1. Part VIII:Medical Exposures in Radiotherapy
IAEA Post Graduate Educational Course on Radiation Protection and Safe Use of Radiation Sources
Part VIII:Medical Exposures in Radiotherapy
Module 4 – Quality Assurance
Lecture 9 : Calibration of Teletherapy units & sources

2. Part III.4.9 : Calibration of Teletherapy units & sources
Objective
To understand the international measurements system
To learn the IAEA calibration protocol
To appreciate the IAEA / WHO calibration network – Traceability to Standard
To familiarise with the method of calibration of a 60Co teletherapy unit.
To understand the calibration procedures for Brachytherapy sources
Part III.4.9 : Calibration of Teletherapy units & sources

3. Part III.4.9 : Calibration of Teletherapy units & sources
The Problem..
Radiation is used to treat cancer. But how do hospitals ensure that this beam of radiation exactly gives the right treatment dose?
The lecturer may use this slide to start the lecture with a question.
Part III.4.9 : Calibration of Teletherapy units & sources

4. Calibration 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
Several protocols are available for dose measurement with Teletherapy units. IAEA protocol is followed by many conutries
Part III.4.9 : Calibration of Teletherapy units & sources

5. Traceability 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
One of the major requirements of absorbed dose measurement is that the dosimeter should have been calibrated and the factor should be traceable
Part III.4.9 : Calibration of Teletherapy units & sources

6. Tractability of calibration to Primary standard
Primary standard laboratories (PSDL) - 15 affiliated PSDLs
Secondary standard laboratories (SSDL) - 62 SSDLs
The traceablility of calibration factor could be explained with the above slide
Hospital Reference dosimeter - SSD meter calibrated against the SSDL standard (End user)
Part III.4.9 : Calibration of Teletherapy units & sources

7. The international Traceability of measurement calibration
PSDL
BIPM
PSDL
SSDL
IAEA
SSDL
The above is taken from the Technical Report series 398 on ‘Absorbed dose determination in External Beam Therapy’ published by IAEA.
The international measurement system (IMS) for radiation metrology, where the Traceability of user reference instruments to primary standards is achieved either by direct calibration in a Primary Standard Dosimetry Laboratory (PSDL) or m ore commonly in a Secondary Standard Dosimetry Laboratory (SSDL) with direct link to the BIPM a PSDL or to the IAEA/WHO network of SSDLs. Most SSDLs from countries not members of the Metre Convention achieve the traceablity of their standards through the IE|AEA. The dashed lines indicate inter-comparisons of primary and secondary standards.
SSDLs
USERS
USERS
USERS
USERS
USERS
Part III.4.9 : Calibration of Teletherapy units & sources

8. Part III.4.9 : Calibration of Teletherapy units & sources
Aim: The dose at reference conditions should be the same all over the world
1Gy
This picture could be used to explain the need for calibration which aims to achieve same dose measurement over all parts of the world.
Ref. T.Korn
Part III.4.9 : Calibration of Teletherapy units & sources

9. What is a primary standard?
An instrument of the highest metrological quality that permits determination of the unit of a quantity from its definition, the accuracy of which has been verified by comparison with the comparable standards of other institution at the same level
Ref: IAEA Technical Report series 398 page 16
Part III.4.9 : Calibration of Teletherapy units & sources

10. What are other classifications of calibration instruments?
Reference instrument: An instrument of the highest metrological quality available at a given location, from which measurements at that location are derived
Field Instrument : A measuring instrument used for routine measurements whose calibration is related to the reference instrument
Part III.4.9 : Calibration of Teletherapy units & sources

11. What are the standard dosimetry laboratories?
Primary Standard Dosimetry Laboratory (PSDL)
A national standardizing laboratory designated by the government for the purpose of developing, maintaining and improving primary standards in radiation dosimetry
Secondary Standard Dosimetry laboratory (SSDL)
A dosimetry laboratory designated by the competent authorities to provide calibration services, and which is equipped with at least one secondary standard that has been calibrated against a primary standard
The lecturer may use this slide to explain the Primary standard and the secondary standard laboratories
Part III.4.9 : Calibration of Teletherapy units & sources

12. 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;
This is an important point. The lecturer may impress the need for calibration immediately after the source change and reinstallation etc.
Part III.4.9 : Calibration of Teletherapy units & sources

