What is the purpose of the MV CoP?
To recommend:
- procedures for calibrating field instruments using a secondary standard (which has been calibrated using the NPL service).
- Procedures for measuring radiation output from Co-60 or a linac.
For MV photons, what are the calibration factors, and measurement conditions for a secondary standard dosemeter calibrated by the NPL service?
What is the estimated uncertainty on the measurement?
D=RN(D)
Where D is the absorbed dose to water (if there were not chamber and sheath)
R is the instrument reading, corrected for T/P and fion.
N(D) is the NPL calibraition factor for the radiation quality.
The radiation quality is TPR20/10.
Measurements should take place in a wwater phantom with the secondary standard in a sheath, at 5cm deep (4-10MV) or 7cm deep (12-19MV), for a 10x10 field.
The uncertainty is +/- 1.5% at 95% confidence level.
Outline the procedure for calibrating a MV photon field instrument.
- Measure the quality index for each beam.
- Use water or perspex phantom (full scatter), 10x10cm field size.
- Sequential or side-by-side irradiation.
- Apply T/P and fion corrections (if required).
Give a basic outline of the NPL calibration process for MV photons
- NPL primary standard measures dose to graphite using a graphite calorimeter in a range of beam qualities.
- Primary standard is used to calibrate at least 3 NPL reference chambers in a graphite phantom.
- NPL reference chamber calibrations are converted from dose to graphite to dose to water.
- User secondary standard chambers are calibrated against NPL reference chambers in water.
Why is graphite used as the MV primary standard?
- High temperature rise per Gy.
- Can be thermally isolated.
- Can be made very ‘pure’.
Briefly outline the processes required for converting dose to graphite to dose to water.
Two methods:
- Involves photon fluence scaling theroem to determine the ratio of the fluences, the mass-energy absorption coeffients.
- Determine the stopping power ratios and a correction factor from MC modelling.
What is the chamber type for MV and kV photon secondary standard?
Thimble chamber type 2611A
What are the correction factors which must be applied to the reading of the MV secondary standard to determine dose to water?
fTp: temperature and pressure correction back to 20 degrees C and 1013 Pa.
fion: accounts for the ion recombination that may occur between the ionisation and detection of ion. Dependant on dose rate and the polarising voltage.
Nelec: takes into account any factor from the electrometer.
fnon-ion: factors in the non-linearity of electrometer.
What are the types of ion recombination? How do you determine fion?
What type of values would you expect from common detectors, and how would this change for FFF?
Initial: from particles which originate on the same track. DR independent, varies with ion density (LET).
General: Particles from different tracks. DR dependent, varies with ion density and in time.
- Dual voltage technique - with V1/V2>3, assuming both in linear region. Accurate if fion<3%
- Jaffe plot - plot 1/M vs 1/V. Takes more time!
- Boag - theoretical, if cannot change V.
Typical values e.g. NE2611 ~0.5%, semiflex 0.125cc ~0.2%, NACP-02 (electrons) ~3.5%. (Here initial re-combination is <0.1%).
For FFF increase of 1-2%.
Give some examples of where adjustments have to be made to the standard CoP for MV photons (3).
- Tomotherapy - cannot set up the standard reference conditions (100cm SSD, 10x10field), calibrated against cFF, Need to correct for beam quality and FS difference, done with an equation.
- Small field dosimetry (e.g. cyberknife, gammaknife)- again calculate conversion to machine specific reference field.
- FFF - issues with TPR20/10, increased ion recombination and non-uniformity. Need equation to determine a beam quality correction factor.
Describe the ideal properties of detectors for measurements of MV photons.
Absolute dosimetry: ion chamber with calibration chain to NPL and following the CoP.
Output/consistency measurements: ion chamber or diode.
Profile: Diode - small volume reduced volume averaging,
PDD: IC - low energy dependence.
What are the categories of kV x-rays as defined in the CoP?
Medium energy (0.5-4mmCu) Low energy (1-8mm Al) Very low energy (0.035-1mm Al)
What information is given in the kV CoP?
Absorbed dose & calibration procedure for the different energies.
Values of the mass-energy absorption coefficient for different HVL.
Backscatter factors (function of FS and SSD)
Chamber correction factor, kch.
Details about the air kerma to water kerma conversion.
Suitable detectors for the different energies (field instrument and SS).
General equipment details.
Info on calculating beam quality (HVL measurement technique).
What is the recommended procedure for measuring absorbed dose to water for medium & low energy x-rays?
Chamber free in air. The dose to water at the surface (Dw,z=0) is given by:
Dw,z=0 = M.N(K).B(W)[(mew(en)/rho)w/air]
Bw is the backscatter factors (FS and SSD dependant), final term is the mass-energy absorption coefficient ratio. Measurement should be corrected for T/P
What is the process for calibrating a field instrument for medium and low energy x-rays?
- Measure the first HVL1 to find the secondary standard calibration factor.
- Simultaneous irradiation in air, at least 5cm from the end of the applicator. FS should be a 7cm diameter circle of 7x7cm sq. Chambers should be orientated so that there is a minimum gradient across the field.
- 3 measurements, then switch, repeat x2.
- T/P correction should be applied.
What is the recommended procedure for measuring absorbed dose to water for very low energy x-rays?
Parallel plate chambers should be used in a block of material to provide backscatter.
Dw,z=0 = M. N(K)k(ch)[mass-energy absorption coefficient ratio]
Kch is a factor to convert from measurement in air and measurement at the surface of the phantom.
What is the recommended procedure for calibrating a field instrument for very low energy measurements?
- Measure HVL1 to find the correct calibration factor.
- Sequentially irradiate the chambers in water equivalent phantom.
- Reading should be corrected for T/P
- three exposures should be given, then swapped, repeated. x2.
Outline the calibration procedure that occurs at the NPL for electrons.
- Define reference depth in water for the specific beam quality.
- Use range scaling to get this depth in graphite.
- Calibrate the chamber against the calorimeter at NPL.
- Theoretically convert from graphite to water.
- Compare the user chamber with the reference chamber.
What are the recommended chambers for electrons?
NE2571 cylindrical graphite farmer or PTW30004 cylindrical graphite farmer for E>9MeV.
NACP or Roos parallel plate.
Describe the procedure for measuring dose for electrons.
- Position appropriate chamber in phantom. FS should be as standard conditions.
- Position chamber so EPOM is at zref.
- Take measurement, accounting for T/P, fion, fpol, electrometer (charge and non-lin).
- Convert to dose using factor.
- Transfer to dmax using depth dose curve.
