Imaging of Lung Nodules and Masses

1

What is a solitary pulmonary nodule (SPN)?

An SPN is a solitary focal lesion in the lung that measures 3 cm or less. A solitary focal lesion that is greater than 3 cm is considered to be a mass, and most masses are malignant. Approximately 150,000 SPNs are detected annually in the United States, often incidentally on imaging. A goal of radiologic evaluation of SPN is to noninvasively assess the likelihood of malignancy within the nodule. SPN is the initial radiographic finding in 30% of patients with lung cancer, and the prognosis depends partly on the stage at presentation.

2

List some causes of pulmonary nodules.

Primary lung cancer and solitary metastasis from extrapulmonary malignancies are common causes of SPNs detected on chest radiography. Pulmonary granuloma and pulmonary hamartoma are other common causes. Box 17-1 provides a more complete list. Many other entities can cause SPNs or multiple pulmonary nodules, including malignancy (e.g., metastatic disease), infections, vasculitis, and inflammatory diseases (e.g., sarcoidosis, rheumatoid arthritis, or inhalational lung disease). Be careful about a “confluence of shadows” or overlap of normal vascular and skeletal structures that can mimic a nodule on a chest radiograph. Nipple shadows can also mimic nodules but often appear bilaterally and symmetrically.

3

What is the general approach to the evaluation of SPN?

The initial step is to determine whether a “nodule” identified on chest radiography is truly a pulmonary nodule or a pseudolesion that mimics a nodule (some of the causes of a pseudolesion are listed in Box 17-1 ). If a “nodule” is actually a pseudolesion caused by a skeletal finding, such as a rib fracture, it exhibits the same anatomic relationship to its apparent bone of origin on multiple radiographic projections. A true pulmonary nodule that overlaps with skeletal structures on one radiographic projection would appear to move away from its apparent bone of origin on other radiographic projections. Radiopaque nipple markers are sometimes helpful in distinguishing nipples from true pulmonary nodules. When a true lung nodule has been confirmed, a more detailed investigation begins.

4

What further diagnostic steps may be implemented in the workup of indeterminate pulmonary nodules?

Comparison with prior imaging studies is the first step, as long-term stability is suggestive of benignity. Thin-section unenhanced computed tomography (CT) is the next most common step for further evaluation of indeterminate pulmonary nodules and is useful for identifying the presence of fat or specific patterns of calcification within a nodule that indicate benignancy. Fluorodeoxyglucose (FDG) positron emission tomography (PET), follow-up chest CT, and tissue sampling are some other options available for the workup of indeterminate pulmonary nodules. The choice is based on multiple factors, including the pretest probability of malignancy, the morphologic features of the nodules, and the patient's clinical history and current status. Although CT nodule densitometry and MRI may also be useful to characterize lung nodules as benign or malignant, they are uncommonly used for this purpose in clinical practice.

5

What are the Fleischner Society guidelines?

The Fleischner Society guidelines provide general recommendations regarding what diagnostic tests should be performed next to evaluate incidentally detected lung nodules on CT. Different guidelines apply depending on whether the nodule is solid or subsolid in attenuation. Exclusion criteria from these guidelines, such as a personal history of prior malignancy, also exist.

6

What are some potential blind spots on chest radiography and CT when trying to detect pulmonary nodules?

On chest radiography, potential blind spots include the lung apices where the clavicles and ribs overlap, the hila and retrocardiac region where superimposed cardiovascular structures are located, and within the lung bases below the level of the anterior portions of the hemidiaphragms where abdominal soft tissue overlaps.

On CT, potential blind spots include the central portions of the lungs (e.g., the hilar regions and the azygoesophageal recess) and the endoluminal portions of the trachea and bronchi.

7

List some morphologic imaging features of nodules assessed on CT.

• Location

• Shape

• Size/volume

• Margins

• Attenuation

• Presence of fat

• Presence and pattern of calcification

• Presence of enhancement

• Change in features over time

8

Describe morphologic imaging findings that are suggestive of a benign SPN.

Small size and smooth, well-defined margins suggest a benign SPN, although 15% and 40% of malignant nodules are less than 1 cm and 2 cm in diameter, respectively, and 20% of malignant nodules have well-defined margins. Intranodular fat is a reliable indicator of a hamartoma, which is a benign lesion ( Figure 17-1 ). Central, diffusely solid, laminated, and “popcorn-like” patterns of nodule calcification are indicative of benignancy, with the first three typically seen in calcified granulomas and the last in a pulmonary hamartoma ( Figure 17-2 ). Indeterminate patterns of calcification exist, and 15% of lung cancers may contain amorphous, stippled, or punctate and eccentric patterns of calcification. Avidly enhancing serpentine or tubular feeding arteries and a dilated draining vein associated with an enhancing nodular lung opacity are pathognomonic of an arteriovenous malformation ( Figure 17-3 ). Small satellite nodules adjacent to a smooth dominant nodule strongly suggest a granulomatous infection. Ground glass opacity surrounding a nodule (“CT halo” sign) may be seen with angioinvasive opportunistic infection, such as by aspergillosis, particularly in the setting of neutropenia ( Figure 17-4 ). Similarly, gas in a crescentic shape along the margin of a nodule (the “air crescent” sign) can be seen in the setting of an angioinvasive fungal infection. A three-dimensional ratio of the nodule's largest axial diameter to the largest craniocaudal diameter greater than 1.78 : 1 (i.e., a flattened configuration) is highly suggestive of benignancy. A peripheral rim of enhancement or the “enhancing rim” sign of a nodule may also suggest a benign SPN.