Dosimetry Flashcards
(25 cards)
Define dosimetry
Measurement of ionising radiation in a given place or in the body
Define exposure and how it is measured
Quantity used to indicate amount of ionisation in air produced by x-rays
Measured by C/kg
Define absorbed dose and how it is measured
Energy absorbed per unit of mass of material
Measured by Gy
Define KERMA and how it is measured
Sum of initial kinetic energies of all charged particles liberated by ionising radiation divided by the mass of the sample.
Measured by Gy
Describe the relationship between KERMA and absorbed dose related to high energy photons
Many highly energetic secondary electrons and X-rays escape region of interest before depositing energy.
KERMA is higher than absorbed dose
Describe the relationship between KERMA and absorbed dose related to low energy photons
The scattered radiation does not travel significant distances, so nearly all the energy is absorbed in the same location.
KERMA and absorbed dose similarly
What is the electronic equilibrium
Energy lost by secondary scattered radiation away +
Energy gained by secondary scattered radiation into point
Define equivalent dose and how it is measured
Absorbed dose x radiation weighting factor
Measured in Sv
Define effective dose and how it is measured
Equivalent dose x tissue weighting factor
Measured in Sv
What do ALARA and ALARP stand for?
ALARA - as low as reasonably achievable
ALARP - as low as reasonably practicable
Describe time with reference to patient care
Patient should be irradiated for as little time as possible
Describe shielding with reference to patient care
Protective viewing window
If in surgery, lead aprons
Patient shielding, eg gonad shields
Describe distance with reference to patient care
Create as much distance as possible with the machine
How do we assess radiation risks for patients?
Dose area product (DAP)
Diagnostic reference levels
Define dose area product and how it is measured
Absorbed dose x area irradiated
Measured by Gycm2
Describe diagnostic reference levels
Ensure dose is optimised due to different sized patients
What impact do DRLs have?
After 20 years, dose reduction was ~10-20%
How are DRLs measured?
Entrance Surface Dose - mGy
Dose area product - Gy cm2
Describe the structure of TLDs and how they work
Lithium Fluoride crystals
Chips, pellets, microrods ~5mm
Absorbs radiation, electrons become excited, then lose energy but some become trapped
How are TLDs read?
Heated to release electrons
Energy released as light pulses
Photoelectric sensitive plate and photomultiplier tube records light pulse
Number of pulses proportional to dose
How are TLDs cleared for reuse?
Heated to 400 degrees for 1.5 hours, 80 degrees for 16 hours
Sensitivity depends on temperature cycle
Describe ion chambers
Used for dosimetric calibration of radiotherapy equipment
Used with a water tank to measure doses
Describe the process of ion chambers
300 to 500 volts applied
Temp rises, air expands, conc of molecules decreases
If atmosphere pressure rises, air compressed, conc of molecules increases
Describe diodes
Silicon doped with impurities
N-type has excess electrons
P-type electron deficient
Thin layers of each placed in contact
