Helping someone with dementia to stop obsessing takes a little time and a great deal of creativity. Each time your Mom starts a conversation about the obsessive subject, it is
With that said, I will share a technique that may seem counter-intuitive at first try. Practice really does make perfect! I want you to try a therapeutic lie or 'fiblet' as I refer to it. Get into Mom's world by being in touch with her feelings when she is obsessing. I'm sure when you tell her that there is no 'cancerous spot' she thinks you are not telling the truth. This sets up a less than ideal interaction. Next time, try agreeing with her - remember it is really her emotion you are connecting to and not the words. You might say something like, "the medication has worked wonders Mom - the spot is completely healed". You may need to go as far as taking a pill (M&M's work great!) in her presence and show her that the 'pills' are almost all gone. Assure her that the Docs tell you the 'spot' is NOT from her smoke but from an allergy you had last year.
Your non-verbal communication style will govern how smoothly you relate. Try an expression of joy - rejoicing in the shrinking of the 'spot'...practice in front of the mirror being aware that dementia folk are adept at reading what we are NOT saying. Be inventive - be creative - but most of all, be her ally in the obsession. You will both feel better for it.
the first time for her. Your body language will be the key to making her feel respected and it will help to keep her unaware of the repetitiveness which is, by the way, a hallmark of this dementia.