TRS 398 (Technical Report Series)

In 2000 IAEA published another International Code of Practice.

“Absorbed Dose Determination in External Beam Radiotherapy” (Technical Report Series No. 398)

Recommending procedures to obtain the absorbed dose in water from measurements made with an ionisation chamber in external beam radiotherapy (EBRT).

1. IAEA TRS - 398 Formalism Output calibration procedures for High Energy Photon and Electron Beams Vinay Desai M.Sc Radiation Physics KIDWAI MEMORIAL INSTITUTE OF ONCOLOGY Bengaluru

2. IAEA International Codes of Practice TRS-277 (1987) TRS-381 (1997) Protocols based on primary standards of air kerma Absorbed dose Determination in Photon and Electron Beams: An International Code of Practice T o obtain the absorbed dose in water from measurements made with an ionisation chamber in external beam radiotherapy The Use of Plane-Parallel Ionisation Chambers in High Energy Electron and Photon Beams: An International Code of Practice T o further update TRS - 277 and complement it with respect to the area of plane parallel ionisation chambers

3. Absorbed dose to water from high energy photon and electron beams can be obtained using the expression, D w ( p eff ) = M N D,air (S/ ) w air p u p cel Where, N D,air = N k (1- g) k m k att = absorbed dose to air calibration factor for the chamber. N k = air kerma calibration factor for the chamber. k m = correction for non-air equivalence of wall material. k att = correction for attenuation of the beam in the cavity. Formalism of Air Kerma Based Protocols

4. Basis of Absorbed Dose to Water Based Formalism The new trend of calibrating ionisation chambers directly in a water phantom in terms of absorbed dose to water was introduced. Many PSDLs already provide calibrations in terms of absorbed dose to water at the radiation quality of Co-60 gamma rays. The development of primary standards of absorbed dose to water for high energy photon and electron beams, and improvements in radiation dosimetry concepts, offer the possibility of reducing the uncertainty in the dosimetry of radiotherapy beams.

5. IAEA TRS-398 In 2000 IAEA published another International Code of Practice. “Absorbed Dose Determination in External Beam Radiotherapy” (Technical Report Series No. 398) Recommending procedures to obtain the absorbed dose in water from measurements made with an ionisation chamber in external beam radiotherapy (EBRT).

6. Salient Features of TRS - 398 Based on standards of absorbed dose to water. endorsed by WHO, PAHO and ESTRO( European Society of Therapeutic Radiology and Oncology ) . Fulfils the need for a systematic and internationally unified approach to the calibration of ionisation chambers in terms of absorbed dose to water and to the use of these detectors in determining the absorbed dose to water for the radiation beams used in RT. Provides methodology for the determination of absorbed dose to water in the low, medium and high energy photon beams, electron beams , proton beams and heavy ion beams used for EBRT.

7. Advantages of Absorbed Dose to Water Based Formalism Reduced Uncertainty: Measurements based on calibration in air in terms of air kerma ( N k ) require chamber dependent conversion factors to determine absorbed dose to water. These conversion factors do not account for differences between individual chambers of a particular type. A more robust system of primary standards: Primary standards of absorbed dose to water are based on a number of different physical principles. There are no common assumptions or estimated correction factors. Therefore, good agreement among these standards gives much greater confidence in their accuracy . Use of a simple formalism: No use of several coefficients, perturbation and other correction factors, unlike N k based formalism.

8. Chamber to chamber variations in the ratio of N D,w /N K for chambers of a given type Not accounted for by air kerma ( N k )based formalism Figure, shows chamber to chamber variations, demonstrated for a given chamber type by the lack of constancy in the ND,w /NK ratio at 60 Co, for a large number of cylindrical ionization chambers commonly used in radiotherapy dosimetry. For a given chamber type, chamber to chamber differences of up to 0.8% have also been reported by the BIPM.( Bureau International des Poids et Mesures ) The ratio of 60 Co calibration factors N D,w /N K is a useful indicator of the uniformity within a given type of chamber.

9. Primary Standards for D w

10. N D,w BASED FORMALISM The absorbed dose to water at the reference depth z ref in water for a reference beam of quality Q o and in the absence of the ionisation chamber is given by D w,Qo = M Qo N D,w,Qo .........(1) where, M Q0 = dosimeter reading under reference conditions (Practical conditions - same as standards lab) N D,w,Qo = absorbed dose to water calibration factor of the dosimeter obtained from standards laboratory

11. However, other than reference beam quality/ (Beam quality Q used is other than the reference quality Q ), D w,Q = M Q N D,w,Qo k Q,Qo .......(2) k Q,Qo = beam quality correction factor (BQCF) k Q,Qo = corrects for the effects of the difference between the reference beam quality Qo

12. Beam Quality Correction Factor k Q,Qo Defined as, ......(3 For 60 Co as calibration quality(Q ), k Q,Qo = k Q Ideally , BQCF should be measured directly for each chamber at the beam quality of that of the user.

