Cancer occurs when normal cells undergo a transformation that causes them to grow and multiply without the normal controls. The cells form a mass or tumor that differs from the surrounding tissues from which it arises. Tumors are dangerous because they take oxygen, nutrients, and space from healthy cells.
Lung tumors almost always start in the spongy, pinkish gray walls of the bronchi -- the tubular, branching airways of the lungs. More than 20 types of malignant tumors that originate in the lung itself -- primary lung caner -- have been identified. The major types are small-cell lung cancer and non-small-cell lung cancer. The more common non-small variety is further divided into squamous cell carcinoma, adenocarcinoma, and large-cell carcinoma. Mixed tumors may also occur.
Types of Lung Cancer
What are the types of lung cancer?
Lung cancers, also known as bronchogenic carcinomas ("carcinoma" is another term for cancer), are broadly classified into two types: small cell lung cancers (SCLC) and non-small cell lung cancers (NSCLC). This classification is based upon the microscopic appearance of the tumor cells themselves. These two types of cancers grow and spread in different ways, so a distinction between these two types is important.
SCLC comprise about 20% of lung cancers and are the most aggressive and rapidly growing of all lung cancers. SCLC are strongly related to cigarette smoking with only 1% of these tumors occurring in non-smokers. SCLC metastasize rapidly to many sites within the body and are most often discovered after they have spread extensively. Referring to a specific cell type often seen in SCLC, these cancers are sometimes called oat cell carcinomas.
NSCLC are the most common lung cancers, accounting for about 80% of all lung cancers. NSCLC has three main types that are named based upon the type of cells found in the tumor. They are:
- Adenocarcinomas are the most commonly seen type of NSCLC in the U.S. and comprise up to 50% of NSCLC . While adenocarcinomas are associated with smoking like other lung cancers, this type is especially observed as well in non-smokers who develop lung cancer. Most adenocarcinomas arise in the outer, or peripheral, areas of the lungs. Bronchioloalveolar carcinoma is a subtype of adenocarcinoma that frequently develops at multiple sites in the lungs and spreads along the preexisting alveolar walls.
- Squamous cell carcinomas were formerly more common than adenocarcinomas; at present they account for about 30% of NSCLC. Also known as epidermoid carcinomas, squamous cell cancers arise most frequently in the central chest area in the bronchi.
- Large cell carcinomas, sometimes referred to as undifferentiated carcinomas, are the least common type of NSCLC.
- Mixtures of different types of NSCLC are also seen.
Other types of cancers can arise in the lung; these types are much less common than NSCLC and SCLC and together comprise only 5-10% of lung cancers:
- Bronchial carcinoids account for up to 5% of lung cancers. These tumors are generally small (3-4 cm or less) when diagnosed and occur most commonly in persons under 40 years of age. Unrelated to cigarette smoking, carcinoid tumors can metastasize, and a small proportion of these tumors secrete hormone-like substances. Carcinoids generally grow and spread more slowly than bronchogenic cancers, and many are detected early enough to be amenable to surgical resection.
- Cancers of supporting lung tissue such as smooth muscle, blood vessels, or cells involved in the immune response can rarely occur in the lung.
As discussed previously, metastastatic cancers from other primary tumors in the body are often found in the lung. Tumors from anywhere in the body may spread to the lungs either through the bloodstream, through the lymphatic system, or directly from nearby organs. Metastatic tumors are most often multiple, scattered throughout the lung and concentrated in the peripheral rather than central areas of the organ.
Lung Cancer Symptoms
One fourth of all people with lung cancer have no symptoms when the cancer is diagnosed. These cancers are usually identified incidentally when a chest x-ray is performed for another reason. The other three fourths of people develop some symptoms. The symptoms are due to direct effects of the primary tumor; to effects of metastatic tumors in other parts of the body; or to malignant disturbances of hormones, blood, or other systems.
Symptoms of primary lung cancers include cough, coughing up blood, chest pain, and shortness of breath.
- A new cough in a smoker or a former smoker should raise concern for lung cancer.
- A cough that does not go away or gets worse over time should be evaluated by a health care provider.
- Coughing up blood (hemoptysis) occurs in a significant number of people who have lung cancer. Any amount of coughed-up blood should cause alarm.
- Chest pain is a symptom in about one fourth of people with lung cancer. The pain is dull, aching, and persistent and may involve other structures surrounding the lung.
- Shortness of breath usually results from a blockage in part of the lung, collection of fluid around the lung (pleural effusion), or the spread of tumor through the lungs.
- Wheezing or hoarseness may signal blockage or inflammation in the lungs that may go along with cancer.
- Repeated respiratory infections, such as bronchitis or pneumonia, can be a sign of lung cancer.
Symptoms of metastatic skin rashes depend on the location and size. About 30-40% of people with lung cancer have some symptoms or signs of metastatic disease.
- Lung cancer most often spreads to the liver, the adrenal glands, the bones, and the brain.
- Metastatic lung cancer in the liver usually does not cause any symptoms, at least at the time of diagnosis.
- Metastatic lung cancer in the adrenal glands also typically causes no symptoms at the time of diagnosis.
- Metastasis to the bones is most common with small cell type cancers but also occurs with other lung cancer types. Lung cancer that has metastasized to the bone causes bone pain, usually in the backbone (vertebrae), the thighbones, and the ribs.
- Lung cancer that spreads to the brain can cause difficulties with vision, weakness on one side of the body, and/or seizures.
Paraneoplastic syndromes are the remote, indirect effects of cancer not related to direct invasion. Symptoms include the following:
- Clubbing of fingers - The depositing of extra tissue under the nails
- New bone formation - Along the lower legs or arms
- Anemia - Low numbers of red blood cells and high calcium level or low sodium level in the blood
- Other effects - Muscle weakness, skin rashes, and degeneration of the brain
- Weight loss
- Low sodium levels
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