AbstractID: 7756 Title: Loss of Beam Flatness Due to the Double Penumbra Effect in
Closely Collimated Electron Applicators and the Necessity of Selecting Oversized Cones
Electron beams from Siemens accelerators exhibit some advantageous features, including, in many applicator configurations, reduced
output dependency on Cerrobend cutout geometry. Certain Siemens electron applicators, however, show an undesired broadening in
the electron beam penumbra due to the close alignment of the mid and lower level trimmers. On Siemens MD model accelerators, for
example, light field observations show that the two trimmers are divergently aligned within 2 mm. A result of this alignment is that
the penumbra from the mid-level trimmer, which is located about 25 cm above the final trimmer, is essentially superimposed on the
final trimmer’s penumbra and causes significant broadening. This effect is particularly problematic for the 5-cm diameter cone, which
is often used for facial lesions where smaller source-to-skin distances are critical. Here the double-penumbra effect, along with the
narrow field, significantly reduces the flatness of the dose profiles. In this work we include comparisons of field isodose profiles
generated with the open 5-cm cone with those of an "identical" field, a 5-cm Cerrobend cutout mounted in a 10x10 cm cone. For 6
and 9 MeV beams, the open 5-cm cone shows isodose widths that are about 1 cm narrower than those of the 5-cm cutout for 80 to
95% isodoses. Hence, for the same target, an additional 5 mm must be added to the margins when the 5-cm open cone is used.
Therefore, whenever patient setup constraints allow, the use of a cutout in a larger, oversized, cone is often preferable on Siemens MD
accelerators.