The risk of developing LM in breast cancer is actually quite high comparatively and yet the authors claim no increased risk across the patient population of cancer patients with brain mets. Compare the number of breast cancer patients in the study with the number of breast cancer patients who developed LM. Infratentorium refers to the posterior fossa region of the brain, where the cerebellum resides, and HER2 already has a demonstrated proclivity to. When you compare the number of breast cancer patients in the study to how many developed LM it is very high but the authors claim across all study groups that it was low, which isn't the case in what I can see -- and for us HER2 gals is deadly wrong.
"Leptomeningeal carcinomatosis developed in 12 cases (14%) and was associated with breast histology and infratentorial cavities (p = 0.024 and 0.012, respectively)"
http://www.ncbi.nlm.nih.gov/pubmed/25434940