K.K.D.RameshJr Medical Physicist

American Oncology InstituteHyderabad

TG-51 PROTOCOL

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

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.

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.

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.

DWQ=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 DWCO60 - Usually absorbed dose calibration factors

will be obtained for reference condition in

a Co60 beam. To obtain calibration factor ( N DWQ)

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 DWQ=N DW

CO60 *KQ(GY/C)

DWQ=M* N DW

CO60 *KQ(GY))

For ë:KQ contains 2 components

KQ=PgrQ *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 DWQcal in to N

DWQ .this values are obtained from the graph for Parallel

plate &Cylindrical chambers.

PgrQ- It is necessary for cylindrical chambers to correct for

gradient effect at the reference depth.

Absorbed dose to Water for ë

KQ=PgrQ *K’R50* Kecal=Pgr

Q*KR50

PgrQ=Mraw(dref+0.5rcav)/Mraw(dref)

DWQ=M*Pgr

Q *K’R50* Kecal* N DWCO60(GY)

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

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

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

Photons reference Dosimetry

10 cm (depth)

100 cm (SSD)

10 x 10 cm2

Electro-

meter

Water Phantom

Ion chamber

Experimental Set-up : SSD

Water

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 thethe 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 interestpoint of the chamber Cylindrical : 0.5 rcyl deeper than the point of interestSSD 100 cm

Field size 10 cm × 10 cm at phantom surface 20 cm × 20 cm - R50 > 8.5 g/cm2

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

waterproofing for ion chamber ( if needed) <1mm PMMA

water phantom (at least 30x30x30 cm3)

lead foil for photons 10MV and above1 mm 20

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 thethe 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