Compulsory PET screening for after cancer treatment

may 2015 my dad was diognosed with a non hogkins lymphoma 

he under went chemo while battling singles  

January 2016 he was given the clear ! And by June 2016 he had been diognosed with a brain tumour and was diognosed terminal after a long battle we lost him 


why wasn't he offered a full body scan after his first cancer when it is proven for cancers to spread I want it compulsory if you have had cancer treatment you are offered a pet scan