Transcript Radiation Protection Duty Holders

IAEA/RCA Kampala
How we set a DRL
An example using CT
David Sutton / Colin Martin
Kampala
Sample
• The survey was launched in January 2012. Data collection
sheets and accompanying instructions were issued to contacts
throughout the five main centres in Scotland to be distributed
to participating CT scanners. During the following nine-month
period data has been collected from a total of 26 scanners
throughout Scotland.
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Scanners in the sample : N=26
Kampala
Choice of exams
• In order to allow comparison of truly similar
examinations conducted for similar purpose and
requiring similar scan technique, you should
specify detailed descriptions of CT procedures,
including a clinical indication (such as CT
abdomen in relation to liver metastases), rather
than simply broad categories of examination
(such as CT abdomen).
• We chose routine CAP, so broke the rule
immediately!
Kampala
Why Chest Abdomen Pelvis ?
• Most common body exam performed
– Quote : “All you need to do to have a CAP
exam in this hospital is walk through the
door!” (one of my Radiologist colleagues)
• All reference levels predate the
widespread introduction of MDCT
• Dose follows technique!
Kampala
Survey Details : 1
• The survey focused on CT CAP examinations
performed on standard (average-sized) patients
using the standard protocol for that centre.
• The standard protocol for CAP exams may vary
from site to site, e.g. number of sequences,
exposure parameters. The standard protocol
used in each was centre was requested as part
of the survey.
Kampala
Survey Details : 2
• The survey requested the DLP to be recorded
for a sample of 30 standard adult patients
(excluding very large and very small patients).
• Centres were not asked to weigh the patients
but to exercise their professional judgement on
what an ‘average’ sized patient is.
• It was requested that the patients positioning
should be the same for all patients participating
in the dose audit according to local protocol.
Kampala
Survey Details 3
• The principle data requested for the CAP
protocol included the number of sequences, the
start/end point of the scan (anatomical location),
exposure parameters: tube voltage, current and
rotation time, pitch, and beam collimation.
• Centres were asked to provide the displayed DLP
for each of the 30 patients participating in the
CT CAP audit.
• Respondents also provided the scanner model
and location.
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Instructions to Centres - Protocols
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Data Collection – Protocol Sheet
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Instructions to Centres – Data Entry
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Data Entry Form
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RESULTS
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Typical protocols 1
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Typical protocols 2
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Protocols
• The number of sequences performed for CAP
examinations was either 1 or 2
• 10 centres used 1 scan sequence (lung apices to
symphysis pubis) 14 used 2 scan sequences
(lung apices to diaphragm, diaphragm to
symphysis pubis).
Kampala
Protocols
• The scan sequences from all centres are broadly
similar.
• All protocols use an applied potential of 120kV,
all use automatic exposure control which
depends on the manufacturer (Smart/Auto mAs
given in the range 100-600, Effective mAs 160200).
• The rotation times varies from 0.5 – 0.8 seconds
and the pitch from 0.797 – 1.375.
Kampala
Data Analysis 1
• Data analyses were conducted using IBM SPSS
Statistics Version 21.
• For each set of data the following parameters
were calculated; mean, median, maximum,
minimum and standard deviation of the DLP.
Kampala
Results for one region
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Data Analysis 2
• The distribution of mean DLPs was determined
• The third quartile value of the distribution of the
mean DLP was then compared to the current
National Diagnostic Reference Level (NDRL).
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Distribution of means
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Results
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N=26
Mean 795 mGycm
SEM 31.7
50th percentile (median) 797 mGycm
75th percentile 880 mGycm
Minimum 521 mGycm
Maximum 1229 mGycm
Proposed DRL 900 mGycm
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Next Step : PACS ?
Kampala