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Penumbra
•
Penumbra is often defined as the distance between
the 20% and the 80% (10% and 90%) isodose lines
•
The penumbra is the region near the edge of the
field where the dose falls off rapidly
Width depends on
Size of ‘source’
SSD/FSD (Lower SSD, higher penumbra)
Energy (Increasing penumbra with increasing
energy: increased field size)