Common Knowledge About Pulmonary Nodules (Part 2)

In the previous section, Common Knowledge About Pulmonary Nodules 1, I introduced some basic facts about pulmonary nodules and mentioned that there are three types: solid nodules, part-solid nodules, and ground-glass density nodules (ground glass nodule, GGN—commonly called ground-glass nodules or ground-glass opacities).

A ground-glass density nodule refers to a hazy nodular opacity in the lung whose density is slightly higher than that of the surrounding parenchyma. A solid nodule is one that is entirely of soft-tissue density and uniform in density. A part-solid nodule contains both ground-glass density and solid soft-tissue density and is non-uniform in density.

Among these three types, part-solid nodules (some doctors tell patients they are mixed nodules) have the highest probability of malignancy, followed by ground-glass density nodules, while solid nodules have the lowest probability of malignancy. If a ground-glass nodule found on a previous examination is later seen on follow-up to be transforming into a part-solid nodule (semi-solid nodule; also called mixed ground-glass nodule, which may be written as mGGN on reports; by contrast, the uniformly frosted-glass-like ground-glass nodule is a pure ground-glass nodule), then the nodule is highly suspected to be cancerous tissue and further examination is needed to determine its nature. If lung cancer is confirmed, treatment should begin as early as possible according to the treatment principles for lung cancer.

A small proportion of lung adenocarcinomas progress through ground-glass opacity (mostly atypical adenomatous lesions) to pure ground-glass nodules (mostly carcinoma in situ), then to mixed ground-glass nodules (mostly minimally invasive adenocarcinoma), and finally to small solid pulmonary nodules (mostly invasive adenocarcinoma). Therefore, if checkup imaging reveals the above features together with other high-risk factors for lung cancer (such as a family history of tumors, a high-risk occupation related to lung cancer, a smoking history, or elevated tumor markers), further tests such as bronchoscopy and PET/CT are needed, and if necessary, CT-guided pulmonary nodule puncture biopsy should be performed to determine the nature of the nodule.

Whether a small pulmonary nodule is malignant is also related to its location. Small pulmonary nodules distributed in the upper lobes of both lungs are more likely to be malignant and more likely to be ultimately diagnosed as lung cancer. The larger the small pulmonary nodule, the higher its degree of malignancy. Tiny pulmonary nodules (diameter ≤5 mm) are mostly benign, and solid nodules within 3 mm in diameter are almost entirely benign. Pulmonary nodules larger than 5 mm are more malignant than tiny nodules. Pulmonary nodules larger than 10 mm basically belong to the intermediate/high-risk category and require periodic reexamination to observe changes in nodule size and shape. Pulmonary nodules larger than 20 mm are mostly malignant.

Whether a nodule is malignant can also be judged from its shape. Malignant nodules generally have the following features: lobulation sign (an irregular nodular contour with local lobulated protrusions); spiculation sign (small spine-like projections at the nodule edge, appearing as fine lines, sometimes dense like a brush); serration sign (small spine-like or small triangular projections at the nodule edge arranged in a zigzag pattern); vacuole sign and cavitation (air-like low-density lucencies within the nodule, often seen in early lung cancer); air bronchogram (fine strip-like air-density lucencies within the nodule); pleural indentation sign (pleura around the lesion drawn inward toward it); and vascular convergence sign (rich blood supply to the lesion, with increased vessels feeding it).

Distinguishing these nodular shapes usually requires high-dose thin-slice CT, but most physical examinations use low-dose CT. Some hospitals in less developed areas still use outdated thick-slice CT, whose diagnostic accuracy is even worse, so routine checkups easily miss early lesions. This is why some lung cancer patients say they had physical examinations every year yet early lung cancer was never detected in time.

However, the above features only indicate a greater likelihood of malignancy; they do not mean that every nodule showing these features is malignant. For example, some benign nodules also have blood supply around them, which may simply be normal intrapulmonary vessels. As I mentioned in the previous section, to accurately diagnose whether a nodule is benign or malignant, the most reliable method is needle biopsy or surgical biopsy. It is just that usually low- and intermediate-risk pulmonary nodules do not require biopsy and can be followed up periodically with low-dose CT to observe changes.

In the next section, I will introduce the routine observation and examination methods for pulmonary nodules.