ABSTRACT
Acoustic emissions (AE) due to microcracking in solid materials permit the monitoring of fracture processes and the study of failure dynamics. As an alternative method of integrity assessment, measurements of electrical resistance can be used as well. In the literature, however, many studies connect the notion of criticality with AE originating from the fracture, but not with the changes in the electrical properties of materials. In order to further investigate the possible critical behavior of fracture processes in rocks and cement-based materials, we apply natural time (NT) analysis to the time series of AE and resistance measurements, recorded during fracture experiments on cement mortar (CM) and Luserna stone (LS) specimens. The NT analysis indicates that criticality in terms of electrical resistance changes systematically precedes AE criticality for all investigated specimens. The observed greater unpredictability of the CM fracture behavior with respect to LS could be ascribed to the different degree of material homogeneity, since LS (heterogeneous material) expectedly offers more abundant and more easily identifiable fracture precursors than CM (homogenous material). Non-uniqueness of the critical point by varying the detection threshold of cracking events is apparently due to finite size effects which introduce deviations from the self-similarity.
ABSTRACT
AIM: To employ the thermal neutron background that affects the patient during a traditional high-energy radiotherapy treatment for BNCT (Boron Neutron Capture Therapy) in order to enhance radiotherapy effectiveness. BACKGROUND: Conventional high-energy (15-25 MV) linear accelerators (LINACs) for radiotherapy produce fast secondary neutrons in the gantry with a mean energy of about 1 MeV due to (γ, n) reaction. This neutron flux, isotropically distributed, is considered as an unavoidable undesired dose during the treatment. Considering the moderating effect of human body, a thermal neutron fluence is localized in the tumour area: this neutron background could be employed for BNCT by previously administering (10)B-Phenyl-Alanine ((10)BPA) to the patient. MATERIALS AND METHODS: Monte Carlo simulations (MCNP4B-GN code) were performed to estimate the total amount of neutrons outside and inside human body during a traditional X-ray radiotherapy treatment. Moreover, a simplified tissue equivalent anthropomorphic phantom was used together with bubble detectors for thermal and fast neutron to evaluate the moderation effect of human body. RESULTS: Simulation and experimental results confirm the thermal neutron background during radiotherapy of 1.55E07 cm(-2) Gy(-1). The BNCT equivalent dose delivered at 4 cm depth in phantom is 1.5 mGy-eq/Gy, that is about 3 Gy-eq (4% of X-rays dose) for a 70 Gy IMRT treatment. CONCLUSIONS: The thermal neutron component during a traditional high-energy radiotherapy treatment could produce a localized BNCT effect, with a localized therapeutic dose enhancement, corresponding to 4% or more of photon dose, following tumour characteristics. This BNCT additional dose could thus improve radiotherapy, acting as a localized radio-sensitizer.
ABSTRACT
PURPOSE: The aim of this study is to investigate radioprotection issues that must be addressed when dedicated accelerators for intraoperative radiotherapy (IORT) are used in operating rooms. Recently, a new version of a mobile IORT accelerator (LIAC Sordina SpA, Italy) with 12 MeV electron beam has been implemented. This energy is necessary in some specific pathology treatments to allow a better coverage of thick lesions. At an electron energy of 10 MeV, leakage and scattered x-ray radiation (stray radiation) coming from the accelerator device and patient must be considered. If the energy is greater than 10 MeV, the x-ray component will increase; however, the most meaningful change should be the addition of neutron background. Therefore, radiation exposure of personnel during the IORT procedure needs to be carefully evaluated. METHODS: In this study, stray x-ray radiation was measured and characterized in a series of spherical projections by means of an ion chamber survey meter. To simulate the patient during all measurements, a polymethylmethacrylate (PMMA) slab phantom with volume 30 x 30 x 15 cm3 and density 1.19 g / cm3 was used. The PMMA phantom was placed along the central axis of the beam in order to absorb the electron beams and the tenth value layer (TVL) and half value layer (HVL) of scattered radiation (at 0 degrees, 90 degrees, and 180 degrees scattering angles) were also measured at 1 m of distance from the phantom center. Neutron measurements were performed using passive bubble dosimeters and a neutron probe, specially designed to evaluate ambient dose equivalent H*(10). RESULTS: The x-ray equivalent dose measured at 1 m along the beam axis at 12 MeV was 260 microSv/Gy. The value measured at 1 m at 90 degrees scattering angle was 25 microSv/Gy. The HVL and TVL values were 1.1 and 3.5 cm of lead at 0 degrees, and 0.4 and 1 cm at 90 degrees, respectively. The highest equivalent dose of fast neutrons was found to be at the surface of the phantom on the central beam axis (2.9 +/- 0.6 microSv/Gy), while a lower value was observed below the phantom (1.6 +/- 0.3 microSv/Gy). The neutron dose equivalent at 90 degrees scattering angle and on the floor plane on the beam axis below the beam stopper was negligible. CONCLUSIONS: Our data confirm that neutron exposure levels around the new dedicated IORT accelerator are very low. Mobile shielding panels can be used to reduce x-ray levels to below regulatory levels without necessarily providing permanent shielding in the operating room.
