DOSIMETRY PROTOCOLS

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DOSIMETRY PROTOCOLS
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I. Introduction

• First generation SCRAD(1971), ICRU14, 21 (1969,
  1972)
• Exposure calibration factor, Nx for ion chambers
  in 60Co beam
• Look up dose conversion factors vs. Nominal
  energy(c?,CE)
• No special considerations in either the type of
  chamber used or the actual quality of the beam
• Omission of these considerations led to errors up
  to 5
• M corrected to the
  standard environment
• Second generation TG-21(1983)
• Third generation TG-51(1999)

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I. Introduction(cont)

• First generationSCRAD(1971), ICRU14, 21(1969,
  1972)
• Second generation TG-21(1983)
• Many of these problems were reduced at the
  expense of added complexity
• Accuracy was better, but it required complex
  calculations
• The chamber dependent factors and their variation
  with beam quality increased potential for errors
  in the clinic
• Third generation TG-51(1999)

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I. Introduction(cont)

• First generationSCRAD(1971), ICRU14, 21 (1969,
  1972)
• Second generation TG-21(1983)
• Third generation TG-51(1999)
• Requires absorbed dose to water calibration
  factors,
• Conceptually easier to understand and simpler to
  implement
• Requires a quality conversion factor, kQ ,change
  in modality, energy, gradient

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II. TG-21 protocol (Med Phys, 1983)

• TG-21 protocol has two major components

Part I How to obtain the calibration factors Nx
(exposure) and Ngas(dose to cavity gas) in a
Co-60 beam. if only Nx is provided by the
standard laboratory, then the user needs to
convert Nx to Ngas. Part II How to use Ngas in a
users beam, which can be any modalities (photon
or electron beams) and energies, and the chamber
can be placed in a plastic medium. Dose to cavity
gas ? dose to medium ? dose to water
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II. TG-51 protocol (Med Phys 26, 1847-70, 1999 )

• The TG-51 protocol is based on absorbed dose to
  water calibration (also in a Co-60 beam)
• The chamber calibration factor is denoted
• The calibrated chamber can be used in any beam
  modality (photon or electron beams) and any
  energy, in water.
• The formalism is simpler than the TG-21, but it
  is applicable in water only.

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II. TG-51 protocol (cont)

• Suitable norminal energy
• photon beams 60Co50MV
• electron beams 450MeV
• Ion chambers calibrated
• in terms of absorbed dose to water in a 60Co
  beam.
• Purpose
• to ensure uniformity of reference dosimetry in
  external beam radiation therapy with high-energy
  photons and electrons.

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Obtaining An Absorbed-dose To Water Calibration
Factor

• Is the absorbed dose to water (in Gy) in
  the calibration laboratorys 60Co beam under
  standard environment.
• first purchased, repaired, redundant checks
  suggest a need, once every two years.
• check sources, regular measurements in a 60C0
  beam, multiple independent dosimetry systems.
• (Gy/C or
  Gy/rdg)
  

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General Formalism

• In a Co-60 beam

• In any other photon beam
• (only cylindrical chamber allowed at present)

• In any electron beam
• (both cylindrical and parallel-plate chambers
  allowed)

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kQ values for cylindrical chambers in photon beams
NRC-CNRC
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Measurement Phantoms

• A water phantom with dimensions of at least
  30?30?30 cm3

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Charge Measurement
(C or rdg)

• Polarity corrections, Ppol
• Electrometer correction factor, Pelec

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Charge Measurement(cont)
(C or rdg)

• Standard environmental condition
• 22oC,101.33kPa,humidity between 2080(variation
  ?0.15)
• PTP one corrects charge or meter readings to
  standard environmental condition.

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Charge Measurement(cont)
(C or rdg)

• Corrections for ion-chamber collection
  inefficiency, Pion
• if Pion ?1.05, another ion chamber should be used
• Voltages should not be increased above normal
  operating voltages. (300V or less)
• 60Co
  pulse/swept beam

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Point of Measurement Effective Point of
Measurement
?
Effective point of measurement
r
point of measurement
rcav
cylindrical
parallel plate
Photon r 0.6 rcav electron r 0.5 rcav
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Beam Quality Specification (Photons)

• For this protocol, the photon beam quality is
  specified by dd(10)x, the percent depth-dose at
  10 cm depth in water due to the photon component
  only, that is, excluding contaminated electrons.
• For low energy photons (lt10 MV with dd(10) lt
  75) dd(10)x dd(10) (contaminated electron is
  negligible)
• For high energy photons (gt10 MV with
  75ltdd(10)lt89) dd(10)x ? 1.267dd(10) 20.0
• A more accurate method requires the use of a 1-mm
  thick lead foil placed about 50 cm from the
  surface. (50?5cm or 30?1cm)
• dd(10)x 0.89050.00150dd(10)pb dd(10)pb
• foil at 50 cm, dd(10)pbgt73

