Assessment of ionization chamber correction factors in photon beams using a time saving strategy with PENELOPE code.

The purpose of this study was to investigate Monte Carlo-based perturbation and beam quality correction factors for ionization chambers in photon beams using a saving time strategy with PENELOPE code. Simulations for calculating absorbed doses to water using full spectra of photon beams impinging the whole water phantom and those using a phase-space file previously stored around the point of interest were performed and compared. The widely used NE2571 ionization chamber was modeled with PENELOPE using data from the literature in order to calculate absorbed doses to the air cavity of the chamber. Absorbed doses to water at reference depth were also calculated for providing the perturbation and beam quality correction factors for that chamber in high energy photon beams. Results obtained in this study show that simulations with phase-space files appropriately stored can be up to ten times shorter than using a full spectrum of photon beams in the input-file. Values of kQ and its components for the NE2571 ionization chamber showed good agreement with published values in the literature and are provided with typical statistical uncertainties of 0.2%. Comparisons to kQ values published in current dosimetry protocols such as the AAPM TG-51 and IAEA TRS-398 showed maximum percentage differences of 0.1% and 0.6% respectively. The proposed strategy presented a significant efficiency gain and can be applied for a variety of ionization chambers and clinical photon beams.

Experimental assessment of gold nanoparticle-mediated dose enhancement in radiation therapy beams using electron spin resonance dosimetry.

In this work, we aim to experimentally assess increments of dose due to nanoparticle-radiation interactions via electron spin resonance (ESR) dosimetry performed with a biological-equivalent sensitive material.We employed 2-Methyl-Alanine (2MA) in powder form to compose the radiation sensitive medium embedding gold nanoparticles (AuNPs) 5 nm in diameter. Dosimeters manufactured with 0.1% w/w of AuNPs or no nanoparticles were irradiated with clinically utilized 250 kVp orthovoltage or 6 MV linac x-rays in dosimetric conditions. Amplitude peak-to-peak (App) at the central ESR spectral line was used for dosimetry. Dose-response curves were obtained for samples with or without nanoparticles and each energy beam. Dose increments due to nanoparticles were analyzed in terms of absolute dose enhancements (DEs), calculated as App ratios for each dose/beam condition, or relative dose enhancement factors (DEFs) calculated as the slopes of the dose-response curves.Dose enhancements were observed to present an amplified behavior for small doses (between 0.1-0.5 Gy), with this effect being more prominent with the kV beam. For doses between 0.5-5 Gy, dose-independent trends were observed for both beams, stable around (2.1 ± 0.7) and (1.3 ± 0.4) for kV and MV beams, respectively. We found DEFs of (1.62 ± 0.04) or (1.27 ± 0.03) for the same beams. Additionally, we measured no interference between AuNPs and the ESR apparatus, including the excitation microwaves, the magnetic fields and the paramagnetic radicals.2MA was demonstrated to be a feasible paramagnetic radiation-sensitive material for dosimetry in the presence of AuNPs, and ESR dosimetry a powerful experimental method for further verifications of increments in nanoparticle-mediated doses of biological interest. Ultimately, gold nanoparticles can cause significant and detectable dose enhancements in biological-like samples irradiated at both kilo or megavoltage beams.

Ionizing radiation-induced gene expression changes in TP53 proficient and deficient glioblastoma cell lines.

The genetic heterogeneity presented by different cell lines derived from glioblastoma (GBM) seems to influence their responses to antitumoral agents. Although GBM tumors present several genomic alterations, it has been assumed that TP53, frequently mutated in GBM, may to some extent be responsible for differences in cellular responses to antitumor agents, but this is not clear yet. To directly determine the impact of TP53 on GBM response to ionizing radiation, we compared the transcription profiles of four GBM cell lines (two with wild-type (WT) TP53 and two with mutant (MT) TP53) after 8Gy of gamma-rays. Transcript profiles of cells analyzed 30 min and 6h after irradiation showed that WT TP53 cells presented a higher number of modulated genes than MT TP53 cells. Our findings also indicate that there are several pathways (apoptosis, DNA repair/stress response, cytoskeleton organization and macromolecule metabolic process) in radiation responses of GBM cell lines that were modulated only in WT TP53 cells (30 min and 6h). Interestingly, the majority of differentially expressed genes did not present the TP53 binding site, suggesting secondary effects of TP53 on transcription. We conclude that radiation-induced changes in transcription profiles of irradiated GBM cell lines mainly depend on the functional status of TP53.

