FIGURE 4.

Points display FLASH modifying factors (FMF) for different mammalian organs/body parts derived from single fraction experiments as a function of single fraction ultra‐high dose rate (UHDR) dose (data reproduced from Ref. [14]). Break‐even normal tissue (NT) sparing factors W _BE as a function of dose per fraction to the NT are displayed as predicted by the linear‐quadratic (LQ) (solid lines) and LQ‐linear (LQ‐L) (dashed lines) models for a generic scenario using α/β‐ratios of (α/β)_NT = 3 Gy and (α/β)_T = 10 Gy for late‐reacting NT and the tumor, respectively, and r = 1.0. W _BE values are displayed for a dose per fraction range from 2 to 40 Gy. An FMF smaller than W _BE (i.e., dark‐shaded area for the LQ model and light‐shaded area for the LQ‐L model) needs to be reached by a hypofractionated UHDR treatment with dose per fraction d, to be advantageous for a tissue region at dose level r, compared to a normofractionated conventional dose rate treatment. A normofractionated treatment with $d=2\mathrm{Gy}$ is indicated by a gray dashed vertical line.