Hh. Liu et al., CALCULATING OUTPUT FACTORS FOR PHOTON-BEAM RADIOTHERAPY USING A CONVOLUTION SUPERPOSITION METHOD BASED ON A DUAL SOURCE PHOTON-BEAM MODEL/, Medical physics, 24(12), 1997, pp. 1975-1985
A realistic photon beam model based on Monte Carlo simulation of clini
cal linear accelerators was implemented in a convolution/superposition
dose calculation algorithm. A primary and an extrafocal sources were
used in this beam model to represent the direct photons from the targe
t and the scattered photons from other head structures, respectively.
The effect of the finite size of the extra-focal source was modeled by
a convolution of the source fluence distribution with the collimator
aperture function. Relative photon output in air (S-c) and in phantom
(S-cp) were computed using the convolution method with this new photon
beam model. Our results showed that in a 10 MV photon beam, the S-c,
S-p (phantom scatter factor), and S-cp factors increased by 11%, 10%,
and 22%, respectively, as the field size changed from 3 x 3 cm(2) to 4
0 x 40 cm(2). The variation of the Sc factor was contributed mostly by
an increase of the extra-focal radiation with field size. The radiati
on backscattered into the monitor chamber inside the accelerator head
affected the S-c by about 2% in the same field range. The output facto
rs in elongated fields, asymmetric fields, and blocked fields were als
o investigated in this study. Our results showed that if the effect of
the backscattered radiation was taken into account, output factors in
these treatment fields can be predicted accurately by our convolution
algorithm using the dual source photon beam model. (C) 1997 American
Association of Physicists in Medicine.