How Can Prostate Cancer Be Treated?

My father recently had his bladder ,prostate and surrounding tissue removed along with 16 Lymphnodes.They were removed due to Bladder Cancer that had moved out to wall and fatty tissue surrounding his bladder..They built a neo bladder from his intestine and he was told all looked good..He was recovering well.Then we got the results from pathology stating they found a small cancer in his prostate that would have been treatable but it has been removed .Then they said they found a granular size cancer cell in 12 of the removed pelviuc lymphnodes..They said the Chemo he had prior to surgery was noir very effective and the probability of it returning was 70 to 80 %.They also said no further Chemo was necessary and would monitor him every 3months..I am confused as to why no Chemo if chances of returning are high.. .