For a 15 cm x 15 cm field, what would you expect the PSF for an 18 MV beam to be?

The Percent Surface Dose (PSD) is influenced by factors like beam energy, field size, and the treatment technique. For an 18 MV photon beam, the percentage surface dose reflects how much of the beam's dose is delivered at the surface of the patient compared to the maximum dose delivered at a specific depth.

For larger field sizes, such as 15 cm x 15 cm, the surface dose tends to be slightly higher than what might be observed for smaller fields due to the flattening filter used in high-energy machines, which expands the beam profile and reduces the effects of beam hardening. The high-energy beams, such as 18 MV, also exhibit increased scatter in larger fields, contributing to a more buoyant elevation of the dose at the surface.

A surface dose assessment of approximately 1.008 is consistent with the well-documented data for high-energy photon beams. Specifically, at an 18 MV energy level, the value reflects how the dose slightly exceeds the 100% reference point (1.00) because of beam characteristics, including scattering and dose build-up in larger field sizes.

Thus, when considering the physics behind high-energy photon therapy and the clinical application of these values, the estimated PSF of 1.008