Rad Prot practical aspects Flashcards
DAP
dose area product
used in diagnostic radiology, EXCEPT CT
starting point for estimating patient effective dose
used to estimate scattered doses from patient
used to audit practice
DLP
dose length product
used for CT
starting point for estimating patient effective dose
used to estimate scattered doses from patient
used to audit practice
DAP calc
= xray beam area x absorbed dose
DAP meter - a parallel plate ionisation chamber mounted at xray tube
independent of distance from xray tube
will increase if field size increased
Gycm^2
look up tables required to convert DAP to organ or effective doses
DLP in CT calc
= CTDIvol * L
L scanned length
no DAP due to geometry
use pencil ionisation chamber
depends on exposure factors and beam width
mGy.cm
measure dose along z axis
CT dose index CTDI
mGy
area under curve/T
CTDIvol
takes into account pitch
measured using a phantom
= CTDIw / pitch
weighted CTDI
CTDIw = 1/3 CTDIcentre + 2/3 CTDIperiphery
mGy
exposure controls
most to least important
engineering controls and design features
safety features and warning devices
systems of work
PPE
Lead PPE
0.25mm will reduce scattered dose by approx 10 times at 100kV
0.35mm: nearly 15 times
scattered beam protection
TVL
tenth value layer
thickness of the material that reduces the dose rate to 0.1 of incident dose rate
eg. 22mm lead
2 TVL
reduces dose by 100 times
annual dose limits
employees
Effective 20 mSv
equiv:
eye lens 20
skin 500
extremities 500
classified person
dose expectations
E >6mSv
eye > 15
skin or extremity >150
estimating staff doses
dosimeter tech
thermo-luminescent dosimeters (TLDs)
optically stimulated luminescent (OSL) dosimeters
variety of electronic devices
must be approved by HSE
effective dose monitoring
measures body and skin dose
TLD or OSL
eye monitors
eye dose
TLD
extremity monitors
skin dose
TLD
DIL
dose investigation level
effective dose level set by employer and given in local rules
when exceeded, a review of working conditions is required to ensure doses are ALARP
(workload, equipment…)
staff monitored
set lower than dose limit
annual level required
equivalent doses
lead aprons
increase distance by 30cm, reduce effective dose by half
thyroid shields
reduction in effective dose by > 5 times with 0.35mm Pb
eye shields
reduction in eye dose limit to 20mSv per year
dose reduction 5-10 times
shielding materials
selection
depends on
radiation type and energy
thickness and weight
permanence
cost
optical transparency
shielding materials
lead sheet
lead glass and lead acrylic
concrete
brick
dose constraints
upper level of dose to an individual used at the design or planning stage
dose constraint < dose limit
0.3mSv for public
1 mSv for occupationally exposed workers
designation of controlled area
designate if dose rate over working day > 7.5 micro Sv per hour
IDR (instantaneous dose rate)> 7.5
if employees untrained in rad prot in area
barrier factor B
barrier transmission factor
dose constraint / (air kerma incident on barrier (t) x occupancy)
t = period over which dose delivered
occupancy = 1 for offices
B = annual dose constraint / annual dose to barrier
sources of radiation (air kerma incident on barrier)
primary beam
secondary/scattered beam
leakage radiation
shielding calculations
primary and scattered beam dose-rates or dap workloads
beam directions
workload (DAPs for xrays and radioactive materials/activity/dose rates for nu med)
floor plan and surrounding rooms: occupancies
distances to barriers
dose constraint
primary beam
if primary beam unattenuated:
inverse square law
beam output data
exposure factors
patient dose data
if attenuated by patient:
use dose to detector
scattered beam
scatter factors
leakage
leakage radiation from tube housing
medical and dental guidance notes require that leakage is less than 1mGy per hour at 1m
below is negligible
assessing facilities
barrier factor:
use radioactive source and appropriate detector:
calibrate for distance, thickness of barrier
check for gaps using source/detector or xray/film
site monitoring
when to carry out
carried out
at installations or after modifications
periodic checks
following incidents
Duty holders under IRMER
referrer
provides relevant clinical details
refers to gp
Duty holders under IRMER
practitioner
weighs up risks
considers alternative methods
decide if the xray is justified
Duty holders under IRMER
operator
ID patients
check exam is justified
check right protocol used
performs the exam
Duty holders under IRMER
employer
put protocols in place
DRLs and SOPs
know who is trained to perform each role
Duty holders under IRMER
MPE
medical physics expert
helps w optimisation
radiation protection
and patient dosimetry
10 day rule
10-day rule stating that the abdominal area (lumbar spine, pelvis, coccyx, and hips) should not be irradiated after the 1st 10 days of one’s menstrual cycle.
