Abstract: The present disclosure relates to the field of radiotherapy, in particular, methods of and apparatus for treating cancer using ultra-high dose rate radiotherapy (FLASH). The apparatus may comprise a device configured to administer to the subject no more than five fractions of proton ultra-high dose rate radiotherapy (FLASH), said fractions having a range of radiation from 1.5 Gy to 60 Gy, collectively. The device may be configured such that the treatment is sufficient to prevent further growth of the tumor for at least 10% longer than standard of care radiotherapy, induce at least 10% more tumor regression than standard of care radiotherapy and/or delay tumor regrowth by at least about 2 months longer than standard of care radiotherapy.