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Summary 2 : 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 penumbra shaping
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.