference in optical density ?(OD), is
evaluated for each pixel applying the corrected
function
?(OD) log10 (GL585,b Nb / GL585,a Na)
?(OD) log10 (GL585,b / GL585,a )
(GL585,b)
(GL585,a)
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THICKNESS DISUNIFORMITIES AMENDMENT
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RESULTS
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RESULTS DOSE PROFILES MEASUREMENTS
Comparison between Fricke gel single layer
measurements and Farmer ionization chamber
(0.6cc) measurements.
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RESULTS DOSE PROFILES MEASUREMENTS
without thickness disuniformities amendment
with thickness disuniformities amendment
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RESULTS IMRT SUPERFICIAL DOSIMETRY
Comparison between TPS calculations, Mapcheck and
Fricke gel-layers measurements.
Treatment Planning System, XIO (CMS Corp., St.
Louis, MO, USA)
Fricke gel-layer dosimeter
Diode detectors array Mapcheck (Sun Nuclear
Corp., Melbourne, FL, USA)
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RESULTS IMRT SUPERFICIAL DOSIMETRY
Dose distributions obtained for one IMRT field
with
Fricke gel-layers
Diode array
TPS
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RESULTS IMRT SUPERFICIAL DOSIMETRY
Comparison between TPS calculations and Fricke
gel-layers measurements
Error (plan vs. gel)
DTA 93.2 of points passing the test with 3mm
threshold 90.1 of points passing the
test with 2mm threshold
Gel-layer dosimetry for dose verification in
intensity modulated radiation therapy. Nucl.
Instr. and Meth. A 580 506-509 (2007)
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RESULTS HDR BRACHYTHERAPY DOSIMETRY
Comparison between TLD and Fricke gel-layers
Ir-192 source
plastic catheter
HDR Microselectron (Nucletron)
1st dosimeter set up (x-z gel)
phantom
2nd dosimeters set up (x-y gel)
TLDs set up
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RESULTS HDR BRACHYTHERAPY DOSIMETRY
Dose profiles on x-y gel-layers compared to TLD
measurements
Gel measurements were in agreement with 30 out of
36 TLDs
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RESULTS HDR BRACHYTHERAPY DOSIMETRY
x
z
The dose profile acquired with the vertical
gel-layer (x-z gel) was in agreement with the
measurements performed at the intersections with
the horizontal gel-layers (x-y gels) and with the
TPS at zgt12mm
Fricke gel-layer dosimetry in high dose-rate
brachytherapy. Submitted for publication on Appl.
Radiat. Isotop.
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RESULTS HDR BRACHYTHERAPY DOSIMETRY
Fricke gel-layer dosimetry in high dose-rate
brachytherapy. Submitted for publication on Appl.
Radiat. Isotop.
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CONCLUSIONS
Gel-layer dosimetry with the thickness
disuniformities amendment algorithm was proven to
be a reliable method for single IMRT fields dose
verification and for HDR brachytherapy dose
distribution measurements (distance from the
source gt12mm). The thickness disuniformities
amendment algorithm represents a fundamental step
for the continuous development of Fricke
gel-layer dosimetry. The aim is to realize
thinner layers (lt1mm) which would give a better
spatial dose resolution.
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Thank you!