Benign
The word benign is often used to refer to a medical condition that will not become life-threatening if it is not treated, but can also mean that something is not harmful.  It is often used in relation to tumours that do not metastasise to other parts of the body.  Tumours that are referred to as being benign may still be life-threatening because of other reasons.  Because of this, the term applies mainly to their biological behaviour.