My mother had fluid in her lungs, and she was told that there were cancer cells in the fluid. The surgeon did the biopsy of a tumor that was in her breast, said it cancerous, and her doctor diagnosed her with Stage IV Breast Cancer. My mother received two courses of chemotherapy (one day every 3 weeks). Her lungs filled with fluid again, but this time when the fluid was tested, there were no cancer cells in the fluid. What does this mean, if anything?