Radiotherapy, apart from its direct tumor cell killing potential, has a contradictory effect on the anti tumor immune response: it augments certain immune parameters, while it depresses others.Treg cells are intrinsically radio resistant due to reduced apoptosis and increased proliferation, which leads to their systemic and/or intratumoral enrichment. While physiologically Treg suppression is not enhanced by irradiation, this is not the case in a tumorous environment, where Tregs acquire a highly suppressive phenotype ,which is further increased by radiotherapy.