Dosimetry

Question 1

How is field width defined?

Answer 1

From a profile, the field width is the FWHM; the lateral distance between the profile lines at the height of the 50% dose.

Question 2

What are the four different sources of dose to the patient during treatment?

Answer 2

Primary beam energy deposition
Scatter from the patient (S_p)
Head/collimator scatter (S_c)
Contaminant charged particle energy deposition

Question 3

How are head scatter measurements made?

Answer 3

Using a columnar mini-phantom and chamber with enough build-up material to absorb all of the contaminating electrons. Measurements made using various field sizes, and the head scatter value is 1.000 for a 10cmx10cm field. The values for other field sizes are worked out accordingly from this.

Question 4

Are scatter contributions considered in the TPS? If so, how?

Answer 4

Yes.

In MU calculations, S_c,p forms part of the denominator.

Question 5

If a linac output at morning run-up had an output of 0.75cGy/MU, what would you do? What might have caused this?

Answer 5

This is outside the 2% tolerance limit but within 3% action limit.

• Check set-up of equipment and re-do measurement to check it’s true.
• See if there had been a trend over the last few days/weeks on this machine.
• Tell an MPE and get them to sign.
• Ensure machine is fixed ASAP but it is still okay for clinical use with the MPE’s agreement.
• If outside 3% then it must immediately be taken out of use.

Question 6

What might be the consequences of an out-of tolerance linac output that remains in use?

Answer 6

• Over- or under- dosing the patient.

- Affects OARs and the tumour - therefore possibly the therapeutic index.

Question 7

Which chamber would you use for electron calibration?

Answer 7

Parallel plate

Question 8

What chamber(s) or measurement devices would you use for small field dosimetry?

Answer 8

Diamond
Diode
Pin-point
Film (radiochromic)
Liquid ionisation chambers
Micro-ionisation chambers

Question 9

What chamber(s) would you use for plan verification?

Answer 9

Farmer chamber or CC13 chamber for VMAT or SABR/SRS etc plan verification, in a phantom e.g. ArcCheck

Question 10

When might diodes be used?

Answer 10

For small field dosimetry

For in-vivo dosimetry e.g with paediatric patients or TBI

Question 11

Other than diodes, what dosimeters might be used for in vivo dosimetry? Why?

Answer 11

TLDs - small, nearly tissue-equivalent, cheap, multiple exposure
Film - thin, can be cut, linear response, can buy film with a large dose-range
EPID - immediate read-out, built-in to linac, easy for gamma analysis

Question 12

Under what conditions can a field be considered “small”?

Answer 12

• Loss of lateral CPE on the beam axis
• Partial occlusion of the primary photon source by the collimating devices on the beam axis
• Size of detector similar to or large compared to beam dimensions

Question 13

What is volume averaging? What causes it?

Answer 13

Detectors average the dose over the volume. The signal produced by a detector is affected by the homogeneity of the absorbed dose over the detector volume. If the dose varies over the volume it gives a different signal to that an infintesimally small detector would.

Question 14

What is the setup for a TPR measurement for photons?

Answer 14

10cmx10cm field. 100cm SCD, full scatter water phantom

Output measured at 20cm and 10cm deep and the ratio taken between the two to get the beam quality.

Question 15

What is a cross-calibration? How do these measurements fit into the calibration chain?

Answer 15

Field chambers are cross-calibrated against secondary standard chambers to calculate the calibration factor (N_D,w) for that field chamber.

The SS (and associated electrometer) was calibrated at the NPL and the calibration coefficient given for this chamber. Therefore the field instrument is traceable to NPL.

Question 16

What considerations need to be taken into account for cross calibrations?

Answer 16

• The phantom; material, size etc
• Temperature/pressure corrections
• Ion recombination, polarity correction, leakage
• Electrometer factors
• Interpolation/extrapolation for energies where N_D,w not defined
• Interpolation/extrapolation for correct values of z_ref and R_50,D (electrons)
• Asymmetry, setup etc.

Question 17

For cross calibrations, the geometric mean is used. Why? Why not the arithmetic mean?

Answer 17

It is used because no bias is introduced to the intercomparison.
The arithmetic mean gives a case where the estimate of the ratio of doses in the two positions to each chamber is not unity.

Question 18

What is the quantity h_m and why is it used?

Answer 18

h_m is the fluence correction factor. It takes into account the difference in fluence between a water phantom and, for example, solid water phantom. It converts the electron fluence in the plastic phantom to the fluence at an equivalent depth in water.

Question 19

When taking measurements with chambers and electrometers, certain corrections have to be applied. What are these, what do they mean? How are they calculated?

Answer 19

Temperature and pressure: T/T0 x P0/P
Ion recombination: 2 Voltage technique
Polarity: Measure with negative and positive polarity, add absolute values, divide by 2M
Electrometer calibration factor: from NPL
Leakage: checked with Sr-90, electrometer nulled

Question 20

What type of dosimeter/chamber would you use for very low, low and medium energy kV measurements?

Answer 20

v low: parallel plate

low: Thimble
medium: Thimble

Question 21

At what reference depth would you measure the absorbed dose for very low, low and medium energy x-rays?

Answer 21

v low: Front face of chamber at surface of full-scatter phantom

low: phantom surface at the same level as the chamber centre
medium: 2cm in a full scatter water phantom OR as low

Question 22

What is the effective point of measurement (EPOM)? Why is it used?

Answer 22

The point in a chamber where the measured dose would arise in the measurement medium in the absence of the chamber.
Correction for the displacement effect. P_eff is shifted towards the radiation source.

Question 23

What is the EPOM for a parallel plate and thimble chamber?

Answer 23

For a parallel plate - the centre of the inside face of the front wall.
Thimble: radius r, shift is 0.6r.

Question 24

What does the volume averaging effect lead to for the dose at the centre of the field and the penumbra?

Answer 24

• The dose in the centre of a small field is underestimated

- The penumbra is “washed out”.

Question 25

What is the difference between a PDI and a PDD for electrons?

Answer 25

PDI = percentage depth ionisation
This is measured directly as electrons entering the medium release more electrons which are detected by the chamber and provide a current reading. This is not dose.
PDD = Percentage depth dose
PDDs are the dose which are calculated from the PDI by multiplying by the appropriate SPR.

Question 26

What does the volume averaging effect lead to for the dose at the centre of the field and the penumbra?

Answer 26

• The dose in the centre of a small field is underestimated

- The penumbra is “washed out”.

Question 27

What are the beam quality specifiers for photon and electron beams?

Answer 27

Photon: TPR20,10
Electron: R_50,D