When pulmonary lesions are mainly nodules or masses, those with a diameter ≤ 3cm are termed nodules, and those with a diameter > 3cm are termed masses. They can be solitary or multiple. Solitary lesion is common in lung cancer, tuberculoma, and inflammatory pseudotumor; while multiple lesions are most common in pulmonary metastatic tumors, and can also be seen in necrotizing granulomas and multiple fluid-containing lung cysts. Nodules and masses are similar in other manifestations except for size.
X-ray
Benign lung tumor
Benign lung tumor has mostly capsule, and is spherical nodule or mass with smooth and sharp edge. Hamartoma may have popcorn calcification.
Malignant lung tumor
Malignant lung tumor grows infiltratively, with blunt edge, and often has short and thin spicules extending peripherally. When close to the pleura, there may be linear, tentiform, or stellate opacities connected to the pleura, forming pleural indentation sign.
The nature of nodules or masses varies, and their manifestations are also different.
Tuberculoma is usually round, may have internal punctate calcification, and often has satellite lesions. Inflammatory pseudotumor is usually a subrounded mass with a diameter of less than 5 cm, often with superior or lateral acuminate protrusion; when the lesion is close to the interlobar pleura or periphery, adhesion and thickening of the adjacent pleura can be seen. Metastatic tumors are often multiple, differently sized, homogeneous or inhomogeneous, mainly in the middle and lower lung fields, with clear edges.
CT
The details of nodules and masses are clearer. Careful analysis of their morphology, internal structure, edge, and other signs often helps in qualitative diagnosis.
Morphology
Lesions may be multiple arcuate protrusions, and the areas between protrusions are invaginated, forming lobulation, termed lobulation sign. If there are spicules on the edges, it is termed spiculation sign.
Internal structure
Sometimes hypodense air opacity with a diameter of 1 - 3mm can be seen in the lesion, which is termed vacuole sign.
Edge
When nodules and masses are adjacent to the pleura, the pleural indentation sign can be formed due to contraction of reactive fibrosis stretching the pleura. Nodules or masses with these signs are common in peripheral lung cancer. Sometimes, nodular or irregular thickening of the peripheral lobular septa can be seen in carcinomatous lymphangitis. Because CT has high density resolution, sometimes fat density opacities (negative CT values) can be found inside nodules or masses, which is helpful for the diagnosis of hamartoma. There are often variously sized, satellite lesions and thick-walled aerated bronchi around pulmonary tuberculomas.
Contrast- enhanced CT is also helpful for qualitative diagnosis. Pulmonary tuberculoma has no enhancement or only mild peripheral annular enhancement. Benign lung tumor may not be enhanced or slightly homogeneously enhanced. Malignant lung tumor often has significantly homogeneous enhancement or central enhancement, mostly transient enhancement. Pulmonary inflammatory pseudotumor may have annular enhancement or mild homogeneous enhancement.
Nodular lesion can be sharply marginated, acinar-sized nodule (less than 1 cm in diameter), and is intraacinar consolidation, commonly in inflammatory or proliferative lesions. Nodular lesion can also be military, nodular opacity (less than 3 mm). Miliary nodules of acute miliary pulmonary tuberculosis have consistent size, density, and distribution; and miliary nodules of carcinomatous lymphangitis are mostly inhomogeneously distributed and accompanied by irregular thickening of the interlobular septa.
Figure 1 nodules and masses
a, b. Lung window and mediastinal window show a pulmonary nodule (↗) with an irregular edge, short spicules, and inhomogeneous density inside the nodule; the nodule is pathologically confirmed lung adenocarcinoma ; c, d. Lung window and mediastinal window show a pulmonary mass (△) with a smooth edge, calcification and fat density can be seen inside the mass; the mass is pathologically confirmed hamartoma.
MRI
Due to the different components of vascular tissue, fibrous connective tissue, muscle tissue, and adipose tissue in nodules or masses, the signal intensity on MRI also varies. Chronic granuloma, caseous tuberculosis, and hamartoma contain some fibrous tissue and calcium, and are predominately hypointense on T2WI. Malignant lesions such as lung cancer or pulmonary metastatic tumor are predominately hyperintense on T2WI. Necrotic lesions in the mass are hypointense on T1WI and hyperintense on T2WI. Cystic changes are hypointense on T1WI and hyperintense on T2WI. Vascular masses such as pulmonary arteriovenous fistula show flow voids or low signal intensities.