13. 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)
It must be mentioned that following any repair related to the source or source head, the unit should be calibrated
Part III.4.9 : Calibration of Teletherapy units & sources

14. Tools required for absorbed dose measurement
Ion chamber with electrometer calibrated for Cobalt beam
0.6cc Capintec ion chamber
Part III.4.9 : Calibration of Teletherapy units & sources

15. Part III.4.9 : Calibration of Teletherapy units & sources
Tools for calibration
Calibrated electrometer
Electrometer may be calibrated separately or with the ion chamber
The lecturer may explain the necessary tools for calibration. The important thing is the dosimeter with an ion chamber. The calibration of both the ion chamber and the electrometer should be traceable to primary standard
Part III.4.9 : Calibration of Teletherapy units & sources

16. Part III.4.9 : Calibration of Teletherapy units & sources
Tools for calibration
Water phantom - Perspex water tank with facility to place ion chamber at the reference depth
Water proof sleeve for ion chamber
Part III.4.9 : Calibration of Teletherapy units & sources

17. Part III.4.9 : Calibration of Teletherapy units & sources
What Phantoms to use?
Water only
Annual calibration (reference dosimetry)
Plastic (unit density or with density correction applied to depth)
monthly
compare with water at annual calibration
Plastic phantom with parallel plate chamber
Part III.4.9 : Calibration of Teletherapy units & sources

18. Part III.4.9 : Calibration of Teletherapy units & sources
Why water as a phantom?
Phantom should be of a material that will absorb and scatter photons in the same way as tissue.
Water and wet tissue absorb photons in almost the same way and hence water is used as phantom material
Part III.4.9 : Calibration of Teletherapy units & sources

19. How to protect chamber from damping?
Ionization chambers show troublesome leakage effects when damp
Chamber protection is necessary
1mm thick acrylic protection sleeve to be used
or use water proof chambers
Part III.4.9 : Calibration of Teletherapy units & sources

20. Part III.4.9 : Calibration of Teletherapy units & sources
Calibration set-up
source
Collimator
Water level
Beam central axis
Reference depth Zref
The lecturer may explain the calibration set-up with this slide. The refernce depth should be 5 or 10cm for cobalt 60
Water Tank
Part III.4.9 : Calibration of Teletherapy units & sources

21. SSD and SAD set-up for calibration
SAD/SCD
The claibraion could be done either with SSD set-up or with SAD set-up
Part III.4.9 : Calibration of Teletherapy units & sources

22. Calibration with lateral beam
Cobalt unit
Ion chamber at 5g/Cm2 depth
IAEA web site
Part III.4.9 : Calibration of Teletherapy units & sources

23. What are other classifications of calibration instruments
Secondary standard : An instrument calibrated by comparison with a primary standard
National Standard :A standard recognized by an official national decision as the basis for fixing the value in a country of all other standards of the given quantity
Part III.4.9 : Calibration of Teletherapy units & sources

24. Reference Conditions for determination of absorbed dose for 60Co
Part III.4.9 : Calibration of Teletherapy units & sources

25. Reference Conditions for determination of absorbed dose for 60Co
The above are taken from the IAEA Technical report series 398 on absorbed dose calibration of extern beam radiotherapy units
*SCD - Source to Chamber distance (SAD set-up)
Part III.4.9 : Calibration of Teletherapy units & sources

26. Cylindrical ion chamber (Thimble type)
Picture of cylindrical ion chamber
Buildup caps
The type of chambers to be used for calibration. Usually a farmer type with 0.6 cc is used.
Part III.4.9 : Calibration of Teletherapy units & sources

27. Part III.4.9 : Calibration of Teletherapy units & sources
137Cs check source
The dosimeter stability and reproducibility should be checked periodically and before calibration. For this purpose a Sr-90 or Cs 137 check source is provided. The idea of using these isotopes for checking is their long half life and hence the output is uniform over a small period.
Part III.4.9 : Calibration of Teletherapy units & sources

28. Reference chamber calibration
Reference chamber should have been calibrated at a standard calibration laboratory
The calibration should be for a reference beam quality (in this case Cobalt 60)
The calibration lab should have calibration traceable to international standard
Part III.4.9 : Calibration of Teletherapy units & sources

