Intensity-modulated radiation therapy (IMRT) delivered with multi-leaf collimator (MLC) in the step-and-shoot mode uses multiple static MLC segments to achieve intensity modulation. For typical IMRT treatment plans, significant numbers of segments are delivered with monitor units (MUs) of much less than 10. Verification of the ability of the linear accelerator (linac) to deliver small MU segments accurately is an important step in the IMRT commissioning and quality assurance (QA) process. Recent studies have reported large discrepancies between the intended and delivered segment MUs. These discrepancies could potentially cause large errors in the delivered patient dose. We have undertaken a systematic study to evaluate the accuracy of the dynamic MLC log files, which are created automatically by our commercial MLC workstation after each delivery, in recording the fractional MU delivered in the step-and-shoot mode. Two linac models were evaluated with simple-geometry leaf sequences and delivered with different total MUs and different nominal dose rates. A commercial two-dimensional diode array was used for the measurement. Large discrepancies between the intended and delivered segment MUs were found. The discrepancies were larger for small MU segments at higher dose rate, with some small MU segments completely undelivered. The recorded fractional MUs in the log files were found to agree with what was delivered within the limits of our experimental uncertainty. Our results indicate that it is important to verify the delivery accuracy of small MU segments that could potentially occur in a patient treatment and that the log files are useful in checking the integrity of the linac delivery once validated. Thus validated log files can be used as a QA tool for general IMRT delivery and patient-specific plan verification.
The performance of a diode array (Profiler) was evaluated by comparing its enhanced dynamic wedge (EDW) profiles measured at various depths with point measurements using a 0.03 cm3 ionization chamber on a commercial linear accelerator. The Profiler, which covers a 22.5 cm width, was used to measure larger field widths by concatenating three data sets into a larger field. An innovative wide-field calibration technique developed by the manufacturer of the device was used to calibrate the individual diode sensitivity, which can vary by more than 10%. Profiles of EDW measured with this device at several depths were used to construct isodose curves using the percentage depth dose curve measured by the ionization chamber. These isodose curves were used to check those generated by a commercial treatment planning system. The profiles measured with the diode array for both 8 and 18 MV photon beams agreed with those of the ionization chamber within a standard deviation of 0.4% in the field (defined as 80% of the field width) and within a maximum shift of less than 2 mm in the penumbra region. The percentage depth dose generally agreed to within 2% except in the buildup region. The Profiler was extremely useful as a quality assurance tool for EDW and as a dosimetry measurement device with tremendous savings in data acquisition time.
This paper focuses on the durability of basalt fiber reinforced polymer (BFRP) laminas exposed to solutions of normal concrete (NC) and seawater sea sand concrete (SWSSC) by acceleration of temperature. Modification of BFRP laminas using different contents of silica nanoparticles and multiwall carbon nanotubes (MWCNTs) was conducted, and the effect of modification on the durability of the lamina was evaluated. The degradation and modification mechanism of BFRP laminas was investigated by scanning electron microscopy (SEM). The results showed that the BFRP laminas exhibited obvious strength degradation after exposure to the NC and SWSSC solutions, while the Young's modulus of the laminas remained nearly unchanged. The tensile strength of BFRP laminas in SWSSC solution declined faster than in NC solution due to the combined effect of alkali ions and saline ions in the SWSSC solution accelerated the corrosion of FRP. Additionally, the durability of MWCNT‐modified specimens has been significantly improved. The SEM images revealed that MWCNTs in the resin matrix improve the interface bonding performance between the fiber and the resin. Furthermore, the diffusion path of OH−, Cl−, and H2O molecules is elongated and hindered by MWCNTs. The silica nanoparticle‐modified BFRP laminas obtained improved corrosion resistance performance only at a content of 1 wt%. Additionally, the long‐term residual tensile strength of modified BFRP laminas under NC and SWSSC environments was predicted.
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