Malignant cells that have broken away from a tumor within the brain or meninges may also be present in spinal fluid. Central nervous system (CNS) involvement is most common with melanoma, breast carcinoma, and lung carcinoma. The image on the right is a Wright-stained smear (1000X) of CSF from a case of metastatic breast carcinoma. The arrow indicates a vacuole present in one of the tumor cells. Dramatic vacuolation is one characteristic of malignant cells in CSF specimens.