Detection of Leptomeningeal Carcinomatosis by Cerebrospinal Fluid in a Dog with a Negative MRI

An 11-year-old female French bulldog was presented for acute onset of seizures and a 2-week history of disorientation. On physical examination, a nodular mass at the fourth mammary gland level was observed. Neurological evaluation showed obtundation and compulsive behavior. Brain MRI study did not reveal any abnormalities. Cerebrospinal fluid (CSF) collected from the cerebellomedullary cistern showed a marked increase of total nucleated cell count (400 cells/μL). Cytological evaluation identified the presence of a monomorphic round cell population characterized by large cell bodies, a single eccentrical located nucleus with high nuclear:cytoplasmatic ratio, and marked atypia with anisocytosis, anisokaryosis, and multiple nucleoli. Leptomeningeal carcinomatosis (LC) was suspected. The dog was euthanatized for worsening of clinical signs. Post-mortem examination identified an anaplastic mammary carcinoma in the nodular mammary mass. Infiltration by neoplastic cells exhibiting the same morphological features was detected along leptomeninges of the telencephalon and cerebellum associated with cortical and subcortical parenchymal micrometastases. To our knowledge, this is the first case of LC in a dog detected by CSF evaluation but without any MRI abnormalities. This finding emphasizes the usefulness of CSF cytology in patients with suspected LC even in the absence of any MRI identifiable lesions.