Beam modeling and VMAT performance with the Agility 160-leaf multileaf collimator.

The Agility multileaf collimator (Elekta AB, Stockholm, Sweden) has 160 leaves of projected width 0.5 cm at the isocenter, with maximum leaf speed 3.5 cms-1. These characteristics promise to facilitate fast and accurate delivery of radiotherapy, particularly volumetric-modulated arc therapy (VMAT). The aim of this study is therefore to create a beam model for the Pinnacle3 treatment planning system (Philips Radiation Oncology Systems, Fitchburg, WI), and to use this beam model to explore the performance of the Agility MLC in delivery of VMAT. A 6 MV beam model was created and verified by measuring doses under irregularly shaped fields. VMAT treatment plans for five typical head-and-neck patients were created using the beam model and delivered using both binned and continuously variable dose rate (CVDR). Results were compared with those for an MLCi unit without CVDR. The beam model has similar parameters to those of an MLCi model, with interleaf leakage of only 0.2%. The verification of irregular fields shows a mean agreement between measured and planned dose of 1.3% (planned dose higher). The Agility VMAT head-and-neck plans show equivalent plan quality and delivery accuracy to those for an MLCi unit, with 95% of verification measurements within 3% and 3 mm of planned dose. Mean delivery time is 133 s with the Agility head and CVDR, 171 s without CVDR, and 282 s with an MLCi unit. Pinnacle3 has therefore been shown to model the Agility MLC accurately, and to provide accurate VMAT treatment plans which can be delivered significantly faster with Agility than with an MLCi

GafChromic (R) RTQA film for routine quality assurance of high-energy photon beams

Self-developing film offers many advantages over conventional radiographic verification film for routine radiotherapy quality assurance (QA). This paper presents results from an initial evaluation of a beam measurement system using GafChromic (R) RTQA film and a flatbed scanner. Variability and energy dependence of the film calibration and accuracy of scanner readout are investigated in the context of QA measurements. For exposures of film between 2 and 4 Gy, the system is adequate for measurement of beam dimensions, as in multi-leaf collimator (MLC) offsets and secondary jaw calibrations, where agreement with conventional film measurements is within 0.5 mm. However, the measurement of absolute dose is subject to errors of about 25 cGy

Characterization of a transmission ionization beam-imager for radiotherapy verification

A system for radiotherapy treatment verification is proposed, using an air-ionization chamber with 1600 simultaneously readable 1 cm pixels. An image of the entire beam may be used to calibrate a portal image, to verify the position of the multi-leaf collimator with respect to delivered dose (either before or during treatment) and to check beam flatness and symmetry. This study characterizes the physical behaviour of such a system. A test chamber has been constructed and its temporal and spatial resolution and noise characteristics are evaluated. Several parameters of the design are varied, and their effects assessed. Temporal resolution is adequate to allow readout between each linear accelerator pulse at 400 pulses per second. Application of low atomic-number build-up and reduction of plate separation were the most effective methods to improve spatial resolution. The full width at half maximum of the line-spread function is shown to be 4.5 mm using a pre-sampling technique. The peak pixel-signal to x-ray quantum noise ratio exceeds 100. Prototype electronics have been tested, demonstrating that electronic noise could be reduced to a level below the x-ray quantum noise. The results of the study allow the simulation of any possible application to evaluate the proposed verification system

A dual modality approach to quantitative quality control in emission tomography

Routine quality control (QC) and optimization of image quality of reconstructed images in single photon emission computed tomography (SPECT) and positron emission tomography (PET) remains a relatively qualitative exercise. With the advent of combined SPECT/CT and PET/CT devices, and accurate post hoc co-registration algorithms, the potential exists to utilize high resolution structural information for QC evaluation in addition to their use for anatomical correlation in clinical studies. The aim of this work was to explore, in principle, the uses of x-ray CT data of QC phantoms used in SPECT and PET to develop more objective assessments of performance of the emission tomographic (ET) devices and reconstructed data. A CT reconstruction of a novel ET QC phantom was segmented into the various compartments it contained. Using software, the voxel values in the different compartments were then altered to correspond to the concentration of the radioactivity in the actual scan of the same phantom on the SPECT system. This produces a high resolution version of a ‘perfect’ ET scan. Image co-registration techniques were then used to spatially align the synthetic high resolution SPECT scan to the measured SPECT scan. Various parameters can then be objectively derived from the registered data, for example, image contrast, spatial resolution, spatial non-uniformity, etc. In this study, we have used this approach to estimate spatial resolution (full width at half maximum, FWHM) and recovered contrast in reconstructed images of a SPECT phantom. Two independent methods were used to measure spatial resolution, obtaining excellent agreement. In conclusion, the ability to produce high resolution synthetic phantoms in emission tomography QC affords an objective approach to assessing system performance and optimizing protocols which is readily automated and quantifiable

Electron-beam induced conduction in a ruthenium carbonyl nanoparticle polymer

A polymer composed of ruthenium carbonyl and of the formula [Ru6C(CO)(15)Ph2PCCPPh2](n) has been synthesized. It is found to behave as a negative electron-beam resist with a sensitivity of 400 C/m(2). Upon exposure to the electron beam, the electrical conductivity of the patterned films is found to vary over seven orders of magnitude according to a power-law dependence on dose. Temperature dependence of the conductivity is studied, and the conduction is attributed to variable-range hopping between ruthenium superclusters in two dimensions. (C) 2000 American Institute of Physics. [S0003-6951(00)01913-6].</p

by Interaction with an Inverted CCAAT Box in Human Cancer Cells

This article is available online a