The symptoms of lung cancer are often sparse and vague such that a series of differential diagnoses may be considered before the diagnosis of lung cancer is made.
An infiltrate on X-ray or CT, may be due to a variety of conditions.
The most common differential diagnoses are:
- infections, including tuberculosis
- fungal infections of the lung
- lung infarction
- benign tumors (hamartomas)
- metastases from other cancer types
- other primary malignant diseases in the mediastinum, for example lymphoma or thymoma
The age of the patient and smoking status will provide a clue to whether lung cancer is the probable cause of an infiltrate on a lung X-ray. For patients under 30 years without a smoking history, this diagnosis is unlikely. In patients over 40 with a history of smoking, lung cancer is a possible diagnosis.
To approach a diagnosis more closely, the first step is to perform a thoracic CT scan. Comparison with earlier images will show whether the finding is new and provide an indication of tumor growth rate.
Massive calcification is unlikely in lung cancer, while common in hamartomas, tuberculomas, and fungal granulomas.
Invasion of neighboring organs, such as the chest wall, is close to pathognomonic of malignant growth. This is commonly observed in the top of the lung (Pancoast tumor).
Cavitation, large size, and multiplicity either intrathoracic or extrathoracic are other signs of malignancy.