Right Sided Effusion in Chest

Hello, my cat has feline leukemia since diagnosed at 7 months old. He is currently 1 year and 9 months old now, he was dealing with anemia on his first birthday but regenerated, and battled anemia again recently but started to regenerate. I brought him into the Vet a couple days ago for an xray because he was wheezing and having trouble breathing, but I assumed it was from low oxygen from anemia. His xray shows fluid on the right side of this chest, they suspect widened cranial mediastinum, but they were unable to give me a 100% that it is cancer. Can you help me locate on his xray where you are able to spot the cancer or tumor? I have read that with felv they are susceptiable to all kinds of infections and he has recently been sick and had on and off fevers. I have a CBC from him on June 23rd and July 24th, but they never mentioned cancer to me from blood work. I have read fluid in chest can come from all types of things such as infections, pneumonia to cancers. Do you think that Lasix would help him with the fluid in his chest? Or possibly draining the fluid out of his chest would help him and they wuold be able to test the fluid for cancer? How painful would this be for my cat? I just want to make sure there is nothing I can do to help him before I let him go. 

Hello!  We do not give second opinions on radiographs on this site, as you will see from the site rules.  As short of context as I am, I would not be happy to make the diagnosis you mention from these radiographs.  To get a proper, formal second opinion, you can have the radiographs sent away with your vets' permission.  This can be done by email these days and we would recommend it to anyone who is unsure about a diagnosis at such a vital stage.