Pulmonary nodules are a very common presentation on chest imaging. In recent years, the number of pulmonary nodules detected by CT scan has increased significantly, but their diagnosis and treatment are felt by many physicians to be overwhelming and even require a lot of time and experience to deliberate, and the final diagnosis and decision making are often based on the clinician’s experience rather than supported by sufficient evidence. In this article, we describe strategies for the diagnosis and management of pulmonary nodules in the context of the recently published British Thoracic Society guidelines for the investigation and management of pulmonary nodules.
A pulmonary nodule is defined as: a single radiopaque shadow, completely surrounded by inflated lung tissue, with well-defined borders, which can be up to 3 cm in diameter.
For descriptive purposes, there have been many classifications of pulmonary nodules based on their size, such as small nodules of <1 cm, large nodules of 1-3 cm, micronodules of <7 mm, and uncountable micro-nodules of 1-3 mm.
The concepts of nodules and masses should be distinguished, with nodules <3 cm and masses >3 cm, and nodules and masses can be different processes of the same disease.
The classification of pulmonary nodules is specifically standardized in the British Thoracic Society guidelines for the management of pulmonary nodules, which recommend applying standardized terminology to pulmonary nodules, first dividing them into solid and subsolid nodules (SSN) based on whether they are solid or not, which in turn can be further divided into partially solid nodules (PSN) and pure ground glass nodules (pGGN). While other classifications such as non-solid nodules, semi-solid nodules, etc. are vague in concept and should be avoided. As shown in the figure below.
Diagnostic ideas
1, for pulmonary nodules, should first understand what are the etiology of nodules, pneumoconiosis, allergic alveolitis, eosinophilic granuloma, nodular disease, pulmonary metastases, alveolar microlithiasis, TB, fungal infection, viral pneumonia, etc. can be the main manifestation of pulmonary nodules.
We can classify them into two categories according to the presence or absence of fever.
Those without fever: mainly pneumoconiosis, allergic alveolitis, eosinophilic granuloma, nodular disease, pulmonary metastases, alveolar microlithiasis, and a few cases of cornual tuberculosis.
Those with fever: seen in cornual tuberculosis, fungal infection, viral pneumonia, etc.
2, Secondly, it should be further clarified whether the nodules are located in the lung or pleura.
The most important thing in the differential diagnosis of nodules is to locate the nodules well. Generally, they are classified into three types according to their distribution in the lung: central distribution, lymphatic distribution, and random distribution. If there is no subpleural nodule, it is central distribution, if the nodule is clearly distributed in the peribronchial interstitium, lobular septum and subpleural area, it is perilymphatic distribution, if the nodule is diffuse and uniform, it is random distribution.
4. If the nodules are centrally distributed, continue to look for the presence or absence of bud signs, if there are bud signs, they are seen in fine bronchial dissemination, such as Tb, ABPA, occlusive fine bronchitis, fine bronchoalveolar carcinoma. The absence of the bud sign is seen in allergic alveolitis, BOOP, pulmonary edema, vasculitis, etc.
5.If the distribution is random, it can be seen in cornified pulmonary tuberculosis, hemorrhagic pulmonary metastases, etc.
6.Lymphatic distribution is mostly seen in nodular disease, lymphatic metastases, silicosis, etc.
Management strategy
In clinical practice, is it important to avoid underdiagnosis of lung cancer? Or to avoid the risk of complications from unnecessary invasive biopsies or surgery in patients with non-cancerous lung nodules? These need to be carefully weighed by clinicians.
1. Solid nodules
The BTS guidelines specifically emphasize that nodules <5 mm in diameter or <80 mm3 in volume do not require further follow-up, based on the findings of a large screening trial suggesting that these nodules are very unlikely to progress to malignancy. Another large multicenter study also showed that such nodules are no more likely to eventually develop malignancy than nodules not found in the lungs. In this way, incidental lung cancers of 〈5 mm will inevitably be missed, but the chance of such an event is very low, and the benefit of continuous surveillance of such patients depends on the assessment of cancer risk rather than on the nodule itself.
In addition, the classification suggests that nodules ≥8 mm in diameter or ≥300 mm3 in volume should be assessed for risk of developing malignancy using a mathematical model from Brooke University, which has an accuracy of 0.9 for predicting malignancy.
Ongoing CT surveillance is recommended for patients evaluated <10% and for nodules 5-8 mm in diameter.
For patients with ≥10%, PET-CT is recommended, and further risk assessment using the Herder model is performed based on the results.
The flow chart of the BTS guidelines for management strategy of solid nodules is as follows.
2. Subsolid nodules
Subsolid nodules also have their own classification for management and have very unique growth characteristics and prognosis. For nodules ≥5 mm, a review of thin-section CT at 3-month intervals is recommended, as more than 1/3 of patients have resorbable lesions.
Risk assessment using the Brock model is still recommended in the BTS guidelines, but risk assessment of these nodules should take into account some of the typical morphologic features suggestive of malignancy, such as the shape of solid nodules, soap bubble-like presentation, and pleural indentation signs, and suggests that CT surveillance should be performed for up to 4 years.
The flow chart of the BTS management strategy for subsolid nodules is as follows.
Brock model and Herder model.
The BTS guidelines for the management of pulmonary nodules specifically point out the importance of using mathematical models to assess the risk rate of malignancy. mathematical models are experimental medicine based on empirical medicine and have the advantages of accurate results, repeatability, and removal of personal influence of the judge, but the application and research in this area in China However, there is a lack of application and research in this area in China. Although it can provide an objective basis for judging the nature of pulmonary nodules, it is only a tool in clinical diagnosis and cannot replace pathological machine diagnosis, so patients with pulmonary nodules should be judged objectively in clinical practice.