2003 Electron Code of Practice

Question 1

How are electron beams specified?

Answer 1

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).

Question 2

What depth are electron beams calibrated at?

Answer 2

zref = 0.6R50,d - 0.1 cm in water

Question 3

What are the designated chambers?

Answer 3

Farmer cylindrical chambers, NACP and Roos parallel plate chambers.

Question 4

What is the electron primary standard?

Answer 4

Graphite calorimeter, yields a direct calibration of an ion chamber in terms of absorbed dose to water

Question 5

Key steps to NPL calibration

Answer 5

1. Reference ion chambers compared to graphite calorimeter at z ref
2. Conversion of calibration factor from graphite to water
3. User chambers compared with reference ion chambers in water at zref

Question 6

What is the uncertainty in the NPL calibration?

Answer 6

1.5% at the 95% CI

Question 7

What chamber is not recommended for absolute electron dosimetry and why?

Answer 7

The Markus chamber because in the trial of different chambers, the behaviour could vary significantly (significant perturbation effects)

Question 8

What is the caveat to using cylindrical chambers in electron dosimetry?

Answer 8

Should only be used where the mean energy at the ref depth is > 5 MeV

Question 9

What is the recommended frequency of secondary standard calibration?

Answer 9

2 yearly as parallel plate chambers are less stable than cylindrical

Question 10

What is the effective point of measurement?

Answer 10

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.

Question 11

Where is Peff taken to be for PP chamber?

Answer 11

At the centre of the inside surface of the chamber front window

Question 12

Where is Peff taken to be for cylindrical chambers?

Answer 12

0.6r towards the radiation source from the centre of the chamber

Question 13

What are the 4 main corrections to raw reading?

Answer 13

1. T and P
2. ion recombination
3. polarity
4. electrometer

Question 14

What are the requirements for the measuring medium in electron dosimetry?

Answer 14

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.

Question 15

How can you get R50 from a PDI curve as opposed to a PDD curve?

Answer 15

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)

Question 16

Why was zref chosen to be what it is?

Answer 16

This set of depths gives better consistency between stopping power ratios, therefore increasing accuracy in the calibration factor.

Question 17

Where is z ref in relation to dmax ?

Answer 17

Low energy beams E0 <9.5 MeV : roughly same

High energy beams E0 >9.5 MeV: zref deeper than dmax.

Question 18

What type of chamber is considered the most accurate for depth dose curves?

Answer 18

Parallel plate

Question 19

How do you convert PDI to PDD?

Answer 19

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.

Question 20

Apart from ion chambers, what other detectors can be used to measure PDD?

Answer 20

Diamond detectors and p-type diodes. These should be compared against ion chambers.

Question 21

Why can’t you use a photon diode for electron measurements?

Answer 21

Photon diodes have a high Z material added close to the sensitive volume to improve relative photon energy sensitivity.

Question 22

What is the best way to minimise meniscus effects in a scanning water tank?

Answer 22

Move chamber from deep to shallow

Question 23

What is the recommended SSD for electron PDDs?

Answer 23

The SSD that is used clinically.

Question 24

What is the equation for the recommended thickness/depth of the phantom for absolute dose?

Answer 24

Thickness = 5 + 1.25 R50,d cm

at least 5 cm

Question 25

Set-up for absolute dose?

Answer 25

Usual conditions of clinically used SSD (around 100 cm), field size of at least 10x10, Peff at z ref.

Question 26

Equation for absorbed dose?

Answer 26

Dw ( zref, w) = Mch Nd,w (R50, d) Gy

where,

Mch = Mraw fTP fpol fion felec

Question 27

Define felec

Answer 27

The factor to convert electrometer reading to absolute charge

Question 28

What are output factors?

Answer 28

OF = dose at dmax in non reference field/ dose at dmax in reference field

Question 29

What is the key equation for getting the calibration factor for a field instrument in intercomparisons?

Answer 29

Nd,w,field = Mref/Mfield *Ndw, ref

Question 30

What is the method for intercomparison in the CoP?

Answer 30

“repeated relative measurements with each chamber (calibrated designated chamber and field) successively placed with its Peff at the reference point”

Question 31

Equation for fpol

Answer 31

fpol = |M+| + |M-|/ 2M where M is the reading when the usual polarity is applied.

Question 32

When is the two voltage technique for fion valid?

Answer 32

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.