13. When no experimental data are available, Correction factors are calculated theoretically, k Q,Qo can be derived comparing D w,Q = M Q N D,w,Qo k Q,Qo with N D,Air formalism, ......(4) which is valid for all types of high energy beams, includes ratios, at the qualities Q and Q o , of Spencer– Attix water/air stopping-power ratios, S w,air , of the mean energy expended in air per ion pair formed, W ai r , and of the perturbation factors P Q . P Q includes p wall , p cav , p cel and p dis (Contd...)

14. Beam Quality Correction Factor In therapeutic electron and photon beams, ( W air ) Q  ( W air ) Qo ......(5) p Q = ( p cav p cel p dis p wall ) Q = overall perturbation factor for an ionisation chamber for in-phantom measurements at beam quality Q. p wall = corrects for non-medium equivalence of the chamber wall. p dis = corrects for replacing a volume of water with the detector cavity when the reference point of the chamber is taken to be at the chamber centre - alternative to p eff . p cel = corrects for the effect of the central electrode during in-phantom measurements. p cav = corrects for effects related to the air cavity, predominantly the in-scattering of electrons.

15. Mean values of k Q at various photon beam qualities for NE 2561and NE 2611 ion chambers open circles - NE 2561 filled circles- NE 2611 line - fit to the expt. data triangle - calculated values normalised to 0.568 (TPR 20,10 of 60 Co)

16. Relation Between N K and N D,w The connection between the N K - N D,air formalism and the N D,w formalism is established for high energy beams by the relationship where, Qo = reference quality ( 60 Co - rays), and p Qo = overall perturbation factor p cel refers exclusively to in-phantom measurements

17. General Practical Considerations Chamber sleeve : Material - PMMA, Wall thickness  1.0 mm Air gap (chamber & sleeve) : 0.1- 0.3 mm  sleeve should not be left in water longer than is necessary to carry out the measurements  The use of a thin rubber sheath is not recommended,

18. Verify stability of the dosimeter system using a check source. Enough time should be allowed for the dosimeter to reach thermal equilibrium. Mains powered electrometers should be switched on at least two hours before use to allow stabilisation Pre-irradiate the ionisation chamber with 2 - 5 Gy to achieve charge equilibrium in the different materials O perate the measuring system under stable conditions whenever the polarity or polarising voltage are modified Measure the leakage current before and after irradiation(< 0.1%) Chamber wall Central electrode P.D. Insulator

19. Correction for Influence Quantities

20. Evaluation of Influence Quantities Atmospheric variations : As all chambers recommended in this report are open to the ambient air, the mass of air in the cavity volume is subject to atmospheric variations. conversion of the cavity air mass to the reference conditions, (generally 101.3 kPa and 20°C) No correction for humidity, if N D,w is referred to a relative humidity (RH) of 50% and is used in 20 - 80% of RH. If N D,w is referred to dry air, apply k h = 0. 997 ( Q o = 60 Co)

21. Polarity effect ( k pol ) : The effect on a chamber reading of using polarizing potentials of opposite polarity must always be checked. For most chamber types the effect will be negligible in photon beams, a notable exception being the very thin window chambers used for low energy X rays. In charged particle beams, particularly electrons, the effect may be significant. True reading is taken to be the mean of the absolute values of readings taken at both polarities. For routine use of a single potential and polarity. Where, M = electrometer reading obtained with the polarity used routinely (+ or - )

22. Ion Recombination( k s ): The incomplete collection of charge in an ionization chamber cavity owing to the recombination of ions requires the use of a correction factor ks . Two separate effects take place: ( i ) the recombination of ions formed by separate ionizing particle tracks, termed general (or volume) recombination, which is dependent on the density of ionizing particles and therefore on the dose rate; and (ii) the recombination of ions formed by a single ionizing particle track, referred to as initial recombination, is independent of the dose rate. depend on the chamber geometry and on the applied polarizing voltage . For beams other than heavy ions, initial recombination is generally less than 0.2%.

