Fig. 2 Representation of the difference between the preclinical beam (SARRP) and the clinical beam. **a** The 3 different spectra (220 kVp, 6 MV FFF, and 6 MV STD) were represented near the k-edge of the Gadolinium (50.2 kV). Calculation were performed with Monte Carlo simulation at 35 cm source–skin distance; 12 mm circular field size for the 220 kVp machine, and at 90 cm source–skin distance; 10 cm depth; 10 × 10 cm^{2} field size for the 6 MV irradiations. **b** Percentage depth dose (PDD) for the 3 photon beams. The maximum dose for the 220 kVp occurs at the surface while it is at approximately 1.5 cm for both 6MV beams