Lung cancer is a malignant tumour that starts in the cells of the lung. Malignant means that it can spread, or metastasize, to other parts of the body. When cancer starts in lung cells, it is called primary lung cancer.
The lung is part of the respiratory system. You use your lungs when you breathe. The lungs are in the chest, one on each side of the heart. The right lung has 3 main parts, called lobes. The left lung is a bit smaller and has 2 lobes. The lungs are cushioned and protected by a thin covering called the pleura.
Cells in the lung sometimes change and no longer grow or behave normally. These changes may lead to benign tumours such as hamartoma and papilloma. Benign tumours are not cancerous. But in some cases, changes to lung cells can cause cancer.
Lung cancers are divided into non–small cell lung cancer (NSCLC) and small cell lung cancer (SCLC) based on the type of cell in which the cancer started.
Non–small cell lung cancer usually starts in glandular cells on the outer part of the lung. This type of cancer is called adenocarcinoma. Non–small cell lung cancer can also start in flat, thin cells called squamous cells. These cells line the bronchi, which are the large tubes, or airways, that branch off from the trachea, or windpipe, into the lungs. This type of cancer is called squamous cell carcinoma of the lung. Large cell carcinoma is another type of non–small cell lung cancer, but it is less common. There are also several rare types of non–small cell lung cancer. These include sarcoma and sarcomatoid carcinoma.
Small cell lung cancer usually starts in cells that line the bronchi in the centre of the lungs. The main types of small cell lung cancer are small cell carcinoma and combined small cell carcinoma (mixed tumour with squamous or glandular cells).
Other types of cancer can spread to the lung, but this is not the same disease as primary lung cancer. Cancer that starts in another part of the body and spreads to the lung is called lung metastasis. It is not treated in the same way as primary lung cancer. Find out more about lung metastasis.
A rare type of cancer called pleural mesothelioma is often mistakenly called a lung cancer. But pleural mesothelioma starts in the pleura covering the lung and is very different from cancer that starts in the lung.