Keywords Computed tomography attenuation correction - incidental findings - low-dose computed
tomography - lung carcinoma - myocardial perfusion scintigraphy
Introduction
An incidental finding is one which is unexpected and unrelated to the primary indication
for an investigation. They are particularly commonly encountered within the thorax
on computed tomography (CT) imaging.[1 ] Any incidental finding which is of clinical concern for the potential to cause harm
or provide benefit to the patient if reported is significant and generally necessitates
further evaluation. The correct approach to dealing with incidental findings is an
increasingly difficult challenge throughout medical imaging. Important steps to ensure
they are managed appropriately include careful review of all acquired images, accurate
reporting of unexpected findings in official imaging reports, clear communication
between medical imaging department and treating teams, and evidence-based further
investigation. Failure or delay at any stage can result in adverse clinical outcomes.
The accuracy of myocardial perfusion scintigraphy (MPS) in the evaluation of myocardial
ischemia can be enhanced with the use of a low-dose CT scan to provide attenuation
correction of the functional images.[2 ] This nondiagnostic CT acquired without breath-holding, or intravenous contrast produces
low-resolution images of a sizeable portion of the thorax and upper abdomen as a by-product.
Potentially significant incidental findings on these CT images have been reported
in 2%–33% of MPS studies, with cases of previously undiagnosed malignancy described
in previous published reviews [3 ],[4 ],[5 ],[6 ] and a solitary case report.[7 ] Despite this, the most appropriate approach to reviewing and reporting on these
CT images and any incidental findings is controversial and has recently been advised
against.[6 ],[8 ]
Case Report
We present the case of a 74-year-old man who underwent MPS for evaluation of increasing
dyspnea as part of a preoperative assessment before potential abdominal aortic aneurysm
repair. He was a retired builder who was an ex-smoker of 25 pack-years with a past
medical history including ischemic heart disease, severe chronic obstructive pulmonary
disease, and tissue aortic valve replacement.
MPS with technetium-labeled hexakis-2-methoxyisobutyl isonitrile (Sestamibi, Cardiolite
®, Lantheus Medical Imaging, North Billerica, MA, USA) as the radiopharmaceutical
and adenosine as a pharmacological stressor showed no evidence of myocardial ischemia,
and this was confirmed with automated analysis. However, a subpleural opacity in the
posterior aspect of the right lower lobe was seen on review of the CTAC images [Figure 1a ]. The corresponding slice on single-photon emission CT (SPECT) fused with CT is also
shown [Figure 1b ]. This measured 10 mm and was noted in the official MPS report. In accordance with
current clinical guidelines for the management of lung nodules, further evaluation
with full-dose diagnostic CT was recommended in the MPS report provided to the referring
physicians.[9 ] Interestingly, there was no radiopharmaceutical (sestamibi) uptake associated with
the tumor.
Figure 1 Selected axial low-dose computed tomography (a) image acquired for attenuation correction
during myocardial perfusion scintigraphy (acquired in prone position). An incidental
10 mm opacity is seen within the right lower lobe. Further evaluation with full-dose
diagnostic-quality computed tomography was recommended. Anatomical equivalent (b)
shown on single-photon emission computed tomography
The patient had undergone multiple CT examinations of the thoracic and abdominal aorta,
which included the region where the incidental CTAC nodule was located, under the
care of the vascular team in the years prior to the MPS taking place (most recently
1 year before MPS), but no lung nodules/opacities had been reported. A further CT
aorta took place 2 months following MPS which reported that the nodule now measured
12 mm and was suspicious for malignancy [Figure 2a ] and [Figure 2b ]. Unfortunately, no further urgent investigation was performed based on the MPS report
or this CT.
Figure 2 Selected axial (a) and coronal (b) computed tomography images from a CT aorta study
Performed 2 months following myocardial perfusion scintigraphy. An incidental right
lower lobe opacity, now measuring 12 mm is seen. Urgent further investigation was
advised due to suspicion for malignancy
The next imaging study performed was another CT scan, which occurred 14 months after
MPS. This revealed that the pleural based opacity in the right lower lobe had grown
to 23 mm and was now associated with multiple smaller opacities within the left lung
and hilar lymphadenopathy [Figure 3a ] and [Figure 3b ]. An urgent biopsy was performed at this point providing a histological diagnosis
of lung squamous cell carcinoma. Staging CT showed diffuse hepatic metastasis [Figure 4a ] and [Figure 4b ]. Emergent chemotherapy was started, but the patient died within weeks of its commencement.
Figure 3 Selected axial (a) and coronal (b) computed tomography images from a CT chest performed
14 months after the initial myocardial perfusion scintigraphy occurred. A 23 mm opacity
was now present within the right lower lobe with associated hilar lymphadenopathy
and multiple smaller opacities within the lung fields. Subsequent computed tomography-guided
biopsy revealed lung squamous cell carcinoma
Figure 4 Selected axial computed tomography images from (a) low-dose attenuation correction
CT acquired during myocardial perfusion scintigraphy (acquired in prone position)
and (b) staging computed tomography performed following lung cancer diagnosis 14 months
later. Within the limits of the lower dose scan, no hepatic lesions are seen at the
time of myocardial perfusion scintigraphy compared with multiple metastatic lesions
on staging computed tomography
Discussion
In this case, there was a delay of over 12 months in fully investigating the incidental
pulmonary nodule seen on CTAC images during MPS and again on CT aorta 2 months later.
This was despite the MPS, and CT reports clearly recommending further investigation.
When initially described incidentally during MPS, the nodule was localized with no
evidence of regional or distant metastasis. Appropriate investigation and earlier
diagnosis may have allowed for greater treatment options, and potentially, a better
outcome as the prognosis of lung cancer is closely linked to disease stage.[10 ] The exact reason for the failure to investigate sooner is unclear.
Lung cancer is the leading cause of cancer death worldwide. As it is often asymptomatic
in the initial stages, it can potentially be diagnosed incidentally on unrelated investigations.[10 ] As lung cancer and ischemic heart disease share common risk factors, particularly
cigarette smoking, patients undergoing MPS already represent a high-risk group. Moreover,
low-dose CT has emerged as an effective screening tool for lung cancer in recent years,
reducing mortality in high-risk groups with a significant exposure to cigarette smoking.[10 ] MPS with CTAC provides a low-dose CT through a considerable portion of the lung
fields in a patient population at higher risk for lung cancer development, and although
it cannot be directly compared to full-dose diagnostic CT or a structured screening
program, can identify early cancers as shown here. This highlights the need for routine
reporting and appropriate follow-up of incidental lung nodules on CTAC images during
MPS.
Conclusion
This case highlights the importance of routine review of CTAC images in MPS and the
crucial role of effective communication between nuclear medicine and treating physicians
following identification of any suspicious incidental finding. Further, referring
physicians must follow-up and appropriately investigate potentially significant incidental
findings promptly, particularly as they can represent undiagnosed malignancy as in
this case. Failure at any step can result in adverse patient outcomes.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms.
In the form the patient(s) has/have given his/her/their consent for his/her/their
images and other clinical information to be reported in the journal. The patients
understand that their names and initials will not be published and due efforts will
be made to conceal their identity, but anonymity cannot be guaranteed.
