How Metastatic Cancer to the Lungs Differs From Primary Lung Cancer

Symptoms, Diagnosis, Treatment, and Prognosis

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radiograph showing secondary lung cancer
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Metastatic cancer to the lungs refers to the spread of a cancer from another region of the body to the lungs. This is also referred to as secondary cancer in the lungs. 

Metastatic cancer to the lungs also referred to as lung metastases, and this can be very confusing. The place where cancer begins is called the primary cancer. For example, if breast cancer begins in a breast and later spreads to the lungs, it would be called breast cancer with metastasis to the lungs, or breast cancer with lung metastases.

It wouldn't be called lung cancer. If you were to look at the cells in the lungs, they would be cancerous breast cells, not cancerous lung cells.

(If you are looking instead for information on cancer which begins in the lungs and spreads to other regions of the body, check out metastatic (stage 4) lung cancer.

Overview

Lung metastases are very common and occur in 30 to 55 percent of advanced cancers. Almost any cancer can spread to the lungs, but some are more likely to do this than others. The most common types of cancer that metastasize to the lungs include:

  • Bladder cancer
  • Breast cancer: Breast cancer commonly spreads to the lungs, and of those who have metastatic breast cancer, roughly 16 percent will have lung metastases.
  • Colon cancer
  • Kidney cancer
  • Prostate cancer
  • Neuroblastoma (usually found in infants and children)
  • Sarcoma
  • Wilm's tumor (often found in children)
  • Melanoma
  • Head and neck cancers
  • Thyroid cancer
  • Testicular cancer

Occasionally, physicians are unable to determine where the primary site of a cancer. In this case, they refer to the cancer as a cancer of unknown origin with metastasis to the lungs. 

(Primary lung cancers most often spread to the bones, brain, liver, and adrenal glands).

How Cancer Spreads

Cancer cells may directly extend to the lungs, such as tumors in the esophagus or chest wall, but most often are carried there indirectly.

There are three possible ways in which cancer cells can spread from the primary site (for example, breast or colon) to the lungs. These include:

  • Bloodstream (hematogenous spread): Cancer cells may "leak" into small blood vessels near the tumors and then be carried to the lungs via the pulmonary arteries.
  • Lymphatics: Tumor cells may leak into the small lymphatic vessels and travel along the lymph pathways (including lymph nodes).
  • Pleural spread and airway spread: This may occur but is often limited to lung tumors and is much less common.

But why do these cells spread? Normal cells can be thought of as being "sticky" and have adhesion molecules that keep them in place. Cancer cells are different, and they these molecules, allowing them to break free and travel.

Symptoms

Sometimes lung metastases are present but do not cause any symptoms. When this is the case, the metastases may be found on a radiological exam done to look for the presence of the spread of cancer. For example, you may have a PET scan after having breast cancer and lung metastases may be found even if you aren't having any symptoms. Symptoms of cancer metastatic to the lungs, when present, are often similar to symptoms of primary lung cancer and can include:

With lung metastases, people often have symptoms related to the primary cancer in addition to lung symptoms. For example, you may have abdominal symptoms related to a primary colon cancer in addition to lung symptoms such as shortness of breath related to lung metastases.

Since metastatic cancer implies that the primary cancer has spread through the body, general symptoms such as fatigue, unexplained weight loss, and decreased appetite are common as well.

Diagnosis

If your doctor suspects that you have lung metastases, there are several tests she may consider. These include:

  • A chest x-ray (a chest x-ray can easily miss small metastases)
  • CT scan of the chest
  • PET scan 
  • Lung biopsy (either a needle biopsy or open lung biopsy)
  • Analysis of pleural fluid if a pleural effusion is present
  • Bronchoscopy

The results of these imaging studies may be evidence enough of metastases, though a biopsy may be needed to confirm the diagnosis.

Treatment

Treatment for cancer metastatic to the lungs is usually determined by the primary cancer, or origin of the cancer. These treatments may include hormonal therapy, targeted therapies, chemotherapy, immunotherapy or a combination of treatments.

Chemotherapy is often the treatment of choice, and is given as palliative therapy; therapy to prolong survival and decrease symptoms but not intended to cure the cancer. In rare instances, particularly with testicular cancer metastatic to the lungs, chemotherapy may be curative.

Occasionally, surgical treatment of lung metastases may be considered. In order for this to be effective, your doctor will want to make sure that your primary tumor is completely removed, that there are only a few (oligometastases) metastases in the lung, and that surgery will be able to completely remove these metastases. When this is the case, metastasectomy may improve your survival.

In addition to surgery, stereotactic body radiotherapy (SBRT), also referred to by terms such as "cyberknife," is sometimes used to treat metastases to the lungs from cancer in other organs. Proton beam therapy may also be considered.

Prognosis

Unfortunately, cancer that has spread to the lungs—by definition stage 4 cancer—is usually not curable. That said, it is often very treatable, and your doctor will talk with you about treatments that may lengthen your life as well as give you the best quality of life possible.

The survival rate varies widely depending on the primary tumor, but 5-year survival rates generally range from 30 to 40 percent. Survival is highest for tumors such as testicular cancer (60 percent 5-year survival) and lowest for tumors such as melanoma which spreads to the lungs.

It's likely that the prognosis for cancer with lung metastases will improve in the near future. Already some stage 4 cancers have responded to treatments such as immunotherapy in ways that were unheard of only a few years ago.

Bottom Line on Cancer Spread to the Lungs (Lung Metastases)

When cancers that originate in another area of the body, such as the breast or bladder, spread to the lungs, people may have symptoms similar to those with lung cancer. Lung metastases, however, are usually treated as part of the treatment for the primary cancer. If a breast cancer spreads to the lungs it will be cancerous breast cells in the lungs, not lung cells.

Metastatic cancers are not usually curable (there are uncommon exceptions) but they are treatable, and treatment may both extend life and control symptoms.

Sources

Miller, K., Siegel, R., Lin, C. et al. Cancer Treatment and Survivorship Statistics, 2016. CA: A Cancer Journal for Clinicians. 2016. 66(4):271-289.

National Cancer Institute. Metastatic Cancer.

U.S. National Library of Medicine. Medline Plus. Lung Metastases.

Wang, H., Zhang, C., Zhang, J. et al. The Prognosis Analysis of Different Metastasis Pattern in Patients With Different Breast Cancer Subtypes: A SEER Based Study. Oncotarget. 2017. 8(16):26368-26379.

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