The use of an aSi-based EPID for routine absolute dosimetric pre-treatment verification of dynamic IMRT fields.

The limited ability to control for the location of a tumour compromises the accuracy with which radiation can be delivered to tumour-bearing tissue. The resultant requirement for larger treatment volumes to accommodate target uncertainty restricts the radiation dose because more surrounding normal tissue is exposed. With image-guided radiotherapy (IGRT) these volumes can be optimized and tumoricidal doses can be delivered, achieving maximal tumour control with minimal complications. Moreover, with the ability of high-precision dose delivery and real-time knowledge of the target volume location, IGRT has initiated the exploration of new indications for radiotherapy, some of which were previously considered infeasible.

Innovations in image-guided radiotherapy.

AAPM Task Group 58 was created to provide materials to help the medical physicist and colleagues succeed in the clinical implementation of electronic portal imaging devices (EPIDs) in radiation oncology. This complex technology has matured over the past decade and is capable of being integrated into routine practice. However, the difficulties encountered during the specification, installation, and implementation process can be overwhelming. TG58 was charged with providing sufficient information to allow the users to overcome these difficulties and put EPIDs into routine clinical practice. In answering the charge, this report provides; comprehensive information about the physics and technology of currently available EPID systems; a detailed discussion of the steps required for successful clinical implementation, based on accumulated experience; a review of software tools available and clinical use protocols to enhance EPID utilization; and specific quality assurance requirements for initial and continuing clinical use of the systems. Specific recommendations are summarized to assist the reader with successful implementation and continuing use of an EPID.

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Clinical use of electronic portal imaging: Report of AAPM Radiation Therapy Committee Task Group 58

Spatial and temporal imaging characteristics of an amorphous silicon flat-panel imager(FPI) were investigated in terms relevant to the application of such devices in cone-beam computed tomography(CBCT) and other x-ray imaging modalities, including general radiography, fluoroscopy, mammography, radiotherapy portal imaging, and nondestructive testing. Specifically, issues of image lag (including the magnitude, spatial uniformity, temporal-frequency characteristics, and dependence upon exposure and frame time) and long-term image persistence (“ghosts”) were investigated. As part of the basic characterization of the FPI, pixel dark signal and noise (magnitude, temporal stability, and spatial uniformity) as well as radiation response (signal size, linearity, gain, and reciprocity) were also measured. Image lag was analyzed as a function of frame time and incident exposure. First-frame lag (i.e., the relative residual signal in the first frame following readout of an exposure) was ∼2–10%, depending upon incident exposure and was spatially nonuniform to a slight degree across the FPI; second-, third-, and fourth-frame lag were ∼0.7%, 0.4%, and 0.3%, respectively (at 25% sensor saturation). Image lag was also analyzed in terms of the temporal-frequency-dependent transfer function derived from the radiation response, allowing a quantitative description of system components contributing to lag. Finally, the contrast of objects as a function of time following an exposure was measured in order to examine long-term image persistence (“ghosts”). Ghosts were found to persist up to 30 min or longer, depending upon the exposure and frame time. Two means of reducing the apparent contrast of ghost images were tested: (i) rapid scanning of the FPI at maximum frame rate, and (ii) flood-field exposure of the FPI; neither was entirely satisfactory. These results pose important considerations for application of FPIs in CBCT as well as other x-ray imaging modalities. For example in CBCT, the magnitude of image lag is such that significant artifacts in tomographic reconstructions may result if strategies are not adopted either to reduce or correct the lag between successive projections (e.g., rapid scanning between projections or iterative correction algorithms, respectively). Similarly, long-term image persistence may necessitate frequent recalibration of offset corrections.

A ghost story: spatio-temporal response characteristics of an indirect-detection flat-panel imager.

Dosimetric properties of an amorphous silicon electronic portal imaging device (EPID) for verification of dynamic intensity modulated radiation therapy (IMRT) delivery were investigated. The EPID was utilized with continuous frame-averaging during the beam delivery. Properties studied included effect of buildup, dose linearity, field size response, sampling of rapid multileaf collimator (MLC) leaf speeds, response to dose-rate fluctuations, memory effect, and reproducibility. The dependence of response on EPID calibration and a dead time in image frame acquisition occurring every 64 frames were measured. EPID measurements were also compared to ion chamber and film for open and wedged static fields and IMRT fields. The EPID was linear with dose and dose rate, and response to MLC leaf speeds up to 2.5 cm s(-1) was found to be linear. A field size dependent response of up to 5% relative to dmax ion-chamber measurement was found. Reproducibility was within 0.8% (1 standard deviation) for an IMRT delivery recorded at intervals over a period of one month. The dead time in frame acquisition resulted in errors in the EPID that increased with leaf speed and were over 20% for a 1 cm leaf gap moving at 1.0 cm s(-1). The EPID measurements were also found to depend on the input beam profile utilized for EPID flood-field calibration. The EPID shows promise as a device for verification of IMRT, the major limitation currently being due to dead-time in frame acquisition.

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Dosimetric properties of an amorphous silicon electronic portal imaging device for verification of dynamic intensity modulated radiation therapy.

We have measured the linearity, spatial resolution (MTF), noise (NPS), and signal-to-noise characteristics (DQE) of an electronic portal imaging device(EPID) based on an amorphous silicon flat-panel array. The array has a 128×128 -pixel matrix and each pixel is 0.75×0.75 mm 2 in dimension so the array covers an area of 96×96 mm 2 . The array acts like a large area light sensor and records the optical signals generated in a metal plate/phosphor screen x-ray detector when the detector is irradiated by a megavoltage x-ray beam. In addition, approximately 0.5% of the total signal is generated by nonoptical processes. The noise measurements show that the device is quantum noise limited with the noise power generated by the x-ray quanta being up to 100 times greater than the noise added by the external readout electronics and flat-panel light sensor itself. However, the flat-panel light sensor does reduce the spatial resolution (compared to a perfect optical sensor with infinitesimal pixel size) because of its moderate pixel size and because optical spread can occur in the transparent glues used to attach the phosphor screen to the flat-panel light sensor. The response of the sensor is very linear and does not suffer from the glare phenomenon associated with TV camera-based EPIDs—characteristics which suggest that the amorphous siliconEPID will be well suited to transit dosimetry. Nevertheless, some limitations need to be overcome before these devices can be used clinically. These include developing larger flat-panel light sensors, the elimination of “noisy” pixels with high dark signal, and improvements in the uniform sensitivity of the sensors. This last requirement is only needed for transit dosimetry applications where it would greatly simplify calibration of the device. In addition, an image acquisition scheme must be developed to eliminate artifacts created by the pulsed x-ray beam generated by linear accelerators. Despite these limitations, our studies suggest that the amorphous siliconEPIDs are very well suited to portal imaging.

D. C. Bouius

Papers

X‐ray quantum limited portal imaging using amorphous silicon flat‐panel arrays