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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 electrometer

  • 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 electrometer

  • kQCo =1

  • FAD = 80cm

  • dmax = 0.5cm

Part 10, Practical 3


Perform measurement in water phantom
Perform measurement in water phantom electrometer

  • 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 electrometer

Part 10, Practical 3


Depth of measurement
Depth of measurement electrometer

  • 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 electrometer

  • 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 chamber

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 chamber

  • 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 chamber

Part 10, Practical 3


Iaea worksheet1
IAEA Worksheet chamber

Part 10, Practical 3



Iaea worksheet2
IAEA Worksheet chamber

Part 10, Practical 3


Please fill in the sheet for your cobalt unit

Please fill in the sheet for ‘your’ Cobalt unit chamber

Conditions and readings on the next page...


Final information
Final information chamber

  • 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? chamber

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 chamber

Can you estimate the uncertainty of this?

Part 10, Practical 3


Uncertainty analysis trs 398
Uncertainty analysis TRS 398 chamber

  • 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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