RapidArcTM treatment verification using polymer gel dosimetry

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RapidArcTM treatment verification using polymer
gel dosimetry

• Sofie Ceberg, Helen Gustavsson, Stine Korreman,
  Joakim Medin, Flemming Kjær-Kristoffersen and
  Sven ÅJ Bäck

DosGel 2008, Heraklion, Crete
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What is RapidArcTM ?
Volumetric Arc Therapy (VMAT) technique

• Irradiation is delivered during one arc
• Varying gantry rotation speed

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What is RapidArcTM ?
Volumetric Arc Therapy (VMAT) technique

• Irradiation is delivered during one arc
• Varying gantry rotation speed
• Varying MLC-positioning
• Varying dose rate

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What is RapidArcTM ?
Volumetric Arc Therapy (VMAT) technique
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So what?

• RapidArc vs IMRT The same target coverage but an
  decreased dose to OAR. Much faster!
• High modulation, small beams, tight margins,
  steep dose gradients, multiple targets etc.
• The advanced inhomogeneous 3D dose distribution
  is difficult to verify with conventional 1D or 2D
  detector system.

DosGel 2008, Heraklion, Crete
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The aim
was to carry out a dosimetric verification of a
RapidArcTM treatment delivery using polymer gel
dosimetry.
Caution! Some polymer gels show a dose rate and
a fractionation dependence (De Deene et al PMB
2006 and A Karlsson et al PMB 2007). However! No
dose rate dependence due to motion in and out of
the beam (S Ceberg et al PMB 2008). No
fractionation due to one arc delivery.
DosGel 2008, Heraklion, Crete
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Material and methods gel preparation

• normoxic Polymer Acrylamide Gel (nPAG) 1.3 l
• 3 AA
• 3 BIS
• 5 gelatine
• 89 deionized water
• 10 mM THP (oxygen scavenger)

DosGel 2008, Heraklion, Crete
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Material and methods RapidArcTM dose delivery

• Treatment planning system Beta version
  RapidArcTM optimiser (Varian) Dose grid 2.5x2.5x3
  mm3
• 18 MV
• Arc rotation of 320 degrees
• Target dose 3.3 Gy
• Dose rate variation 200-600 MU/min

DosGel 2008, Heraklion, Crete
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Material and methods magnetic resonance imaging

• 1.5 T MRI unit (Siemens)
• 32-echo multi spin echo sequence
• 25 ms inter-echo spacing
• 4000 ms repetition time
• 1 x 1 x 3 mm3 voxel size

DosGel 2008, Heraklion, Crete
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Results justification of relative evaluation
Approx. linear dose response covering the used
dose range
DosGel 2008, Heraklion, Crete
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Results Absorbed dose measurements, gel vs TPS
DosGel 2008, Heraklion, Crete
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Results Difference image (TPS-gel)
Deviations within 3
DosGel 2008, Heraklion, Crete
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Results 3D iso-surfaces
DosGel 2008, Heraklion, Crete
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Results 3D iso-surfaces overlay
DosGel 2008, Heraklion, Crete
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Results The distribution of the deviations
112 071 voxels (0.93.1)
76 877 voxels (1.42.8)
DosGel 2008, Heraklion, Crete
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Conclusions

• Gel measurements agreed well with the treatment
  planning system within the treated volume.
• Within analysed isodose volumes (80 and 95),
  90 of the voxels showed a deviation lt 5.
• The larger deviations were observed particularly
  in the low dose volumes outside the treatment
  volume.

DosGel 2008, Heraklion, Crete
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Discussion...
Clive Are the gel detectors useful? Sofie Oh
yes! 3D nature and high resolution etc. For
instance compare 3 mm MLCs to the 2D detector
array with smallest center-to-center distance of
5 mm. Clive Why are they not in the
clinic? Sofie The complete detector system
must be user friendly! For instance if the system
is not more environmental friendly than film or
cant be used for doses less than 2 Gy its not
of interest for the radiotherapy clinic!
DosGel 2008, Heraklion, Crete
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Acknowledgement
The Swedish society for Radiation Physics
DosGel 2008, Heraklion, Crete