ABSTRACT

Many health systems have been using coronary CT angiography (CCTA) as a first-line examination for ischaemic heart disease patients in various countries. The rising number of CCTA examinations has led to a significant increase in the number of reported incidental extracardiac findings, mainly in the chest. Pulmonary nodules are the most common incidental findings on CCTA scans, as there is a substantial overlap of risk factors between the population seeking to exclude ischaemic heart disease and those at risk of developing lung cancer (i.e., advanced age and smoking habits). However, most incidental findings are clinically insignificant and actively pursuing them could be cost-prohibitive and submit the patient to unnecessary and potentially harmful examinations. Furthermore, there is little consensus regarding when to report or actively exclude these findings and how to manage them, that is, when to trigger an alert or to immediately refer the patient to a pulmonologist, a thoracic surgeon or a multidisciplinary team. This pictorial essay discusses the current literature on this topic and is illustrated with a review of CCTA scans. We also propose a checklist organised by organ and system, recommending actions to raise awareness of pulmonologists, thoracic surgeons, cardiologists and radiologists regarding the most significant and actionable incidental findings on CCTA scans.

Keywords: Incidental findings; Cardiac-gated imaging techniques; Coronary angiography; Lung neoplasms.

RESUMO

Muitos sistemas de saúde têm utilizado a angiotomografia coronariana (ATC) como exame de primeira linha para pacientes com doença cardíaca isquêmica em diversos países. O crescente número de exames de ATC tem levado a um aumento significativo do número de relatos de achados extracardíacos incidentais, principalmente no tórax. Nódulos pulmonares são os achados incidentais mais comuns nas imagens de ATC, pois há uma substancial sobreposição de fatores de risco entre a população que busca a exclusão de doença cardíaca isquêmica e aqueles em risco de desenvolver câncer de pulmão (por exemplo, idade avançada e tabagismo). No entanto, a maioria dos achados incidentais é clinicamente insignificante e seu seguimento ativo pode ter um custo proibitivo e submeter o paciente a exames desnecessários e potencialmente prejudiciais. Além disso, há pouco consenso sobre quando relatar ou ativamente excluir esses achados e como manejá-los, ou seja, quando acionar um alerta ou imediatamente encaminhar o paciente para um pneumologista, um cirurgião torácico ou uma equipe multidisciplinar. Este ensaio pictórico discute a literatura atual sobre esse tópico e é ilustrado com uma revisão de imagens de ATC. Também propomos uma lista de verificação organizada por órgão e sistema, recomendando ações para a conscientização de pneumologistas, cirurgiões torácicos, cardiologistas e radiologistas sobre os achados incidentais mais significativos e acionáveis nas imagens de ATC.

Palavras-chave: Achados incidentais; Técnicas de imagem de sincronização cardíaca; Angiografia coronária; Neoplasias pulmonares.

INTRODUCTION
 
Coronary CT angiography (CCTA) has recently been included in the guidelines for the diagnosis and management of coronary artery disease by several international cardiological societies, such as the American Heart Association/American College of Cardiology, the European Society of Cardiology and the European Association for Cardio-Thoracic Surgery.(1) In addition, the National Institute for Health and Care Excellence also recommends CCTA as a first-line test for evaluating stable angina based on cost-effectiveness and diagnostic accuracy.(2,3)
 
Numerous incidental findings (IFs) can be documented by CCTA, despite the small field of view (FOV) and optimised protocol for cardiac anatomy and function. A systematic review by Kay et al.(4) reported IFs in 45% of CCTA scans (7-100%).
 
Most IFs are clinically insignificant and pursuing them can add unnecessary costs and occasionally harmful evaluations. However, some IFs might present an alternative explanation for symptoms often misinterpreted as ischaemic heart disease (IHD).(5) In addition, the opportunity for dual screening (i.e., screening for IHD and lung cancer) in a population that shares risk factors of both diseases (e.g., advanced age and smoking habits) is appealing and could be achieved by using the full FOV.(6)
 
CCTA services have been successfully implemented worldwide, being most often the result of a partnership between radiologists and cardiologists. However, the service provision, multidisciplinary support, referral pathways and even access to relevant clinical information or previous examinations are very dependent on local practice, expertise and resources.(2) Therefore, clear guidelines for reporting and managing IFs are challenging to be implemented, but the need for multidisciplinary collaboration, including pulmonologists and thoracic surgeons, is widely recognised.
 
