Volume of tumour

Hello,

My mum is undergoing treatment for triple negative breast cancer.

She has finished chemo now and is waiting surgery in the new year.

She had a scan halfway through chemo and sadly the tumour does not appear to have shrunk by much, however, she was told that the volume inside the tumour has changed.

What would this indicate? Can the inside of a tumour dissolve but the outside remain the same circumference?

Thanks x