Dosimetry Flashcards
What is the IC Profiler? And why is it used for monthly checks?
The ICP profiler is a 2-D array of 251 ION CHAMBER detectors
Allows for simultaneous measurement on 4 linear arrays: X, Y and two diagonals
In a single, multi-purpose measurement you get constancy checks for output and beam quality – plus flatness, symmetry, field size and penumbra width.
Why must the IC profiler be calibrated before use?
An array calibration must be carried out to eliminate the difference in response between detectors and indicated by the IC Profilers instruction manual should be as close to measurement set up as possible
How is Photon Flatness measured by the IC Profiler?
Flatness calculation by VARIANCE:
A method of calculating flatness based on the average between the maximum value of a chamber in the field region (80% of field size) and the minimum value of a chamber in the field region.
How is Photon Symmetry measured by the IC Profiler?
Symmetry calculation by AREA AVERAGE:
A method of calculating symmetry based on the area used by Siemens.
Approx: SYM = [(Dose_Mx - Dose_Mn)/(Dose_Mx + Dose_Mn)]*100
If a beam profile was out of tolerance, what could be done to correct it?
There are annual QC sessions on each machine where we use the water tank to do more thorough flatness and symmetry measurements (can get different field sizes, measurement depths, scan with higher resolution, measurements are in water).
If these measurements confirm the beam is unacceptable it could be adjusted and re steered to bring it back in line with machine performance during commissioning and to better match the data used in the TPS to create the beam model.
How is Electron Flatness measured by the IC Profiler?
Flatness calculation by RATIO:
It determines the flatness of the radiation profile, expressed as a ± percent value and calculated over a portion of the field size.
The relevant field sizes for this calculation are:
The distance from 50% left of CAX to 50% right CAX
The Ratio (IEC) flatness calculation is:
+/- [(max value of chamber in region)/(min value of chamber in region)]*100
How is field size of the beam profile defined in the IC Profiler?
The field size is the width in cm @ 50% of the max intensity
How is the Penumbra region of the field defined in the IC Profiler?
The penumbra region is the region encompassing 20% to 80% intensity
How is Electron Symmetry measured by the IC Profiler?
Symmetry calculation by Point Ratio:
Point Ratio (IEC) symmetry is defined by IEC Standard 976. It finds the dose ratio of all symmetric detectors in a portion of the specified field size, always using the larger of the two numbers as the numerator. The maximum value in this series is the IEC number. Electron Symmetry: FS [ 90% CAX ] – 2 cm
The Point Ratio (IEC) symmetry calculation is:
SYM = (D_MX/D_MN) *100
Define tolerance and action levels, and how are they set?
Tolerance level: performance to within the tolerance level gives acceptable accuracy in any
situation.
IPEM 81: The tolerance level normally equates roughly with a 1 SD requirement on the particular equipment or process under consideration.
Action level: performance outside the action level is unacceptable and demands action to
remedy the situation.
IPEM 81: Action levels are often set at approximately twice the corresponding tolerance level.
NB:
The QC measurement is expected to give the best estimate of the particular measured parameter. However, this will have an associated uncertainty, dependent upon the measurement technique. The tolerance set for the parameter must take into account the uncertainty of the measurement technique employed.
Tolerances should be set with the aim of achieving the overall uncertainties desired. Variances can be combined in quadrature for combined factors and this can be used to determine specific tolerance limits for individual parameters.
How are baseline values set in radiotherapy?
This is done through COMISSIONING:
Following acceptance of equipment, a full characterisation of its performance for clinical use over the whole range of possible operation should be undertaken. This is referred to as commissioning.
Depending on the type of equipment, acceptance and commissioning may partially overlap. Together they will establish the baseline-recorded standards of performance to which all future performance and QC tests will be referred.
What correction factors are applied to the mean instrument reading to convert this into an output for photon beams? (5)
The mean instrument reading is divided by the MU and is taken as the product with various correction factors to give the output in cGy/MU. These are:
- Absorbed dose to water calibration factor for the field instrument; (N_{D,f})
- Ion Recombination correction factor; (p_{ion})
- Phantom Correction Factor; (PCF)
- Temperature and pressure correction factor; (TPCF), and
- Wedge Factor; (WF)
What correction factors are applied to the mean instrument reading to convert this into an output for electron beams? (7)
The mean instrument reading is divided by the MU and is taken as the product with various correction factors to give the output in cGy/MU. These are:
- Absorbed dose to water calibration factor for the secondary standard chamber; (N_{D,w})
- Ion Recombination correction factor; (p_{ion})
- Polarity Correction Factor; (f_{pol})
- Fluence Correction Factor; (h_{m})
- Uncorrected Intercomparison ratio (secondary standard: field instrument: M_{uncorr})
- Percentage depth dose at the reference point; (PDD), and
- Temperature and pressure correction factor; (TPCF)
Why are temperature and pressure correction factor used while calculating dose using ion chambers?