This pictorial essay reviews the most common IFs on CCTA scans, organised by organ and structure (Table 1) and discusses their clinical significance and proposed management.

 
Lung
 
IFs of the lung are the most common ones, with pulmonary nodules (PNs) or masses occurring in 14% to 38% of CCTA scans.(4,5) Multiple international societies provide guidelines for investigating and managing PNs that exceed 5 mm in diameter (or 80 mm3 in volume) or show suspicious features.(7-11) Suspicious features of malignancy in PNs include the diameter-volume ratio, growth, distance from the pleura (if more than 10 mm), spiculation, ground-glass appearance, pleural indentation, vascular convergence, circumference-diameter ratio (roundness), upper lobe location, presence of air bronchogram, presence of lymphadenopathy and cavity wall thickness. Benign features include some patterns of nodule calcification (diffuse, central, laminated or popcorn pattern), smooth border, cavitation, satellite lesions and perifissural location.(8)
 
The British Thoracic Society(8,9) recommends the Brock model for estimating the risk of lung cancer in any solid nodule greater than 8 mm in diameter (or 300 mm3 in volume) and in subsolid nodules larger than 5 mm if they are stable after three months. The decision between CT surveillance or further characterisation (i.e., PET-CT, biopsy, excision or non-surgical treatment) depends on this risk estimate, so the report should include it.(9) However, physicians should note that the Brock model was validated for low-dose CT scans and not for CCTA scans.(12,13) For solid, noncalcified PNs measuring between 5 and 8 mm in diameter, their growth rate is better at distinguishing malignancy from benign pathology than are morphological features. Because of this, the Fleischner Society recommends performing a baseline low-dose chest CT as soon as possible and a subsequent follow-up low-dose chest CT for any indeterminate PNs between 5 and 8 mm in diameter.(10,11)
 
The prevalence of IFs of malignancy in all patients undergoing CCTA is small (estimated at 0.7%), and most of them (72%) will be lung cancer (primary or secondary).(4,5) Even in patients with known cancer elsewhere, incidental small PNs are often benign and should not preclude treatment for the primary malignancy until proven to be metastases. Likewise, second primary lung cancer may have a better prognosis than may metastatic lung cancer and still be a candidate for treatment. The report should include any incidental and previously undocumented lung lesions larger than 5 mm and a proposed follow-up schedule. Lesions larger than 8 mm in diameter, growing or presenting suspicious features (Figure 1), should trigger an alert and referral for a lung cancer team so that they can be reviewed by a pulmonologist, an oncologist and a thoracic surgeon.(11)

 
Pulmonary emboli (Figure 2) are rare, identified in just 0.2% of CCTA scans, but should be reported and trigger an urgent referral, as the patient will benefit from a timely start of therapy.(14)

 
Pulmonary consolidation should trigger a referral to the respiratory team and a post-treatment imaging reassessment, as the differential diagnosis is vast and includes infection, alveolar haemorrhage, organising pneumonia and malignancy, among others.(15)
 
Interstitial lung abnormalities (ILAs) are imaging findings potentially compatible with interstitial lung disease (ILD) in patients with no prior history of ILD.(16) These findings are unexpected and incidental, common in the older (above 60 years of age) smoking population (4-9%). ILAs are often asymptomatic but may be related to mild ILD with potential functional impairment, risk of progressing disease and increased mortality risk.(17) Some imaging patterns, such as subpleural reticulations, basal predominance and honeycombing, are more strongly associated with progression.(16) Others, such as centrilobular nodules, are less likely to progress. Likewise, imaging patterns of pulmonary fibrosis are related to increased all-cause mortality. There are no clear guidelines for reporting or managing ILAs. Still, the Fleischner Society proposes that patients with respiratory symptoms, physiological abnormalities, gas transfer abnormalities and extensive CT changes should be referred for pulmonary evaluation and to a respective multidisciplinary team if available (Figure 3).(16) Follow-up of ILAs may still be appropriate after the exclusion of ILD but in the presence of risk factors for progression, even if the optimal interval for follow-up CT scanning is unknown.(16,18)

