Radiobiology/Radiation Safety

Question 1

Absorbed Dose (D)

Answer 1

• the energy deposited by ionizing radiation per unit mass. Measured in Gray.
  
  • (will not take into account organ or the type of radiation)
  • Absorbed dose is always > air kerma, because when radiation interacts with tissue/matter there is back-scatter radiation
    
    • Measures deterministic effects.

Question 2

Equivalent dose (H)

Answer 2

• The absorbed dose weighted for the radiation type, which has a radiation weighting factor (W_R) proportional to its Linear Energy Transfer (LET). Measured in Sieverts.
  
  • • N.b. Xrays have W_R of 1, protons have 5, alpha particles have 20
      
      • X-rays/gamma rays have low LET, alpha particles have high LET
        
        • This makes sense as we know alpha particles are bad-ass energy-imparting mofos.
          
          • Absorbed dose of 1 Gy of Xrays = equivalent dose of 1 Sv
          • Absorbed dose of 1Gy of alphas = equivalent dose of 20Sv

Question 3

Effective dose (E)

Answer 3

• the sum of the equivalent doses for each organ, each weighted for the sensitivity of the organ to radiation by a tissue weighting factor, W_T. Measured in Sieverts.
  
  • Effective dose, E
    
    • = the sum of (abdosrbed dose,Dx radiation weighting factor,W_Rx tissue weighting factor,W_T)
    • • Effective dose accounts for non-uniform irradiation of the body, and for the different sensitivity of bits to radiation. Is measured in Sieverts.
    • N.B. you do NOT use the effective dose to calculate for individual patients! Why?
      
      • Effective dose is calculated for phantoms, NOT real people
      • We can’t use monte-carlo software for every patient
      • Effective dose gives a braod indication of the detriment to health from an exposure to ionosing radiation, but will NOT account for:
        
        • Patient weight
        • • Lead shielding used etc.
            
            • i.e. lots of errors, the biggest of which is that it is for a phantom!
    • Measures stochastic effects
  • I.e. the risk to different parts of the body varies depending on how suscpentible (radiosensitive) the organ is to the effects of radiation). Tissue weighting factors account for this.

Question 4

• Somatic vs hereditary

Answer 4

• Somatic = affects the exposed individual
• Hereditary – affects subsequent generations. Damage comes from irradiation of gonads.

Question 5

• Deterministic / harmful tissue reactions

Answer 5

• A somatic effect that increases with severity as you get increased absorbed dose.
• Tends to have a threshold dose under which an effect is not seen, and then the severity increases after you exceed this dose. (i.e. non-random)
• Are rare – usually confied to high dose, high time fluoroscopy (e.g. screening times of 2-3 hours and/or long acquisition runs, tube current > 10mA)
• Examples: radiation burns, cataracts, blood vessel damage

Note how it seems to have a cutoff of 2 gray before we observe tissue reactions. That means that the practical threshold dose for use in diagnostic radiology is 2gray, at which point you may see erythema of the skin (deterministic)

Question 6

Stochastic / cancer and heritable effects

Answer 6

• Is an effect in which the probability of an occurrence increases with an absorbed dose, but the severity of the effect does not depend on the magnitude of the absorbed dose
• Is an all-or-none phenomenon, and has no dose threshold (coz even at high doses it is not certain that cancer/genetic damage will happen, and no known safe dose where cancer won’t happen) – i.e. is random!

Example: cancer. Genetic effects. (i.e. stochastic effects show up years after exposure

Question 7

Linear energy transfer

Answer 7

• the amount of energy deposited per unit path length.
  
  • LET increases with ion charge, decreases with velocity
  • In general, high LET radiations (alpha particles etc) are more damaging to tissue than low LET radiations (e.g. XRs)
  • Has a relationship to tissue weighting factor in that a more susceptible tissue to stochastic radiation effects will suffer more damage in that

Question 8

Tissue weighting factor

Answer 8

a probability for stochastic radiation effects in various organs and tissues., gonads > bone marrow, colon, stomach> bladder/eosophagus / everything else

Question 9

Air kerma

Answer 9

• the sum of initial kinetic energies of all ions created by radiation per mass of air (Kerma = kinetic energy released per unit mass) –
  
