Fig. 8

The SPR of the EPID under different conditions. a Shows the central axis SPRs of different thicknesses (3, 10, 20, 30, and 40 cm) and different field sizes (3–20 cm). b Shows the off-axis SPRs of different thicknesses and different off-axis distances (0–10 cm) when the field size is 20 × 20 cm². c Shows the central axis SPRs of different exit gaps and different field sizes (3–20 cm) when the thickness is 20 cm