SU-E-T-122: Dose Response Analysis of Radiochromic Films in Regions of Low Dose Using Separation Color Components.

PURPOSE: To evaluate the dose response of EBT2 films in regions of low dose using the decomposition of the image's color channels (RGB, Red, Green and Blue). METHODS: Doses ranging from 1 Gy to 60 Gy were used to calibrate the dose response of Gafchromic ® EBT2 films irradiated in 6MV photons beams. Segments of film with dimensions of 8.5 cm × 8.5 cm were used. Another segment of film with dimensions 8.5 cm × 20.25 cm was also irradiated with a maximum dose of 4Gy to determine the percentage depth dose (PDD). The films were digitized by a LaserJet M1132 MFP - HP ® scanner in standard resolution of 150dpi and analyzed by a routine created in MatLab to convert the image to gray levels as well as assess the desired color components from the image. RESULTS: The green component presented the higher sensitivity (17.8 a.u./Gy) when the separated color channels and the shades of gray analysis are compared. The red component presented the highest signal to noise ratio in the low dose range (63% at 1Gy). The blue component presented low sensitivity (0.66 a.u./Gy) in the entire dose range. A linear fitting (r=0.998) was used to the green and gray components until a dose of 4 Gy. The red component presented a non-linear behavior in the entire dose range. The useful dose range found was from 1 Gy to 15 Gy. The maximum differences between the reference PDD, measured with ionization chamber in a water phantom, and the PDDs determined with film were 6%, 9% and 14% for the green, gray and red components, respectively. CONCLUSIONS: This work results show that the use of radiochromic films on planning verification procedures in low dose ranges can be benefit from the analysis of the image's separated color components.

SU-E-T-278: Study of MAGIC-F Gel and PENELOPE Code Simulation Response for Clinical Electron Beams.

PURPOSE: To evaluate the response of MAGIC-f gel through dose response curves, percentage depth dose (PDD) and beam profile for clinical electron beams. METHODS: Glass tubes (Vacutainer ®), with 6 cm length and 0.5 cm radius, with MAGIC-f were positioned inside a water phantom to study the gel response with doses from 0.5 Gy to 20 Gy in electron beams of 6, 9 e 12 MeV. Glass tubes of 20 cm length and 1 cm radius and PMMA phantoms of 10 × 5 × 5 cm3 were used to PDD and beam profiles determinations, respectively, with a maximum dose of 2 Gy to the gel. The samples were analyzed through magnetic resonance imaging (MRI) with a 3 T tomography using a head coil, multiple spin echo sequence with 16 echos, TE 15ms and TR 4000ms. The MAGIC-f response was simulated with PENELOPE Monte Carlo code in the same geometry used in the irradiations. The results obtained with MAGIC-f and PENELOPE were compared with clinical data. RESULTS: Calibration curves for MAGIC-f showed a linear behavior, with correlation coefficient of 0.99, for all energies. The PDD and beam profile curves obtained with MAGIC-f presented differences lower than 1.5% and 3.0%, respectively, when compared to clinical data. Results obtained by PENELOPE and clinical data showed differences up to 1.0% and 1.5%, respectively, for PDD and profile curves. CONCLUSIONS: The dosimetric parameters for electron beams obtained experimentally with MAGIC-f and with PENELOPE code showed similar results to the clinical data. From the results it can be inferred that MAGIC-f can be used as a complementary dosimetric tool for electron beams due to its characteristics of high spatial resolution and the ability to construct tridimensional dose distributions. Also PENELOPE can be used to study MAGIC-f gel response in electron beams.

SU-E-T-178: Optically Stimulated Luminescence (OSL) Dosimetry: A Study of A-Al2O3:C Assessed by PENELOPE Monte Carlo Simulation.