  • i.e. how much ionized junk does the radiation create.
  • If the ions created deposit within the material, and bremmstrahlun losses are neglibigle, then air kerma is approximately close to absorbed dose
    
    • Absorbed dose is normally > air kerma due to of backscatter

Question 10

Linear-quadratic dose response

Answer 10

• – the relationship between dose and biological response that is curved
  
  • This implies that the rate of change in response is different at different doses – therefore the response may change slowly at low doses, for example, but rapidly at high doses. (i.e. a linear relationship at low doses but a quadratic relationship at high doses)
• the relationship between dose and biological response that is curved
  
  • This implies that the rate of change in response is different at different doses
    
    • At low doses, cancer incidence is believed to be very low from low-LET radiation – either cells aren’t being hurt, or cells are more likely to repair damage at low doses
    • Assumes that risk increases with increasing dose
    • The ‘plateau’at the top is because at this point the cells have been fried – they are dead, so can’t develop a cancer!
  • Note: the IRCP still takes the conservative vie that the linear no-threshold best represents risk, even though it overestimates risk at low doses – the reasoning “even if we are being conservative, we are being conservative on the side of safety!”

Question 11

Latent period

Answer 11

• the time for stochastic effects to become apparent after exposure
• leukaemia after 2 years, all other cancers after about 10 years

is dependent on the age of the person – children are more radiosensitive

Question 12

Absolute risk

Answer 12

• (the additive model) predicts a constant excess of induced cancer throughout life unrelated to radiation exposure. Assumes there is a constant risk of cancer.

Question 13

Relative risk

Answer 13

• (the multiplicative model) predicts the excess of indiced cancers will rise with age in constant proportion to the natural rate of cancer.
  
  • Preferred for radiology
    
    • Considers how the normal risk changes for age

Question 14

Diagnostic reference level

Answer 14

• is the benchmark radiation dose measured for a given radiation wto which radiology departments may compare their measured doses (and investigate should their measurements consistently exceed the DRLS).
  
  • Are a quality assurance tools. Are NOT a dose limit.
  • Can be used to international comparative dosimetry.

Question 15

Genetically significant dose

Answer 15

the gonadal equivalent dose weighted by the probability of future offspring

Question 16

Background radiation

Answer 16

• Cosmic radiation,(very high energy i.e. > 10BeV)
• Internal radionuclides (diet)
• Radionuclides in air (mostly radon)
• External gamma radiation (mostly from rocks)
• PLUS radiation from man-made sources such as smoke detectors, nuclear industry, medical radiation (!)
  
  • Total dose = approx. 2 mSv per annum
    
    • Often, we can explain our examinations to patients in terms of background equivalent radiation time (BERT)

Sievert used for effective dose and equivalent dose. The absorbed dose is given in Gray.

Question 17

RADIATION AND PREGNANCY

Answer 17

Key figure to keep in your head – 100mSv!

• The foetal threshold dose for deterministic effects is >100mSv
• Termination of pregnancy is NOT recommended for foetal doses less than 100mSv

Question 18

Deterministic Effects of foetal irratiation

Answer 18

Gestation stage

Time period

Potential effects

Pre-implantation

0-10 days

• ‘All or nothing”
• If lethal exposure – get undetectable embryonic death.
• If non-lethal exposure – damaged cells are repaired, malformations don’t happen (ie. it goes on living)

Organogenesis

2-8 weeks

• Organ malformation (CNS is most sensitive)
• Foetal death
• Growth retardation

Fetal growth

8weeks-term

• Severe mental retardation

(reduction of 30 IQ points per Sv)

• (and the stochastic risk of cancer)

Question 19

Stochastic effects of foetal irratiation

Answer 19

• No threshold known (obviously)
• We assume children are more radiosensitive than adults.
• So, the cancer calculations per Sv we used (review the ones I have written above) are changed to be
  
  • 10% per Sievert
  • Therefore, 1 in 10,000 per mSv
• (to put in perspective, child has 1 in 500 risk of cancer). Also,
  
  • 4% of liveborn embryos have congenital defects

10% of fetuses have inherited disorders

Question 20

DRL

Answer 20

• The standard exposure for a standard procedure that is expected in an standard human body as calculated from a sample of patients
  
  • “standard x, standard y, standard z!”
• It’s a quality assurance indicator
  
  • Can be set at a national level or local level
    
    • Get sites to submit data on radiation, then calculate the data at the 75 percentile (not the mean)
• Have to be set in a readily measurable quality
  
  • CT = CTDIvol, DLP
  • DR, Fluoro – DAP, ESD
  • effective dose would be unsuitable coz we can’t monte carlo model everything!
• Apply to imaging only (not radiotherapy)
• Still need to achieve OPTIMISATION
• Is a legal requirement to audit our doses, and use DRLs as a comparative figure of good practice
• Why don’t we have diagnostic reference levels for patient examinations?
  
