PATHOLOGY AND DIAGNOSTICS
Patient dosimetry audit
for nuclear medicine
and radiotherapy planning CT
Auditing patient doses can and should be done, but with care to ensure
that all of the contributing factors are fully considered
Patient dosimetry audit is a legal requirement in the
UK under the Ionising Radiation (Medical Exposure)
Regulations (IRMER) 1 in order that that diagnostic
reference levels (DRLs) can be established and used
for X-ray imaging examinations. The principles are
also applicable internationally as IRMER is itself
based on European legislation and
recommendations of the International Commission
on Radiological Protection (ICRP). DRLs represent
radiation dose levels that would be considered
typical for a standard patient. They are a means of
monitoring patient doses and are a guide to ‘good
and normal practice’, 2 allowing consistently high
doses to be identified and investigated. Patient
dosimetry audit is the analysis of data relating to
patient doses to calculate average dose indicator
values (usually dose length product (DLP) for CT),
check adherence to existing DRLs and set new ones
where there is enough data.
Patient dosimetry audit is a well established
practice in diagnostic radiology. In the UK,
National DRLs (NDRLs) are set by Public Health
England (PHE) 3–5 and guidance on establishing
Local DRLS (LDRLs) has been in place for around
15 years. 2 The use of electronic systems such as
computed radiological information systems (CRIS)
has had a profound impact on the scale and
efficiency of this process. 6,7 Our local system at
RRPPS (the Radiation Protection Services of
University Hospitals Birmingham NHS Foundation
Trust, UK) is based on CRIS downloads analysed
using an in-house python software to give average
dose indicators by examination and room.
Figure 1 demonstrates how LDRLs are used and
established, and gives example data from an audit
of CT lumbar spine examinations at a large UK
hospital. Mean (average) doses for individual
rooms can be compared against an LDRL, which is
set as the mean of individual room means. In this
case there is clearly a problem with scanner 4, on
which the LDRL is consistently exceeded. Having
identified that an issue exists via patient
dosimetry audit, investigations and corrective
action can be implemented. In this case the
investigation revealed that whilst all of the
scanners were equipped with tube-current
modulation, scanner 4 had been set up with
a much high reference tube current than the
others. This was rectified to harmonise the
protocols across all scanners. Additional cases
FIGURE1
Example data for a CT patient dose audit of lumbar spine examinations
Mean (average) dose length products (DLPs) are broken down by room
The red line indicates the local diagnostic reference level (LDRL)
Matthew Gardner
MSci CSci RRPPS
Medical Physics,
University Hospitals
Birmingham NHS
Foundation Trust, UK
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