When cancer has started to spread from its original location, it is called metastatic cancer. Metastatic lung cancer denotes a lung cancer that has begun to spread. Metastasis is the process by which cancer cells detach themselves and travel through the body via the bloodstream or lymph system.
Image Credit: Yok_onepiece/Shutterstock.com
Types of metastatic lung cancer
Metastasis does not usually occur early on, with most metastatic lung cancers being stage 3 or 4. The spread of cancerous cells to other areas of the body typically starts with the invasion of nearby areas, such as from one part of the lung to the other. When this occurs, it is referred to as local metastasis.
In more extreme cases, cancer cells can travel through the lymph nodes to other parts of the body. This can lead to the formation of secondary tumors in other organs. When this occurs, it is known as distant metastasis. Spread via the lymph system typically takes longer than spread via the bloodstream.
The most common places a lung cancer would metastasize to are the brain, bones, and adrenal glands. On the other hand, the most common types of cancer tumors that metastasize to the lungs are breast, colon, prostate, and bladder cancer, neuroblastoma, sarcoma, and Wilm’s tumor. While these are the most common ones, almost any cancer is capable of spreading to the lungs.
Steps of cancer metastasis
There are four general steps to metastatic lung cancer: tumor establishment, vascular invasion, extravasation, and creation of the distant metastatic focus. The first of these, tumor establishment, focus on the growth of the tumor, including organizing vascular supply, escaping the immune system, and movement. Once established, the cells of the primary tumor can adopt a different morphology to facilitate movement to new metastatic centers.
Vascular invasion can be both lymphatic and hematologic. Invasion into lymph nodes is easier to do than into blood vessels. This is related to the thinner thickness of the walls of the lymph vessels. However, when in the lymph system, cancer cells are faced with the immune system, which they can avoid in the blood vessels. The type of vascular invasion can also dictate where the metastasis will set – along with the lymph nodes or along the blood vessels.
After having entered the lymph or blood system, the cancer cells need to escape circulation and establish themselves. However, the process by which this is done is not well understood. It is believed that blood flow must be reduced to allow the tumor cells to express adhesion molecules, which then find their specific receptors. The cells then poke holes between endothelia to facilitate migration out of the lumen of the vessel.
Not many tumor cells survive the process of moving in the circulatory system, and even fewer successfully establish metastatic nodules. There are many outstanding questions about when metastasis occurs, if the cells move randomly or how the homing mechanisms work. The establishment of the distant metastatic focus differs depending on the location of the new site, and there is therefore no general mechanism in metastatic lung cancers.
Treatment and prognosis
Metastatic lung cancer can be difficult to treat since it is a sign that cancer has spread. Removal of visible tumors does not usually improve the treatment outcome. Instead, chemotherapy is the most common treatment used.
If the metastatic lung cancer is local, surgery can be an option and can lead to full recovery and cure, but this is rare otherwise. Other treatment options include radiation therapy, placing stents in the airways, and laser therapy.
Unfortunately, the prognosis for most forms of metastatic lung cancer is not good. Survival for more than 5 years is rare with metastatic lung cancer. The median survival after diagnosis of metastatic lung cancer is only 5 months, with other studies putting the survival time at around 3 to 12 months. If the original form of the cancer was lymphoma or testicular cancer, there is improved prognosis when using chemotherapy.
Studies looking at patterns in metastatic lung cancer find that the metastatic sites and chances of survival are dependent on sex, the type of original cancer, and the age at which the patient is diagnosed. The study found that the survival of metastasis in the nervous system was better than that of liver metastases from metastatic lung cancer. The most common type of lung cancer that gave rise to metastatic lung cancer was adenocarcinoma of the squamous cell type.
- UCLA Health. 2020. Lung Metastases (Metastatic Cancer). [online] www.uclahealth.org/.../liver-cancer-metastatic
- Popper, H., 2016. Progression and metastasis of lung cancer. Cancer and Metastasis Reviews, 35(1), pp. 75-91.
- Riihimäki, M., Hemminki, A., Fallah, M., Thomsen, H., Sundquist, K., Sundquist, J. and Hemminki, K., 2014. Metastatic sites and survival in lung cancer. Lung Cancer, 86(1), pp. 78-84.
- Moffitt Cancer Center. 2020. What Is Metastatic Lung Cancer?.