Περίληψη:
A durable recommendation for brachytherapy treatment planning systems to
account for the effect of tissue, applicator and shielding material
heterogeneities exists. As different proposed approaches have not been
integrated in clinical treatment planning routine yet, currently
utilized systems disregard or, most commonly, do not fully account for
the aforementioned effects. Therefore, it is of interest to evaluate the
efficacy of current treatment planning in clinical applications
susceptible to errors due to heterogeneities. In this work the effect of
the internal structure as well as the shielding used with a commercially
available cylindrical shielded applicator set (Nucletron part #
084.320) for vaginal and rectum treatments is studied using
three-dimensional Monte Carlo simulation for a clinical treatment plan
involving seven source dwell positions of the classic microSelectron HDR
Ir-192 source. Results are compared to calculations of a treatment
planning system (Plato BPS v. 14.2.7), which assumes homogeneous water
medium and applies a constant, multiplicative transmission factor only
at points lying in the shadow of the shield. It is found that the
internal structure of the applicator (which includes stainless steel,
air and plastic materials) with no shield loaded does not affect the
dose distribution relative to homogeneous water. In the unshielded side
of the applicator with a 90degrees, 180degrees, or 270degrees tungsten
alloy shield loaded, an overestimation of treatment planning system
calculations relative to Monte Carlo results was observed which is both
shield and position dependent. While significant (up to 15%) at
increased distances, which are not of major clinical importance, this
overestimation does not affect dose prescription distances by more than
3%. The inverse effect of approx. 3% dose increase at dose
prescription distances is observed for stainless steel shields.
Regarding the shielded side of the applicator, it is shown that the
default treatment planning system transmission factors for tungsten
alloy result in a consistent dose overestimation thus constituting a
safe approach given the nature of associated clinical applications.
Stainless steel is shown to be an ineffective shielding material with
transmission factors reaching up to 0.68 at increased distances
irrespective of shield geometry. (C) 2004 American Association of
Physicists in Medicine.
Συγγραφείς:
Lymperopoulou, G
Pantelis, E
Papagiannis, P
Rozaki-Mavrouli,
H
Sakelliou, L
Baltas, D
Karaiskos, P