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Beam Quality Specification (electrons)
Percent depth ionization to be measured at SSD
100 cm for field size ? 10?10 cm2 (or ?20?20 cm2
for Egt20 MeV).
Parallel-plate chamber measured curve
II. Cylindrical chamber measured curve I, needs
to be shifted by 0.5 rcav to get curve II. Curve
II is the percent ionization curve.
R50 1.029I50 0.06 (cm) for 2?I50 ? 10
cm R50 1.059I50 0.37 (cm) for I50 gt 10 cm
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Beam Quality Specification

• depth-dose at 10 cm depth for curve shifted
  upstream by 0.6 rcav (photon), 0.5 rcav (e-)

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Beam Quality Specification(cont)
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kQ values for cylindrical chambers in photon beams
NRC-CNRC
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kQ values for cylindrical chambers in photon beams
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Reference conditions

• e- dref 0.6 R50 - 0.1 cm
• photon dref 10 cm

photon source
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Photon Beam Dosimetry
where M is the fully corrected chamber
reading, kQ is the quality conversion factor,
is the absorbed dose to water chamber
calibration factor
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Electron Beam Dosimetry
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Electron Beam Dosimetry ( )
depends on users beam must be measured in clinic.
Cylindrical chamber
Same as shifting the point of measurement
upstream by 0.5rcav.
parallel plate chamber
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Electron Beam Dosimetry ( Kecal )
Kecal is the photon-to-electron conversion factor
for an arbitrary electron beam quality, taken as
R50 7.5 cm. The values of Kecal for a number of
cylindrical and parallel-plate chambers are
available in the TG-51 protocol.
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Electron Beam Dosimetry ( )
is the electron quality conversion factor
converting from Qecal to Q.
for a number of cylindrical and parallel-plate
chambers are available in Figs. 5-8 in the TG-51
protocol. It can also be calculated from the
following expressions
Cylindrical
Parallel-plate
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kR50 for Cylindrical Chambers
NRC-CNRC
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kR50 for Parallel Plate Chambers
NRC-CNRC
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Worksheet

• Sample
• Worksheet A Photon Beam(lt 10MV)
• Worksheet A Photon Beam(gt lt 10MV)
• Worksheet B Electron Beams - Cylindrical
  Chambers
• Worksheet C for plane-parallel
  chambers
• Worksheet D Electron Beams using Plane-Parallel
  Chambers

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Using Other Ion Chambers

• Finding the closest matching chamber for which
  data are given.
• Critical features are in order
• Wall material
• Radius of the air cavity
• Presence of an aluminum electrode
• Wall thickness

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III. Discussion

• TG-51 emphasis in primary standards laboratories
• move from standards for exposure or air kerma to
  those for absorbed dose to water
• clinic reference dosimetry is directly related to
  the quantity
• absorbed dose can be developed in linear
  accelerator
• absorbed dose to water have an uncertainty (1s)
  of less than 1 in 60Co25MV
• TG-51 must be performed in a water phantom
• measurements in plastics, including
  water-equivalent plastics, are not allowed, does
  not preclude the use of plastic material for QA.
• in-water calibrations must be performed at least
  annually.

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III. Discussion (cont)

• TG-21 combined both the theory and practical
  application
• TG-51 serves only as a how to
• The results of clinical reference dosimetry in
  photon beam will not change more than 1 compare
  to TG-21

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Relation betweenabsorb-dose and air-kerma
standards
A theoretical relationship between absorbed-dose
and air-kerma standards (or calibration factors)
can be derived by comparing the absorbed-dose to
water determined for a 60Co beam using the TG-51
and TG-21 protocols. Using kQ 1.0 and assuming
Ppol1.0, the following relationship for a 60Co
beam can be obtained
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Relation betweenabsorb-dose and air-kerma
standards (cont)
kQ 1.0 Ppol1.0
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Relation betweenabsorb-dose and air-kerma
standards (cont)
Note that no measured value is necessary. All the
required numerical values can be obtained from
the TG-21 report. The ratio also be directly
determined using the absorbed-dose and air-kerma
calibration factors measured at the calibration
laboratory.
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Compare Dose Kerma Standards
The equation used to determine ND,W/NK is as
follows
- where Prepl, Pwall and L/? values were taken
from TG-21 data and Ngas/Nx comes from Gastorf et
al.
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TG-51
TG-21
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Photon Equations
The equations used to determine absorbed dose
for photons are as follows
TG-51
TG-21
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Electron Equations
The equations used to determine dose for
electrons are as follows Both values are
absorbed dose at dref.
TG-51
TG-21
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TG-51 Photon Measurements
Search for dmax (apply a 0.6 rcav shift to
effective point of measurement). Place chamber
at 10cm 0.6 rcav to determine PDD at 10 cm.
Determine kQ value using the data from TG-51
report. Move chamber to calibration depth, 10
cm (without a 0.6 rcav shift). Make
measurements for Polarity Correction ( 300 and -
300 Volts) and Pion (- 150 Volts). Polarity
Correction was less than 0.2 for all chambers
studied.
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TG-51 Electron Measurements

