Cancer cells and their ability to avoid the immune system
There are several types of cancer immune evasion.
Such as releasing immunesupressive cytokines, downregulation of MCH and comprising mutiations which are not immunogenic. 
If the cancer at a tissue is evading immune system by immunosupressive cytokines it may not be affected from auto-immune attack.
However if the cancer is not attacked by the immune system just because it is not "different" enough from the nearby healthy tissues, then at this time auto-immune attack to the tissue may technnically attack the cancer also.