27 research outputs found

    Functional intercomparison of intraoperative radiotherapy equipment – Photon Radiosurgery System

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    BACKGROUND: Intraoperative Radiotherapy (IORT) is a method by which a critical radiation dose is delivered to the tumour bed immediately after surgical excision. It is being investigated whether a single high dose of radiation will impart the same clinical benefit as a standard course of external beam therapy. Our centre has four Photon Radiosurgery Systems (PRS) currently used to irradiate breast and neurological sites. MATERIALS AND METHODS: The PRS comprises an x-ray generator, control console, quality assurance tools and a mobile gantry. We investigated the dosimetric characteristics of each source and its performance stability over a period of time. We investigated half value layer, output diminution factor, internal radiation monitor (IRM) reproducibility and depth-doses in water. The half value layer was determined in air by the broad beam method, using high purity aluminium attenuators. To quantify beam hardening at clinical depths, solid water attenuators of 5 and 10 mm were placed between the x-ray probe and attenuators. The ion chamber current was monitored over 30 minutes to deduce an output diminution factor. IRM reproducibility was investigated under various exposures. Depth-dose curves in water were obtained at distances up to 35 mm from the probe. RESULTS: The mean energies for the beam attenuated by 5 and 10 mm of solid water were derived from ICRU Report 17 and found to be 18 and 24 keV. The average output level over a period of 30 minutes was found to be 99.12%. The average difference between the preset IRM limit and the total IRM count was less than 0.5%. For three x-ray sources, the average difference between the calculated and actual treatment times was found to be 0.62% (n = 30). The beam attenuation in water varied by approximately 1/r(3). CONCLUSION: The x-ray sources are stable over time. Most measurements were found to lie within the manufacturer's tolerances and an intercomparison of these checks suggests that the four x-ray sources have similar performance characteristics

    Refining Conditions of Fatigue Testing of Hot Mix Asphalt

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    The beam fatigue test of hot-mix asphalt (HMA) has been used for nearly a half century. However, several conflicting results have been recently reported. This study attempts to refine test conditions such as waveform type (haversine versus sinusoidal), incorporating rest periods between loading cycles, and the effect of rest period on the healing of the HMA to minimize (eliminate) gross errors in the data analysis of the fatigue test results. In the deflection-controlled haversine test [ASTM D7460, 2010, “Standard Test Method for Determining Fatigue Failure of Compacted Asphalt Concrete Subjected to Repeated Flexural Bending,” Annual Book of ASTM Standards, Vol. 04.03, ASTM International, West Conshohocken, PA, pp. 1–14] permanent deformations lead to a new equilibrium neutral position of the beam and the force output follows a sinusoidal waveform. This tends to bend the beam in both directions similar to the deflection-controlled sinusoidal test. This would produce erroneous fatigue results since the test assumptions do not match the actual test conditions. In contrast, the deflection-controlled sinusoidal test [AASHTO T-321, 2012, “Standard Method of Test for Determining the Fatigue Life of Compacted Hot Mix Asphalt (HMA) Subjected to Repeated Flexural Bending,” Annual Book of AASHTO Standards, Vol. 32, Washington, DC, pp. T321-1–T321-11] is more consistent than the deflection-controlled haversine test [ASTM D7460, 2010, “Standard Test Method for Determining Fatigue Failure of Compacted Asphalt Concrete Subjected to Repeated Flexural Bending,” Annual Book of ASTM Standards, Vol. 04.03, ASTM International, West Conshohocken, PA, pp. 1–14]. When tests, with and without rest periods, are compared for healing studies, it is even more important to use a deflection-controlled sinusoidal test in order to obtain a fair comparison and accurate healing results. Since neither the haversine waveform nor the sinusoidal waveform in the lab exactly simulates field conditions, it is important to use a sinusoidal waveform in order to obtain consistent results. It is recommended that ASTM changes the ASTM D-7460 designation and test procedure to require a deflection-controlled sinusoidal waveform instead of haversine. Implementing the recommended test conditions is a crucial step in studying the concept of HMA healing and; as a result, estimating the endurance limit which plays an important role in designing sustainable pavements
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