Imaging of solitary pulmonary nodule—a clinical review

Current widespread use of cross-sectional imaging has led to exponential rise in detection of solitary pulmonary nodules (SPNs). Whilst large numbers of these are benign ‘incidentalomas’, lung cancers presenting as SPNs are often early disease, which have good prognosis. Therefore, there is rising demand and expectation for more accurate, non-invasive, diagnostic tests to characterize SPNs, aiming to avoid missed or delayed diagnosis of lung cancer. There are wide differential diagnoses of benign and malignant lesions that manifest as SPNs. On conventional imaging, the morphological features supporting benignity include stable small nodule size, smooth demarcated margins, and calcifications. Lack of significant contrast enhancement is also more suggestive of benign nodules. With improved understanding of tumor biology, for instance neo-vascularization and increased vascular permeability, imaging techniques such as dynamic contrastenhanced computed tomography (CT) provide details on contrast uptake and wash-out kinetics, which is more closely reflecting the physiological and pathological phenomena. Positron emission tomography (PET) using 18fluorine-fluoro-deoxyglucose (18F-FDG) is a well-established functional imaging technique, for which one of the most common indications is differentiating between benign and malignant SPNs. Combined PET-CT integrates the anatomical, morphological and metabolic aspects in a single examination, improving overall diagnostic accuracy. Semi-quantitative analysis in FDG-PET imaging is based on measurement of maximum standardized uptake values (SUVmax). SUVmax analysis may become more useful as an assessment of tumor biology in future risk stratification models for cancers. Dual-time point FDG-PET imaging, dual-energy CT, perfusion CT, magnetic resonance (MR) imaging using dynamic contrast enhancement or diffusion-weighted imaging (DWI) techniques, are among the growing armamentarium for diagnostic imaging of SPNs. Provided there is no unacceptably high procedural or operative risk, tissue diagnosis by resection or percutaneous biopsy of SPN should be advocated in those patients identified as at moderate or high risk of malignancy, based on clinical stratification.