Fluid in the lungs after treatment- Please help

My father has had leukemia for 7 years now and finished his treatemnt end of last year. Everything looked good. Then he got what looked like a cold as my mother was sick, and doctors thought it was pneumonia now say its not. They have no idea what it is and its been almost 2 months. He has had a CT scan, an x ray, blood work, 165 ml of fluid drained from his lungs and they are now doing tests on the fluid. They have not found anything. He has shortness of breath, heavyness in the chest, cough, and the worst has low appatite and has lost 12 pounds. They will be doing a biopsy soon but I am concerned that it could be lung cancer now and it would get worst fast. If anyone has had similar experiences we would greatly appreciate it.