Figure 5.

Relative output as a function of field size for the 6 and 18 MV photon beams measured with the MapCHECK and MatriXX, and compared with the measurement using a Farmer‐type ion chamber. Measurement was done in a solid water phantom at 100 cm source to detector distance (SDD) and 10 cm depth. Field sizes varied from $4\times 4\hspace{0.17em}{\text{cm}}^2$ to $22\times 22\hspace{0.17em}{\text{cm}}^2$.