A type of radiotherapy that produces particularly precise targeting of tumour tissue, thus reducing damage to surrounding unaffected cells. It has been shown to be effective for tumours in the eye, such as intraocular melanomas, tumours at the base of the skull and those of the spinal cord where conventional radiotherapy may exceed a safe radiation dose. Certain other tumours may also be better treated with this device but for many for example prostate cancer, there is not yet sufficient evidence of superiority over other methods.