Part VIII:Medical Exposures in Radiotherapy Calibration of Teletherapy units & sources Lecture 9 :...

Part VIII:Medical Exposures in RadiotherapyPart VIII:Medical Exposures in Radiotherapy

Lecture 9 : Calibration of Teletherapy units Calibration of Teletherapy units & sources& sources

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

Module 4 – Quality Assurance

Part III.4.9 : Calibration of Teletherapy units & sourcesCalibration of Teletherapy units & sources 2

ObjectiveObjective

• To understand the international To understand the international measurements system measurements system

• To learn the IAEA calibration protocolTo learn the IAEA calibration protocol• To appreciate the IAEA / WHO calibration To appreciate the IAEA / WHO calibration

network – Traceability to Standardnetwork – Traceability to Standard• To familiarise with the method of calibration of To familiarise with the method of calibration of

a a 6060Co teletherapy unit.Co teletherapy unit.• To understand the calibration procedures for To understand the calibration procedures for

Brachytherapy sourcesBrachytherapy sources


The Problem..The Problem..

• Radiation is used to treat cancer. But how do hospitals ensure that this beam of radiation exactly gives the right treatment dose?


Calibration of Telecobalt unitCalibration of Telecobalt unit

• The output measurement i.e.the dose rate at the SSD or SAD for a reference field size (10 x 10 Cm2).

• To be carried out by a qualified person

• Necessary to follow a protocol – to attain with other radiotherapy centres


Traceability of Calibration factorTraceability of Calibration factor

Measurement Traceability refers to the chain of calibrations between an instrument used to make a reading in terms of some physical unit and the basic realization of that unit. On a national scale, the physical realization of a unit is known as a national measurement standard


Tractability of calibration to Primary Tractability of calibration to Primary standardstandard

Primary standard laboratories (PSDL) - 15 affiliated PSDLs

Secondary standard laboratories (SSDL) - 62 SSDLs

Hospital Reference dosimeter - SSD meter calibrated against the SSDL standard (End user)


The international Traceability of The international Traceability of measurement calibrationmeasurement calibration

PSDL BIPM PSDL

IAEASSDL SSDL

SSDLs

USERS USERS USERS USERS USERS


Aim: The dose at reference conditions should be the same all Aim: The dose at reference conditions should be the same all over the worldover the world

1Gy

1Gy1Gy

1Gy

1Gy

1Gy1Gy

1Gy

1Gy

1Gy

Ref. T.Korn


What is a primary standard?What is a primary standard?

• An instrument of the highest metrological quality An instrument of the highest metrological quality that permits determination of the unit of a quantity that permits determination of the unit of a quantity from its definition, the accuracy of which has from its definition, the accuracy of which has been verified by comparison with the comparable been verified by comparison with the comparable standards of other institution at the same levelstandards of other institution at the same level

Ref: IAEA Technical Report series 398 page 16


What are other classifications of What are other classifications of calibration instruments?calibration instruments?

• Reference instrumentReference instrument: An instrument of the : An instrument of the highest metrological quality available at a given highest metrological quality available at a given location, from which measurements at that location, from which measurements at that location are derivedlocation are derived

• Field Instrument : A measuring instrument used Field Instrument : A measuring instrument used for routine measurements whose calibration is for routine measurements whose calibration is related to the reference instrumentrelated to the reference instrument


What are the standard dosimetry What are the standard dosimetry laboratories?laboratories?

• Primary Standard Dosimetry Laboratory (PSDL)Primary Standard Dosimetry Laboratory (PSDL)– A national standardizing laboratory designated by the A national standardizing laboratory designated by the

government for the purpose of developing, maintaining government for the purpose of developing, maintaining and improving primary standards in radiation dosimetry and improving primary standards in radiation dosimetry

• Secondary Standard Dosimetry laboratory (SSDL)Secondary Standard Dosimetry laboratory (SSDL)– A dosimetry laboratory designated by the competent A dosimetry laboratory designated by the competent

authorities to provide calibration services, and which is authorities to provide calibration services, and which is equipped with at least one secondary standard that has equipped with at least one secondary standard that has been calibrated against a primary standard been calibrated against a primary standard


When is the right time for calibration?When is the right time for calibration?

