RADIATION PROTECTION IN RADIOTHERAPY Part 10: Optimization of protection in External Beam Radiotherapy PRACTICAL EXERCISE IAEA Post Graduate Educational

RADIATION PROTECTION IN RADIATION PROTECTION IN RADIOTHERAPYRADIOTHERAPY

Part 10: Optimization of protection in Part 10: Optimization of protection in External Beam RadiotherapyExternal Beam Radiotherapy

PRACTICAL EXERCISEPRACTICAL EXERCISEIAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources

IAEA Training Material on Radiation Protection in Radiotherapy

Part 10, Practical 2 2Radiotherapy

Objectives of Part 10Objectives of Part 10 Be familiar with the ‘design considerations’ as Be familiar with the ‘design considerations’ as

stipulated by appendix II in the BSSstipulated by appendix II in the BSS Be able to apply these design considerations Be able to apply these design considerations

in the context of radiotherapy equipmentin the context of radiotherapy equipment Be aware of relevant international standards Be aware of relevant international standards

and other documents which provide and other documents which provide specification for external beam radiotherapy specification for external beam radiotherapy equipmentequipment

Part 10 : Part 10 : External Beam External Beam RadiotherapyRadiotherapy

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

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

IAEA Training Material on Radiation Protection in Radiotherapy


ContentsContents Rationale for calibrationRationale for calibration Step by step procedure to be followed Step by step procedure to be followed

for calibration of a photon beam from a for calibration of a photon beam from a medical linear accelerator following medical linear accelerator following IAEA TRS 277IAEA TRS 277

Interpretation of resultsInterpretation of results


IAEA TRS 277IAEA TRS 277

Assumes user has a Assumes user has a calibration factor for calibration factor for exposure Nexposure NXX or air- or air-

KERMA NKERMA NKK for the for the

ion chamber/ ion chamber/ electrometer electrometer combination in usecombination in use

Determines absolute Determines absolute dose to waterdose to water


What Minimum Equipment is What Minimum Equipment is Needed?Needed? Linear accelerator with front pointer Linear accelerator with front pointer Water phantom, spirit levelWater phantom, spirit level Calibrated ionization chamber and Calibrated ionization chamber and

electrometer combinationelectrometer combination IAEA TRS 277 protocolIAEA TRS 277 protocol


BackgroundBackground Calibration ChainCalibration Chain

Primary Standard Lab: Primary Standard Lab: Calibration Cobalt BeamCalibration Cobalt Beam

Secondary Standard Secondary Standard Lab: Transfer of Lab: Transfer of calibration factor to the calibration factor to the user’s instrument using user’s instrument using Cobalt radiation in airCobalt radiation in air

User: Determination of User: Determination of dose in water in user’s dose in water in user’s beambeam


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

Chamber volume Chamber volume 0.6cc0.6cc

Internal radius Internal radius 3.15mm3.15mm

Internal length 24mmInternal length 24mm Get KERMA factor:Get KERMA factor: NNkk = 9.08 10 = 9.08 10-3-3 Gy/div Gy/div


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

NNDD = N = NKK (1-g) k (1-g) kattattkkmm

with with g the fraction of g the fraction of

brehmsstrahlung brehmsstrahlung generated in water for generated in water for 6060Co = 0.003Co = 0.003

kkattatt attenuation in wall attenuation in wall

correctioncorrection kkmm material ( material (i.e.i.e. non-air) non-air)

correction for wall and correction for wall and build-up capbuild-up cap

If Exposure factor NIf Exposure factor NX X

is known:is known:

NNKK = N = NXX (W/e) (1 - g) (W/e) (1 - g)-1-1


TRS 277 TRS 277 work sheetwork sheet


Want to calibrate a 6MV XWant to calibrate a 6MV X R Ray ay beambeam

SAD = 100cmSAD = 100cm DDmax max = 1.5cm= 1.5cm

Elekta


Require Require beam beam qualityquality

To be specified To be specified as TPRas TPR2020

10 10 = ratio = ratio

of dose at of dose at isocentre with isocentre with 20cm attenuation 20cm attenuation to the same with to the same with 10cm attenuation10cm attenuation




TPRTPR20201010 = 0.67 = 0.67

Elekta


Effective point of measurement in Effective point of measurement in chamberchamber Up stream of the Up stream of the

physical centrephysical centre


Perform measurement in water Perform measurement in water phantomphantom

Fill with water to Fill with water to correct depthcorrect depth

Let temperature Let temperature equilibrate (>1 hour)equilibrate (>1 hour)

Level phantomLevel phantom Insert chamberInsert chamber Ensure linac Ensure linac

settings and beam settings and beam orientation correctorientation correctPTW small water phantom


