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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 2

part 10 external beam radiotherapy

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

contents
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

Part 10, Practical 2

iaea trs 277
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

Part 10, Practical 2

what minimum equipment is needed
What Minimum Equipment is Needed?
  • Linear accelerator with front pointer
  • Water phantom, spirit level
  • Calibrated ionization chamber and electrometer combination
  • IAEA TRS 277 protocol

Part 10, Practical 2

background
Background
  • Calibration Chain
    • Primary Standard Lab: Calibration Cobalt Beam
    • Secondary Standard Lab: Transfer of calibration factor to the user’s instrument using Cobalt radiation in air
    • User: Determination of dose in water in user’s beam

Part 10, Practical 2

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 KERMA factor:
  • Nk = 9.08 10-3 Gy/div

Part 10, Practical 2

first step conversion of kerma in air factor from ssdl to absorbed dose to air chamber factor n d
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

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

Part 10, Practical 2

trs 277 work sheet
TRS 277 work sheet

Part 10, Practical 2

want to calibrate a 6mv x r ay beam
Want to calibrate a 6MV X Ray beam
  • SAD = 100cm
  • Dmax = 1.5cm

Elekta

Part 10, Practical 2

require beam quality
Require beam quality
  • To be specified as TPR2010 = ratio of dose at isocentre with 20cm attenuation to the same with 10cm attenuation

Part 10, Practical 2

want to calibrate a 6mv x r ay beam13
Want to calibrate a 6MV X Ray beam
  • SAD = 100cm
  • Dmax = 1.5cm
  • TPR2010 = 0.67

Elekta

Part 10, Practical 2

effective point of measurement in chamber
Effective point of measurement in chamber
  • Up stream of the physical centre

Part 10, Practical 2

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 2

reference conditions
Reference conditions

Part 10, Practical 2

want to calibrate a 6mv x r ay beam17
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

Part 10, Practical 2

need correction for
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

Part 10, Practical 2

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
  • 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)

Part 10, Practical 2

corrections of electrometer reading
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

Part 10, Practical 2

calculation of absorbed dose to water d w at effective point of measurement p eff
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

Part 10, Practical 2

stopping power ratios
Stopping power ratios
  • From TRS 277
  • Energy dependent

Part 10, Practical 2

perturbation correction
Perturbation correction
  • From TRS 277 Fig14
  • depends on chamber wall material
  • for 2505/3A material is graphite
  • pu = 0.993 for TPR2010=0.67

Part 10, Practical 2

s et up of chamber
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

Part 10, Practical 2

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

Part 10, Practical 2

iaea worksheet
IAEA Worksheet

Filled in for 60-Co !!!

Part 10, Practical 2

please fill in the same sheet for your 6mv linac

Please fill in the same sheet for ‘your’ 6MV linac

Conditions and readings on the next page...

final information
Final information
  • T = 22oC, p = 99.3kPa
  • Uncorrected readings: 84.5, 84.2, 84.3 and 84.3

Part 10, Practical 2

questions29
Questions?

Let’s get started...

Part 10, Practical 2

result 0 858 gy per 100 monitor units
Result: 0.858 Gy per 100 monitor units

What is your reaction? Shut down the unit?

Part 10, Practical 2

need to find out what the dose normalisation conditions are
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

Part 10, Practical 2