FIGURE 3
Mean DLP data for octreotide SPECT/CT examinations in four body
regions for the moderate dose mode on the T16 SPECT/CT scanner 15
(mGycm)
Thick black markers indicate the mean
Thin lines inside the boxes indicate the median
Dashed line indicates the proposed NDRL for
localisation (moderate dose mode)
Boxes indicate the fi rst and third quartiles
Whiskers indicate the smallest and largest values
800
700
600
500
400
300
200
100
0
Abdo
(15)
Abdo/Pelvis
(14)
Chest/Abdo/Pelvis
(32)
Head/Chest/Abdo/Pelvis
(10)
Body Region (Number of Patients)
It is certainly
possible to
develop systems
for patient
dosimetry audit
in nuclear
medicine CT,
but auditors
must be aware
of the additional
complications
that may not
be familiar to
those primarily
working in
diagnostic
radiology
not found in conventional CT, and so there was
more to consider. In discussions between
diagnostic radiology and nuclear medicine
physicists, two additional considerations were
identifi ed; dose mode and body region. Dose
mode accounts for the fact that different CT scans
had different purposes; for some the CT data was
simply used for attenuation correction and so a
lower CT dose is needed. Others are full diagnostic
quality CT scans, needing higher doses. Table 1
summarises the four dose modes in use in our
hospital’s nuclear medicine department. Body
region arises due to the use of SPECT-guided CT, in
which only the part of the body of interest on the
SPECT scan undergoes CT. This is to be encouraged,
as it means that the patient’s exposure to X-rays is
limited, but does also mean that CT scan lengths
vary according to what is scanned and we begin to
see descriptions which resemble diagnostic
radiology CT; head, t-spine, abdomen and so on as
well as the dose mode and examination type. So
we must break down the data in order to audit and
compare like-for-like. Unfortunately not all of
these additional data are captured as standard in
CRIS downloads, our standard source of data for
patient dosimetry audit. CRIS records captured the
nuclear medicine examination type and DLP, so
paper records completed after each scan had to be
modifi ed to capture dose mode, body region and
the scanner used.
Figures 2 and 3 give selected results from this
initial audit, 15 which highlight some important
points. Figure 2 clearly shows that the different
dose modes result in signifi cantly different doses,
and so they must be considered separately in
5
HHE 2019 | hospitalhealthcare.com
order to complete a reliable audit. This also
highlights that the NDRLs are currently
somewhat limited in that they do not fully
consider the range of scan purposes, although of
course it must be acknowledged that there are
limited data currently available and this will
hopefully improve in time.
While Figure 3 should be treated with some
caution as the numbers of patients are quite low,
there is still some useful insight to be gained.
There does appear to be a trend that the CT scans
covering more than the chest, abdomen and
pelvis or more give notably higher doses (and
consistently exceed the proposed NDRL). This is to
be expected, as longer scans of course give larger
DLPs. This raises the question of is there a
genuine concern about this exceeded NDRL? This
NDRL is set for octreotide examinations, and
would therefore represent a collation and average
across all body regions scanned. It seems unfair
therefore to apply it to longer scans, as they are
achieving their clinical aim but will inherently
struggle to work within DRLs set based on shorter
scans. Further study of the NDRL publication 13
reveals that the octreotide data used in this
national audit are based primarily on CT scans of
the abdomen, meaning that it may not be
appropriate to apply the NDRL for longer scans.
This highlights the need to consider body regions
separately when carrying out the audit.
Overall there are a number of challenges in
patient dosimetry audit for nuclear medicine CT.
Far fewer nuclear medicine CT scans are carried
out compared with radiology CT, and the need to
subdivide the data into dose modes and body