Incidental findings in medical imaging

Abstract

The success of medical imaging as a diagnostic tool has resulted in a continuing increase in its use. Technological advances mean that images are now acquired at higher resolution and in greater volumes than ever before. This has led to an increase in the detection of findings which do not appear to be related to the primary purpose of the examination and have been termed “incidental”. Many of these will be harmless but some will carry significant implications for the patient’s health. Determining which of these findings are significant and which may be safely disregarded is an increasing problem in radiology practice. Radiologists should familiarise themselves with the more common incidental findings in order to make the best possible estimation of their importance in each case and to allow them to make appropriate recommendations for further investigation where this is indicated. The decision to advise further investigation carries implications for the patient and the service as a whole and requires careful consideration.

It is one of the paradoxes of modern medical imaging that the source of our greatest triumph – the ability to image the human body in ever greater detail – is also the source of one of our greatest challenges. Our ability to detect ever more subtle departures from what is considered a normal appearance has outstripped our understanding of what many of these abnormalities mean. Sometimes it seems that our tests are too good. ¹

Definitions are important. For the purpose of this commentary, incidental findings (IF) are considered to include any findings not directly related to the primary purpose for which the imaging examination was undertaken. Even this broad definition can be problematic. Medical imaging is increasingly used as a diagnostic sieve or triage tool, performed in advance of formal clinical assessment with history and examination. If there is no clearly formulated clinical question, then it becomes very difficult to determine what is incidental. In fact in this context there are no “incidental findings”, only “findings” which may turn out to be of greater or lesser importance. The development and potential widespread introduction into clinical practice of blood tests to detect circulating tumour DNA may add another layer of complexity. ²

Although in many contexts, the term incidental carries implications of irrelevance or harmlessness, that is not necessarily the case with medical imaging. If incidental findings were always insignificant, there would be no need for us to be concerned with them. But the detection of a renal mass on an MRI study performed for back pain or a lung cancer on a cardiac CT scan may have far greater implications for the patient’s health than the disease process responsible in each case for the symptoms which prompted referral.

Why are we discussing this now? This is not a new phenomenon, nor is it one entirely confined to radiology. The clinician who observes a skin cancer on the face of a patient attending for a different problem is taking advantage of an incidental finding on clinical examination. Radiologists have long been aware of the potential to make an opportunistic diagnosis in this way. In previous decades, an abdominal radiograph performed to look for renal calculi might demonstrate calcification in the pancreas or in the wall of an aortic aneurysm, incidental to the purpose of the study, but potentially important for the patient.

However, it is the advent and subsequent growth of cross-sectional imaging techniques, principally CT and MRI, which has really brought IF to prominence as an issue central to radiology practice. ³ The exquisite detail provided by modern imaging techniques has completely changed our understanding of how frequently and in how many different respects the human body departs from our traditional concept of “normal”. As we undertake more and more imaging at ever greater resolution, the number of abnormalities we detect continues to rise. The harder we look, the more we find.

This increased rate of detection brings with it a number of problems. As previously noted, our understanding of the meaning or importance of some of these findings has not kept up with our ability to identify them. Sometimes the images themselves may include features which allow us to be reasonably confident that a particular finding is either important or not - site, size, morphology, attenuation or signal characteristics may all be helpful. In many other cases there will be doubt and a decision must be made about how best to manage this uncertainty. If it is decided that a particular lesion cannot be dismissed as irrelevant, further imaging or other more invasive tests may be recommended. The impact on the patient can range from anxiety and minor inconvenience to real harm in the event of a complication from an invasive procedure such as biopsy or endoscopy.

Much has been written about the concept of overdiagnosis – the detection and subsequent treatment of disease which if left alone would not cause problems in the patient’s lifetime. ⁴ Although the term is most commonly used in relation to screening programmes, it applies equally to IFs found in symptomatic patients. The narrative of early diagnosis is seductive, but the term cancer – as currently used - covers many very different diseases including some indolent lesions which would be over treated by traditional therapeutic strategies. ⁵ It is hoped that developments in Artificial Intelligence will help us in the future ⁶ to better stratify these patients to different management strategies, some of which may involve observation rather than intervention. For now, there is still a significant risk that the detection and reporting of an IF will result in over treatment.

Aside from the impact on the individual patient, there are significant implications for radiology services, particularly in a constrained tax-funded system such as the NHS. The direct cost of follow-up investigations is one consideration ⁷ but an even greater risk is the potential that an increase in the number of studies performed to follow-up incidental findings will inevitably make services harder to access for other patients, some of whom may have greater need.

So where does all this leave us as radiologists? How should we balance the risk between causing anxiety and even harm by reporting findings which may otherwise have no implication for the patient’s health against the risk of dismissing findings which may turn out to be important? Firstly we must accept that given the uncertainties inherent in radiology practice and the limitations of the tests we use, we will not always get it right. Next we should put ourselves in a position to make the best possible assessment as to the likely relevance of each finding. We should acquaint ourselves with the appearances of the common IF in each organ as described in the articles which follow, together with the features which in each case give the best possible steer as to their likely importance.

Finally we must recognise that choosing to mention any particular finding in a radiology report is not a neutral act – it carries consequences for the patient, for the service and for other patients. For the patient, we are potentially committing them to further tests, some of which may cause concern or even real harm. For the service we are imposing a burden in the shape of additional workload at a time at which in the UK at least services are at full stretch.

Is it acceptable not to mention certain findings in a report? Of course – we do it all the time. Few of us include mention of every simple renal cyst or bone island in our CT reports, partly to keep our reports of manageable length and partly because we do not wish to distract the reader from the important abnormalities. It becomes harder when there is more doubt as to the nature of the lesion detected but even then, an active decision is required as to what to include in a report. Although guidelines produced by professional bodies can help, the decision itself can and should vary depending on the circumstances of the individual patient. Opinions will differ among radiologists as to how far our role should extend into this domain. ⁸ Some will feel uncomfortable at the prospect of making a judgment of this nature and will prefer simply to list a series of findings and leave it to the referring clinician to decide whether or how to proceed. My contention would be that in the model of clinical radiology which we have established over the last half century it is not only our right to be active participants in these decisions, it is actually our responsibility. The primary duty of the radiologist is the same as that of any other doctor – to improve the health of the patient. Activities which do not contribute towards that goal should not form part of our practice.