In brachytherapy, what is meant by 'scatter build-up'?

In the context of brachytherapy, 'scatter build-up' refers to the increase in dose due to secondary radiation interactions. In brachytherapy, a radioactive source is placed close to or within the tumor, which results in the emission of radiation. As this radiation travels through tissue, it interacts with atoms in the surrounding medium, causing additional ionization and the production of secondary radiation, such as scattered photons.

This scattered radiation can contribute to an increased dose to the surrounding tissues, above what would be expected from the primary radiation emitted by the source alone. Understanding this phenomenon is crucial for accurate dose calculations and treatment planning, as it helps ensure that adequate treatment is delivered to the tumor while minimizing unwanted exposure to healthy tissues.

The other options do not accurately capture the concept of scatter build-up in this specific context of radiation therapy. While the proximity to the radiation source and attenuation effects are important considerations in dosimetry, they do not directly define the term 'scatter build-up' as it relates to secondary radiation resulting from interactions within the tissue.