FIGURE 1.

Diagram illustrating the principal research question addressed in this work. For hypofractionated (HF) ultra‐high dose rate (UHDR) radiotherapy (RT) to be favorable compared to normofractionated (NF) conventional dose rate (CONV) RT with the same relative dose distribution (i.e., the same geometric sparing r), the FLASH effect needs to reduce radiobiological damage (RD) to critical normal tissue (NT) regions more than a NF CONV treatment would do. This minimum RD reduction is quantified in this work by the “break‐even NT sparing weighting factor” W _BE and is obtained by requiring that RD to the tumor (T) and the NT are the same for the NF CONV treatment and the HF UHDR treatment. Symbols are defined in Table 1.