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Summary:
equivalence between square, rectangular,
circular or irregular fields
• Output in air ratio, S
c
, in air is not symmetric in X and Y
➢
Equivalent squares with individual weighting of collimator elements can lead
to sufficiently accurate approximation
➢
For elongated fields less accurate
• Phantom (volume) scatter factor, S
p
, is symmetric in X and Y
➢
Traditional equivalent square formula lead to sufficiently accurate
approximation for S
p
, TPR, RDD, PDD
➢
For elongated fields less accurate
• Irregular blocked or MLC shaped fields require more sophisticated models