• Before the first medical use of the unit;

• Following replacement of the source or following reinstallation of the teletherapy unit in a new location;


When is the right time for calibration?When is the right time for calibration?

• Following any repair of the teletherapy unit that includes removal of the source or major repair of the components associated with the source exposure assembly; and

• At regular intervals as per the policy of the department (not exceeding 6 months preferable to have monthly)


Tools required for absorbed dose Tools required for absorbed dose measurement measurement

• Ion chamber with electrometer calibrated Ion chamber with electrometer calibrated for Cobalt beam for Cobalt beam

0.6cc Capintec ion chamber


Tools for calibrationTools for calibration

• Calibrated electrometerCalibrated electrometer

• Electrometer may be Electrometer may be calibrated separately or calibrated separately or with the ion chamber with the ion chamber


Tools for calibrationTools for calibration

• Water phantom - Perspex Water phantom - Perspex water tank with facility to water tank with facility to place ion chamber at the place ion chamber at the reference depthreference depth

• Water proof sleeve for ion Water proof sleeve for ion chamberchamber


What Phantoms to use?What Phantoms to use?

• Water onlyWater only– Annual calibration (reference Annual calibration (reference

dosimetry)dosimetry)• Plastic (unit density or with density Plastic (unit density or with density

correction applied to depth)correction applied to depth)– monthlymonthly– compare with water at annual compare with water at annual

calibrationcalibration

Plastic phantom with parallel plate chamber


Why water as a phantom?Why water as a phantom?

• Phantom should be of a material that Phantom should be of a material that will absorb and scatter photons in the will absorb and scatter photons in the same way as tissue.same way as tissue.

• Water and wet tissue absorb photons in Water and wet tissue absorb photons in almost the same way and hence water almost the same way and hence water is used as phantom materialis used as phantom material


How to protect chamber from damping? How to protect chamber from damping?

• Ionization chambers show troublesome Ionization chambers show troublesome leakage effects when dampleakage effects when damp

• Chamber protection is necessaryChamber protection is necessary– 1mm thick acrylic protection sleeve to be 1mm thick acrylic protection sleeve to be

usedused– or use water proof chambersor use water proof chambers


Calibration set-upCalibration set-up

Reference

depth Zref

source

Collimator

Beam central axis

Water Tank

Water level


SSD and SAD set-up for calibrationSSD and SAD set-up for calibration

SAD/SCD


Calibration with lateral beamCalibration with lateral beam

IAEA web site

Ion chamber at 5g/Cm2 depthCobalt unit


What are other classifications of What are other classifications of calibration instrumentscalibration instruments

• Secondary standardSecondary standard : An : An instrument calibrated by instrument calibrated by comparison with a primary comparison with a primary standardstandard

• National StandardNational Standard :A standard :A standard recognized by an official national recognized by an official national decision as the basis for fixing decision as the basis for fixing the value in a country of all other the value in a country of all other standards of the given quantitystandards of the given quantity


Reference Conditions for determination of Reference Conditions for determination of absorbed dose for absorbed dose for 60Co

Influence Quantity Refence value or Reference Characteristics

Phantom Material Water

Chamber type Cylindrical or parallel

Measurement Depth 5g/cm2 (or 10g/cm2)


Reference Conditions for determination of Reference Conditions for determination of absorbed dose for absorbed dose for 60Co

Reference point of the chamber

For cylindrical chambers, on the central axis at the centre of the cavity volume. For Plane-parallel chambers, on the inner surface of the window at its centre

Position of the reference point of the chamber

For cylindrical and plane-parallel chmabers, at the measurement depth zref

SSD or *SCD 80cm or 100 cm

Field size 10 x 10 cm2

*SCD - Source to Chamber distance (SAD set-up)


Cylindrical ion chamber (Thimble type)Cylindrical ion chamber (Thimble type)

• Picture of cylindrical ion chamberPicture of cylindrical ion chamber

Buildup caps


137137Cs check source Cs check source


Reference chamber calibration

• Reference chamber should have been Reference chamber should have been calibrated at a standard calibration laboratorycalibrated at a standard calibration laboratory

• The calibration should be for a reference The calibration should be for a reference beam quality (in this case Cobalt 60)beam quality (in this case Cobalt 60)

• The calibration lab should have calibration The calibration lab should have calibration traceable to international standard traceable to international standard


How to determine the absorbed dose to How to determine the absorbed dose to water?water?

