Electron beam therapy at extended SSD: a Monte Carlo investigation of output fom a Siemens Oncor linear accelerator
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Date
2008Author
Foley, Mark J.
O'Shea, T. P.
Downes, Patrick A.
Shearer, Andrew
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O'Shea, T., Foley,M.J., Rajasekar,D., Downes,P.A.,Van der Putten,W., Moore,M., Shearer,A. (2008) 'Electron beam therapy at extended SSD: a Monte Carlo investigation of output fom a Siemens Oncor linear accelerator'. Journal Of Applied Clinical Medical Physics, 9 (4).
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Abstract
Electron-beam therapy is used to treat superficial tumors at a standard 100 cm
source-to-surface distance (SSD). However, certain clinical situations require the use of an
extended SSD. In the present study, Monte Carlo methods were used to investigate clinical electron
beams, at standard and non-standard SSDs, from a Siemens Oncor Avant Garde (Siemens Healthcare,
Erlangen, Germany) linear accelerator (LINAC). The LINAC treatment head was modeled in BEAMnrc
for electron fields 5 cm in diameter and 10 × 10 cm, 15 × 15 cm, and 20 × 20 cm; for 6 MeV, 9 MeV,
and 12 MeV; and for 100 cm, 110 cm, and 120 cm SSD. The DOSXYZnrc code was used to calculate
extended SSD factors and dose contributions from various parts of the treatment head.
The main effects of extended SSD on water phantom dose distributions were verified by Monte Carlo
methods. Monte Carlo¿calculated and measured extended SSD factors showed an average difference of
±1.8%. For the field 5 cm in diameter, the relative output at extended SSD declined more rapidly
than it did for the larger fields. An investigation of output contributions showed this decline was
mainly a result of a rapid loss of scatter dose reaching the d(max) point from the lower scrapers
of the electron applicator. The field 5 cm in diameter showed a reduction in dose
contributions; the larger fields generally showed an increased contribution from the scrapers with
increase in SSD. Angular distributions of applicator-scattered electrons have shown a large number
of acute-angle electron tracks contributing to the output for larger field sizes, explaining the
shallow output reduction.