A small photon-intensity element used in IMRT is called a:

In the context of intensity-modulated radiation therapy (IMRT), a photon-intensity element is referred to as a beamlet. This term specifically describes the individual units of the radiation beam that are modulated in intensity across the treatment area. Each beamlet can be controlled to deliver a specific dose of radiation to targeted tissues while minimizing exposure to surrounding healthy tissue.

The choice of the term "beamlet" is crucial, as it conveys the idea of a small part of the overall radiation beam that can be adjusted independently. This allows for highly precise treatment planning, essential in targeting tumors accurately while preserving adjacent normal tissues. The use of beamlets is fundamental in the optimization algorithms that define the treatment plans.

In contrast, the terms pixel and voxel have different meanings; a pixel is related to two-dimensional digital images, while a voxel pertains to three-dimensional space (Volume Element) in imaging or treatment planning contexts. The term "segment" often refers to a distinct part or section of a treatment plan but does not specifically denote the small photon-intensity elements. Therefore, beamlet is the appropriate term for this context.