Beam profile and penumbra

Beam penumbra tend to be larger for electron beams than for photon beams. For electron beams, dose falls to 90% of central axis value approximately 1 cm inside the geometric field edge for depths near the dose maximum; a 10 x 10 cm2 beam, for instance, produces an

'effective' field size of only 8 x 8 cm2. The corresponding distance for a photon beam is only about 0.5 cm. Thus, a larger electron beam is required to cover a given target to a clinically useful dose. This property of electron beams makes abutting of photon and electron beams problematical since a uniform dose across a field junction cannot be achieved at all depths.

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