Primary and Secondary Lung Cancer

The terms primary lung cancer, secondary lung cancer, and second primary lung cancer are used often in oncology and can be confusing to people living with the disease. A primary lung cancer refers to an original cancer in the lungs that is unrelated to any previous cancer. In other words, it is not metastases from another cancer. A secondary lung cancer can mean different things. It may be used to refer to a second primary lung cancer, which is a second and unrelated cancer in the lungs in someone who had a previous lung cancer. It may also be used to refer to metastases to the lungs from a cancer such as breast cancer, or the spread of lung cancer to other regions. Some people may also hear the term "unknown primary" in referring to cancer. We will take a better look at what these terms mean here.

Definition of Primary Lung Cancer

A primary lung cancer is a cancer that begins in the lungs. Most commonly, primary lung cancer is referred to simply as lung cancer, without adding the word primary. Understanding the difference between primary and secondary lung cancer can help explain why this description may be used.

A primary lung cancer can refer to any type of lung cancer, whether non-small cell lung cancer, small cell lung cancer, carcinoid tumor, or other uncommon types. The cause is also not important in using the term primary lung cancer.

Metastases From the Lungs to Other Regions

If a primary lung cancer spreads to other regions of the body (known as metastasis) it is referred to primary lung cancer metastatic to the location of the spread. For example, cancer that begins in the lungs and spreads to the brain would be termed “primary lung cancer metastatic to the brain.”

Definition of Secondary Lung Cancer

The term secondary lung cancer may refer to either metastatic cancer, or a second primary lung cancer.

Metastases to the Lungs

Cancers that begin in other regions of the body and spread to the lungs are not primary cancers. Sometimes they are described as “secondary cancers” but most often they are defined first by the site of the cancer followed by “metastatic to the lungs.” For example, a breast cancer that spreads to the lungs may be called “breast cancer metastatic to the lungs,” “metastatic lung cancer from the breast,” or "a primary breast cancer with secondary lung involvement." This is important as there are many cancers which do not begin in, but spread to the lungs.

On the other hand, if someone with breast cancer developed another unrelated cancer that began in the lungs, that tumor would be considered a primary lung cancer. The person, in the this case, would have a "second primary cancer.

Second Primary Lung Cancer vs. Metastases

When someone who has had lung cancer develops a second lung cancer, the second lung cancer could be one of two things.

Most commonly, a second area of cancer in the lungs occurring in someone who has had lung cancer is a metastasis—that is, the spread of the lung cancer already present (even if removed in the past by surgery) to another area in the lungs. This would be referred to as a lung metastasis from the primary lung cancer, but the term secondary cancer is also sometimes used to describe this finding. The description you might read on a report may say something like: “primary lung cancer metastatic to another region in the lungs.”

In contrast, and not uncommonly, people may develop a second primary lung cancer. This refers to a completely new lung cancer unrelated to the first lung cancer. It may be the same type of lung cancer but would differ from the first in its molecular characteristics. This second primary lung cancer would have started with a different cluster of cancer cells than the first tumor.

A second primary lung cancer may be found shortly after a first diagnosis, or many years later.

Two Primaries

It may seem surprising that people would develop two separate lung cancers that are unrelated, but as noted above, this is not uncommon. It can help to think about the risk factors for lung cancer and causes. Whatever caused one lung cancer to begin could also damage other cells in the lungs in a way that could lead to cancer. For example, people who carry a genetic risk for breast cancer sometimes develop two separate and unrelated breast cancers due to a gene mutation they carry. Likewise, people who have a genetic risk for lung cancer or are exposed to substances that cause lung cancer such as tobacco or radon in the home, may also at times develop two unrelated lung cancers.

Are They Separate? 

Sometimes two separate cancers develop in the lungs at the same time. In this scenario, both tumors would be considered primary lung cancers. It can be difficult at first glance to know whether two cancers like this are unrelated, or if in fact one of the cancers is metastases due to the original and only primary cancer. With advanced testing, such as molecular profiling of lung cancers, it is possible most of the time to determine if the cancers are unrelated.

Tumors With Unknown Primary 

On rare occasions, a tumor is found in the lungs and doctors can’t be certain where it originated—the original site of the cancer is unknown. Some cancers are only discovered after they have spread to many parts of the body, including the lungs. In this case, the cancer would be called “metastatic cancer to the lungs of unknown origin," not a primary lung cancer. The tumor cells in these cancer are often very "undifferentiated" meaning that it is challenging to tell what type of cell they were originally before they became malignant.

A Word From Verywell

There are many challenging terms in oncology, and most people weren't interested in learning this new language before this diagnosis. The best option is to ask a lot of questions, and ask again if the answers aren't satisfying to you. Oncology is changing so rapidly that being your own advocate in your care and learning about your disease can sometimes make a difference in outcomes.

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