The basic equation for dose to water for beam The basic equation for dose to water for beam quality Qquality Q

DDw,Q w,Q = M= MQQNND,w’ D,w’ KKQQ

Where Where MMQQ is the Corrected meter readingis the Corrected meter reading

NND,w’D,w’ is the calibration factor for reference is the calibration factor for reference beam beam quality (quality (6060Co beam)Co beam)

kkQQ is the chamber specific factor which corrects is the chamber specific factor which corrects for for the difference between reference beam quality the difference between reference beam quality and and the actual beam quality (hence forthe actual beam quality (hence for

6060Co KCo KQQ is 1) is 1)


What are the corrections to be applied to What are the corrections to be applied to the meter reading M?the meter reading M?

The Meter Reading MThe Meter Reading M needs to be needs to be

corrected forcorrected for

1.1. Pressure, temperature & HumidityPressure, temperature & Humidity

2.2. Electrometer Calibration kElectrometer Calibration kelecelec

3.3. Polarity correction kPolarity correction kpolpol

4.4. Recombination correction kRecombination correction kss


Why should the pressure & temperature Why should the pressure & temperature correction be applied?correction be applied?

• Most ion chambers are open to the Most ion chambers are open to the ambient air, the mass of air in the ambient air, the mass of air in the cavity volume is subject to atmospheric cavity volume is subject to atmospheric variations. The correction factor to variations. The correction factor to convert the cavity air mass to the convert the cavity air mass to the reference condition should be appliedreference condition should be applied


How to correct for Pressure & How to correct for Pressure & Temperature?Temperature?

kkTPTP = (273.2+T) P = (273.2+T) Po o / (273.2+T/ (273.2+Too)P)PWhere T & P are the Temperature & Pressure Where T & P are the Temperature & Pressure

during measurementduring measurement

TTo o & P& Po o are the Temperature & Pressure at are the Temperature & Pressure at reference condition (usually 101.3kPa and reference condition (usually 101.3kPa and 2020ooC)C)

Remember the correction factor due to pressure Remember the correction factor due to pressure could be as high 15% at high altitudescould be as high 15% at high altitudes


What is electrometer calibration?What is electrometer calibration?

• If the electrometer and the ion chamber If the electrometer and the ion chamber are calibrated separately a correction are calibrated separately a correction factor kfactor kelec elec for the electro meter should for the electro meter should

be appliedbe applied


What is Polarity effect on meter reading?What is Polarity effect on meter reading?

The effect on a chamber reading of The effect on a chamber reading of using polarizing potentials of using polarizing potentials of opposite polarity must always be opposite polarity must always be corrected forcorrected for

kkpolpol = |M = |M++| + |M| + |M--||

Where MWhere M++ & M & M-- are the meter reading with +ve are the meter reading with +ve

& -ve polarities and M is the meter reading with & -ve polarities and M is the meter reading with polarity used routinelypolarity used routinely

2M

+ & -ve bias voltage


What is Ion recombination correction?What is Ion recombination correction?

• The incomplete collection of charge in an The incomplete collection of charge in an ionization chamber cavity owing the ionization chamber cavity owing the recombination of ions requires the use of a recombination of ions requires the use of a correction factor kcorrection factor kss

• kkss = (V = (V11/V/V22))2 2 - 1- 1(V(V11/V/V22))2 2 – (M– (M11/M/M22))

VV1 1 & V& V2 2 are the lower and higher bias voltages, are the lower and higher bias voltages,

MM1&1&MM22 are the reading taken with these voltages are the reading taken with these voltages


And finally the absorbed dose isAnd finally the absorbed dose is

DDw,Q w,Q = M= MQQNND,w’ D,w’ kkQQ

Where Where

MMQQ= M k= M kp,Tp,T k kpolpol,k,kelec elec kkss


Have you corrected for timer error?Have you corrected for timer error?