Reference conditionsReference conditions




TPRTPR20201010 = 0.67 = 0.67

d = 5cmd = 5cm FS 10x10cmFS 10x10cm22

effective point of effective point of measurement 0.75r measurement 0.75r upstreamupstream

Elekta


Need correction forNeed correction for

TemperatureTemperature (the higher the less molecules (the higher the less molecules in chamber)in chamber)

PressurePressure (the higher the more molecules in (the higher the more molecules in chamber)chamber)

PPTpTp = P = P00/P (T + 273.2)/(T/P (T + 273.2)/(T00 + 273.2) + 273.2) with P and T the measured pressure (in kPa) and with P and T the measured pressure (in kPa) and

temperature (in temperature (in ooC) and PC) and P0 0 = 101.3kPa and T = = 101.3kPa and T =

2020ooC as reference conditionsC as reference conditions


Need also correction for Need also correction for recombination of ions in the chamberrecombination of ions in the chamber

Effect depends on radiation quality, dose rate Effect depends on radiation quality, dose rate and high voltage applied to the chamberand high voltage applied to the chamber

Protocol provides a method to calculate it Protocol provides a method to calculate it based on two chamber readings with different based on two chamber readings with different high voltages applied:high voltages applied:

assume here: passume here: pss = 1.003 ( = 1.003 (i.e.i.e. wwe lose 0.3% of the e lose 0.3% of the generated ions due to recombination)generated ions due to recombination)


Corrections of electrometer Corrections of electrometer readingreading

MMuu = M = Mrawraw p pTPTP k khh p ps s withwith MMuu and M and Mrawraw the corrected and the raw reading the corrected and the raw reading ppTP TP and pand pss the temperature, pressure and the temperature, pressure and

recombination correctionrecombination correction kkhh a humidity correction - in most circumstances k a humidity correction - in most circumstances khh

can be assumed to be 1can be assumed to be 1

Please note that in electron beams also a Please note that in electron beams also a polarity correction is requiredpolarity correction is required


Calculation of absorbed dose to water, DCalculation of absorbed dose to water, Dw w

at effective point of measurement Pat effective point of measurement Peffeff

DDw w (P(Peffeff) = M) = Muu N NDD s sw,airw,air p puu

with Mwith Muu N NDD the corrected reading and the absorbed dose the corrected reading and the absorbed dose

to air factor as discussed beforeto air factor as discussed before

ssw,airw,air the stopping power ratio between water and air to the stopping power ratio between water and air to

correct dose to air to dose to watercorrect dose to air to dose to water

ppuu a perturbation correction factor a perturbation correction factor


Stopping Stopping power ratiospower ratios

From TRS 277From TRS 277 Energy dependentEnergy dependent


Perturbation correctionPerturbation correction From TRS 277 From TRS 277

Fig14Fig14 depends on depends on

chamber wall chamber wall materialmaterial

for 2505/3A for 2505/3A material is material is graphitegraphite

ppuu = 0.993 for = 0.993 for

TPRTPR20201010=0.67=0.67


SSet-up of chamber et-up of chamber

Focus Chamber Focus Chamber Distance (PDistance (Peffeff) = 100cm) = 100cm

Depth = 5cm waterDepth = 5cm water FS 10x10cmFS 10x10cm22 TPRTPR2020

1010 = 0.67 = 0.67 NE 2505/3A chamberNE 2505/3A chamber 100 monitor units100 monitor unitschamber

95cm

5cm


Questions:Questions:

Where is PWhere is Peffeff compared to the geometric compared to the geometric

centre of the chamber?centre of the chamber? What is the stopping power ratio?What is the stopping power ratio?


IAEA IAEA WorksheetWorksheet

Filled in for 60-Co !!!

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

Conditions and readings on the next Conditions and readings on the next page...page...


Final informationFinal information

T = 22T = 22ooC, p = 99.3kPaC, p = 99.3kPa Uncorrected readings: 84.5, 84.2, 84.3 Uncorrected readings: 84.5, 84.2, 84.3

and 84.3and 84.3


Questions?Questions?

Let’s get started... get started...


Result: 0.858 Gy per 100 Result: 0.858 Gy per 100 monitor units monitor units What is your reaction? Shut down the What is your reaction? Shut down the unit?unit?


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

The centre has used as reference The centre has used as reference conditions a depth of 10cm (as conditions a depth of 10cm (as recommended recommended e.g.e.g. by several planning by several planning systems)systems)

TPR ratio between 10 and 5cm depth: TPR ratio between 10 and 5cm depth: TPRTPR1010

55 = 0.847 = 0.847 Therefore, the dose at reference point Therefore, the dose at reference point

for the centre is 1.013 Gy per 100mufor the centre is 1.013 Gy per 100mu

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RADIATION PROTECTION IN RADIOTHERAPY Part 10: Optimization of protection in External Beam Radiotherapy PRACTICAL EXERCISE IAEA Post Graduate Educational