Can anyone explain this medical term pls?

Hi  my dad was diagnosed with terminal cancer and has been sent home with pallative care until he passes  - i have read his discharge notes and I just wondered if anyone can explain what one phrase means please? 

A CT Brain showed a large necrotic lesion within the right hemisphere. 

They said he had a brain tumour and lung cancer both diagnosed from CT scans only - all I can establish is that necrotic means dead tissue / cells - it has just puzzled me thats all - if anyone can shed any light xx