There are doctors that won't do it but far more that will. I've been through cancer treatment as stage4 recently 2 friend I met going through treatment sadly passed away. One had enough with treatment, 4 years 96 rounds of chemo. He chose to stop. And the other did chemo until the end. In both cases it was completely their choice. If your told they will no longer treat you although that is rare, you just make an appointment with a different oncologist.