Intraoperative SolidState Based Urethral Dosimetry in Low Dose Rate Prostate Brachytherapy

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Intraoperative Solid-State Based Urethral
Dosimetry in Low Dose Rate Prostate Brachytherapy
D.L. Cutajar1, A.B. Rosenfeld1, G.J. Takacs1,
M.L.F. Lerch1, J.A. Bucci2, L.J. Duggan2, K.E.
Enari2, M. Zaider3, M. Zelefsky3

• 1. Centre for Medical Radiation Physics,
  University of Wollongong, Australia
• 2. St George Cancer Care Centre, Kogarah,
  Australia
• 3. Dept of Medical Physics, Memorial Sloan
  Kettering Cancer Center, NY, USA

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Low Dose Rate Prostate Brachytherapy

• Involves the implantation of between 50 and 150
  radioactive seeds into the prostate as a
  treatment for prostate cancer
• The seeds are implanted using guiding needles, in
  desired locations designated by the treatment
  plan
• These seeds emit low energy photons with a short
  range in tissue
• The prostate receives a high dose whereas the
  surrounding organs receive a low dose
• The prostate wraps around the urethra
• The dose to the urethra needs to be minimised to
  reduce complications

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The Urethra

• The urethra passes through the prostate, near to
  the centre at the apex, but forward of centre at
  the base

Bladder
Urethra
Rectum
Urethra
Rectum
Side View
Top View
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The Urethra

• The urethra passes through the prostate, near to
  the centre at the apex, but forward of centre at
  the base

Peripheral seed loading is recommended to reduce
the urethral dose
Bladder
Urethra
Rectum
Urethra
Rectum
Side View
Top View
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Why Use Online Mini Dosimetry?

• Seeds are not always placed according to the
  treatment plan as
• Needles may diverge when penetrating different
  layers of tissue
• Seeds may drift along the needle paths
• Seeds may drift due to blood flow
• The prostate volume may vary due to oedema

There is a need to monitor the urethral dose in
real time to minimize the risk of problem
development. The urethral dose should also be
measured post implant for treatment planning
verification.
X-ray image of treatment volume
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The Urethra Mini Dosimeter

• The Centre for Medical Radiation Physics have
  developed an intraoperative dosimetry system for
  LDR prostate brachytherapy using
  Spectroscopic Dosimetry

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Spectroscopic Dosimetry

• LDR brachytherapy seeds contain isotopes that
  emit low energy photons (27 keV for I125)
• In low Z materials (water, tissue), most energy
  loss for photons of these energies is due to the
  photoelectric effect
• The Compton cross-section is significant, but
  energy loss due to scattering is minimal
• As photons are attenuated, the peaks in the
  energy spectra are not be shifted. They are
  attenuated in intensity only
• Low energy tails forming on peaks are
  insignificant
• The number of counts corresponding to photon
  peaks in a measured spectrum is proportional to
  the dose rate at the point of measurement

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The Urethra Mini Dosimeter

• The urethra mini dosimetry system consists of a
  urethra probe connected to a data acquisition and
  display unit
• The urethra probe contains a silicon mini
  detector of less than 1mm3 active volume,
  connected to a preamplifier through a 30cm
  catheter

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Operation of the Urethra Mini Dosimeter

• The Urethra Mini Dosimeter obtains a spectrum of
  the measured radiation and measures the number of
  counts corresponding pulses from photon peaks
• The dose rate and total dose after full decay are
  calculated using a predetermined algorithm that
  takes into account the average dose deposited in
  tissue per photon and the detector efficiency

I125 seed spectrum (Amersham 6711), measured with
the Urethra Mini Dosimeter, 37ºC
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Isotropy Measurements (Azimuthal)

• The dose rate of an 125I seed was measured, 1cm
  from the detector in a Perspex phantom, at
  varying azimuthal angles about the probe axis
• The urethra probe shows an isotropic response
  about the probe axis, within the errors of
  measurement (5)

f
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The Gel Phantom

• A phantom was constructed at St George Cancer
  Care Centre
• The phantom contains an artificial urethra
  between two needle grids, above an ultrasound
  cavity
• The phantom may be filled with a tissue
  equivalent gel
• Seeds may be implanted into desired locations
  within the gel using the needle grids
• Online in vivo dosimetry of the urethra may be
  simulated

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The Gel Phantom
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Depth-Dose Measurements

• The depth-dose profile along the transverse axis
  of the detector and seed

The expected dose was obtained using the VariSeed
7.0 dose planning system The measured dose was
obtained using the Mini Dosimetry system The
results are within an error of 5
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Isotropy Measurements (polar)

• Measurements were obtained for various polar
  angles about the detector
• At each measurement point, a comparison was made
  between the measured dose and the expected dose
  obtained using the VariSeed 7.0 dose planning
  system

The measured doses do not agree with the dose
planning system at large polar angles
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Isotropy Measurements

• The VariSeed 7.0 dose planning system was set to
  use point source approximations to calculate the
  doses from brachytherapy seeds
• Seed anisotropy needs to be taken into account
  when calculating the dose at any angle away from
  the transverse axis

q
6711 seed, I-125 Amersham Health
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Isotropy Measurements

• Monte Carlo simulations were performed using
  EGSnrc V4 to calculate the anisotropy correction
  for the Variseed 7.0 determined doses
• The expected doses after anisotropy correction
  and the measured doses were plotted vs. polar
  angle

The measured doses at varying polar angles now
correspond to the planned doses
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Multiple seed Measurements

• 12 seeds were placed within the phantom in a 3D
  geometry around the mini dosimeter
• Variseed 7.0 predicted a dose of 79.8 Gy, without
  anisotropy correction
• After anisotropy correction, this became 73 Gy
• The measured dose was 69.7 Gy - 0.5 Gy
• The measurement was within 5 of the calculated
  dose
• One possible source of error is the difficulty in
  positioning each seed. Localization errors of the
  order of millimetres are significant, as the
  photons have a short range in tissue

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Conclusion

• The Urethra Mini Dosimetry system
• Uses Spectroscopic Dosimetry
• Is operational at body temperature
• Has an isotropic response to radiation at all
  angles
• Measures the combined dose from multiple seeds
  with an accuracy within 5
• Provides online dosimetry
• Is small enough to be used in vivo
• May be used as a dose monitoring system during
  implantation
• May be used as a dose planning verification
  system post implant

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The Future

• Conduct clinical trials at St George CCC, Sydney,
  and at Memorial Sloan Kettering Cancer Centre,
  NY?
• Integrate with a dose planning system?
• Can we use Spectroscopic Dosimetry in other
  areas?

Thank You
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University of Wollongong
Australia