Dose Course 2018_Flipping book

Summary: TPS modelling of small & composite fields

• Modelling of the direct beam source becomes important in narrow collimated modulated fields due to source occlussion

• In small and modulated fields the MLC is more involved in shaping primary energy fluence, so modelling this becomes important

• Explicit modelling of the direct beam source size and the effects due to the MLC is not common in most TPS. Implementations (approximations) vary.

• Fine resolution is required in dose but also energy fluence calculations for small field modelling

• Only at very high beam energies (or in low density regions) lateral electron transport alone dominates dose in the penumbra → a beam model that accounts for finite source size and collimating jaw effects plays a primary role in the dose calculated by the TPS

• Most TPSs do not require small field measured data as default input to their beam configuration engines. 48