Multiple Primary Cancers

When most people think about cancer, they picture it as a single, life-changing diagnosis. But it’s possible for someone to develop more than one type of cancer over their lifetime. Doctors call this situation multiple primary cancers — meaning the second cancer is a new and unrelated disease, not a recurrence or spread of the first.

How Common is it?

In the general population, the lifetime risk of developing any cancer in the U.S. is about 40% for men and 38% for women. While most people who are diagnosed with cancer will only face it once, research shows that around 5–8% of cancer survivors develop a second, unrelated cancer during their lifetime. For cancer survivors, the odds are higher than average. In fact, today about 1 in 6 new cancer diagnoses is in someone who has already had cancer before.

Why Does This Happen?

There are several reasons why someone may develop two different cancers:

• Shared risk factors: Certain habits or exposures can raise the risk of multiple cancers. For example, smoking increases the likelihood of lung, bladder, and head and neck cancers.

• Genetic predisposition: Some inherited conditions, such as BRCA mutations or Lynch syndrome, make people more susceptible to more than one type of cancer.

• Effects of treatment: Chemotherapy and radiation can be lifesaving, but in some cases they slightly raise the risk of a new cancer years later.

• Living longer after cancer: Advances in early detection and treatment mean survivors are living longer, which gives more time for a new, unrelated cancer to develop.

Breast Cancer and Second Cancers

Breast cancer is one of the most common cancers where doctors see survivors develop another, different cancer later in life. The risks vary depending on age, genetics, and treatment history.

• Second breast cancer: For most breast cancer survivors, the risk of a new cancer in the opposite breast is about 0.2–0.4% per year — roughly 4–8% over 20 years, but higher for those with BRCA mutations.

• Ovarian cancer: Women with BRCA1 mutations have a 39–44% lifetime risk of ovarian cancer; those with BRCA2 mutations have an 11–17% risk. Without these mutations, the average woman’s lifetime risk is less than 2%.

• Uterine (endometrial) cancer: Women who take tamoxifen for 5 or more years have about a 2–3 times higher risk of endometrial cancer. Even with that increase, the overall risk remains fairly low (around 1–2% over 10 years of use).

• Leukemia: Chemotherapy drugs such as anthracyclines or alkylating agents can increase the chance of leukemia. The risk is small — generally less than 1% of survivors.

• Other cancers: Studies show breast cancer survivors may also have a slightly elevated risk of colon, thyroid, and lung cancers compared to the general population, though these risks are less precisely measured.

Because of these risks, breast cancer survivors are often advised to:

• Continue regular mammograms (and sometimes MRI for high-risk women).

• Stay up to date with gynecologic and colon screenings.

• Consider genetic counseling if they have a strong family history of breast or ovarian cancer.

What Does This Mean for Survivors?

Most survivors will not develop a second cancer, but it is something to be aware of. Doctors typically recommend ongoing follow-up care not only to check for recurrence, but also to monitor for new cancers. Healthy lifestyle choices — such as avoiding tobacco, maintaining a balanced diet, exercising regularly, and staying up to date on screenings — can help lower risk.

The Bottom Line

While the idea of facing two different cancers can feel overwhelming, it’s important to remember that this is still relatively uncommon. Thanks to better treatments and survivorship care, many people who do experience a second cancer are diagnosed earlier and live longer, healthier lives.