Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UMLS:C0162871 (abdominal aortic aneurysm)
 8,664 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

A novel transmission detector (IBA Dosimetry, Germany) developed as an IMRT quality assurance tool, intended for in vivo patient dose measurements, is studied here. The goal of this investigation is to use Monte Carlo techniques to characterize treatment beam parameters in the presence of the detector and to compare to those of a plastic block tray (a frequently used clinical device). Particular attention is paid to the impact of the detector on electron contamination model parameters of two commercial dose calculation algorithms. The linac head together with the COMPASS transmission detector (TRD) was modeled using BEAMnrc code. To understand the effect of the TRD on treatment beams, the contaminant electron fluence, energy spectra, and angular distributions at different SSDs were analyzed for open and non-open (i.e. TRD and block tray) fields. Contaminant electrons in the BEAMnrc simulations were separated according to where they were created. Calculation of surface dose and the evaluation of contributions from contaminant electrons were performed using the DOSXYZnrc user code. The effect of the TRD on contaminant electrons model parameters in Eclipse AAA and Pinnacle(3) dose calculation algorithms was investigated. Comparisons of the fluence of contaminant electrons produced in the non-open fields versus open field show that electrons created in the non-open fields increase at shorter SSD, but most of the electrons at shorter SSD are of low energy with large angular spread. These electrons are out-scattered or absorbed in air and contribute less to surface dose at larger SSD. Calculated surface doses with the block tray are higher than those with the TRD. Contribution of contaminant electrons to dose in the buildup region increases with increasing field size. The additional contribution of electrons to surface dose increases with field size for TRD and block tray. The introduction of the TRD results in a 12% and 15% increase in the Gaussian widths used in the contaminant electron source model of the Eclipse AAA dose algorithm. The off-axis coefficient in the Pinnacle(3) dose calculation algorithm decreases in the presence of TRD compared to without the device. The electron model parameters were modified to reflect the increase in electron contamination with the TRD, a necessary step for accurate beam modeling when using the device.
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PMID:A Monte Carlo investigation of contaminant electrons due to a novel in vivo transmission detector. 2128 80

The dosimetric leaf gap (DLG) is a parameter used by Eclipse to model the rounded leaf ends of Varian MLCs. The DLGs were determined for the Millennium (M120) and High-Definition (HD120) model MLCs and taken as the difference between measured (0.6mm diode, IBA) and nominal MLC-defined profile FWHM values. Configuring the Eclipse pencil beam algorithm with the measured DLG gave poor agreement between measured and calculated IMRT dose distributions for the HD120 but not the M120. Agreement was optimized by adjusting the DLG for the HD120; 0.3mm changes in DLG were enough to cause significant variations in field dose agreement. Optimal DLG values of 0.04cm and 0.05cm were found for the 6MV HD120 and 10MV HD120, respectively, and 0.135cm 0.175cm for the 6MV M120 and 18MV M120, respectively. Agreement between measured and calculated dose distributions worsened for the AAA algorithm indicating separate DLG values may be required. A leaf calibration software upgrade also reduced agreement by changing the physical leaf position for a given location value. The change was detected using film and the picket fence MLC-pattern which places the two banks of opposing leaves at the same position but at different times. The DLG value can be adjusted from its measured physical value to improve the dosimetric accuracy of Eclipse IMRT plans and compensate for the effects of treatment planning algorithm and varying leaf calibrations. Since leaf calibrations are variable it is important to define the dosimetric leaf gap for each accelerator and clinic.
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PMID:Poster - Thurs Eve-06: Maximizing eclipse IMRT dose accuracy by adjusting the dosimetric leaf gap parameter. 2851 26