Rad Safety Flashcards
List the categories of exposures according to the International Commission on Radiological Protection.
Situations:
1. Planned
2. Existing
3. Emergency
Individuals:
Occupations
Public
Medical
Eye dose
20mSv in a year over 5 years, with no single year exceeding 50mSv.
Or 0.5Gy threshold
Pregnant staff
Carry out a risk assessment - nil need to cease working
Dose limit 1mSv to foetus
Types of dosimeters
Film Badge
TLD
OSL
EPD
Film badge
Cheap, permanent record
TLD
Thermoluminescent dosimeter - reusable and more sensitive than film
OSL
Optically stimulated luminescence - stimulated by light, read by a laser, no dose-fading
EPD
Electronic personal dosimeter - silicone diode detector that allows instant reading, but expensive and fragile
3 key points of radiation protection
- Time (reduce exposure time)
- Distance (from rad source)
- Shielding
Name 3 techniques for X ray protection
- X ray tube position - scatter towards floor
- Positioning - out of way of rays
- Shielding
3 techniques for Nuclear Medicine protection
- Time: reduce close contact with pt (post injection of radioisotope)
- Distance: maximise distance from pt
- Shielding: structural shielding (lead aprons not effective in NM)
What is ‘radiation incident’
unplanned or unexpected irradiation of at least 5mSv for staff or 1mSv for public.
Must be reported and investigated, inform relevant stakeholders.
Usually still indicates under control.
What is ‘radiation accident’
Any of:
- use of radioactive substance 50% more than prescribed
- use of wrong radioactive substance
- unintended irradiation due to equipment malfunction
- Radioactive substance given to wrong pt or wrong part of body
Emergency exposure
Uncontrolled irradiation, usually indicating a larger scale (20-100mSv, more people)
Diagnostic Reference Levels
Ensures irradiation from usual procedures are within an expected and acceptable limit.
Name 3 methods to monitor radiation dose in patients.
- Direct measurement
a) surface dosimetry (TLD or OSLD) or ionisation chamber at the end of tube (Dose-Area Product meter)
b) Cumulative Air Kerma - for peak skin dose estimation, considers backscatter factor
c) CT Dose Index + Dose Length Product - Calculating effective doses
Typical X ray doses
Chest + thoracic spine <0.1mSv
Rest ~0.3mSv
MMG 0.44mSv
Typical CT doses
CT head - 2mSv
Chest - 7mSv
CTAP ~17mSv
List 4 safety concerns in MRI
- Static magnetic field interacting with metallic implants - MRI safety sheet
- Gradient coils creating noise - ear protection
- RF field - heating in conductive leads could cause burns
- Gd contrast - nephrogenic system fibrosis in renal impairment)
MRI and pregnancy
No evidence of safety concerns to foetus
Small amount of GD can reach breast milk, but not absolutely contraindicated
US safety
Non ionising however still deposits energy in tissues - heat/thermal index (potential cell damage), bubbles in biological fluid can resonate and increase in size > collapse to cause shock wave > microvascular damage
Mechanical Index used to indicate potential risk of Cavitation
What is KERMA
Kinetic Energy Released per unit MAss
Air KERMA usually in the context of QC for measuring tube output
Absorbed dose
Amount of energy deposited per unit matter (Gray = Joule/kg)
Equivalent dose
Measure to assess the biological effect of irradiation, takes the type of rad into consideration.
H = DxWr in Sv
Effective dose
Considers irradiation to different body parts according to their tissue types/susceptibility. Unit: Sv
Flaw of MGD
Assumes 4.2cm breast tissue thickness, 50% adipose and 50% glandular tissue
Real life usually 6cm thick and <20% glandular
MMG safety
Must not exceed 3mGy (legal)
Should not exceed 2mGy (best practice)
Diff between absorbed dose, equivalent dose, effective dose
Absorbed dose - energy deposited in a standard mass of tissue (Gy)
Equivalent dose - tissue sensitivity to irradiation (Sv)
Effective dose - dose that represents overall risk from irradiation (Sv)
Deterministic effect
Known threshold = non-random in occurrence
Higher dose = higher severity
2000mGy = erythema
Stochastic effect
Random
No known threshold
Probability of effect occurring increases with increasing dose
Risk estimation
Linear No Threshold (LNT) - higher dose = higher probability of stochastic effects
Linear Quadratic Response (LQR) - low dose = more likely to repair damage, higher risk at higher doses (sigmoid shaped relationship)
LNT said to overestimate potential harm
Skin dose at which erythema and epilation occur
2-5Gy
Pts who exceed 2Gy should be followed up
Skin dose at which moist desquamation occurs
> 15Gy
