Understanding the brain a bit, with an emphasis on the cerebellum. Please add if you can.
The brain is divided up by the dura mater into three regions: the left and right hemispheres (supratentorial) and the posterior fossa region also referred to as infratentorial. The dura mater has lots or nerve endings and can feel pain, in contrast the brain does not have such nerve endings so unless determined by brain scans, many brain mets, lesions and tumors won't be found until symptoms manifest.
The tentorium cerebelli (
https://en.m.wikipedia.org/wiki/Tentorium_cerebelli) is the structure that isolates the cerebellum from the rest of the brain. This is the rigid membrane that they refer to tumor being above (supratentorial) or below (infratentorial). Surgeries should take place in either compartment but not to the tentorium as its structure can't be repaired or replaced.
The cerebellum is responsible for movement coordination though people who have done a lot of movement study will find that cerebellar coordination activities are distributed across the brain and the cerebellum is more resistant to showing issues.
The cerebellum has a large surface area, plus a greater risk of developing LM after a craniotomy. Hesitation about treating the cerebellum increases the risk of spread and decreases the patient's chance of rapid treatment to stop LM spread.
https://en.m.wikipedia.org/wiki/Anat...the_cerebellum
Much more detailed information about the cerebellum and its structures:
http://nba.uth.tmc.edu/neuroscience/m/s3/chapter05.html
Overview and good diagram showing the different areas of the whole brain
http://www.brainwaves.com