What's the downside of pet scan? is it needed before you are administered with chemo

Low risk: PET scans are low risk. There is a very small potential risk of an allergic reaction to the radioactive tracer that is used in the scan. The PET scans can show metabolic function in a way that other scans can not. They are often done before chemo so that after chemo they can track the progress of the treatment.

PET scan ...: Depending on the type of cancer, PET scans are often performed for initial staging evaluation. Once the initial stage is determined then a treatment plan can be formulated. Follow-up PET scans can be used for restaging. The initial PET scan is often helpful as a baseline study. The downside is radiation exposure but the scan should be performed if the benefits outweigh the risks.