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a. The concept of equivalent square for quantities describing

phantom scatter

Equivalent field

is defined as 'the standard’ (i.e. circular or square)

field that has the same central axis depth dose characteristics as the

given non-standard field

(Day and Aird 1996, BJR25, 138).

The equivalency between standard and non-standard fields is determined by the

requirement that the contribution to the dose along CAX from scattered photons

for the two fields be equal.

Namely, that the quantity describing phantom scatter

(e.g

scatter factor

) in the standard and non-standard field at the point of

calculation is equal.