Radiation Risk and Dose Calculation Flashcards
Describe the ICRU Sphere.
The ICRU sphere is 0.3 m in diameter with a density of 1000 kg m−3 and a mass composition equivalent to tissue of 76.2% oxygen, 11.1% carbon, 10.1% hydrogen and 2.6% nitrogen. The field is ‘expanded’ so that it encompasses the sphere and ‘aligned’ so the quantity is independent of the angular distribution of the radiation field. In effect this defines an instrument with a uniform isotropic response.
What is the Ambient Dose Equivalent (H*(10))?
This is the absorbed dose which would be generated in the associated oriented and expanded radiation field at a depth of 10 mm on the radius of the ICRU sphere which is oriented opposite to the direction of incident radiation.
An oriented and expanded radiation field is an idealised radiation field which is expanded and in which the radiation is additionally oriented in one direction.
When would H’(0.07) be used?
For low energy (i.e. Mammography) X-Rays.
How are H*(10) and H’(0.07) converted into Personal Dose equivalences (Hp(10), Hp(0.07))?
Corrected for anticipated backscatter.
How is the Effective Dose Equivalent Defined?
The average absorbed dose to each organ multiplied by its radio-sensitivity.
How is absorbed dose converted into equivalent dose?
Multiplied by radiation weighting factor.
What are the radiation weighting factors according to ICRP-103?
Photons and Electrons (All Energies) - 1
Neutrons (All energies) - Continuous function bewteen 2 and 20.
Protons (>20MeV) - 2
Alpha particles, fission fragments, heay nulcei - 20
What are the radiation weighting factors according to ICRP-60?
Photons and Electrons (All Energies) - 1
Neutrons (20MeV) - 5
Protons (>20MeV) - 5
Alpha particles, fission fragments, heay nulcei - 20
How do stochastic effects change with dose?
Probability of effect is proportional to dose, no threshold.
Evidence from Hiroshima/Nagasaki/Chernobyl survivors, early radiation workers, radium dial painters, Uranium miners.
How do deterministic effects change with dose?
No effect below threshold dose.
Severity of effect increases with dose above threshold.
In ICRP-103, which tissues/organs have a Tissue Weighting Factor of 0.12?
Red bone marrow Colon Lung Stomach Breast Remainder of organs not mentioned. (adrenals, small intestine, kidney, muscle, pancreas, spleen, thymus, uterus, extrathoracic region, gall bladder, heart, lymph nodes, oral mucosa, prostate, cervix)
In ICRP-103, which tissues/organs have a Tissue Weighting Factor of 0.04?
Bladder
Liver
Oesophagus
Thyroid
In ICRP-103, which tissues/organs have a Tissue Weighting Factor of 0.08?
Gonads
In ICRP-103, which tissues/organs have a Tissue Weighting Factor of 0.01?
Skin
Bone surface
Brain
Salivary glands
What are the four components that the Tissue weighting factor depends on?
Probability of fatal cancer.
Probability of severe genetic effects
Relative length of life lost
Weighted probability of non-fatal cancer.
What is the average annual radiation dose to a person in the UK from all sources?
2.7mSv
What percentage of the annual dose arises from medical exposures?
14%
What percentage of the annual dose arises from Radon Gas?
50%
What percentage of the annual dose arises from food and drink and other naturally occurring radiation in the body?
10%
What is the risk of life-time fatal cancer for the adult population?
5%/Sv
What is the risk of lifetime fatal cancer for paediatric examinations?
6%/Sv
What is the risk of lifetime fatal cancer for newborns?
15%/Sv
What is the threshold lethal dose for of an embryo/foetus?
0.1-1Gy
What is the threshold dose for gross malformations in an embryo/foetus?
- 2Gy at 2-5 weeks
0. 5Gy at 5-7 weeks
What is the threshold dose for abnormal brain development in an embryo/foetus?
No threshold at 8-15 weeks
0.6-0.7Gy at 16-25 weeks
What are the advantages of measuring patient surface dose?
Easy to measure (TLD on patient)
Easy to calculate from radiographic factors (Tube output, and SSD)
What are the disadvantages of measuring patient surface dose?
No indication of volume irradiated
Non-additive if beam position changes
How can patient surface dose be calculated?
Surface Dose = Tube output x mAs x inverse square law correction x backscatter factor
What are the advantages of patient organ dose measurement?
Can be calculated from surface dose if organ position is known.
Table of normalised organ dose data available
What are the disadvantages of patient organ dose measurement?
Difficult to measure directly
Need to know attenuation of beam at organ depth (PDD)
What assumptions are made when using DAP as a patient dose measurement?
All energy is absorbed by patient
DAP-meter is larger than field
Define the CTDI?
CTDI = 1/nT * INT(D(z)dz) n=number of simultaneous slices T=slice thickness D(z)=dose profile along z-axis Usually integrated between -7T and +7T
Define the CTDI-100?
CTDI corrected by L/(nT) where L is the chamber length (100mm), n is the number of simultaneous slices, T is the slice width
Define the CTDI-w?
CTDI-w = 1/3 CTDI-100, centre + 2/3 CTDI-100, periphery
Define the CTDI-vol?
CTDI-w corrected for pitch (couch increment) and mAs
Define the Dose-Length Product?
CTDI-vol x irradiated length