 
Other findings in the lung parenchyma include bronchial wall thickening in patients with COPD, emphysema, bronchiectasis and atelectasis (Figure 4).(4)

 
Intrapulmonary lymph nodes (IPLNs) are common on chest CT (prevalence of up to 66%) but under-represented on IF studies and do not require follow-up. The morphological criteria for IPLNs (Figure 5) are solid, homogeneous and noncalcified nodules with less than 12 mm in diameter. IPLNs may have an oval, lentiform, or triangular shape, have regular and smooth margins, and be located primarily in the middle or lower lobes within 15 mm of the pleura.(19) Half of the cases show a connection between IPLNs and the pleura. In addition, IPLNs may present single or multiple attachments with veins but not with arteries, which can be useful in differentiating adenocarcinomas from IPLNs.(19)

 
Pleural space
 
Pleural effusion should be reported and related to heart failure in cardiac patients. Pleural plaques (Figure 6) are also common and under-reported IFs, linked to asbestos exposure and increased risk of mesothelioma. (5) The report should differentiate them from pleural metastases in patients with lung cancer. In addition, the presence of pleural plaques is also a marker for increased mortality in a history of asbestos exposure, and the report should document them for surveillance and legal compensation.(4,5,20)

 
Pneumothorax is rarely seen or reported on CCTA scans, likely because of the reduced FOV of CCTA scans and the outpatient setting. However, its clinical presentation can range from asymptomatic to life-threatening, and, when present, it should be reported and the patient should urgently be referred to the respiratory team or emergency room.
 
Mediastinum
 
Mediastinal lesions have a comprehensive list of differential diagnoses, including benign pathology (e.g., pericardial, bronchogenic or oesophageal duplication cysts; diving goitre) and malignancy (e.g., thymoma, thyroid malignancy, germ cell tumours, neurogenic tumours, oesophageal cancer and lymphoma; Figures 7 and 8).(4,5,21)

 
Most lesions in the anterior mediastinum will have attenuation compatible with a soft tissue lesion, with larger lesions more likely representing early-stage thymic epithelial tumours and smaller lesions likely expressing benign cysts.(22) On follow-up evaluation, most lesions are stable or slowly growing, and the absence of growth cannot distinguish between benignity and malignancy. While long-term follow-up may be appropriate, a purely cystic lesion is most commonly a benign thymic cyst and does not need follow-up. Thoracic MRI scanning is far superior to CT in distinguishing simple or complex cystic lesions from solid lesions, identifying fatty, cystic or necrotic components within solid lesions, as well as septations or soft tissue components within cystic lesions. In addition, MRI may be appropriate to alleviate patient anxiety.(22)
 
Mediastinal teratoma is the most common mediastinal germ cell tumour. Mature teratomas usually present multiple densities, including fat, cystic spaces, homogeneous soft tissue and calcification. Conversely, immature teratomas usually present as solid heterogeneous lesions. Mature and most immature teratomas are benign but some immature teratomas may have a malignant germ cell tumour component and even mature teratomas may undergo malignant transformation of non-germ cell components (usually squamous component).
 
