A basilar predominance is typically noted due to preferential blood flow to the lung bases. Individual nodules may have “feeding vessels” consistent with their hematogenous origin. On HRCT scans, a connection between nodules and the adjacent pulmonary vessels (ie, the mass-vessel sign) may be seen in approximately 75% of cases. Nodules may also be either cavitary or surrounded by a “halo” of ground-glass attenuation, which is typical of hemorrhagic metastases such as those due to choriocarcinoma. Features of lymphangitic cancer may also be present, which again is consistent with a hematogenous origin of disease.
It should be noted that the reported incidence of malignant disease as a cause of multiple pulmonary nodules has been shown to vary greatly, from as low as 10% to as high as 58% in some surgical series. In 133 patients with a known malignancy who underwent video-assisted thoracoscopy for multiple pulmonary nodules, 64% proved to have at least one malignant nodule. A number of malignancies can result in a miliary pattern, rendering differential diagnosis more problematic. This includes tumors, such as renal cell carcinoma, head and neck cancers, and testicular tumors, that have their primary venous drainage in the lungs. itat on
The differential diagnosis includes a number of additional entities that result in random nodules. The most important of these is miliary infection (Fig 10)., In fact, while differentiation between miliary infection and a miliary tumor may be impossible to determine by imaging features alone, in general, close correlation with the clinical history renders these diagnoses relatively straightforward. Miliary metastases are frequently due to metastatic thyroid cancer, renal cancer, and melanoma, among other cancers, while larger less profuse metastases tend to be adenocarcinomas in adults, typically originating from the lung, breast, or the GI tract., Less commonly, diffuse nodules may be identified in patients with septic emboli, invasive fungal infections, and pulmonary vasculitides. These entities frequently result in cavitary nodules, some with a distinct “halo” of ground-glass attenuation, and have even been described in patients with organizing pneumonia. Despite similarities between these entities and routine metastatic disease, it should be emphasized that the numbers of nodules identified in these cases usually fail to meet the criterion of “too many nodules to count,” with the differential diagnosis again further aided by close clinical correlation.
Figure 10. Random nodules: miliary tuberculosis. A magnified HRCT scan image through the right upper lobe shows innumerable tiny nodules throughout the lungs extensively involving the pleural surfaces (black arrowheads) and along bronchovascular structures (arrows). Numerous unattached nodules are also identifiable. This pattern is typical of a random, miliary distribution. While typically resulting from either metastatic disease or infection, clinical correlation is usually diagnostic. Case courtesy of Nestor Muller, MD, Vancouver, BC, Canada.