It is also referred to as shutter errorIt is also referred to as shutter error

Timer error Timer error = M = MAAttA A –M–MBBttBB/ (nM/ (nMAA-M-MBB))Where MWhere MAA is the reading in a time t is the reading in a time tAA

MMBB is the reading in ‘n’ short exposures of time t is the reading in ‘n’ short exposures of time tBB

MMl l = M= MA A /(t/(tAA++))


Calibration of High energy X ray beamsCalibration of High energy X ray beams

• Points to rememberPoints to remember– First the energy index First the energy index

Q(TPRQ(TPR20,1020,10) will have to be ) will have to be determined determined

– kkQ Q has to be obtained from has to be obtained from the table for the energy indexthe table for the energy index

– Measurement depth for Q>.7 Measurement depth for Q>.7 is 10 g/cmis 10 g/cm22


What is the purpose of IAEA & WHO What is the purpose of IAEA & WHO network?network?

• Facilitate uniformity of radiation dosimetry Facilitate uniformity of radiation dosimetry measurements around the worldmeasurements around the world

• Maintain the consistency and accuracy of therapeutic doses by exercising a national and international inter-comparison programme as well as providing calibration services to the end users, mainly radiotherapy departments in hospitals.


IAEA / WHO inter-comparison programmeIAEA / WHO inter-comparison programme

• Postal dose inter-comparison with TLDPostal dose inter-comparison with TLD

• TLD capsules sent to different centersTLD capsules sent to different centers

• Exposed to 2Gy Exposed to 2Gy


WHO/IAEA photon Dose Quality AuditWHO/IAEA photon Dose Quality Audit

TLD capsules

Level 1 Dose Quality Audit:Dose in Reference Conditions

FS 10x10, d5cm

Calibration of brachytherapy sourcesCalibration of brachytherapy sources

Gamma and Gamma and sources sources


BSS appendix II.19BSS appendix II.19

• Calibration: “Registrants and licensees Calibration: “Registrants and licensees shall ensure that: shall ensure that: (c) (c) sealed sources used for brachytherapy be calibrated sealed sources used for brachytherapy be calibrated

in terms of activity, reference air kerma rate in air or in terms of activity, reference air kerma rate in air or absorbed dose rate in a specified medium, at a specified absorbed dose rate in a specified medium, at a specified distance, for a specified reference date; ...distance, for a specified reference date; ...

(e) the calibrations be carried out at the time of commissioning (e) the calibrations be carried out at the time of commissioning a unit, after any maintenance procedure that may have an a unit, after any maintenance procedure that may have an effect on the dosimetry and at intervals approved by the effect on the dosimetry and at intervals approved by the Regulatory Authority. “Regulatory Authority. “


Specifications of activitySpecifications of activity

• The recommended quantity for the The recommended quantity for the specification of the gamma sources is specification of the gamma sources is the reference air kerma rate to air, in air, the reference air kerma rate to air, in air, at a reference distance of one meter at a reference distance of one meter corrected for attenuation and scattercorrected for attenuation and scatter


Specification for Specification for sources sources

• Recommended quantity for specification Recommended quantity for specification of beta ray sources in the reference of beta ray sources in the reference absorbed dose rate in water at a absorbed dose rate in water at a reference distance from the sourcereference distance from the source

• Reference distance varies with type of Reference distance varies with type of sourcesource


Calibration at SSDL & Hospital levelCalibration at SSDL & Hospital level

• Traceability in calibrations at SSDLsTraceability in calibrations at SSDLs

• Detector: Well type chamber calibrated Detector: Well type chamber calibrated against the PSDL or at an ADCL or the against the PSDL or at an ADCL or the IAEA dosimetry laboratoryIAEA dosimetry laboratory


