Dosimetric evaluation of pair production enhanced radiotherapy (PPER) with the use of high-z organometallics
Tsiamas P, Isrow D, Chetty I, Kim J, and Brown S. Dosimetric evaluation of pair production enhanced radiotherapy (PPER) with the use of high-z organometallics. Med Phys 2017; 44(6):2997.
Purpose: The current study evaluates dosimetric and spectral effects when platinum-based chemotherapeutics and less toxic tungstophosphoric-acid (TPA) organometallics are present during X-ray radiotherapy. We hypothesize that dose enhancement from the high Z elements, when 6 and 18MV beams used, will increase tumor cell kill due to greater pair production, especially for the 18MV beam. Methods: Dosimetry of Pt and TPAwas modeled using EGSnrc code with pair production turned on and off. EGSnrc GUI was used to create homogeneous Pt and TPA mixtures with water. Angular distribution, spectral distribution and dose was determined as a function of different mixture concentrations (100lmolar, 1mmolar), depths (0, 10 cm), mixture thicknesses (5 mm, 5 cm) and X-ray energies (6, 18MV). Results: As expected, angular distribution calculated for the 511 keV annihilation photons was isotropic. However, the highest fluence of particles was observed at 5.50. This peak was attributable to bremsstrahlung produced at the target since it was not observed when modeling excluded the target and included only particles produced in the mixture area in the phantom. Relative increase of 511 keV photons was between 10-13% (TPA-1mmolar) and 1- 2.5% (TPA-100lmolar) for both 18 and 6MV. Fluence increases for mixtures (TPA, Pt) were ∼14% (1mmolar) and 1-2% (1lmolar). Dose Enhancement due to photoelectric, with pair production turned off was up to 10% (TPA- 1mmolar, 6MV) and 26% (TPA-1mmolar, 18MV). Including pair production in the simulations increased the dose by another 30% (18MV, TPA-1mmolar) and ∼10% (6MV, TPA-1mmolar). Conclusion: Simulations showed that there is a dose enhancement effect when high-z organometallics are used with high-energy X-ray beams. While part of the effect is attributable to photoabsorption, a significant contribution of dose comes from pair production suggesting a new strategy for dose enhancement using organometallics.