June 1, 2017 -- Proton therapy is considered by some as the optimal radiation delivery modality -- with the finite range of protons enabling highly conformal dose targeting and reduced dose to normal tissue. Image guidance, using cone-beam CT (CBCT) for example, should increase the accuracy and improve outcomes further. But alongside, recent years have seen the emergence of online MR-guided radiotherapy, promising unparalleled soft-tissue image contrast and the ability to "see what you treat." In a theatrical debate at the recent European Society for Radiotherapy and Oncology (ESTRO) meeting, speakers considered whether proton-guided photons will be superior to photon-guided protons.