Does radiotherapy cause brain fog?

Yep, brain overload as much as everything else, @rose, possibly even affected from the anaesthetic as well .... I didn't have chemo but did have rads - & also have memory/speech issues just as you've experienced.  

I just apologise in advance now for forgetting everyone's names & all my friends know to email me with things I am supposed to follow up on later on, as I'll have forgotten about them by the time I get home!  Mind you, my husband has been diagnosed with dementia, so maybe it has rubbed off onto me too??  I hope not!

take care x