The decision to recommend radiotherapy is never trivial, and is based on careful assessments of the potential benefits and associated risks. Re-treatment using photon-based Radiotherapy, although less commonly used, requires additional care in terms of patient selection, as either a radical or palliative approach to recurrent or metastatic tumour, or for new cancers occurring in an adjacent site. Parts of the central nervous system (CNS) are irradiated not only when treating tumours that originate in the CNS, but also when treating cancers that originate in other tissues.