Electron beam dose calculation: Hybrid pencil beam model extended for small fields

The hybrid electron pencil beam model (HPBM) can usually be applied to middle or large fields and get good results, whereas it is sometimes not so successful to apply HPBM to small fields. In this note we report some results obtained by using an extended hybrid electron pencil beam model for small fields. Considering the condition of small fields, we redefine the central-axis depth dose for mono-energetic electrons in small fields and calculate the fitted spectrum of incident electron beam for these small fields. Through these improvements, we can make the extended hybrid electron pencil beam model applicable to small fields and obtain good results. And at the same time, the extended HPBM retains the calculation accuracy and efficiency for middle and large fields.