29. How to determine the absorbed dose to water?
The basic equation for dose to water for beam quality Q
Dw,Q = MQND,w’ KQ
Where MQ is the Corrected meter reading
ND,w’ is the calibration factor for reference beam quality (60Co beam)
kQ is the chamber specific factor which corrects for the difference between reference beam quality and the actual beam quality (hence for 60Co KQ is 1)
Part III.4.9 : Calibration of Teletherapy units & sources

30. What are the corrections to be applied to the meter reading M?
The Meter Reading M needs to be corrected for
Pressure, temperature & Humidity
Electrometer Calibration kelec
Polarity correction kpol
Recombination correction ks
Part III.4.9 : Calibration of Teletherapy units & sources

31. Why should the pressure & temperature correction be applied?
Most ion chambers are open to the ambient air, the mass of air in the cavity volume is subject to atmospheric variations. The correction factor to convert the cavity air mass to the reference condition should be applied
The lecturer may emphasise the need to measure the pressure accurately.
Part III.4.9 : Calibration of Teletherapy units & sources

32. How to correct for Pressure & Temperature?
kTP = (273.2+T) Po / (273.2+To)P
Where T & P are the Temperature & Pressure during measurement
To & Po are the Temperature & Pressure at reference condition (usually 101.3kPa and 20oC)
Remember the correction factor due to pressure could be as high 15% at high altitudes
Part III.4.9 : Calibration of Teletherapy units & sources

33. What is electrometer calibration?
If the electrometer and the ion chamber are calibrated separately a correction factor kelec for the electro meter should be applied
It is a practice in some calibration centres to provide factors for ion chamber and the electrometer separately. In such cases, both the factors should be used in absorbed dose determination
Part III.4.9 : Calibration of Teletherapy units & sources

34. What is Polarity effect on meter reading?
The effect on a chamber reading of using polarizing potentials of opposite polarity must always be corrected for
kpol = |M+| + |M-|
Where M+ & M- are the meter reading with +ve & -ve polarities and M is the meter reading with polarity used routinely 2M
+ & -ve bias voltage
Part III.4.9 : Calibration of Teletherapy units & sources

35. What is Ion recombination correction?
The incomplete collection of charge in an ionization chamber cavity owing the recombination of ions requires the use of a correction factor ks
ks = (V1/V2)2 - 1
(V1/V2)2 – (M1/M2)
V1 & V2 are the lower and higher bias voltages,
M1&M2 are the reading taken with these voltages
Part III.4.9 : Calibration of Teletherapy units & sources

36. And finally the absorbed dose is
Dw,Q = MQND,w’ kQ
Where
MQ= M kp,T kpol,kelec ks
Part III.4.9 : Calibration of Teletherapy units & sources

37. Have you corrected for timer error?
It is also referred to as shutter error
Timer error  = MAtA –MBtB/ (nMA-MB)
Where MA is the reading in a time tA
MB is the reading in ‘n’ short exposures of time tB
Ml = MA /(tA+)
IT is important to measure the timer error particularly in the case of cobalt units. There is a time lag for the source to come ON and this could affect the output measured. Hence a correction should be applied for the timer error
Part III.4.9 : Calibration of Teletherapy units & sources

38. Calibration of High energy X ray beams
Points to remember
First the energy index
Q(TPR20,10) will have to be determined
kQ has to be obtained from the table for the energy index
Measurement depth for Q>.7 is 10 g/cm2
Part III.4.9 : Calibration of Teletherapy units & sources

39. What is the purpose of IAEA & WHO network?
Facilitate uniformity of radiation dosimetry measurements 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.
The IAEA/WHO has a postal dose inter-comaprison programme. The lecturer may explain the need and the usefulness of such a programme.
Part III.4.9 : Calibration of Teletherapy units & sources

40. IAEA / WHO inter-comparison programme
Postal dose inter-comparison with TLD
TLD capsules sent to different centers
Exposed to 2Gy
The stand and the TLD capsules used for postal dose inter-comparison. Usually IAEA/WHO sends 4 TLD capsules and a stand. The Stand is placed in water phantom and the TLD is inserted in the stand as shown. This ensures a depth of 5cm if the water level is up to the top of the stand.
Part III.4.9 : Calibration of Teletherapy units & sources

41. WHO/IAEA photon Dose Quality Audit
TLD capsules
The slide shows the TLD capsules distributed by the IAEA and the holder which allows to place the TLD capsules at 5cm depth in a water phantom. The most simple water phantom is shown in the right lower corner.
Level 1 Dose Quality Audit:
Dose in Reference Conditions
FS 10x10, d5cm
Part III.4.9 : Calibration of Teletherapy units & sources

