How we set a DRLAn example using CT

David Sutton / Colin Martin

Kampala

IAEA/RCA 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

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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!

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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!

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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.

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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.

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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).

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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 160-200).

• The rotation times varies from 0.5 – 0.8 seconds and the pitch from 0.797 – 1.375.

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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.

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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

• 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 ?

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