The natural course of incidentally detected, small, subsolid lung nodules—is follow-up needed beyond current guideline recommendations?

Subsolid lung nodules are defined as focal lung lesions on computed tomography (CT) smaller than 3 cm with a ground glass component (1,2). This category includes nodules with only ground glass attenuation, called “pure ground glass nodules” or “nonsolid nodules”, and nodules with both a ground glass and a solid component, called “part-solid nodules” (1,2). Subsolid nodules on CT can represent various pathological entities, from benign to malignant lesions, in particular from the adenocarcinoma spectrum (3,4). However, in an overall perspective, malignant subsolid nodules often grow slowly, have a low metastatic potential and a good prognosis (5-10). Their risk of histological aggressiveness is related to a larger nodule size and both the presence and the size of a solid nodule component, which, in malignant lesions, often represents the invasive tumor component (4,11). To avoid over-management that could potentially cause harm to the patient and an economic burden on health care systems, the risk of malignancy of subsolid nodules is often assessed with longitudinal CT examinations for temporal changes (12). If a nodule is transient, it is assumed to be benign. If a nodule increases in size, develops a new solid component or an existing solid component increases in size, it is suspicious for malignant progression and, as a consequence, calls for action (1,8). The most commonly used guidelines for incidentally detected subsolid nodules, the “Fleischner guidelines”, recommend follow-up with CT examinations for at least five years for subsolid nodules with an average diameter of 6 mm or more that are stable without an increase in nodule size or development of a new solid component (1). For subsolid nodules smaller than 6 mm, follow-up is not recommended. The minimum of five years results from the fact that there is limited data available on the natural course of subsolid nodules that are stable for a prolonged time after discovery (1)