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Using an Electronic Portal Imaging Device (EPID) Instead of Film

Varian EPID

We are told by Varian that PortalVision 6.1.13 supports a mode of image acquisition which integrates during the entire beam-on period of the accelerator. In PV 6.1.13 the mode is installed only upon customer request and requires a service call from Varian. Version 7.1.2003.95 has an AM Maintenance mode which allows for integration. Varis/Aria can now manage the integration of fields during a dry run without the patient.

Use the link here for an example of using the Varian EPID in a difficult case with two MLC carriage positions.

PTW 729 Ion Chamber Array

We can read files from the PTW 729 ion chamber array. Comparision of using the PTW 2D versus a higher resolution EPID.

Map Check Diode Array

We can read files from the Map Check diode array. Map Check versus EPID example.

IBA Matrixx Ion Chamber Array

We can read files from the IBA (Scanditronix-Welhofer) Matrixx ion chamber array.

Elekta iViewGT

Elekta introduced a software feature in iViewGT 3.1 that enables the iViewGT EPID to integrate. The iViewGT acquires images by accumulating frames into a composite image. That image is then renormalized so that all images have the same brightness. In 3.1 the renormalization factor, or scaling factor, for each image is saved. The user can now recover this factor. When the scaling factor is divided into the image pixel data, the integrated value can be recovered.

We have written a utility called IviewToDicom that will assemble all the segment files for each beam that is the result of an integration. See link to online user manuals.

We have a paper:
"A method for deconvolution of integrated electronic portal images to obtain incident fluence for dose reconstruction", JACMP Vol. 6, No. 4, Fall 2005, pp. 22-39, www.jacmp.org.

No Film

Using an EPID eliminates the handling of film. All that is required is a dry treatment run without the patient, with each beam portal integrated and saved to a file. Those images are then readily available for input to Dosimetry Check. About a half hour to an hour of processing is then required to confirm a treatment delivery. By downloading the plan as well, dose comparison tools can be used to compare the reconstructed plan to that intended.

Other Devices: Ion chamber and diode arrays

The same technique used above for deconvolution of an EPID devce applies to diode and ion chamber arrays used to integrate field images. Please contact us if you are interested in some other device. We will need only a series of integrated square field sizes to provide a deconvolution method to convert the integrated image back to in air fluence.