RADIATION PROTECTION IN RADIOTHERAPY

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RADIATION PROTECTION IN RADIOTHERAPY
IAEA Training Material on Radiation Protection in
Radiotherapy

• Part 10 Optimization of protection in External
  Beam Radiotherapy
• PRACTICAL EXERCISE

IAEA Post Graduate Educational Course Radiation
Protection and Safe Use of Radiation Sources
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Objectives of Part 10

• Be familiar with the design considerations as
  stipulated by appendix II in the BSS
• Be able to apply these design considerations in
  the context of radiotherapy equipment
• Be aware of relevant international standards and
  other documents which provide specification for
  external beam radiotherapy equipment

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Part 10 External Beam Radiotherapy
IAEA Training Material on Radiation Protection in
Radiotherapy

• Practical 3 Calibration of a 60-Co unit using
  TRS 398

IAEA Post Graduate Educational Course Radiation
Protection and Safe Use of Radiation Sources
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Contents

• Differences between TRS 277 and TRS 398
• Step by step procedure to be followed for
  calibration of a photon beam from a 60-Co unit
  following IAEA TRS 398
• Interpretation of results

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What Minimum Equipment is Needed?

• 60-Co unit with front pointer
• Water phantom, spirit level
• Calibrated ionization chamber and electrometer
  combination
• IAEA TRS 398 protocol

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IAEA TRS 398

• Assumes user has a calibration factor for
  exposure ND for the ion chamber/ electrometer
  combination in use
• Determines absorbed dose to water

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IAEA TRS 398

• Published in 2000
• Very general - can be used for photons (kV, MV),
  electrons, protons and heavy ions
• Straight forward process

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Advantages of absorbed dose calibration
The exposure/ KERMA way

• Easier for the user
• Less factors required
• Get NDw directly - only conversion for beam
  quality required

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Assume you have a NE 2505/3 3A ion chamber and
Farmer electrometer

• Chamber volume 0.6cc
• Internal radius 3.15mm
• Internal length 24mm
• Get absorbed dose to water factor - usually
  provided by the SSDL for a Cobalt reference beam
• ND,w 9.95 10-3 Gy/div

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The formalism

• DwQ (zref) MQ NDCo kQCo with
• DwQ (zref) - the dose in the users beam quality Q
  at reference location zref
• MQ - the corrected chamber reading
• NDCo - the absorbed dose to water factor for
  Cobalt as provided by the SSDL
• kQCo - a correction for beam quality difference
  between Cobalt and the users beam

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Want to calibrate a Cobalt unit

• kQCo 1
• FAD 80cm
• dmax 0.5cm

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Perform measurement in water phantom

• Fill with water to correct depth
• Let temperature equilibrate (gt1 hour)
• Level phantom
• Insert chamber
• Ensure linac settings and beam orientation correct

PTW small water phantom
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Reference conditions for 60-Co
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Depth of measurement

• Measurement depth 5cm in water
• Chamber position with geometric centre of the
  chamber at measurement depth
• No correction for the effective point of
  measurement is applied - this is different from
  TRS 277!

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Need correction for

• Temperature (the higher the less molecules in
  chamber)
• Pressure (the higher the more molecules in
  chamber)
• kTp P0/P (T 273.2)/(T0 273.2)
• with P and T the measured pressure (in kPa) and
  temperature (in oC) and P0 101.3kPa and T
  20oC as reference conditions

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Need also correction for recombination of ions in
the chamber

• Effect depends on radiation quality, dose rate
  and high voltage applied to the chamber
• Use two voltage method - normal voltage V1 and
  reduced voltage V2 (reduced voltage should be
  smaller than 0.5V1) with readings M1 and M2 ,
  respectively
• ks ((V1/V2)2 - 1)/ ((V1/V2)2 - (M1/M2))

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Corrections of electrometer reading

• MQ Mraw kTP kelec kpol ks with
• MQ and Mraw the corrected and the raw reading
• kTP and ks the temperature, pressure and
  recombination correction
• kelec a factor allowing for separate calibration
  of the electrometer - here 1
• kpol (M M- )/ 2M a polarity correction with
  M being the reading at normal polarity

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Absorbed dose in 60-Co

• Dw (zref) MQ NDCo with
• Dw (zref) - the dose in the users beam quality Q
  at reference location zref
• MQ - the corrected chamber reading
• NDCo - the absorbed dose to water factor for
  Cobalt as provided by the SSDL

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IAEA Worksheet
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IAEA Worksheet
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(No Transcript)
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IAEA Worksheet
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Please fill in the sheet for your Cobalt unit

• Conditions and readings on the next page...

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Final information

• Want to calibrate dose to dmax
• Percentage depth dose for 10x10cm2, SSD 80cm at
  d5 78.8
• T 28oC, p 100.3kPa
• Uncorrected readings for 1min exposure 184.5,
  184.2, 184.3 (for normal polarity) and 185.0,
  184.7, 184.6 (for - polarity)
• Mean reading for 1/3 voltage 182.1
• Assume time is corrected for on/off effect
  (timer error)

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Questions?

• Lets get started...

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Result 2.47 Gy per minute at depth of maximum
dose

• Can you estimate the uncertainty of this?

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Uncertainty analysis TRS 398

• Uncertainty from SSDL 0.6
• User uncertainties
• stability of dosimeter 0.3
• establishment of reference conditions 0.5
• dosimeter reading relative to timer 0.1
• correction factors used 0.3
• Total 0.9