How is the depth of the 80% isodose line for a clinical electron beam of energy E (MeV) approximately defined?

The depth of the 80% isodose line for a clinical electron beam is defined based on the energy of the electron beam in megavolts (MeV). In clinical practice, it is commonly accepted that the depth of the 80% isodose line occurs at approximately 1/3 of the energy of the electron beam. This relationship is based on empirical data collected from radiotherapy practices, which indicates that as electron beam energy increases, the penetration capability of the beam also increases, directly affecting the depth of key isodose lines such as the 80%.

By understanding this relationship, medical dosimetrists can accurately predict where optimal radiation delivery occurs within the tissue, ensuring that the tumor receives adequate radiation while minimizing the dose to surrounding healthy tissues. This depth estimation helps in planning the treatment fields and consequently contributes to effective patient care in radiation therapy.