Imaging dose from megavoltage cone beam computed tomography (MVCBCT) can be significantly reduced without loss of image quality by using an imaging beam line (IBL), with no flattening filter and a carbon, rather than tungsten, electron target. The IBL produces a greater keV-range x-ray fluence than the treatment beam line (TBL), which results in a more optimal detector response. The IBL imaging dose is not necessarily negligible, however. In this work an IBL was dosimetrically modeled with the Philips Pinnacle3 treatment planning system (TPS), verified experimentally, and applied to clinical cases. The IBL acquisition dose for a 200 degrees gantry rotation was verified in a customized acrylic cylindrical phantom at multiple imaging field sizes with 196 ion chamber measurements. Agreement between the measured and calculated IBL dose was quantified with the 3D gamma index. Representative IBL and TBL imaging dose distributions were calculated for head and neck and prostate patients and included in treatment plans using the imaging dose incorporation (IDI) method. Surface dose was measured for the TBL and IBL for four head and neck cancer patients with MOSFETs. The IBL model, when compared to the percentage depth dose and profile measurements, had 97% passing gamma indices for dosimetric and distance acceptance criteria of 3%, 3 mm, and 100% passed for 5.2%, 5.2 mm. For the ion chamber measurements of phantom image acquisition dose, the IBL model had 93% passing gamma indices for acceptance criteria of 3%, 3 mm, and 100% passed for 4%, 4 mm. Differences between the IBL- and TBL-based IMRT treatment plans created with the IDI method were dosimetrically insignificant for both the prostate and head and neck cases. For IBL and TBL beams with monitor unit values that would result in the delivery of the same dose to the depth of maximum dose under standard calibration conditions, the IBL imaging surface dose was higher than the TBL imaging surface dose by an average of 18%, with a standard deviation of 8% (p = 2 x 10(-6)). The IBL can be modeled with acceptable accuracy using a standard TPS, and accounting for IBL dose in treatment plans with the IDI method is straightforward. The resulting composite dose distributions, assuming similar imaging doses, are negligibly different from those of the TBL. The increased IBL surface dose relative to the TBL is likely clinically insignificant.
The aim of this study was to develop a phantom and analysis software that could be used to quickly and accurately determine the location of radiation isocenter to an accuracy of less than 1 mm using the EPID (Electronic Portal Imaging Device). The proposed solution uses a collimator setting of 10×10cm2 to acquire EPID images of a new phantom constructed from LEGO blocks. Images from a number of gantry and collimator angles are analyzed by automated analysis software to determine the position of the jaws and center of the phantom in each image. The distance between a chosen jaw and the phantom center is then compared to the same distance measured after a 180° collimator rotation to determine if the phantom is centered in the dimension being investigated. Repeated tests show that the system is reproducibly independent of the imaging session, and calculated offsets of the phantom from radiation isocenter are a function of phantom setup only. Accuracy of the algorithm's calculated offsets were verified by imaging the LEGO phantom before and after applying the calculated offset. These measurements show that the offsets are predicted with an accuracy of approximately 0.3 mm, which is on the order of the detector's pitch. Comparison with a star‐shot analysis yielded agreement of isocenter location within 0.5 mm. Additionally, the phantom and software are completely independent of linac vendor, and this study presents results from two linac manufacturers. A Varian Optical Guidance Platform (OGP) calibration array was also integrated into the phantom to allow calibration of the OGP while the phantom is positioned at radiation isocenter to reduce setup uncertainty in the calibration. This solution offers a quick, objective method to perform isocenter localization as well as laser alignment and OGP calibration on a monthly basis.PACS number: 87.55.Qr
The LS method is superior to the BLI and QLI methods, and correction algorithm effectiveness decreases as imaging dose increases. All correction methods failed first due to ring artifacts and second due to MTF drop. If ring artifacts in axial slices within a 4 mm longitudinal distance from phantom section interfaces are acceptable, statistically significant loss in spatial resolution does not occur until over 25% of the EPID is covered in randomly distributed dead detectors, or NRDDs of 4 mm diameter are present.
Purpose: CT imaging of patients for radiotherapy treatment planning frequently includes anatomy that extends beyond the 50cm nominal field‐of‐view (nFOV): 28.35% of the 575 CT scans we acquired from July–December 2006. The purpose of this study was to evaluate 1) the degradation of Hounsfield units (HU) in the extended field of view (eFOV), and 2) the dosimetric impact of ignoring or correcting this degradation within the treatment planning system (TPS). Method and Materials: CT images were acquired at maximum FOV (82cm) on a diamond‐shaped 30×30×13cm solid‐water phantom, challenging the reconstruction algorithm's ability to model the truncated projection data adequately. A unique dataset was acquired for 19 phantom locations (1cm intervals outside of the nFOV) and each imported into a TPS. Each phantom was contoured using a threshold of −200HU. A single treatment beam with isocenter placed at the center of the phantom was planned to deliver 100cGy to isocenter for each CT data set; dose map comparisons with and without homogenous correction for density (HU>−700 =1gm/cc) were performed relative to the control phantom within the nFOV. Results: Significant variation of HU was observed as a function of phantom displacement outside the nFOV (range +513 to −873HU); most dramatically ∼5cm beyond the nFOV border. If uncorrected, these changes in HU produced significant dose errors. Plans were compared to control plans and dose difference maps were generated; uncorrected images 3cm outside the nFOV demonstrated >5% difference, where overriding HU values above −700HU maintained <5% error for phantom positions ∼10cm beyond nFOV. Conclusion: HU values differ significantly for anatomy 2cm outside the nFOV, can be visualized and should be corrected for dose calculations. These results show <5% dose error can be accomplished for anatomy extending ∼10cm beyond the nFOV (which accounts for 98% of eFOV patients here).
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