Abstract: A method is proposed for choosing electron cutoff energy,which is self-adaptive with cell size,background material and electron position.As residuary range of an electron defined by total stopping power is less than distance to the nearest cell boundary,the electron is killed.Compared with the CUT model used in MCNP program,efficiency and precision of self-adaptation method is higher.The most important is that cutoff energy of electron is selected automatically in program in all models,without consideration of energy of particles or dimension of model.

Key words: energy deposition, X-rays, radiotherapy, cutoff