\\ recently underwent 'routine' gallbladder surgery and they have also found cancer.\\\
That doesn't indicate whether it is malignancy in the Gall Bladder OR the adjacent liver........a biopsy would help in that situation and that would give us some indication of the prognosis.
\\\\Is this the normal procedure - the lumbar test\\\
Yes.....perfectly normal following an abnormality on a brain MRI.
The "something" may well have nothing to do with the cancer, but it certainly is suspicious and a LP may well aid in diagnosis.
The brain is not a common site for gall bladder cancer spread (if it is gall bladder cancer) buit if the cancer is in the liver AND the brain then we are hunting for another site of primary cancer.
Too early to say more at the moment, but will be interested in any further developments.