Internal Calibration of gel dosimeters: A feasibility study

1
Internal Calibration of gel dosimeters A
feasibility study

• Jamie Trapp, Tanya Kairn, Scott Crowe, Andrew
  Fielding
• Plus
• Kerry Williams, Rick Franich, Michael Taylor,
  Matthew So

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Introduction

• Gels are slowly(?) being used more often in the
  clinics.
• Issues preventing wider usage
• Manufacturing difficulty? (can be purchased
  commercially)
• Readout? (scanner access/purchase)
• All in one services available (e.g. Imagel in
  UK, MGS?)
• User confidence/trust (Accuracy and precision -
  calibration)

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Gel Calibration

• Principle
• Place vessel of gel in air or in a water phantom
• Irradiate with a given field
• Assume the dose is equivalent to that in standard
  measurement conditions (i.e. in water)
• Match points of specific dose with values
  obtained from MRI, CT, optical, ultrasound etc.
• Construct calibration curve

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Gel Calibration

• Calibration Methods
• Many Vials
• Single Flask
• Test tube (long axis)
• In phantom from planning system

Image from Hilts et al 2000
Image from Oldham et al 1998
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Gel Calibration Problems

• Assume dose in gel is the same as standard
  measurement conditions (or that planning system
  is accurate)
• Taylor et al1,2 showed that dosimetry accurate
  to within 1 for large flask, 1-2 for many
  vials, 2-4 for test tube long axis
• Require separate calibration gel (except for In
  Phantom)
• Same history of temperature, background etc?
• 1. Taylor, M.L., R. Franich, P.N. Johnston, M.
  Miller, and J.V. Trapp, Systematic variations in
  polymer gel dosimeter calibration due to
  container influence and deviations from water
  equivalence. Phys. Med. Biol., 2007. 52 p.
  3991-4005.
• 2. Taylor, M.L., R.D. Franich, J.V. Trapp, and
  P.N. Johnston, A comparative study of the effect
  of calibration conditions on the water
  equivalence of a range of gel dosimeters, in 2007
  IEEE Nuclear Science Symposium Conference Record,
  B. Yu, Editor. 2007, IEEE Piscataway, NJ, USA.
  p. 3170-3175.

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Solution

• Directly obtain absolute dose points from within
  the phantom gel itself by placing dosimeters at
  key locations

(Figures unrelated for descriptive purposes
only)
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Problem with the solution

• Placing a dosimeter inside the phantom gel
• Tissue or water equivalence of ion chambers, TLDs
  etc
• Perturbs the dose downstream of the dosimeter,
  thus destroying the very dose distribution in the
  gel phantom that was being measured

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Solution to the problem with the solution

• Organic Plastic Scintillators
• Density 1.032 g/cc (good for applications that
  require water equivalency)
• Good efficiency (25-30 of NaI(Ti) scintillators)
• Can be machined to very small dimensions (few
  cubic millimetres)
• Recently used with diode instead of PM tube.
• Williams, K., N. et al, A portable organic
  plastic scintillator dosimetry system for low
  energy X-rays A feasibility study using an
  Intraoperative X-ray unit as the radiation
  source. Journal of Medical Physics, 2007. 32(2)
  p. 73-76.

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Aim

• To determine whether an OPS can be used inside a
  gel dosimeter without perturbing the dose
  distribution
• Monte Carlo
• Experimentally

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Monte Carlo

• EGSnrc/BEAMnrc
• 20 20 20 cm volume - PAG
• Embedded with 2 2 0.25 cm volume scintillator
• Pre-commissioned model of Elekta Precise linac
• 6MV nominal photon energy
• SSD 90 cm
• Voxel size was 0.25 0.25 0.25 cm

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Monte Carlo

• MAGIC and PAG
• Scintillators in centre of field at depths of
  1.5, 10, 18.5 cm
• Composition from manufacturer data (see table in
  abstract)
• Scintillator only
• Reflective paint and lightproof covering mixed
  throughout scintillator volume
• Second gel without scintillators simulated for
  comparison

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Monte Carlo Results PAG Central Axis
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location of profile (below), in terms of
depth-dose
lateral profile at 10.375 cm depth, normalised to
dose in gel-only phantom on CAX
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location of profile (below), in terms of
depth-dose
lateral profile at 10.625 cm depth, normalised to
dose in gel-only phantom on CAX
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location of profile (below), in terms of
depth-dose
lateral profile at 10.875 cm depth, normalised to
dose in gel-only phantom on CAX
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location of profile (below), in terms of
depth-dose
lateral profile at 11.075 cm depth, normalised to
dose in gel-only phantom on CAX
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location of profile (below), in terms of
depth-dose
lateral profile at 11.325 cm depth, normalised to
dose in gel-only phantom on CAX
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Monte Carlo Results PAG
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Monte Carlo Results - MAGIC
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location of profile (below), in terms of
depth-dose
lateral profile at 10.375 cm depth, normalised to
dose in gel-only phantom on CAX
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location of profile (below), in terms of
depth-dose
lateral profile at 10.625 cm depth, normalised to
dose in gel-only phantom on CAX
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location of profile (below), in terms of
depth-dose
lateral profile at 10.875 cm depth, normalised to
dose in gel-only phantom on CAX
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location of profile (below), in terms of
depth-dose
lateral profile at 11.075 cm depth, normalised to
dose in gel-only phantom on CAX
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Monte Carlo Results - MAGIC
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Experiment

• MAGIC Gel
• Scintillator
• BC430 (Saint Gobain)
• 2mm diam 7.5cm length
• Irradiation
• Varian linac
• 1000 MU, 6MV
• 10 x 10cm field
• 95cm SSD
• Scanning
• Quasar Vista Optical CT

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Experimental Results
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Conclusions

• Monte Carlo
• Dose perturbation of up to 1 immediately
  following scintillator (completely gone within
  about 1cm)
• Dose perturbation of up to -1 immediately before
  scintillator (less than 0.5cm)
• Experiment
• didnt work have to try again
• Scintillator didnt interfere too much with
  optical scanning.

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Acknowledgements

• Australian Research Council Linkage Grant Number
  LP0562315
• National Health and Medical Research Council
  Grant Project Grant
• RMIT Foundation Maxwell Eagle Grant
• QUT Faculty of Science Research Career
  Development Program Grant

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QUT (We have openings for researchers coming up)