On pet scan the phrase "central photopenia suggesting necrosis" what does this mean?

Usually necrosis: PET scan uses F18 FDG (a glucose metabolite) to localize active metabolic processes such as tumors. The uptake of the radiopharmaceutical is considered hot when increased over normal tissues or background Sometimes central portion of tumor is photogenic or cold reflecting necrosis or death of some of tumor. Necrosis can be related to chemotherapy or on presentation when tumor outgrows blood supply