Sampling considerations for intensity modulated radiotherapy verification using electronic portal imaging

A model has been developed to describe the sampling process that occurs when intensity modulated radiotherapy treatments (delivered with a multileaf collimator) are imaged with an electronic portal imaging device that acquires a set of frames with a finite dead-time between them. The effects of the imaging duty cycle and frame rate on the accuracy of dosimetric verification have been studied. A frame interval of 1 s with 25%, 50% and 75% duty cycle, and a 50% duty cycle with frame intervals of 1, 2, 4, 8, and 16 s have been studied for a smoothly varying hemispherical intensity profile, and a 50% duty cycle with frame intervals of 1, 2, 4, and 8 s for a pixellated distribution. In addition an intensity modulated beam for breast radiotherapy has been modeled and imaged for 0.33 s frame time and 1, 2, and 3 s frame separation. The results show that under sparse temporal sampling conditions, errors of the order of 10% may ensue and occur with an oscillatory pattern. For the beams studied, imaging with a 1 or 2 s frame interval resulted in small errors at the 1%-2% level, for all duty cycles shown.

IMRT verification with a camera-based electronic portal imaging system

An evaluation of the capabilities of a commercially available camera-based electronic portal imaging system for intensity-modulated radiotherapy verification is presented. Two modifications to the system are demonstrated which use a novel method to tag each image acquired with the delivered dose measured by the linac monitor chamber and reduce optical cross-talk in the imager. A detailed performance assessment is presented, including measurements of the optical decay characteristics of the system. The overall geometric accuracy of the system is determined to be +/-2.0 mm, with a dosimetric accuracy of +/-1.25 MU. Finally a clinical breast IMRT treatment, delivered by dynamic multileaf collimation, is successfully verified both by tracking the position of each leaf during beam delivery and recording the integrated intensity observed over the entire beam.

Optical scattering in camera-based electronic portal imaging

An investigation of scintillation light scattering within camera-based electronic portal imaging devices is presented. A simple analytical scatter model is adapted for the precise geometries of two different camera-based imaging systems and the results of modelling and experimental measurements are compared. The results of the study strongly suggest that the main source of optical scattering is multiple reflection between the scintillation screen and 45 degree mirror in both systems. The scattered light has a highly non-uniform distribution, which is strongly dependent upon field size. For large radiation fields the scatter contribution can be over 20% of the primary signal scintillation light intensity in the centre of the field. A purely optical method of removing the scattered light signal using a louvre grid on the surface of the scintillation screen is then presented and experimentally demonstrated to be effective.

A large-area ionization chamber for portal image calibration

Methods of removing the effects of linear accelerator (linac) output fluctuation from electronic portal images are described and compared. The output of the linac is measured using a specially constructed large-area ionization chamber during imaging and recorded with the image. The use of a dose-rate signal directly from the linac monitor chamber is discussed. Various versions of a quadratic thickness calibration scheme are tested, incorporating linac output data measured by the ionization chamber. Experimental results are presented showing that the incorporation of data from the ionization chamber gives improved absolute calibration accuracy and flatness. Immediately after calibration, the mean systematic thickness error in calibration of a uniform 136.8 mm water-equivalent slab was shown to be no more than 0.6 mm with a thickness variation within each image also of no more than +/-0.8 mm. This was true even when imaging with an unstable linac beam giving mean thickness errors between images of 8.8 mm and variations within each image of +/-4.9 mm without the ionization chamber correction. Up to one month after calibration, use of the ionization chamber to remove short-term linac fluctuations is shown to still keep mean thickness errors to less than 1.6 mm with variations within each image of no more than +/-1.4 mm.

Practical implementation of compensators in breast radiotherapy

BACKGROUND AND PURPOSE

A method of using electronic portal imaging to design compensators for tangential breast irradiation has been developed. We describe how this has been implemented.

MATERIALS AND METHODS

The compensator design method generates wedged and unwedged beam weights, in conjunction with templates for multiple lead-sheet compensators and pseudo-CT outlines. The latter describe the breast and lung profiles in a set of transverse slices. The layers of the compensator and pseudo-CT outlines are transferred to a treatment planning system for verification. The accuracy of the planning system for the high transmission blocks used to describe the compensators has been verified using a plotting tank system. Dose volume histogram data and transaxial and sagittal plan slices have been compared for both standard and compensated treatments for a sample set of five patients.

RESULTS

The planning system predicted the dose at depths of 1.5 and 5 cm to within 2% for the compensators tested. The biggest source of discrepancy was a consequence of the planning system requiring blocks to have integer percentage transmission. For all patients studied, the compensated treatment resulted in a significant reduction in the percentage volume outside the 95-105% dose, with an average reduction of 10.2%. The percentage volume outside the 95-107% dose was also reduced by typically 3.4%. The implementation was found to yield a convenient automatic method of designing compensators using electronic portal imaging and verifying the results using a planning system.

CONCLUSIONS

These results indicate that this method of implementation can be used in practice. The dosimetric accuracy of the treatment planning system is limited by the requirement that blocks should be of integer transmission, but this effect is small.

Rapid portal imaging with a high-efficiency, large field-of-view detector

The design, construction, and performance evaluation of an electronic portal imaging device (EPID) are described. The EPID has the same imaging geometry as the current mirror-based systems except for the x-ray detection stage, where a two-dimensional (2D) array of 1 cm thick CsI(Tl) detector elements are utilized. The approximately 18% x-ray quantum efficiency of the scintillation detector and its 30 x 40 cm2 field-of-view at the isocenter are greater than other area-imaging EPIDs. The imaging issues addressed are theoretical and measured signal-to-noise ratio, linearity of the imaging chain, influence of frame-summing on image quality and image calibration. Portal images of test objects and a humanoid phantom are used to measure the performance of the system. An image quality similar to the current devices is achieved but with a lower dose. With approximately 1 cGy dose delivered by a 6 MV beam, a 2 mm diam structure of 1.3% contrast and an 18 mm diam object of 0.125% contrast can be resolved without using image-enhancement methods. A spatial resolution of about 2 mm at the isocenter is demonstrated. The capability of the system to perform fast sequential imaging, synchronized with the radiation pulses, makes it suitable for patient motion studies and verification of intensity-modulated beams as well as its application in cone-beam megavoltage computed tomography.

An electronic portal imaging device for transit dosimetry

An electronic portal imaging device has been designed and constructed. It consists of an array of 128 CsI scintillation crystals coupled to photodiodes which is scanned across the field in 4 seconds. The linac is operated at a dose rate of 400 cGy min-1 and the dose delivered for image acquisition is approximately 27 cGy. The data acquisition controller is a stand-alone STE computer located within the scan arm. Sample images are presented showing contrast and spatial resolution of the system together with a humanoid phantom image and a clinical image of a breast cancer patient. The phantom images show the detector has a contrast resolution of 0.3% (at 15 mm diameter) and a spatial resolution of 2.5-3.2 mm. Images of uniform Perspex blocks have also been calibrated for thickness, indicating the system can measure radiological thickness to an accuracy of 2-3 mm of water. These results indicate the detector may be used for transit dosimetry applications including compensator design.