ABSTRACT
Volumetric-modulated arc therapy (VMAT), a form of intensity-modulated arc therapy (IMAT), has become a topic of research and clinical activity in recent years. As a form of arc therapy, portal images acquired during the treatment fraction form a (partial) Radon transform of the patient. We show that these portal images, when used in a modified global cone-beam filtered backprojection (FBP) algorithm, allow a surprisingly recognizable CT-volume to be reconstructed. The possibility of distinguishing anatomy in such VMAT-CT reconstructions suggests that this could prove to be a valuable treatment position-verification tool. Further, some potential for local-tomography techniques to improve image quality is shown.
Subject(s)
Image Processing, Computer-Assisted/methods , Radiotherapy, Computer-Assisted/methods , Radiotherapy, Intensity-Modulated , Tomography, X-Ray Computed/methods , Esophageal Neoplasms/diagnostic imaging , Esophageal Neoplasms/radiotherapy , HumansABSTRACT
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 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.
Subject(s)
Film Dosimetry/instrumentation , Film Dosimetry/methods , Radiotherapy, High-Energy/instrumentation , Calibration , Humans , Models, Theoretical , Photons , Quality Control , Radiometry , Radiotherapy Dosage , Radiotherapy Planning, Computer-Assisted , Radiotherapy, Conformal , Reproducibility of Results , Time FactorsABSTRACT
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.
