Dosimetry in Radiotherapy Flashcards
State and explain the three ways to measure ionising radiation used in radiotherapy.
Ionisation - collect ion-pairs produced in air.
Calorimetry - ionising particle shares its energy with many others and eventually ions recombine. Energy ends up as heat.
Chemical effects - Free radicals produced by ionising particles cause chemical changes.
How does a Free-Air ionisation chamber measure dose?
- Photon beam passes between parallel plates with a high polarising voltage between then
- Voltage must be high enough to separate +ve and -ve ions before they recombine.
- X-Ray produce electrons which cause ionisation
- Mas f air in collecting volume, dm, defined by guard plate and beam geometry
- Ion (not electrons0 are collected and measured.
- Energy to create ion pair is known, so dose can be calculated.
What will happen if the voltage between the plates is not high enough?
The measured charge will be proportional to the voltage.
What beam energies are Free-Air ionisation chambers suitable for?
kV beam qualities. Rance of photo- and Compton electrons from MV energies would require a 4m plate separation to reach electronic equilibrium.
Where is the primary standard kept?
At the NPL
What does a primary standard do?
A primary standard makes and absolute measurement from first principles. All other devices are calibrated to the primary standard through a traceability chain.
What advantage does a thimble chamber have compared to a free-air chamber when measuring MV beam qualities?
Smaller (air shell “compressed” to solid “air-equivalent” graphite) so more practicable.
What are large ionisation chambers used for and why?
Used for environmental monitoring.
Very sensitive, but poor spatial resolution.
What are small ionisation chambers used for and why?
Used for fine resolution scanning.
Good spatial resolution, but small signal.
When would a parallel plate chamber be used?
Parallel plate chambers are thin in the gradient direction for better resolution, but with a large detecting volume for decent signal.
Used for measurements in high dose gradient fields (e.g. electron beams, kV beams, build up regions of MV beams)
Thimble chamber would be too big and would measure contributions from other parts of the high gradient field.
How do calorimeters measure dose?
Dose = Energy/Mass = E/m
E=mc x dT
E/m = specific heat capacity x temperature rise = Dose
Temperature rise measured using very sensitive thermistors.
What advantage do calorimeters have over ionisation chambers?
Directly measure absorbed dose.
What type of calorimeter does the NPL use for their primary standard?
Graphite
Why isn’t a water calorimeter used?
- Specific heat capacity too high (1Gy ~ 0.24mK temperature increase)
- Difficulties with impurities adding heat defects.
How is the dose in the graphite calorimeter converted into a dose in water?
Uses ratio of electron densities (Photon fluence scaling theorem)
What temperature rise is caused by a dose of 1Gy in a graphite calorimeter?
1.5mK
Describe one modern way of measuring the temperature rise in a graphite calorimeter.
Core temperature kept constant by reducing current in a heating circuit.
D=VIt/M
What is a secondary standard?
High-quality dosemeter owned by hospitals that is sent to be calibrated against the NPL primary standard
NPL provide correction factor to convert dose readings made with secondary standard to accurate dose measurements
All other field instruments in the hospital are cross-calibrated against this
How is a field instrument cross-calibrated?
- Field instrument and secondary standard placed at 10cm in same perspex phantom
- Both irradiated with the same beam (should receive the same dose)
- Ratio of reading from field instruments and secondary standard taken
- Repeated 12 times with position of the chambers swapped after each three readings
What is the quality dependent factor?
Due to chamber construction, response varies with beam energy (or quality). The secondary standard chamber is calibrated against the NPL chamber at a number of different beam qualities and a calibration curve is produced
What is the quality index of a beam?
Same as the TPR (20/10) (i.e. Tissue Phantom Ratio a depths of 20cm and 10cm)
What other correction factors are applied to calibrations of chambers?
k(tp) - correction for temperature and pressure (1013.25/P x T/293.15)
k(ion) - correction for ion recombination
k(elec) - correction for electometer reading
k(lin) - correction for linearity wrt dose
How is k(ion) quantified?
Measure charge collected at different polarising voltages as recombination increases at low voltages
How do TLDs record the absorbed dose?
- TLDs contain lattice impurities which give additional energy levels to the electrons in the lattice
- These energy levels are meta-stable so act as traps for the electrons
- When irradiated the electrons jump from the valence to the conduction band and then some fall into the traps
- Heating the material releases the electrons from the traps where they initially move to the conduction band before falling into the valence band and emit light
- The light recorded from the TLD is proportional to the absorbed dose