• Search for Imax and I50 (apply a 0.5 rcav shift
  to effective point of measurement).
• - From these determine R50 and dref.
• - From R50 kR50 is determined.
• Move center of chamber to the calibration
  depth, dref (without a 0.5 rcav shift).
• Measure Polarity Correction ( 300 and 300
  Volts) and Pion (- 150 Volts).
• Move center of chamber to dref 0.5rcav and
  calculate the gradient correction.
• Polarity correction was less than 0.2 for all
  chambers studied.

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RESULTS
Comparison Between Absorbed Dose and Air Kerma
Calibration
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RESULTS
Comparison Between Absorbed Dose and Air Kerma
Calibration
The 1.2 discrepancy between the measured and
calculated values represents a basic discrepancy
between the Air Kerma and Absorbed dose standards
and the TG-21 formalism used to convert Air Kerma
into Dose. NIST and NRC Canada are aware of a
0.7 difference between their two Air Kerma
standards. This difference is in the right
direction to suggest that ND,w / NK for NRC
Canada should be closer to unity.
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RESULTS
Comparison Between TG-51 and TG-21 Calibrations
(photons)
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RESULTS
Comparison Between TG-51 and TG-21 Calibrations
(electrons)
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Conclusions
Comparison Between TG-51 and TG-21 Calibrations
(electrons)
Standards
There is an apparent 1 discrepancy between the
Absorbed Dose Standard and the Air Kerma Standard
(converted to dose using TG-21).
Photons
Institutions, which calibrate at dmax for
TG-21, may see a different TG51/TG21 ratio, due
to the difference in the determination of dd for
TG-51 and TG-21. The 1 discrepancy in the
standards is reflected in a 1 discrepancy
between TG-51 and TG-21 for x-rays.
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Conclusions
Comparison Between TG-51 and TG-21 Calibrations
(electrons)
Electrons
In addition to the 1 discrepancy in standards
there is an additional discrepancy for electrons,
partly due to new stopping power data used for
the TG-51 protocol. The magnitude of changes in
electron beam output may depend on the electron
energy.
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Experience
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IV. Conclusions

• Omissions of the type of chamber used or the
  actual quality of the beam in SCRAD led to errors
  up to 5.
• There are some trends which indicate that the
  AAPM protocol improves the consistency of the
  calculation of dose for the various dosimetry
  system use.
• TG-21is more consistently than SCRAD in the
  various chamber and phantom combinations.

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IV. Conclusions (cont)

• The results of clinical reference dosimetry in
  TG-51 photon beam will not change more than 1
  compare to TG-21.
• This protocol represents a major simplification
  compares to the AAPMs TG21 protocol in the sense
  that large tables of stopping-power ratios and
  mass-energy absorption coefficients are not
  needed and the user does not need to calculate
  any theoretical dosimetry factors.

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IV. Conclusions (cont)

• The comparison shows approximately a 1
  discrepancy between measured and calculated
  ratios. This discrepancy may provide a reasonable
  measure of possible changes between the
  absorbed-dose to water determined by TG-51 and
  that determined by TG-21 for photon beam
  calibrations.

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IV. Conclusions (cont)

• The typical change in a 6 MV photon beam
  calibration following the implementation of the
  TG-51 protocol was about 1, regardless of the
  chamber used, and the change was somewhat smaller
  for an 18 MV photon beam. On the other hand, the
  results for 9 and 16 MeV electron beams show
  larger changes up to 2, perhaps because of the
  updated electron stopping power data used for the
  TG-51 protocol, in addition to the inherent 1
  discrepancy presented in the calibration factors.
  The results also indicate that the changes may be
  dependent on the electron energy.

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IV. Conclusions (cont)

• It is recommended that a first-time-TG-51-user
  should obtain both the absorbed dose and
  air-kerma calibration actors on the same chamber,
  and perform an initial comparison as described in
  this study to estimate the inherent discrepancy
  expected from the implementation of the TG-51
  protocol