There are several different types of lung cancer, and these types are important to understand as they can respond differently to treatment. Non-small cell lung cancer is most common, and can be broken down into lung adenocarcinomas, squamous cell carcinomas, and large cell carcinomas. Small cell lung cancers are the second most common form and are named based on the appearance of the cells under the microscope. Carcinoid tumors are one form of neuroendocrine tumor of the lungs, and other tumors, such as sarcomas and lymphomas may also occur in the lungs. Mesothelioma is a type of cancer that does not arise in the lungs, but rather in the membranes surrounding the lungs (the pleura).
Primary vs. Metastatic Cancer
Primary lung cancers are cancers that begin in lung tissues. If you look at these cancers under the microscope, you will see cancerous lung cells. Yet cancer that is found in the lungs can sometimes be metastatic cancer to the lungs. In this case, the cells in the tumor will be of other tissues. For example, when breast cancer spreads to the lungs, it is referred to as breast cancer with lung metastases, not lung cancer. Under the microscope, the cells in the lung will be breast cancer cells.
The difference between primary lung cancers and lung metastases is important, as treatments will be different. (Breast cancer metastatic to the lungs will be treated as breast cancer, not lung cancer.) When a single mass is noted in the lungs, it is often a primary lung cancer (though it could be a benign tumor or a single metastasis). When lung metastases occur, they often appear as several areas of tumors in different regions of the lungs.
Every Cancer is Unique
While lung cancers are separated into the categories noted above, there are other differences on a molecular level, and no two lung cancers are exactly alike. Two lung cancers that are the same type and subtype as well as the same stage, may behave very differently. Fortunately, molecular profiling is now available that can further characterize lung cancers based on genetic changes in the tumor, allowing treatment with medications that specifically target those changes.
Let’s take a look at the different types and subtypes of lung cancer separately.
Non-Small Cell Lung Cancers
Non-small cell lung cancers account for 80 percent of lung cancers, and broken down into three types that can differ in the types of symptoms found, causes, and treatment options. These include:
- Adenocarcinoma (roughly 50 percent)
- Squamous cell carcinoma (roughly 30 percent)
- Large cell carcinoma (roughly 10 percent)
Some tumors have characteristics of more than one of these types, such as adenosquamous tumors.
Adenocarcinoma of the Lung
lLung adenocarcinomas are the most common type of non-small cell lung cancer overall, and are also the type most commonly found in people who have never smoked, in women, and in young adults with lung cancer. These tumors often begin in the periphery (outer parts) of the lungs, and may be present for a long time before they are diagnosed. The first symptoms are often shortness of breath (with exercise at first) and fatigue, which can easily be dismissed as due to something else. In addition, since these tumors are often found in people in who lung cancer is not suspected, they may be missed for some time.
Everyone who is diagnosed with a lung adenocarcinoma should have genetic testing (molecular profiling) done to look for the presence of treatable genetic changes in the tumor. Treatments designed to target the specific abnormality or pathway that drives the growth of the tumor are available for many people with these tumors, and can significantly extend life even with stage 4 cancers when used.
Squamous Cell Carcinoma (Epidermoid Carcinoma)
Squamous cell carcinomas of the lung are the second most common type of non-small cell lung cancer. In contrast to adenocarcinomas, these lung cancers often start in the bronchial tubes in the central part of the lungs and can cause symptoms early on, especially hemoptysis (coughing up blood), recurrent respiratory tract infections, and a persistent cough. Squamous cell carcinomas used to be the most common form of lung cancer, but the incidence has decreased. It’s thought that the decrease is squamous cell cancers and increase in lung adenocarcinomas may be related to the adoption of filters in cigarettes, as with filters, the smoke is inhaled more deeply into the lungs (the region where adenocarcinoma begins). Squamous cell lung cancers are much more likely to be associated with smoking than lung adenocarcinomas.
Large Cell Carcinoma
Large cell carcinomas is the least common form of non-small cell lung cancer and are named for the appearance of large round cells when examined under the microscope. Large cell cancers often occur in the outer regions of the lungs and tends to grow rapidly.
Small Cell Lung Cancer
Small cell lung cancer accounts for around 15 percent of lung cancers, and it is the type of lung cancer most strongly associated with smoking. Small cell lung cancer usually grows in the central areas of the lungs, but most people have few symptoms until just before they are diagnosed. Small cell cancers usually grow and spreads very rapidly, with the majority of people having inoperable cancer at the time of diagnosis. Even though most of these cancers cannot be cured by surgery, small cell lung cancer often responds well, for a time to chemotherapy and radiation. Since these cancers tend to spread (metastasize) early, the first symptoms may be related to the spread of the cancer to the brain (brain metastases), and can include headaches, weakness, visual or speech changes, or new onset seizures.
Carcinoid tumors account for up to 5 percent of lung cancers, but not all lung carcinoid tumors are malignant (cancerous). These tumors are made up of cells called neuroendocrine cells, and along with small cell cancers are also referred to as neuroendocrine tumors.
In contrast to other lung cancer types, carcinoid tumors are often found in younger people, and are the most common type of lung tumor found in children. They do not appear to be related to smoking and tend to grow slowly. Surgery is the treatment of choice. Due to their slower growth, treatment options other than surgery are often different than those for other forms of lung cancer. It’s now thought that many carcinoid tumors may actually be misdiagnosed as being lung adenocarcinomas.
Other Lung Tumors
In addition to the tumors above, cancerous growths in the lungs may include:
Rare Tumors That Present in the Lungs
Tumors that begin in tissue other lung tissue are occasionally found in the lungs. Some tumors that can present in the lungs include sarcomas, hamartomas (the most common type of benign lung tumor), and lymphomas.
Secondary Lung Cancer (Metastatic Cancer of the Lungs)
Cancer that has spread to the lungs from other regions of the body, for example, the breast, may be called secondary lung cancer. In this example, cancer begins in breast tissue, not lung tissue, and would be referred to as breast cancer metastatic to the lungs, rather than lung cancer.
There are many types of cancer that can metastasize to the lungs, with the most common being breast cancer, colon cancer, melanoma, kidney cancer, and bladder cancer.
Mesothelioma is not actually cancer that develops in the lungs, but rather begins in the mesothelium, a membrane that surrounds the lungs. Only about 2,000 cases are diagnosed per year in the United States, but the incidence is increasing worldwide. Most cases of mesothelioma are due to exposure to asbestos on the job.