Cells don’t usually become cancerous at the flip of a switch. Instead, they go through several phases in which their appearance and growth patterns become increasingly abnormal. The cells in precancerous lung tissue are different from cells in surrounding healthy lung tissue, but they haven’t yet become full cancer cells.
Abnormal precancerous cells grow on the surface lining of the airways and alveoli (air sacs) in the lungs. They haven’t yet invaded the surface below them. They may later transform into cancerous cells and become invasive, growing into deeper tissue layers and forming a tumor.
If precancerous lung tissue turns cancerous, it usually becomes non-small cell lung cancer (NSCLC). Small cell lung cancer usually develops quicker and is rarely found as precancer. Precancerous lung tissue may also be called carcinoma in situ, CIS, in situ carcinoma, preinvasive lesions, or precancer. Doctors may diagnose precancer as stage 0 NSCLC.
There are two primary subtypes of precancerous lung tissue: atypical adenomatous hyperplasia (AAH) and squamous cell carcinoma in situ (CIS). Precancers of the lung are categorized by which type of lung cell is affected.
AAH tissue can turn into adenocarcinoma, a type of NSCLC. AAH, also called adenocarcinoma in situ, develops in glandular tissue. Glandular tissue makes the mucus that helps clear germs and foreign particles out of the airways. AAH is usually found deeper within the lungs, in smaller airways, and alveoli.
AAH is linked to lung cancer. Up to 1 out of 5 people with lung cancer have AAH in other parts of the lung.
Squamous cell CIS may develop into another type of NSCLC called squamous cell carcinoma. Squamous cell CIS forms from the squamous cells that make up the lining of the airways. It is often found in the larger airways toward the center of the lung.
Abnormal lung cells may go through other steps before they become squamous cell CIS. These steps may include:
Smoking and infections can lead to inflammation and lung damage. If a person stops smoking, or if their infection heals, the cells may recover and begin growing normally again. If the damage continues, squamous cell hyperplasia, metaplasia, or dysplasia may develop. Squamous hyperplasia and squamous metaplasia are not precancer, but they may become precancerous over time. Doctors consider squamous dysplasia to be an early type of precancer.
Sometimes, lung cancer develops after cells go through each of these steps. In other cases, cells skip some steps. For example, low-grade lesions (less severe abnormal areas) like metaplasia can quickly turn cancerous.
These types of precancerous lesions can all increase the risk of lung cancer. High-grade (more severe) lesions are more likely to lead to cancer than low-grade lesions. In one study of 164 people, 25 percent of people with hyperplasia, squamous metaplasia, or mild or moderate dysplasia developed lung cancer within five years. Additionally, 39 percent of people with severe dysplasia or CIS were diagnosed with cancer five years or less after diagnosis. Lung cancer may develop either in the area with precancerous tissue or in a different part of the lung.
Doctors sometimes find precancer during imaging tests such as CT scans. CT scans are used for lung cancer screenings. Doctors recommend yearly screening for people between the ages of 55 and 80 who have a history of smoking and currently smoke or quit within the last 15 years.
Bronchoscopy is another imaging technique that can detect cancer or precancer. During this procedure, a thin tube with a camera on the end is guided down the windpipe and into the lungs. Doctors can perform a bronchoscopy to find lesions inside the lungs or to diagnose the cause of lung symptoms. They can also perform a biopsy — a procedure in which a small sample of tissue is removed so that cells can be examined more closely under a microscope.
Doctors may find precancerous cells during a biopsy or an examination of larger tissues, like a tumor or a lobe of a lung. Finding precancerous cells may be a sign that there is cancerous or precancerous tissue somewhere else in the lung. Early detection of precancer or early-stage cancer may lead to quicker treatment and better outcomes.
If you are diagnosed with AAH, you may not need any treatment. Most of these precancerous lung lesions do not become cancerous. Some doctors may recommend observation with regular CT scans. The scans can show whether the precancerous tissue is growing. For example, doctors recommend that people with AAH get follow-up CT scans every six months.
Some cases of AAH and squamous cell CIS require surgery. Surgery may involve removing a small piece of a lung, an entire lobe, or multiple lobes. How much lung needs to be removed depends on whether the doctor thinks that there may be lung cancer in the nearby tissue.
Other treatments can also be used to remove lesions. Some of these treatments lead to fewer side effects than traditional surgery. They may be used when surgery isn’t a good option, such as for people with other lung conditions like chronic obstructive pulmonary disease. Other treatments may include:
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