for high dose > 10 mGy
28 day rule
missed period
replace 10 day rule with a 28-day rule. This means that radiological examination, if justified, can be carried throughout the cycle until a period is missed. Thus the focus is shifted to a missed period and the possibility of pregnancy.
low dose exams
typical fetal doses
0.001 - 50mGy
ensure doses are <100mGy to avoid deterministic effects
mental retardation
loss of 30IQ points per Gy
fetal irradiation and cancer
risk coefficient of inducing childhood cancer from radiation exposure is 10^-4 mGy^-1
1 in 10000 per mGy
after first 3 weeks of pregnancy
procedures for people of reproductive potential
establish pregnancy
period overdue- treat as pregnant
12-55 years
ask children under 16
for exposures where the primary beam may irradiate the pelvis
accidental exposure
person receives an exposure in error, no exposure of any kind intended
reportable:
1mSv effective dose or above for child
3mSv or above for adult
all modalities
due to systematic/human/procedural/equipment error
eg. electronic referrals, ID error, operator errors
unintended exposures due to equipment malfunction
reported under IRMER
equipment that delivers radiation AND ancillary equipment that influences dose to the patient eg. software, contrast injectors
unintended exposures risks
stochastic effects: increased cancer risk
deterministic tissue injuries
psychological harm
reporting unintended exposures
notification no later than 2 weeks after discovering incident
final report no later than 12 weeks
report to CQC
report:
what happened
estimate of doses received
root causes and contributory factors
previous incidents similar
if local procedure was followed
corrective measures adopted
how to prevent in future
average annual radiation dose in UK
natural background
2.3mSv per year
average annual dose
2.7mSv per year
how legislation keeps doses ALARP
dose efficient equipment
equipment QA programmes
optimising scans
training
TRLs and patient dose audits
DRLs
diagnostic reference levels
radiation employer sets DRLs
a dose level for typical examinations for groups of standard sized patients for broadly defined types of equipment
periodic audit of doses to be carried out and actions taken if consistently exceeded
setting DRLs
National DRLs published based on 3rd quartile of data
local DRLs based on local audits
(based on median now)
stochastic effects risk factors
risk of fatal cancer
5% per Sv
higher risk for children:
more radiosensitive
more time for effect to be expressed
alternative to DAP meter
ESD
entrance surface dose
absorbed dose to air on beam axis at point of entry to the patient
ESD = incident absorbed dose x backscatter factor (BSF)
BSF
back scatter factor
depends on
xray spectrum
xray field size
thickness of patient
HVL
A material’s half-value layer (HVL), or half-value thickness, is the thickness of the material at which the intensity of radiation entering it is reduced by one half
higher beam HVL, higher BSF
patient dose calculations
needed for individual patient if involved in radiation incident/found to be pregnant
info needed:
tube output data and filtration, kV, mAs, ..
DAP,
patient height and weight if useful
IRMER17 vs IRR17
IRMER17:
protection of patients
referrer,practitioner,operator,MPE
DRLs
training
regulated by CQC
IRR17:
protection of staff and public
RPA,RPS
local rules
annual dose limits
training
regulated by HSE
