Without sounding like a broken record I suspect Marek's Disease. After seeing the photo I am almost sure. Marek's is a form of cancer that is triggered by the herpes virus. Every organ or any organ on or in a chickens' body can be involved. Some common examples are a chickens eyes, their nerves, or their skin. Each type of cancer may well present itself in a different fashion so it is very hard to be sure of a diagnosis of Marek's Disease.