23. For pulsed beams, it is recommended in this Code of Practice that the correction factor ks be derived using the two voltage method, This method assumes a linear dependence of 1/ M on 1/ V. The recombination correction factor ks at the normal operating voltage V1 is obtained from, Where, M 1 = electrometer reading at polarising voltage V 1 (Normal Voltage). M 2 = electrometer reading at polarising voltage V 2 (Lower Voltage) . (M 1 and M 2 are corrected for k pol at their respective voltages). a , a 1 and a 2 = quadratic fit co- efficients for pulsed and scanned beams V 1 /V 2 = 3.

24. For continuous radiation ( 60 Co gamma rays), Ion Recombination( k s ): Contd...

25. Influence quantity Reference value/characteristics Phantom material Water Chamber type Cylindrical or plane parallel (PP) Measurement depth, z ref 5 or 10 g/cm 2 Reference point For cylindrical chambers, on the of the chamber central axis at the centre of the cavity volume. For pp chambers, on inner surface of the window at its centre Position of the reference At the measurement depth z ref point of the chamber SSD or SCD 80/100 cm Field size 10 cm × 10 cm 60 C0 -Rays :Reference Dosimetry

26. 5 cm (depth) 80 cm (SSD) 10 x 10 cm 2 Electro- meter Water Phantom Ion chamber Experimental Set-up : SSD Water 60 C0 -Rays :Reference Dosimetry

27. The absorbed dose to water at z ref in water, in the user 60 Co beam and in the absence of the chamber , D w ( z ref ) = MN D,w Gy/min where , M = reading of the dosimeter corrected for temperature and pressure , electrometer calibration, polarity effect, ion recombination and timer error. M = M unc k TP k elec k pol k s /(t   t) & t = time of irradiation (min) Absorbed dose at z max : For SSD Set-up, D w ( z max ) = D w ( z ref )x 100/PDD( z ref ) For SAD Set-up, D w ( z max ) = D w ( z ref )/TMR( z ref ) 60 C0 -Rays :Reference Dosimetry

28. High Energy X-rays:Reference Dosimetry

29. Choice of beam quality index For high energy photons produced by clinical accelerators the beam quality Q is specified by the tissue phantom ratio TPR 20,10 . This is the ratio of the absorbed doses at depths of 20 and 10 cm in a water phantom, measured with a constant SCD of 100 cm and a field size of 10 cm × 10 cm at the plane of the chamber. The most important characteristic of the beam quality index TPR 20,10 is its independence of the electron contamination in the incident beam.

30. High Energy X-rays: Measurement of QI (TPR 20 10 ) Influence quantity Reference value/characteristics Phantom material Water Chamber type Cylindrical or plane parallel (PP) Measurement depths 20 and 10 g/cm 2 Reference point of For cylindrical chambers, on the the chamber central axis at the centre of the cavity volume. For PP chambers, on the inner surface of the window at its centre Position of the reference At the measurement depths point of the chamber SCD 100 cm Field size at SCD 10 cm × 10 cm

31. Experimental Set-up for QI Water Chamber = 100 cm

32. High Energy X-rays: Reference Dosimetry Influence quantity Reference value/characteristics Phantom material Water Chamber type Cylindrical Measurement depth z ref For TPR 20 10 < 0. 7, 10 (or 5) g/cm 2 For TPR 20 10 = 0. 7, 10 g/cm 2 Reference point of On the central axis at the centre of the the chamber cavity volume Position of the reference At the measurement depth z ref point of the chamber SSD/SCD 100 cm Field size 10 cm × 10 cm

33. 10 cm (depth) 100 cm (SSD) 10 x 10 cm 2 Electro- meter Water Phantom Ion chamber Experimental Set-up : SSD Water High Energy X-rays:Reference Dosimetry

34. High Energy X- rays:Reference Dosimetry Absorbed dose to water at the reference depth z ref D w,Q ( z ref ) = M Q N D,w k Q Gy/MU Where, M Q = M unc k TP k elec k pol k s = Corrected Electrometer reading Absorbed Dose to water at z max D w,Q ( z max ) = 100 D w,Q ( z ref )/PDD ( z ref ) Gy/MU - SSD D w,Q ( z max ) = 100 D w,Q ( z ref )/TMR ( z ref ) Gy/MU - SAD

35. Calculated Values of K Q For HE Photon Beams High Energy X- rays:Reference Dosimetry

36. Calculated values of k Q for various cylindrical ionisation chambers High Energy X- rays:Reference Dosimetry

37. High Energy Electrons: Determination of Beam Quality (R50)

38. Choice of beam quality index: Beam quality index is the half-value depth in water R50. This is the depth in water (in g/cm 2 ) at which the absorbed dose is 50% of its value at the absorbed dose maximum, measured with a constant SSD of 100 cm. Field size at the phantom surface of at least , 10 cm × 10 cm for R50 ≤ 7 g/cm 2 ( Eo ˂ 16 MeV ) and 20 cm × 20 cm for R50 > 7 g/cm 2 ( Eo ˃16 MeV ).

