Topic 9: radiation dosimetry Flashcards
how much radiation comes from medical in the UK
12%
what are the dominant interactions between XR and gamma ray photons and tissue
- photoelectric absorption
- Compton scattering
4 possibilities of effects of radiation on DNA
- no damage
- fully repaired damage to one DNA strand
- faulty repair of one DNA strand
- beyond repair - cell death
3 types of radiation effects
- somatic
- genetic
- teratogenic
stochastic effects of radiation
- risk of effect occuring increases with dose but severity doesnt
- effects may take years to show
- involves DNA mutation but not cell death
deterministic effects of radiation
- predictable
- occur only above threshold dose which is high
- severity increases with dose above threshold
- effects usually occur quickly
- often involves cell death
radiation teratogenesis
effects on embryo or fetus in utero
younger people are more at risk to radiation because
- their cells are dividing faster
- they will live longer giving more time for cancer to appear
what is dosimetry
measurement of dose caused by radiation deposited in a medium
importance of dosimetry
- monitoring of exposures
- identification of variation
- legislation
- protection of pts and staff
X
exposure
K
kerma
D
absorbd dose
Ht
equivalent dose
E
effective dose
what measure is intensity
measure of the number of photons in the beam
what does air ionisation chamber measure
- measures exposure (X) in C/Kg and can also measure absorbed dose in air
KERMA
- kinetic
- energy
- released
- mass of
- absorber
KERMA unit
1K = 1 Joule/ Kg
absorbed dose
- measure of average energy absorbed from the radiation beam into medium
what is the stage 1 process of KERMA and absorbed dose
ionising photons transfer some of their energy to particles within mass and cause ionisation and charges particles
(KERMA, K)
what is stage 2 of the process of kerma and ABSORBED DOSE
the charged particles deposit their energy within the mass (absorbed dose)
where does the incoming energy from X-ray or gamma ray photons go?
- part is used to create ion pairs
- part is transfer to kinetic energy of the electron
what does WR (weighting factor) account for
relative damaging effects of different types of radiation
what is effective dose
sum of equivalent dose for all organs/ tissues weighed by a detriment-related tissue weighting factor
what does the ICRP publication define tissue weighting factor to take into account
how radiosensitive each organ is
important properties of radiation dosimeters
- linear response
- wide dynamic range
- high sensitivity
- high specificity
- accuracy
- isotropy (not affected by direction of incoming radiation)
- fast response
- low background noise
- versatile
- reusable
- energy-dependent response
the two type of dose-meters
- absolute
- practical
features of absolute dose meters
- very specialised equipment and techniques
- kept at the national physical laboratory (NPL)
- used under carefully controlled conditions
features of practical dose-meters
- smaller, more compact
- used in hospitals/ universities
- calibrated regularly against absolute dose-meter
- sensitive to external factors, e.g. pressure
types of dose meters
- air-filled ionisation chambers
- solid state detectors
- radiographic film
what do air-filled ionisation chambers do
- measure charge created in a known volume of air
what do air-filled ionisation chambers measure
measure exposure (X) which infers D and K
examples of applications of air ionisation chambers
- as detectors in automatic exposure devices
- measure exposure (X) accurately
- measure xr output and perform QA test on XR equipment
how much energy is needed to create ion pair in air
34 eV
what kind of process is creating an ion pair in an pair in air ionisation chambers
enclosed volume of air
applications of semiconductor detectors
- field dosimetry
- quality control
- personnel monitoring
what are semiconductors used for
used to monitor staff exposure in interventional procedures
advantages of semiconductor detectors
- wide dynamic range
- high sensitivity
- small footprint allow excellent spatial resolution
- fast response
- versatile
disadvantages of semi conductor detectors
- energy dependent
- expensive
- sensitive to environmental conditions (temperature)
what are TLDs
- thermoluminescent dosimeters
- passive dosimeters
how do TLDs work?
- some crystals absorb radiation energy when exposed to radiation and absorb it for long periods of time
- this energy stored is released by heating the crystal
- amount of light emitted is proportional to absorbed dose
- calibration of TLDs allows absorbed dose to be determined
advantages of TLDs
- wide dynamic range
- high sensitivity
- reliable
- small in size
- reusable
- versatile
- no saturation
- clean and quick processing
disadvantages of TLDs
- passive
- storage instability
- fading
- light sensitivity
- cant identify type of radiation
- requires dedicated equipment
what does radiographic film contain
contains an emulsion that absorbs radiation energy and stores it permanently
- chemical processing reveals this energy stored as optical density
advantages of radiographic film
- permanent record
- can determine quality of radiation with filters
- identify gross non- uniformities
- visual inspection
disadvantages of radiographic film
- limited dynamic range
- non-linear response
- energy-dependent
- wet chemical processing
- fogging
- doses unreliable in cases of gross overexposure
the main purpose of a personal dosimetry programme
- assess whether or not the exposure received by staff exceeds the dose limits
- assess the effectiveness of strategies being used for dose optimisation
challenges in measuring dose
- measuring absorbed dose in tissues is not feasible
- effective dose is an abstract quantity and for a population and should not be used to describe individual experience
Hp (10)
effective dose ( whole body )
Hp (3)
dose to lens of the eye
Hp (0.07)
dose to the skin
what is Hp (d)
- quantity defined in the body
- also cannot be measured directly
- vary from person to person
- vary according to location on the body where it is measured
what is absorbed dose measured in
grays
what is equivalent dose measured in
sieverts
what is equivalent dose
the sum of absorbed doses from different types of radiation to a particular organ or tissue multiplied by respective radiation weighting factors
what is effective dose
sum of equivalent doses to tissues/ organs irradiated X their respective tissue Weighting factors
