This is a very difficult questions to answer without seeing the rash and knowing a bit more about your uncle's case. I will say that there are many different types of skin reactions that can be associated with various internal cancers. These range from causing changes in the color of the skin, to causing the skin to peel, to provoking true rashes. Each of these changes tends to be associated with a different type of cancer.
Based on the symptoms that you are describing to me, especially the rash over the eyes and neck and the associated joint and muscle pain
, I would say that the most likely of the many possibilities is that this is something called dermatomyositis. Dermatomyositis is an autoimmune condition of the body that is associated sometimes, but not always with cancer. People with dermatomyositis have probably a 5 times increases risk of developing various cancers, especially lung, bladder, pancreas and stomach.
Despite this most people with dermatomyositis do not end up having cancer. It sounds, however, that in your uncles case they may have found a cancer. Therefore, he will need to follow up closely with his cancer doctor
and other doctors
to make sure he gets the best treatment possible.