retrocerebellar arachnoid cysts are a cyst that is filled with fluid. The cyst in this case is located in the retrocerebellar area which is behind the cerebellum. The cerebellum is the part of the brain that has to do with motor function.

Infants are most susceptible to developing arachnoid cysts, although cyst formation can occur up through adolescence. Arachnoid cyst development in adults occurs much less frequently. Arachnoid cysts occur predominantly in males.

Arachnoid cysts can be classified according to their location and by the type of tissue making up the cyst wall (arachnoid connective tissue or glioependymal tissue).

It can be: arachnoid cyst colloid cyst enlarged perivascular space dermoid cyst choroid plexus cyst pineal cyst Rathke cleft cyst cystic neoplasm parasites (neurocysticercosis) DW malformation

Arachnoid cysts can cause chronic remodeling and thinning of the adjacent skull. So--while I've personally never seen a case of skull fx directly overlying an arachnoid cyst (general diagnostic radiologist, 6 yrs in practice)--it certainly would seem that the involved portion of skull would be relatively more susceptible to fracture in the event of trauma.