Brain right ventricle 8mm lesion -cyst or tumor?

Hard to say: If the interpretation on the MRI by the radiologist is that it is a cyst, then it is likely benign. It also depends if there is any area of enhancement that would suggest a tumor. Also if you have serial MRI scans, you can tell if it is getting bigger or changing which is more likely a tumor. The location in the ventricle is important too. If it obstructs the foramen it could be a problem.

Grey area: Where in the right ventricle is it located? Is it near the 3rd ventricle? Would see a neurosurgeon.