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AbstractID: 9348 Title: Dose sensitivity modification in polymer gel dosimeters improves the accuracy of
optical CT scanning for gel dosimetry
Optical CT scanning is a convenient, bench-top method of imaging 3D dose distributions in gel dosimeters such as BANG®. These scanners offer medical physicists a
powerful tool for treatment plan validation in IMRT and conformal RTP. However, one of the limitations of optical CT is the dynamic range of light intensity
measurement. A/D converters in plug-in data acqusition boards have at most 16-bit resolution. This, along with the requirement that the signal-to-noise ratio be at least
100, limits the dynamic range to the factor of 216/100 = 655. This limits the maximum optical density increment •ODmax throughout all projections in all planes to
log(655) = 2.8 optical density units. The maximum optical density increment depends on the dose distribution D(r), on the dose at the point of dose maximum, and on
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the sensitivity of the gel, or the k-factor in the equation A(D) = A(0) + kD +lD + mD , where A(D) is the unit-length optical density of the gel irradiated to dose D. The
k-factor (or the proportionality constant of the linear term) will largely determine the maximum optical density increment for any given treatment plan delivered to the
gel. Therefore, the utilization of the full dynamic range of the scanner requires control of the k-factor. Such a control is made now possible by the introduction of
polymer gel kits with chemical modification of sensitivity (BANGkit™). We present examples of OCT – derived dose maps from various IMRT plans delivered to
sensitivity-modified BANGkit gel phantoms.
Supported by NIH grant R44HL59813.
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