Cancer isn’t always a one-time event. It can be a chronic (ongoing) illness, much like diabetes or heart disease. Cancer can be closely watched and treated, but sometimes it never completely goes away. This is often the case with certain cancer types, such as ovarian cancer, chronic leukemias, and some lymphomas. Sometimes cancers that have spread or have come back in other parts of the body, like metastatic breast or prostate cancer, also become chronic cancers.
Source and full text: American Cancer Society http://www.cancer.org/treatment/survivorshipduringandaftertreatment/when-cancer-doesnt-go-away