Towards two-dimensional brachytherapy dosimetry using plastic scintillator: New highly efficient water equivalent plastic scintillator materials

Plastic scintillator (PS) has been proposed for both one- and two-dimension al (1D and 2D) dose measurements for radiation therapy applications. For lo w-energy photon modalities (e.g., brachytherapy), an efficient water equiva lent scintillator is needed. To perform 2D measurements, a high localizatio n of the scintillation process is required. Guided by comparison of the mas s energy absorption coefficients as a function of energy and of the dose di stribution as a function of distance from the radioactive source, as modele d by Monte Carlo photon transport simulation, a small quantity of medium at omic number (Z) atoms (4% Cl) was incorporated in a polyvinyl toluene (PVT) based PS to approximate closely (within 10%) the radiological properties o f water in the 20-662 keV energy range. However, the scintillation efficien cy of commercial PS mixtures drops as much as 70% when loaded with high ato mic number additives. We developed experimental techniques to;assess the sc intillation efficiency and locality of 15 new PS mixtures. These mixtures d iffer by the type of the scintillation dyes and the type of compound contai ning the medium Z atoms (chlorine). To achieve higher material stability, 4 -chlorostyrene was used as a loading compound to ensure polymerization with the PVT base. Two of the new PS materials exhibited scintillation efficien cies within 30% of one of the most efficient commercially available product s (BC-400), which is not water equivalent at such low energies. These new s cintillator materials are promising candidates for the development of an ac curate and efficient radiation dosimetry method not only for brachytherapy, but also for superficial and diagnostic applications. (C) 1999 American As sociation of Physicists in Medicine. [S0094-2405(99)01708-3].