  • Because you have to give what you have to give! The DRL can’t really help if we need to zap a really obese patient, or if you have a really complicated INR case, etc etc.
  • Also, you do not look at each patient dose after their scan and go ‘crap, this is above the DRL’. It is about using a sample size from your practice to calculate your department’s radiation exposures, and see if you have areas to improve
• Use of DRLs can highlight
  
  • Poor technique
  • Old equipment
  • Lax radiation safety / high dose protocls
    
    • Therefore – a quality assurance protocol!

Question 21

Dose Limits

Answer 21

• For occupational radiation workers
  
  • Effective dose 20mSv per year, averaged over 5 yearsOR
  • An effective dose 50mSv in a single year
• For occupational radiation workers deterministic effect prevention:
  
  • Lens of eye - Equivalent 20mSv in any year to prevent eye deterministic effects
  • Skin - Equivalent dose 500 mSv to extremities to stop skin deterministic effects
    
    • Note how these are quoting equivalent doses? That’s coz we are looking at one point of the body. Effective dose looks at the SUM of all equivalent doses.
• For the public
  
  • An effective dose 1mSv per year
• For the public prevention of deterministic effects
  
  • Lens of eye – 15mSv per year (equivalent)
  • Skin – 50mSv per year(equivalent)

Question 22

Factors to minimise dose

Answer 22

• TIme
• Shielding
• Distance (inverse square law, bitches)
• Technical stuff
  
  • Collimation

BEAM QUALITY

Question 23

Effect of decreasing time on dose

Answer 23

• Minimize acquisition runs (main source of radiation)
• Use pulsed fluoroscopy, and use a rate ALARA
• If equipment permits, also use
  
  • Last image hold (!)
  • Acquisition replay
    
    • This is for fluoro stuff, this ‘time’ component

Question 24

Effect of distance on dose

Answer 24

• All about the inverse square law:
  
  • Radiation from a point source decreases by the square of the distance (ie. dose is proportional to 1/(distance)²
    
    • Ie twice the distance will ¼ the dose, but half the distance will give 4 times the dose
• So re distance things for patients:
  
  • Keep detector as CLOSE to the patient as possible
  • Keep xray tube AS FAR away from the patient as possible
  • Keep patient as close to Image intensifier as possible (minimize air gap)
• So re distance thing for staff:
  
  • If they aren’t needed for the exam, why are they in the room?
  • If staff are in the room, maximize your distance
    
    • Note: most radiation is back-scattered (ie. scattered back towards the tube).

Question 25

Effect of shielding on dose

Answer 25

• Patients AND staff
  
  • Apron / eye goggles
  • Make sure any anatomy we aren’t interested in isn’t in the FOV (ie. move arm out of the way for a lateral CXR)
  • • Consider lead shields

Question 26

Effect of technical factors on dose

Answer 26

• BEAM QUALITY
  
  • High kV
  • Additional filtration
    
    • But, may lose contrast
• • Decreases scatter = greater contrast
    
    • Lower patient effective dose (i.e. lower DAP), but same ESD
    • Lower staff dose (less scatter)

Question 27

Technical Factors affecting dose in fluoroscopy

Answer 27

• Patient close to II, far from XR tube (SSD is big)
• Optimise kVp and mA. Consider ABC
• Have largest possible FOV with II (then collimate very well to maximisie minification)
• Collimate well – to reduce scatter
• Filtration
• Pulsed fluoro (use a low pulse rate ALARA)
• Magnificaiton – avoid if possible
• Last image hold – account for registrar muppetry
• Grid – will reduce scatter (and increase contrast) but will increase dose. Is it needed?
• Dose-area-product (DAP) meter – to monitor skin dose and plan/react
• ABC – use high or low dose setting