The lesion could be a neoplasm, bacterial or viral infection. It is obvious that the fish has an advanced case of HLLE. You should be making some big changes in what you feed the fish in the way of quality, variety and supplements. I believe that anyting that is a chronic stressor contributes to HLLE. The biggest factors are probably dietary, water quality issues and stress related. Jay may very well be right that the fish has tuberculosis. This would not be surprising since the fish's immune function has probably been surpressed for quite a while now.
Terry B