My spouse has stage IV which has metastasized to the brain last June. He went in for whole brain radiation in September and I did notice a bit of forgetfulness, and occasional confusion. The front lobe (near the forehead) caters to short term yet I imagine it would depend on the location of the tumor.
My husband just finished the gamma ray for 8 lesions and so far has done tremendous, a bit tired yet coherent. He does repeat himself sometimes.
With the brain, I was told to watch carefully for confusion, ability to move hands, legs, hands numbing and tingling sensations in the fingertips. Finally, if walking began to be a problem this was also a 'check-up' sign.
It never hurts to talk to the doctor away from your cancer patient... sometimes this really helps you define the questions the patient may not be able to ask. This has helped me anyway.
Best of luck.
Demetria