PURPOSE: To use the Monte Carlo code PENELOPE to study attenuation and tissue equivalence properties of a-Al2O3:C for OSL dosimetry. METHODS: Mass attenuation coefficients of α-Al2O3 and α-Al2O3:C with carbon percent weight concentrations from 1% to 150% were simulated with PENELOPE Monte Carlo code and compared to mass attenuation coefficients from soft tissue for photon beams ranging from 50kV to 10MV. Also, the attenuation of primary photon beams of 6MV and 10MV and the generation of secondary electrons by α-Al2O3 :C dosimeters positioned on the entrance surface of a water phantom were studied. RESULTS: A difference of up to 90% was found in the mass attenuation coefficient between the pure \agr;-A12O3 and the material with 150% weight concentration of dopant at 1.5 keV, corresponding to the K-edge photoelectric absorption of aluminum. However for energies above 80 keV the concentration of carbon does not affect the mass attenuation coefficient and the material presents tissue equivalence for the beams studied. The ratio between the mass attenuation coefficients for \agr-A12O3:C and for soft tissue are less than unit due to the higher density of the \agr-A12O3 (2.12 g/cms ) and its tissue equivalence diminishes to lower concentrations of carbon and for lower energies due to the relation of the radiation interaction effects with atomic number. The larger attenuation of the primary photon beams by the dosimeter was 16% at 250 keV and the maximum increase in secondary electrons fluence to the entrance surface of the phantom was found as 91% at 2MeV. CONCLUSIONS: The use of the OSL dosimeters in radiation therapy can be optimized by use of PENELOPE Monte Carlo simulation to provide a study of the attenuation and response characteristics of the material.

SU-E-T-145: MRI Gel Dosimetry Applied to Dose Profile Determination for 50kV X-Ray Tube.

PURPOSE: The aim of this study was to use MRI gel dosimetry to determine the dose profile of 50kV MAGNUM® X-ray tube, MOXTEK Inc., in order to calibrate small solid dosimeters of alanine, tooth enamel and LiF-TLDs, commonly used in clinical quality assurance and datation dosimetry. METHODS: MAGIC-f polymer gel was kept in two plastic containers of 100mL, avoiding attenuation of the primary beam trough the wall. Beam aberture of 3mm and dose rate of 16.5Gy/min were set, reproducing irradiation conditions of interest. The dose rate was assumed based on data of the vendor information of the tube and dose of 30Gy was delivered at the surface of the gel. MAGIC-f gel was irradiated at source-surface distances(SSD) of 0.1cm and 1.0cm. After 24hours of irradiation, gel was scanned in an Achieva® 3T Philips® MRI tomography using relaxometry sequence with 32 Echos, Time-to-Echo(TE) of 15.0ms, Time-to-Repetition(TR) of 6000ms and Field-of-View(FOV) of 0.5×0.5×2.0mm. Dose map at the central plain of irradiation was calculated from T2 relaxometry map. RESULTS: The gel dosimetry results evidenced a build-up depth of 0.13cm for SSD=0.1cm and no build-up was detected for SSD=1.0cm. However, the dose profile evidenced high gradient of dose in SSD=0.1, decreasing the dose from 100% to 30% in 1.4cm depth inside the gel; In turn, the dose distribution is homogeneous after 0.4cm deth for SSD=1.0cm. CONCLUSIONS: MRI gel dosimetry using MAGIC-f presented as feasible technique to determine dose profiles for kilovoltage x-rays tubes. The results evidenced that the calibration of small solid dosimeters can be performed using SSD of 1.0cm in the 50kV MAGNUM® X-ray tube using 0.4cm/g/cm3 filter. This work was funded supported by CNPQ, CAPES and FAPESP.

MO-F-BRB-06: Gold Nanoparticle Modify Density of Ionizations inside Cells Submitted to Radiation Therapy: Microscopic Track Analysis of Secondary Electrons Using Monte Carlo.

