Optimal configuration for detection of gold nanoparticles in tumors using Kβ X-ray fluorescence line

Effects on the accelerating electron bunches due to the presence of sulfur hexafluoride or air in the linac waveguide

Sulfur hexafluoride gas (SF6) is used as a dielectric insulator in the acceleration process of certain medical linear accelerator waveguides. Nevertheless, some innovative development and investigation cases require intervention in the linear accelerator or, specifically, on the waveguide, which could affect the sealing of the device. In this regard, vacuum sealing systems can be compromised, affecting the properties of the radiation beams produced. The presence of sulfur hexafluoride or air inside the VARIAN 6/100 waveguide was investigated under different pressure conditions and non-uniform electric fields, adapting Monte Carlo simulation techniques for modeling radiation transport coupled with electric fields. Obtained results indicated the suitability of the proposed approach, while comparisons with theoretical approaches and experimental evidence supported the model's consistency.

Dosimetry of tumor targeting imaging by convergent X-ray beam as compared with nuclear medicine

During decades nuclear medicine procedures, based on radiolabeled agents, have proved to be efficient for diseases diagnosis and treatment. Radiation emerging from patient is detected aimed at localizing radiotracer distribution that is further correlated with biochemical/metabolic physiological processes. However, a significant drawback associated with current nuclear medicine procedures implementing radionuclide infusion regards to the inherent absorbed dose as well as radiopharmaceuticals' production, storage and elimination from patient body, thus representing a risk at patient and public health level. In the recent years, alternative methods have been proposed to reduce/eliminate radionuclides in some nuclear medicine procedures. The combination of high atomic number nanoparticles infused within patient body with incident X-ray beam, like tumor targeting and treatment, appears as a potential alternative method capable of theranostics. The process is based on inducing X-ray fluorescence and secondary electrons emission in high atomic number nanoparticles by means of excitation with an external X-ray beam, avoiding employing radioactive substances. The present work reports on the dosimetry performance of both methods, comparing whenever the external convergent X-ray beam alternative may involve less or larger radiation dose levels, according to comparable signal/image quality during the procedure. To this aim, a simplified theoretical model is proposed and associated Monte Carlo simulations are performed in order to compare typical case of nuclear medicine imaging with potential performance of an innovative method, called OXIRIS (Orthovoltage X-ray Induced Radiation and Integrated System), based on convergent X-ray beam exciting high atomic number nanoparticles infused in patient. The obtained results support the proposed alternative method's feasibility, once demonstrated that patient absorbed dose levels are relative similar to those currently used by nuclear medicine procedures, whereas dose to targeted region (tumor) are significantly higher, which may be useful for treatment purposes.

Assessment of FLUKA, PENELOPE and MCNP6 Monte Carlo codes for estimating gold fluorescence applied to the detection of gold-infused tumoral volumes

Different kinds of nanoparticles have been widely studied for biomedical purposes, including applications like dose enhancement in radiotherapy treatments and contrast agent in radiological studies. Recent work suggests that gold nanoparticles can be used as contrast agents in K-edge imaging and X-ray Fluorescence Computed Tomography, mainly due to their high K-edge energy value and good biocompatibility. However, the gold X-ray fluorescence (XRF) signal obtained in these procedures is relatively week when compared with Compton or bremsstrahlung radiation emitted in the surrounding tissues, mainly because it is not possible to achieve large gold nanoparticles concentrations within biological tissues added to the XRF is attenuated by other tissues when leaving the patient body. This work presents a feasibility study on implementation of FLUKA, PENELOPE and MCNP6 Monte Carlo codes to model the detection of gold XRF emitted by a small volume containing different gold concentrations and located at different depths in a tissue-equivalent phantom. Results indicate that there is good agreement between PENELOPE and FLUKA for gold Kα and Kβ lines estimations when highly symmetric simulation scenario and kilovoltage X-ray beam were used, achieving differences lower than 2%; however, differences up to 6 times were observed between FLUKA and MCNP6 under the same conditions. In addition, remarkable differences were obtained when megavoltage X-ray beam was used, being up to 11 times between PENELOPE and FLUKA and up to 4 times between FLUKA and MCNP6 for gold Kα and Kβ lines estimations. In this regard, a suitable normalization method was proposed and implemented to perform cross-comparisons of XRF estimations obtained from the Monte Carlo codes. By means of the proposed method, FLUKA, PENELOPE and MCNP6 can be successfully implemented to assess which configuration (gold concentration and target volume depth) leads to a better detection of gold XRF, despite differences in XRF estimation between the codes.

Experimental determination of the gadolinium dose enhancement in phantom irradiated with low energy X-ray sources by a spectrophotometer -Gafchromic-EBT3 dosimetry system

This work reports the experimental determination of dose enhancement produced in phantoms containing target volumes doped with Gadolinium and irradiated with low-energy X-ray beams. EBT3 Gafchromic films were immersed into 5 ml target volumes to simulate tumor filling with 100% ultra-pure water (blank sample) and ultra-pure water infused with Gd solution (Omniscan®) in different concentration (9-24 mg/ml). The dose enhancement due to excitation of Gd K-edge (50.2 keV) was evaluated in terms of the increment in optical density, obtained by a Spectrophotometer-Gafchromic-EBT3 dosimetry system calibrated in terms of the dose-response for 1-8 Gy range. The dose enhancement was evaluated in two condition: a beam quality with spectrum above the absorption edge in a medium with and without Gd; and two beam qualities (above and below the absorption edge) in a medium with a fixed Gd concentration (18 mg/ml). The obtained results confirmed increments in relative dose enhancement according to Gd concentrations up to 18 mg/ml, with a dose enhancement of 1.1 Gy and an average percentage enhancement of 28.4%. For higher values of concentration, the attenuation interaction of the primary beam is more relevant instead of dose enhancement process. On the other hand, the dose enhancement obtained to comparison the spectra above and below the absorption edge, shown results up to 3.3 Gy of enhancement and average percentage enhancement of 88%.