42. Calibration of brachytherapy sources
Gamma and  sources

43. Part III.4.9 : Calibration of Teletherapy units & sources
BSS appendix II.19
Calibration: “Registrants and licensees shall ensure that:
(c) sealed sources used for brachytherapy be calibrated in terms of activity, reference air kerma rate in air or absorbed dose rate in a specified medium, at a specified distance, for a specified reference date; ...
(e) the calibrations be carried out at the time of commissioning a unit, after any maintenance procedure that may have an effect on the dosimetry and at intervals approved by the Regulatory Authority. “
The sealed sources received by the hospital should always be checked for activity and the BSS requirement for calibration is given above
Part III.4.9 : Calibration of Teletherapy units & sources

44. Specifications of activity
The recommended quantity for the specification of the gamma sources is the reference air kerma rate to air, in air, at a reference distance of one meter corrected for attenuation and scatter
Part III.4.9 : Calibration of Teletherapy units & sources

45. Specification for  sources
Recommended quantity for specification of beta ray sources in the reference absorbed dose rate in water at a reference distance from the source
Reference distance varies with type of source
Part III.4.9 : Calibration of Teletherapy units & sources

46. Calibration at SSDL & Hospital level
Traceability in calibrations at SSDLs
Detector: Well type chamber calibrated against the PSDL or at an ADCL or the IAEA dosimetry laboratory
The above is a Kethly electrometer with a well chamber for calibration of HDR sources and iodine seeds used in brachytherapy
Part III.4.9 : Calibration of Teletherapy units & sources

47. Calibration at the hospital level
Recommended that photon and intravascular brachytherapy sources be calibrated with an appropriately calibrated well type chamber
Intravascular  applicator
Part III.4.9 : Calibration of Teletherapy units & sources

48. Part III.4.9 : Calibration of Teletherapy units & sources
What are the maintenance standards for photon sources and intravasuclar sources
Well type chambers should be calibrated regularly (at recommended interval)
Calibration at 137Cs reference
The calibration point of well type chamber is the point at which the centre of source is positioned during calibration
This point may vary depending on source strength
In the above picture a Capintec isotope calibrator with a well type chamber is shown.
Part III.4.9 : Calibration of Teletherapy units & sources

49. Part III.4.9 : Calibration of Teletherapy units & sources
Isotope calibrator
A means of verifying the source strength of radioactive sources. This could be a set-up for the direct determination of air-KERMA in HDR sources or just a well counter for radioactive wire, seeds or needles.
They require a calibration factor that can be traced to a national standard facility.
It should also be noted that different calibration factors may be required for different sources.
Part III.4.9 : Calibration of Teletherapy units & sources

50. Calibration of HDR Source
To be calibrated every time a source is changed in the unit
The set up for HDR source calibration is shown above
Electromenter
Well type chamber
Part III.4.9 : Calibration of Teletherapy units & sources

51. Part III.4.9 : Calibration of Teletherapy units & sources
Summary
All dose measuring instruments should be calibrated with their calibration traceable to the primary standard
IAEA protocol Tech Report Series 398 provides method of determining absorbed dose in External Beam Radiotherapy
All Radiotherapy centers should have a field instrument with its calibration traceable to primary standard
Part III.4.9 : Calibration of Teletherapy units & sources

52. Part III.4.9 : Calibration of Teletherapy units & sources
Summary
For reference dosimetry water is the only phantom to be used with cylindrical ion chamber (or Parallel Plate) chamber at 5cm (or 10 cm) for cobalt beam
IAEA /WHO network provides uniformity of dose over different centers in the world
Part III.4.9 : Calibration of Teletherapy units & sources

53. Part III.4.9 : Calibration of Teletherapy units & sources
Questions
What is primary standard?
What is a secondary standard lab?
What is a secondary standard dosimeter?
What is the aim of IAEA/WHO postal dose inter-comparison ?
State the correction factors used in absorbed dose determination in cobalt beam
Part III.4.9 : Calibration of Teletherapy units & sources

54. Where to find more information?
Absorbed dose Determination in External Beam Radiotherapy
Technical Reports Series No 398 IAEA publication
Technical Reports Series No 277 IAEA publication
Basic Safety Standards
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
Part III.4.9 : Calibration of Teletherapy units & sources