The ion chambers used in measuring the adsorbed dose are generally vented ion chambers. Thus the mass of the air inside the sensitive volume of the chamber varies with respect to the density and volume. As the volume of the chamber remains constant, the density varies with respect to pressure and temperature.
As the pressure increases the density increases(i.e.more air molecules) {Boyel’s law} , hence more interaction occurs. This results in more number of charge collection. The dose measured in this case is overestimated from that measured in standard condition
If the temperature increases the density decreases{Charle’s Law}. In this case, the number of interactions are less and the dose measured is under estimated from the measured dose under standard conditions.
Define the Ion Recombination factor
The ion recombination correction factor, P_{ion}, is defined to account for incomplete collection of charges and it is a function of dose per pulse in a linear accelerator
Define the phantom correction factor
The phantom scatter correction factor (Sp) accounts for the contribution of the radiation scattered in the phantom material to the dose at the point at depth dref on the central axis that in the reference irradiation set-up
Define the wedge factor
As the wedge filter reduces the beam intensity and can result in large errors in delivering dose; the precise wedge transmission factor or wedge factor (WF) needs to be determined and accounted for
Define the polarity correction factor
The polarity correction factors are given as a function of mean electron beam energy at the depth of measurement and the ion recombination corrections are tabulated against the operating dose per pulse from linear accelerators.
Define the Fluence Correction Factor
The electron fluence correction factor is needed in conversion of dose measured in plastic to dose in water.
The electron fluence correction factor, which is the ratio of the electron fluence in the water to that at the scaled depths in a plastic phantom. (AAPM TG-25)
Why use a ROOS chamber for electron measurements?
The Roos electron chamber is used as a reference electron chamber. It is recommended by the IAEA-2 for high precision electron dosimetry in radiation therapy
Plane parallel type ionisation chambers are the recommended dosimeter for reference dosimetry in clinical electron beams
Why do we use Zref as a measurement depth in Electrons?
The depth of calibration for megavoltage
x-ray beams is 10 cm, while for electron beams it is at a reference depth zref.
The recent dosimetry protocols based on in-water calibration by the AAPM (TG-51) have endorsed this approach, and all data are
expressed in terms of R50. The reference depth zref for electron beam calibration in
water is expressed in terms of the R50 as follows:
zref = 0.6 R50 (in cm) − 0.1 cm, where R50 isdefined as the depth at which the electron beam depth dose decreases to 50%
of its maximum value.
What is R50?
R50 isdefined as the depth at which the electron beam depth dose decreases to 50%
of its maximum value.
Where would you find the equation for Zref?
Check the American Association for Physicists in Medicine (AAPM) reports (TG-51: protocol for clinical reference dosimetry of high-energy photon and electron beams (1999)) for guidance
What is the reason for calibrating against a secondary standard?
The use of calibrated radiotherapy dosimeters with ionization chambers, traceable to primary standards directly or through secondary standards, is essential for the accurate evaluation of patient radiation dose delivery in radiotherapy (making sure 1Gy delivered in Dundee is the same as 1Gy anywhere else)
Explain the calibration chain to primary standards
The National Physical Laboratory (NPL) develops and maintains the primary standards for radiation dosimetry for the UK, including those which are for external beam radiotherapy.
The radiotherapy treatment machines have dose traceable to the NPL primary standard graphite calorimeter which ensures accuracy and consistency.
Treatment machines are usually calibrated against a tertiary dosimeter which has been calibrated against a secondary standard ionisation chamber (the NPL designed NE 2611 ionisation chambers) which itself has been calibrated at the NPL through comparison with the graphite calorimeter primary standard
Once calibrated, the accuracy of delivered dose is assessed through independent audit from another centre, or from NPL.
Reference dose audit acts to verify that treatment machines have been calibrated correctly using the relevant UK Code of Practice (CoP) for megavoltage photon (MV) beams, electron (MeV) beams or kilovoltage (kV) beams, as used by all National Health Service (NHS) centres.
Why are some correction factors applied in an inter-comparison measurement, but not others? i.e., we include ion recombination but not temp/pressure.
The ion-recombination correction factor is used to correct the response of an ionization chamber for the lack of complete charge collection, which is due to the recombination of ions exhibiting opposite charges during transit to each electrode. This will vary between the detectors, however:
Temperature and pressure are the same for each chamber as they are being measured at the same time, and so these differences are cancelled out as it is an inter-comparison of response of each chamber under the same conditions
