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Phantom scatter: equivalent square of a rectangular field

Area over perimeter method

(Sterling et al 1964 BJR 37, 544; Patomaki 1968, BJR 41,381 etc)

"

4"

PA

or

Perimeter

Area

S

2

4 2

2

S

a b

S

a b

a b

S

a b

Method not based on sound physical principles, BUT used for years, with surprisingy

accurate results (<1%) for rectangular fields of length <20cm and length/width<4

(Day

and Aird 1996 BJR25, 138,McDermott, MedPhys 25(11), 2215, 1998).