Rapid two-dimensional dose measurement in brachytherapy using plastic scintillator sheet: linearity, signal-to-noise ratio, and energy response characteristics.

Because of the large dose gradients encountered near brachytherapy sources, an efficient, accurate, low-atomic number areal detector, which can record dose at many points simultaneously, is highly desirable. We have developed a prototype of such a system using thin plates of plastic scintillator as detectors. A micro-channel plate (MCP) image intensifier was used to amplify the optical scintillation images produced by radioactive 125I and 137Cs sources in water placed 0.5-5.7 cm distance from the detector. A charge-coupled device (CCD) digital camera was used to acquire 2-D light-intensity distributions from the image intensifier output window. For both isotopes, a small area (2 x 3 mm2) PVT detector yields a CCD net count rate that is linear with respect to absorbed dose rate within +/- 3% out to 5.7 cm distance. Acquisition times range from 1.5-400 sec with a reproducibility of 0.5-5.5%. If a large-area (6 x 20 cm2) PVT detector is used, a four-fold increase in count rate and large deviations from linearity are observed, indicating that neighboring pixels contribute light to the signal through diffusion and scattering in PVT and water. A detailed noise analysis demonstrates that the image intensifier reduces acquisition time 10000-fold, reduces noise relative to signal 200-fold, and reduces amplifier gain noise as well.