What does the term 'photon energy' relate to in the context of brachytherapy sources?

The term 'photon energy' in the context of brachytherapy sources is directly associated with the ability to penetrate tissues. Higher photon energy allows for greater penetration through bodily tissues, which is crucial in ensuring that the radiation can reach the cancerous cells effectively while minimizing exposure to surrounding healthy tissues. In brachytherapy, the delivered radiation needs to penetrate to a sufficient depth to target tumors while being confined to a specific area, thus relying heavily on the energy of the photons emitted by the radioactive sources. This factor significantly influences the effectiveness of the treatment and is a key consideration in the selection and application of brachytherapy sources.