Dose Course 2018_Flipping book

9

About σ

❑ The lateral spread (or σ ) increases with depth until a maximum spread is achieved. ❑ Beyond this depth there is a rapid loss of electrons as their larger lateral excursion causes them to run out of energy. ❑ σ increases with depth indefinitely using Fermi-Eyges theory, which is contrary to what is observed experimentally in a narrow-beam dose distribution. ❑ σ is also over-estimated since electrons who have been thru large angle scatter events at shallower depths and lost due to loss of energy are ignored (Barry Med Phys 1982).

Dublin 2018