Calibration at the hospital levelCalibration at the hospital level

• Recommended that photon and Recommended that photon and intravascular brachytherapy sources be intravascular brachytherapy sources be calibrated with an appropriately calibrated with an appropriately calibrated well type chambercalibrated well type chamber

Intravascular applicator


What are the maintenance standards for What are the maintenance standards for photon sources and intravasuclar sourcesphoton sources and intravasuclar sources

• Well type chambers should be Well type chambers should be calibrated regularly (at recommended calibrated regularly (at recommended interval)interval)

• Calibration at Calibration at 137137Cs referenceCs reference• The calibration point of well type The calibration point of well type

chamber is the point at which the chamber is the point at which the centre of source is positioned during centre of source is positioned during calibration calibration

• This point may vary depending on This point may vary depending on source strength source strength


Isotope calibratorIsotope calibrator

• A means of verifying the source strength A means of verifying the source strength of radioactive sources. This could be a of radioactive sources. This could be a set-up for the direct determination of air-set-up for the direct determination of air-KERMA in HDR sources or just a well KERMA in HDR sources or just a well counter for radioactive wire, seeds or counter for radioactive wire, seeds or needles.needles.

• They require a calibration factor that can They require a calibration factor that can be traced to a national standard facility. be traced to a national standard facility.

• It should also be noted that different It should also be noted that different calibration factors may be required for calibration factors may be required for different sources. different sources.


Calibration of HDR SourceCalibration of HDR Source

• To be calibrated every time a source is To be calibrated every time a source is changed in the unitchanged in the unit

Well type chamberElectromenter


Summary Summary

• All dose measuring instruments should be All dose measuring instruments should be calibrated with their calibration traceable to calibrated with their calibration traceable to the primary standardthe primary standard

• IAEA protocol Tech Report Series 398 IAEA protocol Tech Report Series 398 provides method of determining absorbed provides method of determining absorbed dose in External Beam Radiotherapydose in External Beam Radiotherapy

• All Radiotherapy centers should have a field All Radiotherapy centers should have a field instrument with its calibration traceable to instrument with its calibration traceable to primary standardprimary standard


SummarySummary

• For reference dosimetry water is the For reference dosimetry water is the only phantom to be used with cylindrical only phantom to be used with cylindrical ion chamber (or Parallel Plate) chamber ion chamber (or Parallel Plate) chamber at 5cm (or 10 cm) for cobalt beamat 5cm (or 10 cm) for cobalt beam

• IAEA /WHO network provides uniformity IAEA /WHO network provides uniformity of dose over different centers in the of dose over different centers in the worldworld


QuestionsQuestions

• What is primary standard?What is primary standard?• What is a secondary standard lab?What is a secondary standard lab?• What is a secondary standard dosimeter?What is a secondary standard dosimeter?• What is the aim of IAEA/WHO postal dose What is the aim of IAEA/WHO postal dose

inter-comparison ?inter-comparison ?• State the correction factors used in absorbed State the correction factors used in absorbed

dose determination in cobalt beamdose determination in cobalt beam


Where to find more information?Where to find more information?

• Absorbed dose Determination in External Beam Absorbed dose Determination in External Beam RadiotherapyRadiotherapy– Technical Reports Series No 398 IAEA publicationTechnical Reports Series No 398 IAEA publication

• Absorbed dose Determination in External Beam Absorbed dose Determination in External Beam RadiotherapyRadiotherapy– Technical Reports Series No 277 IAEA publicationTechnical Reports Series No 277 IAEA publication

• Basic Safety Standards Basic Safety Standards – Safety series No 115 IAEA publication Safety series No 115 IAEA publication

• Design and implementation of a radiotherapy programme: clinical, medical physics, radiation protection and safety aspects – - IAEA Tec Doc 1040

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Part VIII:Medical Exposures in Radiotherapy Calibration of Teletherapy units & sources Lecture 9 :...

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