RADIATION PROTECTION IN RADIOTHERAPY

1
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
2
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

3
Part 10 External Beam Radiotherapy
IAEA Training Material on Radiation Protection in
Radiotherapy

• Practical 2 Calibration of a megavoltage photon
  beam using TRS 277

IAEA Post Graduate Educational Course Radiation
Protection and Safe Use of Radiation Sources
4
Contents

• Rationale for calibration
• Step by step procedure to be followed for
  calibration of a photon beam from a medical
  linear accelerator following IAEA TRS 277
• Interpretation of results

5
IAEA TRS 277

• Assumes user has a calibration factor for
  exposure NX or air-KERMA NK for the ion chamber/
  electrometer combination in use
• Determines absolute dose to water

6
What Minimum Equipment is Needed?

• Linear accelerator with front pointer
• Water phantom, spirit level
• Calibrated ionization chamber and electrometer
  combination
• IAEA TRS 277 protocol

7
Background

• Calibration Chain
• Primary Standard Lab Calibration Cobalt Beam
• Secondary Standard Lab Transfer of calibration
  factor to the users instrument using Cobalt
  radiation in air
• User Determination of dose in water in users
  beam

8
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 KERMA factor
• Nk 9.08 10-3 Gy/div

9
First step conversion of KERMA (in air) factor
from SSDL to Absorbed dose to air chamber factor
ND

• ND NK (1-g) kattkm
• with
• g the fraction of brehmsstrahlung generated in
  water for 60Co 0.003
• katt attenuation in wall correction
• km material (i.e. non-air) correction for wall
  and build-up cap

• If Exposure factor NX is known
• NK NX (W/e) (1 - g)-1

10
TRS 277 work sheet
11
Want to calibrate a 6MV X Ray beam

• SAD 100cm
• Dmax 1.5cm

Elekta
12
Require beam quality

• To be specified as TPR2010 ratio of dose at
  isocentre with 20cm attenuation to the same with
  10cm attenuation

13
Want to calibrate a 6MV X Ray beam

• SAD 100cm
• Dmax 1.5cm
• TPR2010 0.67

Elekta
14
Effective point of measurement in chamber

• Up stream of the physical centre

15
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
16
Reference conditions
17
Want to calibrate a 6MV X Ray beam

• SAD 100cm
• Dmax 1.5cm
• TPR2010 0.67
• d 5cm
• FS 10x10cm2
• effective point of measurement 0.75r upstream

Elekta
18
Need correction for

• Temperature (the higher the less molecules in
  chamber)
• Pressure (the higher the more molecules in
  chamber)
• PTp 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

19
Need also correction for recombination of ions in
the chamber

• Effect depends on radiation quality, dose rate
  and high voltage applied to the chamber
• Protocol provides a method to calculate it based
  on two chamber readings with different high
  voltages applied
• assume here ps 1.003 (i.e. we lose 0.3 of the
  generated ions due to recombination)

20
Corrections of electrometer reading

• Mu Mraw pTP kh ps with
• Mu and Mraw the corrected and the raw reading
• pTP and ps the temperature, pressure and
  recombination correction
• kh a humidity correction - in most circumstances
  kh can be assumed to be 1
• Please note that in electron beams also a
  polarity correction is required

21
Calculation of absorbed dose to water, Dw at
effective point of measurement Peff

• Dw (Peff) Mu ND sw,air pu
• with Mu ND the corrected reading and the absorbed
  dose to air factor as discussed before
• sw,air the stopping power ratio between water and
  air to correct dose to air to dose to water
• pu a perturbation correction factor

22
Stopping power ratios

• From TRS 277
• Energy dependent

23
Perturbation correction

• From TRS 277 Fig14
• depends on chamber wall material
• for 2505/3A material is graphite
• pu 0.993 for TPR20100.67

24
Set-up of chamber

• Focus Chamber Distance (Peff) 100cm
• Depth 5cm water
• FS 10x10cm2
• TPR2010 0.67
• NE 2505/3A chamber
• 100 monitor units

95cm
chamber
5cm
25
Questions

• Where is Peff compared to the geometric centre of
  the chamber?
• What is the stopping power ratio?

26
IAEA Worksheet
Filled in for 60-Co !!!
27
Please fill in the same sheet for your 6MV linac

• Conditions and readings on the next page...

28
Final information

• T 22oC, p 99.3kPa
• Uncorrected readings 84.5, 84.2, 84.3 and 84.3

29
Questions?

• Lets get started...

30
Result 0.858 Gy per 100 monitor units

• What is your reaction? Shut down the unit?

31
Need to find out what the dose normalisation
conditions are!

• The centre has used as reference conditions a
  depth of 10cm (as recommended e.g. by several
  planning systems)
• TPR ratio between 10 and 5cm depth TPR105
  0.847
• Therefore, the dose at reference point for the
  centre is 1.013 Gy per 100mu