Keytruda side effects (melanoma stage IV)

Hello everyone

Thank you so much for being so helpful in the past. I have another question regarding your experience with treatment. 

My mother has stage IV Melanoma, mets in brain (3) and in the lung (1). 

There was swelling in her brain caused by two of the mets (liquid around them) which was taken care of during the operation. Two of the mets were taken out during the op and 1 is still there. 

We are preparing to move her to another hospital, but doctors are considering giving her first round of treatment now. 

We have mentioned Keytruda to them which they know about and think it may be helpful, but they are saying that Keytruda may produce another swelling (liquid) in her brain. 

Is that a known side effect of Keytruda and other immunotheraphy drugs?

Thank you