The annoying pulmonary nodule on CT

Lung cancer is a frequent disease worldwide. Although squamous cell carcinoma has decreased, adenocarcinoma has increased along with lung cancer in nonsmokers and adenocarcinoma is now the most common cell type accounting for nearly half of the cases. For the year 2012 an increase in lung cancer death of 7% was predicted among women while a decrease was expected for men. With a 15% overall 5-year survival rate the prognosis has been the same for several decades, however, much better in a certain subgroup of stage I carcinoma after radical surgery (1,2). Incidentally detected pulmonary nodules at routine CT of the thorax or abdomen generate an increasing and resource consuming activity with multiple followup examinations or other diagnostic measures. Growing medical research and implementation of different screening programs fuel this process (3). In certain risk groups, the prevalence was in the range of 8–51% (4–6). Detection rates also vary with slice thickness, MIP (maximum intensity projection) technique, and optional computer-assisted image analysis with in turn a possible risk of viewers fatigue and time restraint (4,5). There are extensive publications describing morphologic parameters and different examination techniques in the diagnostic approach of a solitary pulmonary nodule (4–7). Still, the summary of the American College of Radiology ends with ‘‘. no single algorithm for workup is generally accepted’’ (8) and the problem with multiple nodules is not always addressed. The Fleischner Society defines a solitary pulmonary nodule as an opacity less than 3 cm in diameter without associated other abnormal findings at chest radiography (5). On CT nodules are further classified as solid, semi-solid, or pure ground-glass, and are commonly multiple. The same society has also described how to differentiate benign nodules from possible malignant lesions (9). Each individual diagnostic method has its own drawback. CT contrast enhancement and PET-CT are less reliable in nodules less than 8mm in diameter and PET-CT may show false-negative results in slow growing adenocarcinoma or carcinoid tumors. Despite scientific proof CT contrast enhancement seems not to have universal acceptance (6,7). Initially the 2-years stability rule was based on old materials, which has been confirmed in more recent research (6,10). It may be unreliable, however, in ground-glass or semi-solid lesions which require longer follow-up (4,11). The radiologist must be aware of measurement errors in two-dimensional measurements and also the limitations of volume measurement, a method which otherwise may be more reliable (7). One author found an inter-observer variation in diameter measurement greater than the increase in diameter expected for a nodule of 5mm which doubles in volume as cited by Beigelman-Aubry et al. There are certain patterns of calcification that differs between benign and malignant nodules with central, diffuse, or laminated calcification in benign lesions. The combination of calcification and fat attenuation suggests a benign hamartoma. The radiologist must also be familiar with the appearance of benign intrapulmonary lymph nodes and other perifissural benign nodules (13). Malignant features are lobulated or spiculated margins, bronchus sign, pleural tag, bubbly lucencies, or air bronchogram also called pseudocavitation (4–6,12). A new classification for lung adenocarcinoma has been implemented with new terminology replacing the former term bronchioloalveolar carcinoma (BAC) with new terms like adenocarcinoma in situ (AIS), minimal invasive adenocarcinoma (MIA), lepedic predominant non-mucinous adenocarcinoma, and invasive mucinous adenocarcinoma. The radiological interpretation according to this new classification is still under investigation. A multidisciplinary approach including imaging, pathology, and genetic knowledge is proposed (11,14–16). A general recommendation is a control within 3 months and then decision for further actions for persistent infiltrates. Nodules larger than 1 cm usually need immediate diagnostic measures unless stability in size for at least 2 years or extensive calcification and no known other