Dose-to-water conversion for the backscatter-shielded EPID: a frame-based method to correct for EPID energy response to MLC transmitted radiation.

PURPOSE: To develop a frame-by-frame correction for the energy response of amorphous silicon electronic portal imaging devices (a-Si EPIDs) to radiation that has transmitted through the multileaf collimator (MLC) and to integrate this correction into the backscatter shielded EPID (BSS-EPID) dose-to-water conversion model. METHODS: Individual EPID frames were acquired using a Varian frame grabber and iTools acquisition software then processed using in-house software developed inMATLAB. For each EPID image frame, the region below the MLC leaves was identified and all pixels in this region were multiplied by a factor of 1.3 to correct for the under-response of the imager to MLC transmitted radiation. The corrected frames were then summed to form a corrected integrated EPID image. This correction was implemented as an initial step in the BSS-EPID dose-to-water conversion model which was then used to compute dose planes in a water phantom for 35 IMRT fields. The calculated dose planes, with and without the proposed MLC transmission correction, were compared to measurements in solid water using a two-dimensional diode array. RESULTS: It was observed that the integration of the MLC transmission correction into the BSS-EPID dose model improved agreement between modeled and measured dose planes. In particular, the MLC correction produced higher pass rates for almost all Head and Neck fields tested, yielding an average pass rate of 99.8% for 2%/2 mm criteria. A two-sample independent t-test and fisher F-test were used to show that the MLC transmission correction resulted in a statistically significant reduction in the mean and the standard deviation of the gamma values, respectively, to give a more accurate and consistent dose-to-water conversion. CONCLUSIONS: The frame-by-frame MLC transmission response correction was shown to improve the accuracy and reduce the variability of the BSS-EPID dose-to-water conversion model. The correction may be applied as a preprocessing step in any pretreatment portal dosimetry calculation and has been shown to be beneficial for highly modulated IMRT fields.

A method for removing arm backscatter from EPID images.

PURPOSE: To develop a method for removing the support arm backscatter from images acquired using current Varian electronic portal imaging devices (EPIDs). METHODS: The effect of arm backscatter on EPID images was modeled using a kernel convolution method. The parameters of the model were optimized by comparing on-arm images to off-arm images. The model was used to develop a method to remove the effect of backscatter from measured EPID images. The performance of the backscatter removal method was tested by comparing backscatter corrected on-arm images to measured off-arm images for 17 rectangular fields of different sizes and locations on the imager. The method was also tested using on- and off-arm images from 42 intensity modulated radiotherapy (IMRT) fields. RESULTS: Images generated by the backscatter removal method gave consistently better agreement with off-arm images than images without backscatter correction. For the 17 rectangular fields studied, the root mean square difference of in-plane profiles compared to off-arm profiles was reduced from 1.19% (standard deviation 0.59%) on average without backscatter removal to 0.38% (standard deviation 0.18%) when using the backscatter removal method. When comparing to the off-arm images from the 42 IMRT fields, the mean γ and percentage of pixels with γ < 1 were improved by the backscatter removal method in all but one of the images studied. The mean γ value (1%, 1 mm) for the IMRT fields studied was reduced from 0.80 to 0.57 by using the backscatter removal method, while the mean γ pass rate was increased from 72.2% to 84.6%. CONCLUSIONS: A backscatter removal method has been developed to estimate the image acquired by the EPID without any arm backscatter from an image acquired in the presence of arm backscatter. The method has been shown to produce consistently reliable results for a wide range of field sizes and jaw configurations.

Development and testing of an improved dosimetry system using a backscatter shielded electronic portal imaging device.

PURPOSE: To investigate the properties of a modified backscatter shielded electronic portal imaging device (BSS-EPID) and to develop a dose model to convert BSS-EPID images to dose in water as part of an improved system for dosimetry using EPIDs. METHODS: The effectiveness of the shielding of the BSS-EPID was studied by comparing images measured with the BSS-EPID mounted on the support arm to images measured with the BSS-EPID removed from the support arm. A dose model was developed and optimized to reconstruct dose in water at different depths from measured BSS-EPID images. The accuracy of the dose model was studied using BSS-EPID images of 28 IMRT fields to reconstruct dose in water at depths of 2, 5, 10, and 20 cm and comparing to measured dose in water from a two-dimensional diode array at the same depths. The ability of the BSS-EPID system to operate independently of detector position was demonstrated by comparing the dose reconstruction of a 10 × 10 cm(2) field using different detector offsets to that measured by a two-dimensional diode array. RESULTS: The shielding of the BSS-EPID was found to be effective, with more than 99% of pixels showing less than 0.5% change due to the presence of the support arm and at most a 0.2% effect on the central axis for 2 × 2 cm(2) fields to fully open 30 × 40 cm(2) images. The dose model was shown to accurately reconstruct measurements of dose in water using BSS-EPID images with average γ pass rates (2%, 2 mm criteria) of 92.5%, 98.7%, 97.4%, and 97.2% at depths of 2, 5, 10, and 20 cm, respectively, when compared to two-dimensional diode array measurements. When using 3%, 3 mm γ criteria, the average pass rate was greater than 97% at all depths. Reconstructed dose in water for a 10 × 10 cm(2) field measured with detector offsets as large as 10 cm agreed with each other and two-dimensional diode array measurements within 0.9%. CONCLUSIONS: The modified BSS-EPID and associated dose model provide an improved system for dosimetry measurements using EPIDs. Several important limitations of the current hardware and software are addressed by this system.

Measurement of coherent scattering form factors using an image plate.

Two new methods for measuring x-ray coherent scattering form factors using an image plate were developed based on a matrix representation of the problem. The methods were tested experimentally using tube potentials of 50, 70 and 92 kV and different filtrations. Water and fat samples were measured and compared to data in the literature. For water, average absolute relative differences between 0.105 and 0.217 were measured when compared to the literature. Literature data for fat show a wide range of values. Our measured values were in the middle of the literature data range with average absolute relative differences between 0.126 and 0.528. The accuracy of the matrix methods was limited by the ill-conditioning of the problem and also experimentally by the limitations of our equipment. The matrix methods developed are shown to be relatively low resolution in momentum transfer parameter and limited to measurements on amorphous materials such as tissues where the attenuation of the x-ray beam is small. These methods, however, can be used to measure form factors with relatively inexpensive, clinical equipment.