PURPOSE: Study density of ionization in cells containing gold nanoparticles (AuNP) submitted to Radiation Therapy. METHODS: Spherical gold nanoparticles with diameters ranging 0-100nm were considered evenly distributed inside a 20mgr;m cubic cell, maintaining the gold concentration of 0.01%, with constant number of gold atoms inside the cell. Monte Carlo simulations were performed using PENELOPE code considering event-by-event transport of secondary electrons with minimum energy of 1keV. Simulated clinical energy spectrum of 250kV and 6MV x-rays;Co-60 and Ir-192 γ-ray sources obtained at each corresponding build-up depths were considered. Density of ionization inside the cell was evaluated counting delta electrons created either in AuNP or cell, excluding electrons attenuated inside the nanoparticles. The dose enhancement resultant from interaction of electrons with few micrometers range was quantified by the factor µDEF as the ratio of doses inside the cell with and without AuNP. RESULTS: Maps of ionization density were obtained at the central plane of the cell illustrating ionizations around and between AuNP. The density of ionization increases in cell medium as the AuNP diameter enlarges, being higher to larger nanoparticles for all energies studied. The total dose deposited in the cell is affected by the fraction of electrons consumed in the nanoparticles, resulting in size-dependence for µDEF. The µDEFs for 250kV are 1.68 to 20nm, 1.83 to 60nm and 1.72 to 100nm; µDEFs for 6MV are 1.14 to 20nm, 1.38 to 60nm and 1.20 to 100nm, therefore presenting an optimum nanoparticle size for clinical applications in Radiation Therapy. CONCLUSIONS: The µDEF describes dose enhancements founded on the effective density of ionizations inside cell medium containing AuNP, considering real electron tracks close to metallic interfaces. The profile of ionizations describes electron spectra of electrons with intracellular range considering dynamics of creation and consumption, hence being directly proportional to potential applicability of AuNP in Radiation Therapy. This work was funding supported by CAPES - Nanobiomed Network.

Influence of dental restorative materials on ESR biodosimetry in tooth enamel.

Using an experimental model and PENELOPE Monte Carlo simulations, the effects of resin and amalgam on the absorbed doses in tooth enamel were studied to evaluate the feasibility of using restored teeth in electron spin resonance (ESR) dose reconstruction. The model consisted of a phantom containing a plate of these restorative materials placed between powered enamel layers exposed to X rays and a 6°Co beam. The experimental results and simulations agreed, showing that the attenuation produced by amalgam and resin with a thickness of 1, 2, and 4 mm is similar to that produced by the enamel itself in the case of the radiation sources employed. For X rays and 6°Co γ radiation the attenuation reached almost 100% and 40%, respectively. These results show that for ESR dose reconstruction, the use of all available enamel of a tooth leads to errors in the estimated dose due to attenuation effects in both healthy and restored teeth. Thus the importance of an enamel selection from different sides of the tooth surface to apply ESR dose reconstruction in the case of a practical situation is shown.

Efficiency of the DNA repair and polymorphisms of the XRCC1, XRCC3 and XRCC4 DNA repair genes in systemic lupus erythematosus.

Impaired DNA repair efficiency in systemic lupus erythematosus (SLE) patients has been reported in some studies, mainly regarding the repair of oxidative damage, but little is known about repair kinetics towards primarily single-stranded DNA breaks. In the present study, we aimed to investigate: (a) the efficiency of SLE peripheral blood leucocytes in repairing DNA damage induced by ionizing radiation and (b) the association of DNA repair gene (XRCC1 Arg399Gln, XRCC3 Thr241Met and XRCC4 Ile401Thr) polymorphisms in SLE patients, considering the whole group, or stratified sub-groups according to clinical and laboratory features. A total of 163 SLE patients and 125 healthy controls were studied. The kinetics of DNA strand break repair was evaluated by the comet assay, and genotyping for DNA repair genes was performed by PCR-RFLP. Compared with controls, SLE leucocytes exhibited decreased efficiency of DNA repair evaluated at 30 min following irradiation. A significant association with DNA repair gene polymorphisms was not observed for the whole group of SLE patients; however, the XRCC1Arg399Gln polymorphism was associated with the presence of anti-dsDNA antibody. The concomitance of two DNA repair polymorphic sites was associated with the presence of neuropsychiatric manifestations and antiphospholipid antibody syndrome. Taken together, these results indicated that SLE leucocytes repair less efficiently the radiation-induced DNA damage, and DNA repair polymorphic sites may predispose to the development of particular clinical and laboratory features.