Researchers continue to make strides as they look for ways to defeat cancer. In many areas of the world, significant progress has been made in regard to improving cancer survival rates.

While cancer treatments continue to evolve and help people fight and ultimately overcome their disease, cancer is a formidable foe that can sometimes return. Understanding why cancer returns can help patients and their loved ones prepare to fight all over again.

Why does cancer come back?

Cancer patients who have finished successful treatment may be disheartened to learn that their cancer has come back. According to Cancer Research UK, cancer can come back if treatment failed to get rid of all the cancer cells. In such instances, the cells left behind may grow into a new tumor. Some cancers also can spread to other areas of the body, forming new tumors elsewhere.

Cancer also might come back if it has become resistant to the drugs used to treat it. Cancer cells are mutated cells in which the genes within them make the cells behave differently from normal, healthy cells. Cell mutations vary, and some might make cancer cells resistant to drugs used during chemotherapy, targeted therapy and hormone therapy. Multi-drug resistance occurs when cancer cells have become resistant to many drugs at the same time.

Does the type of treatment matter?

Cancer can come back regardless of which type of therapy or therapies doctors initially employ to get rid of it.

Surgery: Cancer can return after surgery if any cancer cells were left behind during the operation. Small cancer cells sometimes break away from the primary cancer prior to surgery, and these are too difficult for surgeons to see and remove during surgery. Surgeons who suspect they left some cancer cells behind may recommend additional treatment to attempt to kill any cancer cells still in the body.

Chemotherapy: Cancer occurs when cells do not die and begin to grow uncontrollably. Chemotherapy aims to kill cells that are attempting to double and form new cells. However, all cells do not divide at the same time. This is why chemotherapy involves a series of treatment sessions, as physicians hope stretching treatment out over time will enable them to catch as many dividing cells as possible. Some cancer cells left behind after chemotherapy may fall victim to the body’s immune system, while others will simply die off. But some may survive and ultimately divide, prompting cancer to return.

Radiation: Radiation therapy aims to prevent cells from growing and dividing by making small breaks in the DNA inside the cells. Radiation therapy is common and effective, but it can leave cancer cells behind. When it does, those leftover cancer cells can lead to a recurrence of cancer.

No one wants to receive a single cancer diagnosis, much less learn that their cancer has returned after treatment. But cancer can return, and when it does, it’s important that patients fight just as hard as they did during their initial treatment sessions.