TRS-398

Introduction

The IAEA Technical Report Series #398, ''Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry based on Standards of Absorbed Dose to Water'' (2004 v.11b), is the absorbed dose protocol used in NCCC and most of Australia.

It is generally considered that the accuracy of planned and delivered dose is ±5% (or more strictly, what is prescribed and what is delivered). However some tumours respond within a very small window of dose and for these ±3% is needed. This creates a need for accurate determinations of doses delivered.

Air Kerma calibrations using NK —> ND,air —> Dw style chains introduce more uncertainties (ie, have more steps)than ND,w —> Dw style calibrations. Air Kerma protocols can have 3-4% uncertainty inherent before even getting to prescribing/delivering dose.

Direct determination of absorbed dose to water can only be obtained by calorimetry, but PSDL's use ionisation methods, chemical or graphite calorimetry to get acceptable dose-to-water determinations from ^60^Co beams. The next step of beam quality correction factors (KQ,Qo) introduces more uncertainty, but to remove this the PSDL/SSDL would need to offer linac ND,w factors.

Why Dose to Water?

* Most biologically relevant
* Reduced uncertainty (less steps. Also, NK factors were for each model of chamber, while ND,w factors can be chamber specific)
* More robust
* NK factors can be perturbed by chamber wall attenuation by up to 0.7%
* NK factors are inherently for ionisation chambers only
* Can be determined by ionisation or calorimetry methods btu results are very similar either way
* Simpler - less perturbation and correction factors needed

Framework

* BIPM/IAEA……………………………………..International Bodies
* PSDL……………………………………..Primary Standard Instruments
* SSDL………………………………Secondary/National Standard Instrument
* Reference Chamber…….Radiation Oncology Department
* Field Chamber…….Radiation Oncology Department

PSDL/SSDL's use the following for absorbed dose determination:

* graphite cavity ionisation chamber in water phantom (Dw from Bragg-Gray corrections)
* graphite calorimeter in graphite phantom (Dw from fluence scaling)
* water calorimeter in water phantom (Dw directly measured by thermistor)
* water calorimeter to Fricke dosimeter (ND,w transfer)
* Fricke standard (ND,w from electron beam characterisation)
* For kV, extrapolation chambers are used to find true dose, convert to ND,w

Nd,w Formalism

'''Dw,Qo = MQ ND,w,Qo kQ,Qo''' under reference conditions.

(1)
\begin{equation} D_{w,Q} = M_Q N_{D,w,Q_0} k_{Q,Q_0} \end{equation}

'''conventional true value''' - value accepted as the best value (ie, average of measurements)

'''reference conditions''' - the set of influence quantities under which calibration is performed without corrections.

'''influence quantities''' - environmental (temp, pressure, humidity), dosimeter (bias, zero drift, aging, warm up) or radiation field (beam quality, FS,dose rate, depth etc)

(if influence factors are independent of each other then a product of factors can be used for correction for each)

'''kQ,Qo''' - Beam Quality Correction Factor


Implementation

An ionometric system is used - one consisting of a chamber assembly (inc. cable), electrometer, phantom assembly and output stability check device.

Ionization Chambers

Cylindrical Chambers

* Can be used for medium and high kV (>80kVp or 2mm Al HVL), 60Co, MV photons and MV electrons for R50>7g.cm^-3^

<tablewidth="924px" tableheight="237px">'''Property''' '''Specification''' '''Notes'''
Cavity Volume 0.1-1cm^3^ able to equilibrate to room temp, pressure quickly
Internal diametre <7mm Bragg-Gray conditions:charge collected from secondary electrons outside chamber volume.
Internal Length <25mm radiation fluence should be uniform along length
Materials tissue equivalent homogenous as possible, allow uniform energy response

Parallel Plate Chambers

* Can be used for all MV electrons (compulsory for R50>7g.cm-3)
* Don't use for high energies, you get backscatter from the rear of the chamber
* Only used for photons if a ND,w,Q is available direct from a PSDL/SSDL
* Guard rings ensure signal is almost entirely from the front window, minimises scatter perterbation making it very good choice for electrons

<tablewidth="924px" tableheight="237px">'''Property ''' '''Specification ''' '''Notes'''
peff inside surface of window for all Q, d
Cavity diametre >= 5mm minimise scatter perterbation
Collecting electrode diametre <= 20mm minimise radially non-uniform effects detected
Guard electrode width >= 1.5 cavity height
Front window thickness <=0.1 g.cm^-3^

Electrometer

* 0.1% resolution (i.e., 4 digits)
* long term stability ±0.5% in a year

Phantoms

* Extend >= 5g.cm^-3^ from field in all directions
* At least 5g.cm^-3^ backscatter beyond measurement point
* Use water wherever possible
* Plastics are OK for electrons (R50<7cm^-3^) if necessary and low low kV (<80kVp)
* There may be inter- and intra-phantom density changes in plastics
* Nominal thickness may not be precise
* Don't use in high energy electron beams as plastic is insulative, creates charge storage which creates an electric field which then affects the fluence reaching the chamber. Use thin slabs to avoid this rather than solid blocks.
* Density of PMMA = 1.190 g.cm^-3^, of RMI 457 (solid Water) = 1.030^^g.cm^-3^

Chamber Position

Reference Point - Centre of cavity volume (cylindrical) or inner surface of front window (parallel)

Effective point of measurement - For cylindrical chambers, the fluence is actually sampled at deff not dref. Apply pdis or shift chamber to d = dref + 0.5r (r= int.chamber radius)

Chamber Calibration

* performed at Standards lab every 2-3 years for Q0 or every 6 years for all qualities
* kV qualities should be calibrated every 2-3 years

Performing Measurements

* Allow chamber to reach thermal equilibrium (~5min)
* allow electrometer to stabilise ( up to 1-2 hrs)
* create charge equilibrium in the measurement setup (ie, irradiate 2-5 Gy)
* check active leakage current < 0.1%
* check passive leakage current <0.1%
* set up monitor chamber where possible to normalise measurements in case of beam fluctuations

Influence Qualities

'''kT,P''' - converts chamber cavity air mass to the equivalent of reference air mass ''''''

'''kh''' - accounts for humidity. Value is unity for 20-80% humidity (ref is 50%). Value is 0.997 for dry air

'''kelec''' - Accounts for differences in meter/chamber from calibration setup (Value is 1 if meter/chamber calibrated together)

'''kpol''' - accounts for changes in readings due to chamber polarity. Important for electrons in particular

'''ks''' - accounts for signal loss due to recombination. For pulsed beams, use the quadratic formula in TRS398. Use bias values in ratio of 3:1 for most accurate correction factor. For parallel plate chambers, plot 1/M vs 1/V and use values where V is linear.


Codes of Practice

High E Photons

High E Electrons

Low E X-rays

Medium E X-rays

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