PET scan before or after chemo?

Hi all. I apologise if my question is stupid, but I was wondering if the PET scan is usually done before or after chemotherapy? Or is the PET scan necessary to determine the type of cancer and stage? My brother has had an orchiectomy almost two weeks ago but pathology will come back only on the 27th, so over two weeks from now. The urologist referred him for a PET scan in the meantime, but the first dates available are in the first half of October. So my question is, will he be able to start chemotherapy before the PET scan? His tumor markers before the orchi were normal, but the CT scan (a day after orchi) showed engrossed lymph nodes in the lumboaortic region, the biggest one of around 3cm.