Solitary pulmonary nodule

(SPN), a well-circumscribed round or oval-shaped lesion measuring less than 3 cm in diameter. SPN is a common finding on routine screening chest radiograph in adults (Fig.1). The vast majority of such incidentally detected lesions are benign. However, SPN is the most common radiological manifestation in patients with asymptomatic lung cancer. Among clinical and historical data the most reliable is assessment of age. SPN are almost always benign in patients under 30. Thus, unless there is a known primary malignancy, SPN in patients younger than 30 may be presumed to be benign, but should be followed for at least 2 years. By contrast, pulmonary malignancy should not be considered rare in the 30-39-year-old age group. At the age of 40 and beyond, the likelihood of malignancy is increased. As most of these lesions may prove resectable, early diagnosis remains essential. The absence of any detectable growth over a 2-year period of observation is a relatively reliable criterion for establishing that a pulmonary nodule is benign.

CT is almost always obtained in the diagnostic work-up of SPN. The radiological factors most often considered useful in determining the nature of a SPN include lesion size and contour, the growth rate and the presence of calcification. The likelihood of malignancy in an indeterminate SPN detected by chest radiography is a direct function of the size of the lesion. The large majority of nodules > 2 cm are malignant while only about 50% of nodules < 2 cm are malignant. Pulmonary malignant nodules often appear ill-defined and/or irregular in shape. A spiculated appearance on HRCT strongly suggests malignancy. Benign nodules are often well-defined, round and smooth in contour. However, solitary pulmonary metastases may have the same appearance. Air bronchograms within a nodule are more common in malignancies than in benign nodules and more suggestive of adenocarcinomas or bronchioalveolar cell carcinomatosis Cavitation is also more common in malignant nodules than benign nodules, but focal benign pulmonary lesions such lung abscess can also cavitate. Although satellite nodules in association with an SPN suggest granulomatous disease, skip metastases around a bronchogenic carcinoma cannot be ruled out. The radiographic presence of calcification within a SPN is generally a reliable sign of benignancy. Benign types of calcification include diffuse (Fig.2), those with a central nidus, laminated or popcorn. By contrast-stippled or eccentric calcification may be present in as many as 10% of malignant lung neoplasms (Fig.3). CT is more sensitive than radiography in the detection of calcification. The measurement of CT number can allow the detection of calcifications invisible on CT scans in some patients with benign nodules (CT nodule densitometry). Pixels denser than 200 HU usually indicate the presence of calcifications. By detecting the presence of fat, CT may be helpful in diagnosing pulmonary hamartoma (see hamartoma, pulmonary (V:1), Fig. 1), or other benign lesions such as lipoid pneumonia. Certain lesions in the lung may display the nonspecific features of an SPN on chest radiography, while CT may identify specific diagnostic features. Such entities include pulmonary arteriovenous malformation, rounded atelectasis, fungus ball, mucoid impaction and pulmonary infarct.

Further radiological assessment is required if the nodule cannot be diagnosed as benign and is not strongly suspected to be malignant on the basis of clinical history, chest radiograph and CT. Several methods can be used including nodule enhancement at CT, PET and needle biopsy. Malignant lesions contain neovascularity and as a result are apt to enhance following injection of contrast medium. The lack of enhancement within a nodule is strongly suggestive of a benign aetiology. It has been shown that bronchogenic carcinomas  tend to enhance on postcontrast CT scans to a greater degree than many benign lesions, although active granulomas, inflammatory lesion and some benign tumours, such as hamartomas can also enhance. This technique may help select patients for surgery if enhancement is present or follow up of enhancement is absent. The value of FDG-PET scans is similar to that of contrast-enhanced CT scans. Presence of uptake in the nodule is relatively nonspecific whereas absence of uptake is strong evidence of benign disease.

Sometimes biopsy of an SPN may be necessary. If the nodule is related to a narrowed or obstructed bronchus or a bronchus is visible within the nodule (positive bronchus sign), or if an endobronchial lesion is detected with CT, bronchoscopy is appropriate and often diagnostic. In such situations, CT can guide the biopsy attempt. When the positive bronchus sign is absent or in the case of a peripheral nodule, percutaneous biopsy is more appropriate (see percutaneous biopsy thoracic). CT can also be used before video-assisted thoracoscopic biopsy or resection to localize the tumour preoperatively using dye or a hooked wire (Fig.4).

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