Firstly we have to know wheather it is a primary or a secondary brain cancer
. Secondly it is very important to have the histological/ cyto pathological details of the tumor
. To have the second opinion
you dont need a referal from your Primary Doctor. it is your life you can take any number of opinions for which you need no permissions.
There are some cancers which have spread to brain can be treated with curative intent with different modalities and you have fair chances of good life expectancy. For this the proper diagnosis is very important.