This document discusses the TG-51 protocol for reference dosimetry. It establishes that dose delivered to tumors must be within ±5% of prescribed to achieve tumor control. TG-51 protocol uses ion chambers calibrated in terms of absorbed dose to water for Co-60 (ND,w) and a conversion factor KQ to determine absorbed dose for other beams. For electron beams, KQ has additional components to account for effects of beam quality and chamber design. Reference conditions are established for photon and electron beam dosimetry.

1. K.K.D.Ramesh
Jr Medical Physicist
American Oncology Institute
Hyderabad
TG-51 PROTOCOL

2. Most of the Clinical studies and Retrospective
data analysis have proved that the dose delivered
to tumor must be within ±5% of the prescribed
dose to achieve meaningful and acceptable tumor
control . This requires all parameters that could
effect the delivered dose must be accurate within
±3% or better.
TG-51 PROTOCOL

3. For this a number of protocols for beam dosimetry based
on the Exposure ,Air Kerma, or Absorbed dose to water
calibration of Ion chambers are available, for
traceability and uniformity.
Among this TG-51 is a Protocol for the practice of
reference dosimetry. It applies to photon with nominal
energies between Co60 to 50 MV and for ë beams of
nominal energies between 4 and 50 MeV.

4. It uses Ion chambers calibrated in terms of absorbed dose
to water in a Co60( N DW
Q).
The absorbed dose to water standard is assumed to be
rugged standard of dosimetry as it is based on Water
Calorimetry, Graphite Calorimetery, Ionometry and
Chemical dosimetry.

5. The absorbed dose to water based formalism is relatively more accurate and simple to use and it is
possible to determine directly the absorbed dose to water from Electrometer reading using a simple
formalism.
DW
Q=M* N DW
CO60 *KQ(GY)
DW
Q – Absorbed dose to water at a user quality beam”Q”(GY)
M - Fully corrected Electrometer reading(c)
N DW
CO60 - Usually absorbed dose calibration factors will be
obtained for reference condition in a Co60 beam. To obtain calibration
factor ( N DW
Q) for user beam,
KQ- Converts the absorbed dose to water calibration factor for a Co60 beam
in to the calibration factor for an arbitrary beam quality ’Q’ which can be
for Ȣ or ë. It is a chamber specific.
N DW
Q=N DW
CO60 *KQ(GY/C)
DW
Q=M* N DW
CO60 *KQ(GY))

6. For ë:
KQ contains 2 components
KQ=Pgr
Q *K’*R50 Kecal=Pgr
Q*KR50
KR50-Chamber specific factor & it depends on the quality for which the absorbed dose calibration factor was
obtained and the user beam quality ‘Q’ as specified by R50.
Kecal- It is a Ȣ to ë conversion factor, is fixed for a given chamber and its value is one for an ë beam quality
Q ecal Value is available from the table for a given chamber at given quality of beamR50 -7.5cm.
K’R50- It is a beam quality dependent and converts N DW
Qcal in to N DW
Q .this values are obtained
from the graph for Parallel plate &Cylindrical chambers.
Pgr
Q- It is necessary for cylindrical chambers to correct for gradient effect at the
reference depth.
Absorbed dose to Water for ë
KQ=Pgr
Q *K’R50* Kecal=Pgr
Q*KR50
Pgr
Q=Mraw(dref+0.5rcav)/Mraw(dref)
DW
Q=M*Pgr
Q *K’R50* Kecal* N DW
CO60(GY)

7. M=Mu*Pion*Ptp*Pelc*Ppol
Pion=1-(v1/V2)2/(M1/M2)-(V1/V2)2 , V2=V1/2
Ptp=(273.2+T/273.2+220C)(1013.2/P)
Pelc= Electrometer calibration factor
Ppol=(M++M-)/2(M+)
M=Mu*Pion*Ptp*Pelc*Ppol

8. Beam Quality Specifications for Photons
Influence quantity Reference value/characteristics
Phantom material Water
Chamber type Cylindrical
Measurement depth zref PDD(10)
Reference point of On the central axis at the centre of the the chamber
cavity volume
Position of the reference At 0.6 rcav above measurement depth
point of the chamber
SSD/SCD 100 cm
Field size 10 cm × 10 cm

9. Reference Conditions for Photon Dosimetry
Influence quantity Reference value/characteristics
Phantom material Water
Chamber type Cylindrical
Measurement depth zref 10 g/cm2
Reference point of On the central axis at the centre of the the chamber
cavity volume
Position of the reference At the measurement depth zref
point of the chamber
SSD/SCD 100 cm
Field size 10 cm × 10 cm

10. Photons reference Dosimetry
10 cm (depth)
100 cm (SSD)
10 x 10 cm2
Electro-
meter
Water Phantom
Ion chamber
Experimental Set-up : SSD
Water

11. Beam quality specification for ë
Influence quantity Reference value/characteristics
Phantom material water
Chamber type PP or cylindrical
Plane parallel (PP)
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 rcyl deeper than the
point of interest
SSD 100 cm
Field size 10 cm × 10 cm
at phantom surface 20 cm × 20 cm - R50 > 8.5 g/cm2

12. When using an ionisation chamber, the measured quantity is R50,ion. The R50 is
obtained using
R50=1.029R50,ion-0.063(2≤ R50,ion ≤10cm)
R50=1.059R50,ion-0.37( R50,ion >10cm)
When using detectors other ion chambers (e. g. diode, diamond, etc.) the measured
quantity is R50

13. waterproofing for ion chamber ( if needed) <1mm PMMA
water phantom (at least 30x30x30 cm3)
lead foil for photons 10MV and above
1 mm  20

14. Reference conditions for ë
Influence quantity Reference value/characteristic
Phantom material water
Chamber type PP or cylindrical - R50  4 g/cm2
Plane parallel (PP) - R50 < 4 g/cm2
Measurement depth zref = (0.6 R50 - 0.1) g/cm2
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 zref
point of the chamber Cylindrical : 0.5 rcyl deeper than zref
SSD 100 cm
Field size 10 cm × 10 cm or
at phantom surface 20 cm × 20 cm - R50 > 8.5 g/cm2