Dosimetric Characteristics of Clinical Photon Beams

Beam Penetration for Irregularly-
Shaped Fields
Concept p of Equivalent q Square: q
�The equivalent field is defined as
that standard (square or circular)
field which has the same centralaxis
depth dose characteristics as
the given non-standard field.
“Day’s Rule”:
S
�� �� �� r
r �
S 1
��r
�
��r
�
r � S 1�
e
�
� � � � � r � e
�
�
Water
S(r) = the central axis scatter in a field of radius r, S∞ = the central axis scatter in
fi field ld of f infinite i fi it radius, di λ iis a scaling li parameter, t and d μ iis a di dimensionless i l shape h
parameter. They computed equivalent square fields for a complete set of
rectangular fields using a value of λ=0.26 cm-1 and μ=0.5.
s
f
d