Addition of diagnostic CT scan does not increase
the cancer risk in patients undergoing SPECT studies.
Comment on Brix et al.: Radiation risk and protection of
patients in clinical SPECT/CT, Published in
The final publication is available at Springer via http://rd.springer.com/article/10.1007/s00259-014-2711-0/fulltext.html or pdf at: http://dx.doi.org/10.1007/s00259-014-2711-0
Dear Sir,
I read with great
interest the article by Brix et al. [1] in which the authors have
expressed concerns regarding the radiation dose to patients from the diagnostic
CT scans that are increasingly being performed as part of SPECT/CT studies, and
the presumed increased risk of cancer to the patients. The purpose of this letter is to examine the reasons
for their concerns, show that such concerns may not be justified, and suggest
that co-registered diagnostic CT scans should be performed, if feasible, to
improve the diagnostic confidence in the SPECT studies.
The authors refer
to the recent BEIR VII report [2] that has recommended the use
of the linear no-threshold (LNT) model to extrapolate the cancer risk from higher
radiation doses to lower doses linearly, with no threshold. The LNT model is however not universally
accepted for estimating radiation cancer risks.
A report contemporary to the BEIR VII report by the French Academy of
Sciences concluded that the use of the LNT model is not justified, as it is not
consistent with radiobiological data and that there may even be possible beneficial
health effects from low dose radiation [3]. The BEIR VII report has on the other hand declared
the atomic bomb survivor data to be the most important data for estimating
radiation health effects, and referring to these data, it said “The arguments
for thresholds or beneficial health effects are not supported by these data”. Another reason given by the BEIR VII report to
support low dose radiation carcinogenic concerns is the 15-country study of
radiation workers that showed a slightly increased risk of cancer among the
radiation workers [4].
Data and
evidence published since the time of the BEIR VII report have however overwhelmingly
contradicted the LNT model [5]. This includes the latest
update to the atomic bomb survivor data, corrections to the data from the 15-country
study of atomic workers, studies showing different cellular responses to high
and low doses of radiation, study of second cancers in radiation therapy patients,
analysis of cancer incidence in Taiwan apartment residents exposed to low dose
radiation from contaminated building materials, and a study of dogs subjected
to chronic radiation over their lifetime, as described in [5]. There
are also recent animal studies that have shown reduced DNA damage (micronuclei)
following repeated CT scans [6]. Another argument against the LNT model is
that the model completely ignores the opposite effects of low dose and high
dose radiation exposures on the immune system.
Since the immune system plays a
crucial role in preventing occult cancers from becoming clinical cancers [7],
again the linear extrapolation of radiation cancer risks from high dose to low
doses would not be justified.
In view of the
above information, notwithstanding the widespread support of many professional and
advisory bodies for the use of the LNT model as mentioned by Brix et al. [1], the use of the LNT model for
estimating the low dose radiation cancer risk appears to be not appropriate, as
there is neither mechanistic nor epidemiological support for the model. Thus, the blanket declaration of the opening
line of the article by Brix et al. [1] which states “Medical imaging
using ionizing radiation always poses some risk of adverse health effects to
the persons examined—especially radiation-related cancer” is unjustified. Also, there is no justification for the
claims of increased cancer risks from the addition of the diagnostic CT scans
to SPECT studies.
Though SPECT
without a co-registered CT scan has been the standard practice for decades, the
addition of the CT scan in the modern SPECT/CT scanners does provide additional
valuable localization and diagnostic information [8], especially in the regions of
anatomy with closely spaced structures.
Since the additional radiation dose from the CT scan should not be of
concern based on the discussions above, acquisition of SPECT/CT studies
whenever feasible could enable evaluation of the patients more accurately. Inclusion of the anatomic information from
the co-registered CT scans can be very useful to physicians in other
specialties by increasing their diagnostic confidence in the SPECT studies,
especially as the modern PACS systems make the diagnostic images readily
available to referring physicians. Finally, raising carcinogenic concerns when
none exist could harm patient health because of evasive actions taken by
physicians of not ordering the appropriate scans or patients refusing the
recommended scans based on such concerns.
In summary, the
carcinogenic concerns expressed by Brix et al. regarding the diagnostic CT
scans are not justified. Hence,
co-registered diagnostic CT scans should be performed, if feasible, to improve
the diagnostic confidence in the SPECT studies.
References
The final publication is available at Springer via http://rd.springer.com/article/10.1007/s00259-014-2711-0/fulltext.html or pdf at: http://dx.doi.org/10.1007/s00259-014-2711-0
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