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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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radiation protection in radiotherapy

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

objectives of part 10
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

Part 10, Practical 3

part 10 external beam radiotherapy

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

contents
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

Part 10, Practical 3

what minimum equipment is needed
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

Part 10, Practical 3

iaea trs 398
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

Part 10, Practical 3

iaea trs 3981
IAEA TRS 398
  • Published in 2000
  • Very general - can be used for photons (kV, MV), electrons, protons and heavy ions
  • Straight forward process

Part 10, Practical 3

advantages of absorbed dose calibration
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

Part 10, Practical 3

assume you have a ne 2505 3 3a ion chamber and farmer electrometer
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

Part 10, Practical 3

the formalism
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 user’s beam

Part 10, Practical 3

want to calibrate a cobalt unit
Want to calibrate a Cobalt unit
  • kQCo =1
  • FAD = 80cm
  • dmax = 0.5cm

Part 10, Practical 3

perform measurement in water phantom
Perform measurement in water phantom
  • Fill with water to correct depth
  • Let temperature equilibrate (>1 hour)
  • Level phantom
  • Insert chamber
  • Ensure linac settings and beam orientation correct

PTW small water phantom

Part 10, Practical 3

reference conditions for 60 co
Reference conditions for 60-Co

Part 10, Practical 3

depth of measurement
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!

Part 10, Practical 3

need correction for
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

Part 10, Practical 3

need also correction for recombination of ions in the chamber
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))

Part 10, Practical 3

corrections of electrometer reading
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

Part 10, Practical 3

absorbed dose in 60 co
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

Part 10, Practical 3

iaea worksheet
IAEA Worksheet

Part 10, Practical 3

iaea worksheet1
IAEA Worksheet

Part 10, Practical 3

iaea worksheet2
IAEA Worksheet

Part 10, Practical 3

please fill in the sheet for your cobalt unit

Please fill in the sheet for ‘your’ Cobalt unit

Conditions and readings on the next page...

final information
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)

Part 10, Practical 3

questions
Questions?

Let’s get started...

Part 10, Practical 3

result 2 47 gy per minute at depth of maximum dose
Result: 2.47 Gy per minute at depth of maximum dose

Can you estimate the uncertainty of this?

Part 10, Practical 3

uncertainty analysis trs 398
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%

Part 10, Practical 3

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