What characteristic of linac photon beams leads to them being non-uniform across field sizes?

Photon beams produced by linear accelerators (linacs) are inherently non-uniform across different field sizes due to the effects of the flattening filter. The flattening filter is a device used in the beam path that serves to modify the intensity profile of the radiation beam.

When linacs produce photons, the beam has a natural tendency to be more intense in the center and less intense towards the periphery, creating a profile that is "peaked." The flattening filter is inserted into the beam path to shape this intensity distribution into a flatter profile, which is desirable for therapeutic doses across a larger treatment area. However, the effectiveness of this flattening filter can vary depending on the field size; larger fields may exhibit varying degrees of flatness or uniformity compared to smaller fields, due to the geometry of the filter and the production characteristics of the beam.

In practice, because larger field sizes require different distributions to achieve the desired homogeneous dose across the planning target volume, the influence of the flattening filter results in varying uniformity across field sizes. Thus, understanding the role of the flattening filter is crucial in medical dosimetry for accurately planning and delivering radiation therapy.