Multi-detector computed tomography coronary angiography: the incidental lung findings
Authors: Khaled Alfakih, Mathew Budoff
Journal: Journal of the Royal Society of Medicine (2011)

Multi-detector computed tomography (CT) coronary angiography (CA) has established itself, in a short space of time, as an important non-invasive tool for the diagnosis of coronary artery disease. In this editorial we discuss the incidental lung findings commonly found on CTCA.

The National Institute for Health and Clinical Excellence (NICE) has recently recommended the use of CTCA as a first-line investigation in patients with stable chest pain and an estimated likelihood of coronary artery disease of 10–29%, acknowledging its excellent negative predictive value (99%).1,2 Used in this group of patients CTCA should reduce the rate of normal coronary arteries at diagnostic invasive coronary angiography, recently reported to be 39% in the American College of Cardiology National Cardiovascular Data Registry.3 Another advantage of CTCA is the fast acquisition time which should translate into low cost. A recent cost-effectiveness analysis found CTCA as an initial test or as an only test to be more cost-effective than single photon emission CT in patients who have stable chest pain without known coronary artery disease with intermediate coronary artery disease prevalence. 4

In terms of safety the radiation dose from CTCA can be as low as 2 mSv with prospective gating, where the acquisition is limited to 10% of the cardiac cycle at end-diastole, but this requires a regular rhythm below 60 bpm.5 Otherwise, retrospective gating with dose reduction techniques can keep the dose relatively low at around 10 mSv.6 Some scanners offer the potential to reduce the dose by a further 40%, with the use of a small bow tie filter, which reduces the scanning field of view (FOV), excluding the lungs.7 The benefit is not limited to dose reduction but it improves the spatial resolution and avoids the plethora of incidental lung findings, the vast majority of which are of no clinical consequence.8 Even on scanners where it is not possible to limit the scanning FOV, there is an argument to reconstruct the small FOV for the heart from the raw data and use that for reporting. The raw data are subsequently deleted. The concept of deleting the raw data is not new and is already used in CT of the spine.

There are two schools of thought on the importance or otherwise of reporting the findings on the full FOV. The advocates state that these are symptomatic patients and hence the cause of their chest pain could be in the lung parenchyma or more importantly they could have an aortic dissection or a pulmonary embolus. The other incidental findings commonly quoted as important include pneumonia, interstitial lung disease, emphysema, pleural effusions, breast cancer, bronchial carcinoma and lung nodules. These alternative diagnoses may well be relevant for patients presenting to the emergency room with chest pain. However, if there is a serious concern about such a possible differential diagnosis, there is a strong argument for scanning the whole chest whereas CTCA only includes the lower half of the chest.

The opposing opinion argues that the clinician, based on the patients symptoms, requested a CTCA and not a lung scan and that reporting the findings in the full FOV turns up a large number of incidentalomas, and in particular small solid lung nodules which are found in up to 69% of smokers over 50 years of age, with a very low incidence of malignancy (1.4%).9 These patients need a referral to a chest physician and further investigation with up to four additional CT scans, based on the Fleishner society recommendations, which incurs significant additional cost, a significant radiation dose and a risk associated with biopsies.8 Furthermore, at the end of a two-year period of follow-up with CT scans, new nodules will have been discovered in 20% of patients necessitating further screening. The National Lung Screening Trial completed the enrolment of 50,000 heavy smokers or ex-smokers over 55 years of age to determine whether there is benefit from screening for lung cancer and will report in three years. Until then the current recommendation from all the major societies is not to screen for lung cancer.

NICE limited their recommendation for CTCA to patients presenting with stable chest pain with an estimated likelihood of coronary artery disease of 10–29%. The patients who meet these criteria are mostly young and/or women and/or have no cardiovascular risk factors. In this group of relatively well and young patients the commonest incidental lung findings are the small solid lung nodules. Even if the national lung screening trial confirms benefit from screening for lung cancer, the evidence would not apply to this low-risk group of patients. This is particularly important in the light of recent data highlighting the ever-increasing cumulative radiation dose to patients from medical imaging and particularly cardiac imaging, and the potential increase in cancer from excessive use of computed tomography.10–12 The risk of future cancer from radiation is higher in younger patients and particularly in younger women and hence it is important to keep their radiation dose from CTCA down to a minimum. This is achieved by both reducing the dose from the CTCA, through the use of dose reduction techniques, as well as the avoidance of unnecessary subsequent CT scans.