Evaluation of the solitary pulmonary nodule: size matters, but do not ignore the power of morphology
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Subsequent to the widespread use of multidetector computed tomography and growing interest in lung cancer screening, small pulmonary nodules are more frequently detected. The differential diagnosis for a solitary pulmonary nodule is extremely broad and includes both benign and malignant causes. Recognition of early lung cancers is vital, since stage at diagnosis is crucial for prognosis. Estimation of the probability of malignancy is a challenging task, but crucial for follow-up and further work-up. In addition to the clinical setting and metabolic assessment, morphological assessment on thin-section computed tomography is essential. Size and growth are key factors in assessment of the malignant potential of a nodule. The likelihood of malignancy positively correlates with nodule diameter: as the diameter increases, so does the likelihood of malignancy. Although there is a considerable overlap in the features of benign and malignant nodules, the importance of morphology however should not be underestimated. Features that are associated with benignity include a perifissural location and triangular morphology, internal fat and benign calcifications. Malignancy is suspected in nodules presenting with spiculation, lobulation, pleural indentation, vascular convergence sign, associated cystic airspace, bubble-like lucencies, irregular air bronchogram, and subsolid morphology. Nodules often show different features and combination of findings is certainly more powerful.
• Size of a pulmonary nodule is important, but morphological assessment should not be underestimated.
• Lung nodules should be evaluated on thin section CT, in both lung and mediastinal window setting.
• Features associated with benignity include a triangular morphology, internal fat and calcifications.
• Spiculation, pleural retraction and notch sign are highly suggestive of a malignant nature.
• Complex features (e.g. bubble-like lucencies) are highly indicative of a malignant nature.
KeywordsComputed tomography Solitary pulmonary nodule Morphology Lung cancer Lung cancer screening
Subsequent to the widespread use of multidetector computed tomography (MDCT) and the growing interest in lung cancer screening, small pulmonary nodules are more frequently detected. Moreover, the global disease burden of lung cancer is on the rise . A solitary pulmonary nodule (SPN) is defined as a rounded opacity in the lung, well or poorly defined, measuring up to 3 cm in diameter . The differential diagnosis for SPNs is extremely broad, including both benign and malignant causes. Recognition of early lung cancers is vital since stage at diagnosis is crucial for prognosis. Estimation of the probability of malignancy is a diagnostic challenge, but is crucial for follow-up or further work-up. First step in this assessment is an evaluation of the clinical parameters such as signs and symptoms, patient age, smoking history, exposure, family history, associated lung diseases, and previous clinical history . Second step is the imaging evaluation. Size, growth, and doubling time are key factors in assessing the malignant potential of a nodule. The likelihood of malignancy positively correlates with nodule diameter: as the diameter increases, so does the likelihood of malignancy. Malignancy, however, is not excluded in small nodules. Lack of growth does not always indicate benignity since adenocarcinomas (in particular those presenting as subsolid nodule) can be slow-growing tumours. Moreover some benign lesions, e.g. intrapulmonary lymph nodes, may show growth and have a volume doubling time in the range of malignant nodules . Although imaging features of benign and malignant nodules show overlap, careful evaluation of morphologic features is an essential element of pulmonary nodule assessment. Nodule morphology should be evaluated on contiguous thin sections in axial, sagittal, and coronal planes. Investigation of nodule metabolism with 18F–fluorodeoxyglucose (FDG) positron emission tomography (PET) can have an additional value, but one needs to keep in mind that small nodules (< 8 mm), adenocarcinoma precursors and invasive adenocarcinomas with lepidic growth, as well as carcinoids can show low or no uptake . In these lesions morphological assessment is crucial in order not to delay diagnosis. A recent study by Chung et al.  on a large set of subsolid nodules from lung cancer screening trials, showed that careful assessment of morphology in subsolid nodules could tremendously increase identification of malignant lesions. This result emphasises the importance of morphology as additional parameter to size and growth in regard to assessing likelihood of malignancy.
Likelihood and odds ratios for malignancy regarding morphological features in solitary pulmonary nodules
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Number of nodules studied
Ground glass: OR 0.74 (CI 0.40–1.35)
Part-solid: OR 1.40 (CI 0.72–2.74)
Ground glass: OR 0.88 (CI 0.48–1.62)
Part-solid: OR 1.46 (CI 0.74–2.88)
LHR 0.293 (smooth, elliptical, polygonal)
LHR 0.735 (minimally lobulated)
LHR 1.888 (deeply lobulated)
Step-wise approach for morphological assessment of the solitary pulmonary nodule
3. Round or oval
4. Triangular or polygonal
11. Pleural retraction
12. Air bronchogram
13. Bubble like lucencies
14. Cystic Airspace
15. Vascular convergence
Assessing the likelihood of malignancy in pulmonary nodules remains a challenging task. Morphological assessment is only one part of the diagnostic puzzle, but its role should not be underestimated. A smooth border, triangular or polygonal shape with perifissural location, fat and popcorn calcifications indicate a benign nature. Features that suggest a malignant nature include a persistent subsolid morphology, spiculation, lobulation, and pleural retraction. More complex findings such as bronchial abnormalities, bubble-like lucencies, an associated cystic airspace and vascular convergence sign are also indicative of a high likelihood of malignancy. In subsolid nodules spiculation, lobulation, and pleural retraction are indicative for an invasive adenocarcinoma rather than a preinvasive lesion.
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Statement of authorship
This manuscript represents original work. Neither this manuscript nor one with substantially similar content has been published or is being considered for publication elsewhere. All authors contributed to this manuscript, read the manuscript and approved the final version of the submitted manuscript.
Conflict of interest
All the authors declare that they have no conflict of interest.
The material in the manuscript has been acquired according to modern ethical standards.
- 17.Yip R, Yankelevitz DF, Hu M, et al (2016) Lung cancer deaths in the national lung screening trial attributed to nonsolid nodules. Radiology 152333. https://doi.org/10.1148/radiol.2016152333
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