Dosimetry Flashcards
How is field width defined?
From a profile, the field width is the FWHM; the lateral distance between the profile lines at the height of the 50% dose.
What are the four different sources of dose to the patient during treatment?
Primary beam energy deposition
Scatter from the patient (S_p)
Head/collimator scatter (S_c)
Contaminant charged particle energy deposition
How are head scatter measurements made?
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.
Are scatter contributions considered in the TPS? If so, how?
Yes.
In MU calculations, S_c,p forms part of the denominator.
If a linac output at morning run-up had an output of 0.75cGy/MU, what would you do? What might have caused this?
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.
What might be the consequences of an out-of tolerance linac output that remains in use?
- Over- or under- dosing the patient.
- Affects OARs and the tumour - therefore possibly the therapeutic index.
Which chamber would you use for electron calibration?
Parallel plate
What chamber(s) or measurement devices would you use for small field dosimetry?
Diamond Diode Pin-point Film (radiochromic) Liquid ionisation chambers Micro-ionisation chambers
What chamber(s) would you use for plan verification?
Farmer chamber or CC13 chamber for VMAT or SABR/SRS etc plan verification, in a phantom e.g. ArcCheck
When might diodes be used?
For small field dosimetry
For in-vivo dosimetry e.g with paediatric patients or TBI
Other than diodes, what dosimeters might be used for in vivo dosimetry? Why?
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
Under what conditions can a field be considered “small”?
- 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
What is volume averaging? What causes it?
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.
What is the setup for a TPR measurement for photons?
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.
What is a cross-calibration? How do these measurements fit into the calibration chain?
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.
What considerations need to be taken into account for cross calibrations?
- 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.
For cross calibrations, the geometric mean is used. Why? Why not the arithmetic mean?
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.
What is the quantity h_m and why is it used?
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.
When taking measurements with chambers and electrometers, certain corrections have to be applied. What are these, what do they mean? How are they calculated?
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
What type of dosimeter/chamber would you use for very low, low and medium energy kV measurements?
v low: parallel plate
low: Thimble
medium: Thimble
At what reference depth would you measure the absorbed dose for very low, low and medium energy x-rays?
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
What is the effective point of measurement (EPOM)? Why is it used?
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.
What is the EPOM for a parallel plate and thimble chamber?
For a parallel plate - the centre of the inside face of the front wall.
Thimble: radius r, shift is 0.6r.
What does the volume averaging effect lead to for the dose at the centre of the field and the penumbra?
- The dose in the centre of a small field is underestimated
- The penumbra is “washed out”.
