Radiation dosimetry and dose boosting produced by gold nanoparticles in tissue-equivalent materials: alanine, 2-methylalanine, asparagine and monosodium glutamate

The interaction of the ionizing radiation with alanine molecules promotes the breakdown of the amine group, forming stable paramagnetic free radicals. In this context, we investigated asparagine, an amino acid composed of two NH2 groups, which could then form more stable free radicals and possibly increase the dosimetric sensitivity. The EPR signal revealed the paramagnetic nature present in asparagine irradiated with X-rays, as well as the dose-dependent behavior of its concentration (EPR intensity). Free radicals were investigated by EPR, FTIR and computer simulation. The computational modeling of the EPR spectrum, using experimentally obtained parameters, revealed the existence of two radiation-induced radicals at room temperature related to the breakdown of the bond between the two NH2 groups thus forming radical I and radical II. The simulated spectrum is in good agreement with the RPE experimental spectrum. The optimization of spectroscopic parameters, signal-to-noise ratio (S / N) and intensity were found for dosimetric purposes with a microwave power of 2 mW, and a modulation amplitude of 1 mT. The dosimetric properties of asparagine are characterized by a linear dose-response in the therapeutic dose range, minimum detectable dose of 0.1 Gy, including low energy dependence (80 KV-6MV) and a dose rate (100-600 cGy / min) independence in a range of clinical relevance. Stability of the EPR signal was also investigated and the signal remains very stable at room temperature, with a loss of 11% over a period of 6 months. Even with good dosimetric properties, asparagine is less sensitive than alanine. Thus, the last part of this work was focused on increasing the dosimetric sensitivity by the insertion of gold nanoparticles in asparagine, monosodium glutamate, alanine and 2-methy-alanine. The results revealed that the gold nanoparticles interact differently for each material due to the intrinsic characteristics of the amino acids, which cause aggregation or stabilization of the nanoparticles. Aggregation occurred in asparagine and monosodium glutamate. There was amplification of the dosimetric signal for all amino acids containing gold nanoparticles. In addition, it presented a dependence with concentration, size and dose deposited. At concentrations higher than 0.5%, the dosimetric gain decreases possibly due to segregation and self-absorption of neighboring nanoparticles. The size also influenced the decrease in the dosimetric gain, since for the aggregated materials, the average size of the gold nanoparticles increased considerably, causing the gold nanoparticles to occupy external spaces of the crystal, increasing the segregation and self-absorption. And finally, the dose dependence, which revealed that, the dose enhancement factor is dependent of the deposit dose, and it is influenced by the ability of each material to recombine. For more sensitive materials the formation of free radicals are superior for the same dose than for less sensitive materials. (author)