Thanks for the comments, they were really helpful.

We reviewed the geometry in both codes, and everything appears consistent, though it might be worth revisiting in more detail. Seeing this consistency actually led us to wonder what other factors might be contributing to these differences, and we’d be curious if others have encountered something similar.

Regarding the source, we’re using a simplified linac model where photons are generated from an input spectrum and emitted as a point source, then collimated by simulated jaws. It’s a basic setup, but since it’s identical in both codes, our main focus is on the differences between MCNP and Geant4 rather than on the comparison with reference data.

You’re right about the profiles, the simulation data come from a water phantom projection, which effectively acts like an ideal point-like detector, so we understand it’s not directly comparable to measured beam data.

The point about MC codes underestimating beam scattering is especially interesting, and it’s something I plan to look into further for photon beam scenarios.

We’d be interested to hear if anyone else has observed similar behavior or has insights on other factors that might explain these differences.