 This is a phase II study to look at the effectiveness and safety of the combination of a new drug called pembrolizumab plus radiotherapy compared to pembrolizumab alone. The purpose of this study is to see if the addition of radiotherapy to pembrolizumab is better than pembrolizumab alone by measuring how long these treatments can control the growth of the cancer. Also it will assess if by adding radiotherapy the investigators can see its effects not only in the tumour that has had radiotherapy but also in other tumours in the rest of the body. 