The term "relapse" means that a cancer has returned after a period of time where it could not be detected.
Cancer can relapse when a few cells remain after treatment. The amount may be too small for doctors to identify them, but over time they multiply, eventually reaching an amount that can be detected with tests.
Relapse is not necessarily a sign that there was something wrong with the treatment that doctors chose for you, or how it was given. Some cancers are just more aggressive and difficult to destroy than others. Each person's response to treatment is different.