Carrier attenuation correction

From the Dose Calc. Options menu


Carrier attenuation correction should be included in dose calculations when the density or atomic composition of the seed carrier differs significantly from water. The COMS plaques use a Silastic silicone carrier. The Silastic material is about 40% Si by weight and has an effective Z of about 10.7 compared to 7.4 for water.

Important: Scatter and flourescence corrections are already included in the carrier factor for COMS plaques that use a silicone carrier because flourescence from the gold will be entirely absorbed in the silicone seed carrier and never reach the eye. Thus, for a COMS plaque that uses a silicone carrier, the carrier factor T(r,d,μ) should be enabled, and the scatter & flourescence factor B(r) should be disabled. For plaques that do not use a seed carrier (e.g. Eye Physics plaques) and for a COMS plaque that uses a gold insert instead of silicone, carrier corrections should be disabled, and the scatter & flourescence factor B(r) factor should be enabled.

or from the carrier control in the Rx Window toolbar:

  • T - The carrier correction factor T is simply a scalar, typically about 0.9 for central axis distances out to about 1 cm (Sou-Tung Chiu-Tsao et al, Dosimetry for I-125 seed (model 6711) in eye plaques, Medical Physics 20(2), 383-389, 1993) for the COMS carriers, which is universally applied without regard to the location of the dose calculation point.
  • T(r) - The correction factor T varies as a function of the distance r between the source center and the calculation point.
  • T(d,μ) - A ray is cast from the center of each source towards the dose calculation point to estimate the primary path length d in the Silastic carrier. The factor μ is a linear attenuation coefficient for the carrier material (roughly 0.1 per mm for Silastic) which accounts for the difference in attenuation in Silastic vs water.
  • T(r,d,μ) - Corrections for both total distance and path length in Silastic are applied (Astrahan M., Improved treatment planning for COMS eye plaques, International Journal of Radiation Oncology, Biology, Physics, 61(4):1227-1242, 2005).
  • no T - Disables carrier correction. The carrier, if one exists, is considered to be water equivalent.