AbstractID: 6572 Title: Central Axis Scatter attenuation and buildup for megavoltage beams: an analytical description for a wide range of geometries with parameters only dependent on beam quality
Monte Carlo simulations show that Central Axis Scatter energy fluence in mega voltage beams is largely directed downstream even at large depths. This observation supports a simple model for transport of scattered radiation, resulting in a formula for Central
Axis Scatter dose with three field size dependent parameters
Α
>
,
Α
<
and
µ s
:
S( d, r ) = exp( -
µ p d ) x
[ Α
>
µ p
/ (
µ s
-
µ p
) x
( 1 - exp( - (
µ s
-
µ p
) d ) ) + Α
<
µ p
/ (
µ s
+
µ p
) ].
µ p
is the known attenuation coefficient of the primary radiation, r is the radius of a circular field, d is depth. The three parameters are described by simple expressions in field size, each with two parameters only depending on beam quality:
µ s
( r ) =
µ p
/
( 1 - exp( -
η
( r+
ρ
) ); Α
>
( r ) =
α
>,0
+
α
>,1 ln(
µ p r );
Α
<
( r ) =
α
<,0
+
α
<,1 ln(
µ p r ).
These expressions are obtained from analysis of Monte Carlo generated data for 60 Co for field radii of 0.2 cm up to 50 cm and depths between the surface and one meter.
Errors in fits to scatter dose are within the statistical errors of the Monte Carlo data over the full range of geometries. page 1

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