Dewan et al. compared the efficacy of three RT regimens: 20 Gy × 1 fraction, 8 Gy × 3, and 6 Gy × 5 in combination with anti-CTLA4 (the latter had no effect on either model when given alone) and concluded that three or five radiation fractions provided a greater immune-stimulating effect at distant, nonradiated sites as compared to a single fraction of radiation.
Source: wiktionary