Determination of volume averaging correction factors using an elliptical absorbed dose model for Gamma Knife Perfexion.

PURPOSE: The purpose of this study is to calculate volume averaging correction factors for detectors used in the dosimetry of Gamma Knife's narrow photon beams, and to determine the impact of volume averaging on the field output correction factor. METHODS: Simulations of different Gamma Knife fields were done using elliptical dose model formalism with newly introduced fit functions. To determine volume averaging correction factors a calculation of the absorbed dose over the volume of the detector was performed. The elliptical dose model was tested with respect to absorbed dose distribution for different volumes and compared with the calculations of Leksell GammaPlan v.11.3.1. RESULTS: The largest differences in absorbed dose calculated by the elliptical model and Leksell GammaPlan are 2.25%, 1.5%, and 0.6% for 16, 8, and 4 mm field sizes, respectively. Volume averaging correction factors were determined for six ionization chambers, five semiconductor detectors, a diamond, and two plastic scintillator detectors. In general, for all examined detectors the impact of volume averaging is more pronounced for smaller field sizes. All studied ionization chambers had a larger volume than other detectors, therefore the volume averaging correction factors for ionization chambers are larger for all investigated field sizes. Besides the fact that plastic scintillator detectors can be considered tissue-equivalent, volume averaging correction factor should be applied. CONCLUSION: Volume averaging correction factors for different detectors are determined and suitable detectors for dosimetry of Gamma Knife's narrow photon beams are recommended. It is shown that volume averaging has a dominant contribution to a field output correction factor.

Magnesium ions reversibly bind to DNA double stranded helix in thin films.

Effects of magnesium (Mg2+) ions on the stability and structural properties of double-stranded DNA are vitally important for DNA folding and functional behavior. Complementing our previous study on highly hydrated thin films of DNA with sodium counterions, with no buffer (pH ≈ 6) and surrounded with Mg2+ cations, here we use Fourier transform infrared spectroscopy and band shape analysis to explore in detail the vibrational signatures of DNA-magnesium interaction in the case when DNA charges are neutralized solely by Mg2+ cations, hereafter called MgDNA. Ion atmosphere has been controlled by the magnesium to phosphate molar concentration ratio r which varied between 0.0067 and 10. For r = 0 we find that spectral features in the base region remain similar as in DNA, whereas changes in the backbone region indicate that the B conformation becomes fully stabilized. With increasing r a pronounced structural reshaping occurs in the phosphate backbone region indicating a blue shift of the asymmetric band, while the symmetric band does not show any displacement in frequency. The band shape analysis of overlapping peaks in the respective phosphate regions demonstrates that the number of constituent modes as well as their positions in frequency do not change, whereas their intensities and bandwidths display disparate changes. The results reflect a variety of local environments at the DNA backbone due to a heterogeneous ion atmosphere with randomly distributed magnesium ions and local patterns of hydrogen bonds which change with increasing r. Remarkably, after crowded r = 10 ion atmosphere is depleted, Mg induced spectral changes vanish and structural features of MgDNA (r ≈ 0) are fully restored. Overall results strongly suggest that in MgDNA on highly hydrated thin films the hydrogen-base pairing remains preserved and that Mg2+ ions, similar to sodium ions, retain their mobility and interact with double helix via water-mediated electrostatic forces.

Possibility of Human Gender Recognition Using Raman Spectra of Teeth.

Gender determination of the human remains can be very challenging, especially in the case of incomplete ones. Herein, we report a proof-of-concept experiment where the possibility of gender recognition using Raman spectroscopy of teeth is investigated. Raman spectra were recorded from male and female molars and premolars on two distinct sites, tooth apex and anatomical neck. Recorded spectra were sorted into suitable datasets and initially analyzed with principal component analysis, which showed a distinction between spectra of male and female teeth. Then, reduced datasets with scores of the first 20 principal components were formed and two classification algorithms, support vector machine and artificial neural networks, were applied to form classification models for gender recognition. The obtained results showed that gender recognition with Raman spectra of teeth is possible but strongly depends both on the tooth type and spectrum recording site. The difference in classification accuracy between different tooth types and recording sites are discussed in terms of the molecular structure difference caused by the influence of masticatory loading or gender-dependent life events.

Sodium and manganese salt DNA thin films: An infrared spectroscopy study.

In this work we have investigated influence of divalent Mn ions on the structure of dsDNA utilizing Fourier transform infrared spectroscopy on DNA thin films obtained from sodium and manganese salt DNA (Na-DNA and Mn-DNA) in manganese chloride solutions and manganese salt DNA in pure water. In the range of low Mn content, 0.0067 ≤ r = [manganese]/[phosphate] ≤ 0.5, the difference between vibrational spectrum of thin films Na-DNA and Mn-DNA is revealed. Former one is more influenced by an increase of Mn content and shows stabilization of B form dsDNA, while in thin films Mn-DNA in MnCl2 and Mn-DNA in pure water, B form is stable even at the lowest Mn content. An increase of Mn content over r > 0.5 induces spectral changes in both base and phosphate region that fully actualize once intrinsic Na+ ions are completely suppressed by divalent Mn2+ ions. Finally, the difference in vibrational spectrum of Na-DNA and Mn-DNA at high Mn concentrations almost completely disappears. The observed results consistently demonstrate that Mn2+ ions interact with both base sites of DNA (primarily C8N7 sites of guanine and adenine) and phosphate groups; both asymmetric and symmetric PO2 vibrations show prominent blue shift in the presence of high Mn content, while B conformation remains stable. Nature of the Mn cation-DNA interaction seems to be electrostatic and water mediated, as demonstrated by almost complete reversal of perturbations in base and sugar-phosphate region in thin films Mn-DNA in pure water.

Effect of magnesium ions on the structure of DNA thin films: an infrared spectroscopy study.

Utilizing Fourier transform infrared spectroscopy we have investigated the vibrational spectrum of thin dsDNA films in order to track the structural changes upon addition of magnesium ions. In the range of low magnesium concentration ([magnesium]/[phosphate] = [Mg]/[P] < 0.5), both the red shift and the intensity of asymmetric PO2 stretching band decrease, indicating an increase of magnesium-phosphate binding in the backbone region. Vibration characteristics of the A conformation of the dsDNA vanish, whereas those characterizing the B conformation become fully stabilized. In the crossover range with comparable Mg and intrinsic Na DNA ions ([Mg]/[P] ≈ 1) B conformation remains stable; vibrational spectra show moderate intensity changes and a prominent blue shift, indicating a reinforcement of the bonds and binding in both the phosphate and the base regions. The obtained results reflect the modified screening and local charge neutralization of the dsDNA backbone charge, thus consistently demonstrating that the added Mg ions interact with DNA via long-range electrostatic forces. At high Mg contents ([Mg]/[P] > 10), the vibrational spectra broaden and show a striking intensity rise, while the base stacking remains unaffected. We argue that at these extreme conditions, where a charge compensation by vicinal counterions reaches 92-94%, DNA may undergo a structural transition into a more compact form.