How are secondary lung tumors differentiated from benign lesions and primary cancers on CT scan?

Updated: Feb 08, 2019
  • Author: Daniel S Schwartz, MD, MBA, FACS; Chief Editor: John Geibel, MD, DSc, MSc, AGAF  more...
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In a patient with a known extrathoracic malignancy and a solitary pulmonary nodule on CT, various scenarios to identify metastatic lesions have been proposed.

With a history of sarcoma or melanoma, the pulmonary nodule is more likely to be a metastasis. In the case of underlying head and neck cancer or breast cancer, a second primary cancer in the lung is more likely. With other malignancies, the nodule is equally likely to be a primary lung cancer or metastatic disease.

Malignant lesions account for 3-10% of CT-detected pulmonary nodules. In an older patient, a solitary nodule is more likely to be malignant (lung cancer, in particular); in a younger patient, multiple nodules are more likely to be metastases. However, the number of pulmonary nodules is generally not helpful in distinguishing between benign and malignant lesions.

Generally, the larger the nodule, the more likely it is to be malignant (80% of solitary nodules >3 cm in diameter were malignant, compared with 20% of nodules < 2 cm), though autopsy data show that 57% of all metastases are 1-5 mm in diameter. Most of the nodules resected at the time of thoracotomy but not seen on a CT scan are small, fibrous lesions.

The mass-vessel sign (ie, a vessel entering the medial aspect of a discrete nodule) indicates hematogenous metastasis. Irregular nodule margins indicate a poor prognosis. An ill-defined margin is observed in choriocarcinoma and in other cancers after chemotherapy, indicating hemorrhage.

Calcification, cavitation, and doubling time

Calcified pulmonary metastases are observed with osteogenic sarcoma, chondrosarcoma, synovial sarcoma, ovarian cancer, breast cancer, colon cancer, and thyroid cancer. Cavitation occurs in pulmonary metastases of sarcomas and squamous cell carcinoma, as well as after treatment.

Patterns of calcification strongly suggestive of a benign nature of a nodule are diffuse homogenous calcification, central calcification, laminated concentric calcification, and popcorn calcification.

A doubling time of between 20 and 400 days is consistent with a malignant lesion. Doubling of the volume means that a nodule 0.5 cm in diameter increases by 0.12 cm in diameter, a 1-cm nodule increases by 0.26 cm in diameter, a 2-cm nodule increases by 0.52 cm in diameter, a 3-cm mass increases by 0.78 cm in diameter, and so forth.

Absence of any changes in size over a 2-year follow-up period is generally accepted as evidence of the benign nature of the nodule. Thin-section CT with three-dimensional (3D) reconstruction of the nodule is a particularly accurate method for assessing size changes.

Lymph nodes

Mediastinal nodes are considered positive on CT on the basis of size criteria—namely, whether the short axis is 1 cm or greater. Nineteen percent of nodes from 0.5-1 cm have been reported positive for micrometastases. Seventy-five percent of lymph nodes with cancer involvement are 1 cm or greater in diameter.

High-resolution CT is the imaging procedure of choice for lymphangitic carcinomatosis. Characteristic findings include thickened septal lines, prominent reticular patterns, nodular thickening of bronchovascular bundles, polygonal lines, and beaded septa. Hilar or mediastinal lymphadenopathy, lung masses, and lung nodules are also commonly identified.

Compared with sarcoidosis (a model of benign interstitial lung disease), lymphangitic carcinomatosis is more commonly unilateral or markedly asymmetric and is associated with fewer nodules and less distortion of surrounding lung parenchyma.

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