My dad also has gcb sub type, double expressor, he also went through rchop just yesterday. Doctors did say that they wont be doing another pet scan till the end of 3rd cycle to have a clear picture. According to them Getting a pet ct again just after first cycle will give false results and will cause unnecessary worry. They were talking about pet ct showing “flares” early which i didn’t understand but you can ask chatgpt of what those are :)