Of course you're right, and that's why the role of DNA in cancer is undeniable. It's possible to argue (what Warburg did in the 1920s) that insult to mitochondria can kick off cancer, but it's almost impossible to maintain that this can occur and progress into malignant cancer without somatic mutations being involved at some point. Without encoding to DNA, how will the cancer continue to behave malignantly across cell generations? A mutation free cancer cell is an oxymoron.