Both inflammatory and malignant diseases may cause mediastinal lymphadenopathy. Examples of the former include tuberculosis, fungal infection, sarcoidosis, silicosis, drug reactions, amyloidosis, Castleman’s disease, ILD and COPD. Examples of the latter include lung cancer, lymphoproliferative disease and metastases (Figure 9). The criteria for lymphadenopathy include a short-axis diameter larger than 10 mm, changes to its usual ovoid shape or usual attenuation, coalescence with adjacent enlarged lymph nodes or an invasive behaviour into the surrounding mediastinal fat.(19) Mediastinal lymph node enlargement is the third most common IF reported in the literature (1.7%) after lung nodules and parenchymal abnormalities.(4,19,23)
 


In the absence of suspicious features, lymph nodes smaller than 15 mm in the short-axis are overwhelmingly reactive lymph nodes and, if few, do not need follow-up. (21) The shape and number of lymph nodes, the presence of a fatty hilum, enhancement or calcifications, as well as previous history of diseases potentially explaining the enlarged lymph nodes are also important when considering follow-up or further characterisation with PET-CT or biopsy.(21)
 
Abnormalities of the aorta (Figure 10) and pulmonary arteries should be considered an integral part of the cardiac or coronary assessment in the context of the cardiovascular disease being evaluated, such as in cases of congenital abnormalities or pre-transcatheter aortic valve replacement.

 
Oesophageal hiatus hernia is very common and may cause chest pain (heartburn) as a confounding symptom behind the CCTA request. These patients will benefit from gastrointestinal evaluation and treatment.(4)
 
Similarly to pneumothorax, pneumomediastinum is infrequent and under-represented in systematic reviews of IFs on CCTA. However, these should be reported and trigger an urgent referral to the respiratory team or emergency department.
 
Chest wall
 
Degenerative bony changes are widespread in older patients and could cause atypical chest pain.(5) Metastatic disease, multiple myeloma, lymphoma, and leukaemia account for more than 99% of malignant bone lesions in the chest wall. Therefore, the report should distinguish them from common benign bony lesions (e.g., bone islands and haemangiomas) that do not require further assessment.(24)
 
The skin, muscles and subcutaneous fat tissue are sometimes the site of metastases and should be reviewed.(5) Despite the low predictive value for breast lesions on CCTA, it may be the first study to demonstrate a previously undiagnosed lesion. Incidental breast lesions first detected on CCTA prove to be cancer in 24-70% of the cases; therefore, any breast lesion not previously demonstrated to be benign (Figure 11) should be reported and trigger an alert for an appointment and further evaluation with a specialist.(25)
 
Abdominal cavity
 
A CCTA study usually includes some slices through the upper abdomen. The most common abdominal IFs are simple hepatic cysts, reported in 5% of CCTA scans as lesions with uniform fluid attenuation, no visible wall and no contrast enhancement.(5) These are benign and require no further imaging. However, the complete characterisation of focal liver lesions is often impossible to be obtained from a CCTA scan and may require a targeted protocol (e.g., triple-phase CT scan or liver MRI).(26) Hence, the report should include any new focal liver lesion apart from simple cysts, and further characterisation should be suggested. Likewise, biliary obstruction, pneumobilia and focal or diffuse thickening of the gallbladder wall should be alerted if previously undiagnosed. Renal cysts are also common and frequently benign IFs that do not require follow-up in the absence of suspicious features (e.g., septations, internal density, enhancement, calcification, and solid components). However, any solid renal nodule or mass should be reported and further characterised.(5,27)
 
Coverage of the adrenal glands, pancreas, spleen, and stomach are limited to the FOV and the patient’s anatomy. However, any previously undiagnosed solid or cystic mass in these organs, splenomegaly, peritoneal disease (e.g., nodules, haziness and omental cake) and ascites should also be reported and further evaluated.(28,29)
 
FINAL CONSIDERATIONS
 
Clinically significant IFs are common in the evaluation of IHD using CCTA. Although their detection has the potential for additional costs and patient harm, it also presents opportunities for intervening to benefit patients. Therefore, radiologists and cardiologists reporting CCTA findings should be familiar with IFs.
 
AUTHOR CONTRIBUTIONS
 
EP and DP: study conception and design, data collection, drafting and review of the manuscript. CM, EM, BH, KI and LTB: review of the manuscript. All authors read and approved the final version of the manuscript.
 
CONFLICT OF INTEREST
 
None declared.
 
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