The image shows the pleural fluid smear of a 72 year-old female patient diagnosed with breast cancer. The cancerous cells have metastasized to the lung and liver. Malignant pleural effusion (MPE) is the build up of fluid and cancer cells that collect between the chest wall and the lung. The MPE forms when tumor cells from lung cancer, breast cancer lymphoma or other types of cancer spread to the pleural space. These cancer cells increase the production of pleural fluid and cause decreased absorption of fluid.