2003 Electron Code of Practice Flashcards
How are electron beams specified?
R50,d: the depth in water along the beam CAX at which the dose is 50% of the maximum, measured at or close to 100 SSD and in a field size that is large enough to ensure that the measured value is independent of field size (at least 10x10).
What depth are electron beams calibrated at?
zref = 0.6R50,d - 0.1 cm in water
What are the designated chambers?
Farmer cylindrical chambers, NACP and Roos parallel plate chambers.
What is the electron primary standard?
Graphite calorimeter, yields a direct calibration of an ion chamber in terms of absorbed dose to water
Key steps to NPL calibration
- Reference ion chambers compared to graphite calorimeter at z ref
- Conversion of calibration factor from graphite to water
- User chambers compared with reference ion chambers in water at zref
What is the uncertainty in the NPL calibration?
1.5% at the 95% CI
What chamber is not recommended for absolute electron dosimetry and why?
The Markus chamber because in the trial of different chambers, the behaviour could vary significantly (significant perturbation effects)
What is the caveat to using cylindrical chambers in electron dosimetry?
Should only be used where the mean energy at the ref depth is > 5 MeV
What is the recommended frequency of secondary standard calibration?
2 yearly as parallel plate chambers are less stable than cylindrical
What is the effective point of measurement?
The depth in a medium where the average energy is is the same as in the chamber. Peff is closer to the radiation source than the centre of the cavity. The Peff must be placed at the depth where dose is required. Calibration factor used for NPL calibration dependent on Peff being at zref - important that the same depth used for intercomparison.
Where is Peff taken to be for PP chamber?
At the centre of the inside surface of the chamber front window
Where is Peff taken to be for cylindrical chambers?
0.6r towards the radiation source from the centre of the chamber
What are the 4 main corrections to raw reading?
- T and P
- ion recombination
- polarity
- electrometer
What are the requirements for the measuring medium in electron dosimetry?
Water, large enough laterally to account for scatter effects (5 cm either side recommended), 5 cm beyond the greatest depth of measurement also recommended (to determine behaviour of Bremstahhlung tail), chamber Peff at desired depth for measurement.
How can you get R50 from a PDI curve as opposed to a PDD curve?
R50,d = 1.029 R50, I - 0.063 cm
R50, d and R50, I are indistinguishable in low energy beams (E0 = 9.5 MeV where E0 is the mean energy at the surface)
Why was zref chosen to be what it is?
This set of depths gives better consistency between stopping power ratios, therefore increasing accuracy in the calibration factor.
Where is z ref in relation to dmax ?
Low energy beams E0 <9.5 MeV : roughly same
High energy beams E0 >9.5 MeV: zref deeper than dmax.
What type of chamber is considered the most accurate for depth dose curves?
Parallel plate
How do you convert PDI to PDD?
Multiple each PDI value by stopping power ratio water to air specific to the beam quality and water depth. Peturbation factors do not need to be considered if PP chamber used.
Apart from ion chambers, what other detectors can be used to measure PDD?
Diamond detectors and p-type diodes. These should be compared against ion chambers.
Why can’t you use a photon diode for electron measurements?
Photon diodes have a high Z material added close to the sensitive volume to improve relative photon energy sensitivity.
What is the best way to minimise meniscus effects in a scanning water tank?
Move chamber from deep to shallow
What is the recommended SSD for electron PDDs?
The SSD that is used clinically.
What is the equation for the recommended thickness/depth of the phantom for absolute dose?
Thickness = 5 + 1.25 R50,d cm
at least 5 cm
Set-up for absolute dose?
Usual conditions of clinically used SSD (around 100 cm), field size of at least 10x10, Peff at z ref.
Equation for absorbed dose?
Dw ( zref, w) = Mch Nd,w (R50, d) Gy
where,
Mch = Mraw fTP fpol fion felec
Define felec
The factor to convert electrometer reading to absolute charge
What are output factors?
OF = dose at dmax in non reference field/ dose at dmax in reference field
What is the key equation for getting the calibration factor for a field instrument in intercomparisons?
Nd,w,field = Mref/Mfield *Ndw, ref
What is the method for intercomparison in the CoP?
“repeated relative measurements with each chamber (calibrated designated chamber and field) successively placed with its Peff at the reference point”
Equation for fpol
fpol = |M+| + |M-|/ 2M where M is the reading when the usual polarity is applied.
When is the two voltage technique for fion valid?
Only if there is a linear relationship between 1/M and 1/V. This is not the case for a number of PP chambers hence NPL calibration service provides a full characterisation of fion for each chamber calibrated.