39. High Energy Electrons : Determination of BQ (R 50 ) Influence quantity Reference value/characteristics Phantom material water - R 50  4 g/cm 2 ( E o  10 MeV ) water or plastic - R 50 < 4 g/cm 2 Chamber type PP or cylindrical - R 50  4 g/cm 2 Plane parallel (PP) - R 50 < 4 g/cm 2 Reference point of PP - on the inner surface of the the chamber window at its centre Cylindrical - on the central axis at the centre of the cavity volume Position of the reference PP - at the point of interest point of the chamber Cylindrical : 0.5 r cyl deeper than the point of interest SSD 100 cm Field size 10 cm × 10 cm - R 50  7 g/cm 2 at phantom surface 20 cm × 20 cm - R 50 > 7 g/cm 2

40. When using an ionisation chamber, the measured quantity is R 50,ion . The R 50 is obtained using, When using detectors other than ion chambers (e. g. diode, diamond, etc.) the measured quantity is R 50 High Energy Electrons: Determination of BQ (R 50 )

41. High Energy Electrons : Reference Dosimetry Influence quantity Reference value/characteristic Phantom material water - R 50  4 g/cm 2 ( E o  10 MeV ) water or plastic - R 50 < 4 g/cm 2 Chamber type PP or cylindrical - R 50  4 g/cm 2 Plane parallel (PP) - R 50 < 4 g/cm 2 Measurement depth z ref = (0.6 R 50 - 0.1) g/cm 2 Reference point of PP - on the inner surface of the the chamber window at its centre Cylindrical - on the central axis at the centre of the cavity volume Position of the reference PP - at z ref point of the chamber Cylindrical : 0.5 r cyl deeper than z ref SSD 100 cm Field size 10 cm × 10 cm or that used for at phantom surface normalisation of output factors

42. z ref = (0.6 R 50 - 0.1) 100 cm (SSD) 10 x 10 cm 2 Electro- meter Water Phantom PP chamber Experimental Set-up : SSD Water High Energy Electrons:Reference Dosimetry

43. Absorbed dose to water at the reference depth z ref D w,Q ( z ref ) = M Q N D,w k Q Gy/MU M Q = M unc k TP k elec k pol k s = Corrected Electrometer reading Absorbed Dose to water at z max , D w,Q ( z max ) = 100 D w,Q ( z ref )/PDD ( z ref ) Gy/MU - SSD High Energy Electrons:Reference Dosimetry

44. High Energy Electrons:Calculated K Q Values

45. Calculated K Q values for PP chambers calibrated in 60 Co High Energy Electrons:Reference Dosimetry

46. Calculated K Q values for Cylindrical chambers calibrated in 60 Co High Energy Electrons:Reference Dosimetry

47. High Energy Electrons:Use of Plastic Phantoms The use of plastic phantom is strongly discouraged, as in general they are responsible for the largest discrepancies in the determinations of absorbed dose in electron beams. Nevertheless , when accurate chamber positioning in water is not possible, or when no waterproof chamber is available, their use is permitted. Plastic phantoms may only be used at beam qualities R 50 < 4 g/cm 2 (E < 10 MeV ). ------------------------------------------------------------------------------------------------- Depth scaling z w = z pl c pl g/cm 2 ( z pl in g/cm 2 ) -------------------------------------------------------------------------------------------------- BQI R 50,ion = R 50,ion,pl c pl g/cm 2 (R 50,ion,pl in g/cm 2 ) -------------------------------------------------------------------------------------------------- Reference Depth z ref,pl = z ref / c pl g/cm 2 ( z ref in g/cm 2 ) -------------------------------------------------------------------------------------------------- D w M Q = M Q,pl h pl z w depth in water, c pl is a depth scaling factor, fluence scaling factor h pl

48. High Energy Electrons:Use of Plastic Phantoms Values of Depth Scaling Factor c pl , Fluence Scaling Factor h pl and Nominal Density p pl for Certain Plastics

49. Thank you. Vinay Desai M.Sc Radiation Physics Radiation Physics Department KIDWAI MEMORIAL INSTITUTE OF ONCOLOGY Bengaluru [email protected] Ppt Reference:SD Sharma