Darren Kahler

University of Florida, Gainesville, Florida, United States

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Publications (28)66.04 Total impact

  • S Lebron · B Lu · G Yan · D Kahler · J Li · B Barraclough · C Liu
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    ABSTRACT: Purpose: In radiation therapy, accurate data acquisition of photon beam dosimetric quantities is important for (1) beam modeling data input into a treatment planning system (TPS), (2) comparing measured and TPS modelled data, (3) a linear accelerator’s (linac) beam characteristics quality assurance process, and (4) establishing a standard data set for data comparison, etcetera. Parameterization of the photon beam dosimetry creates a portable data set that is easy to implement for different applications such as those previously mentioned. The aim of this study is to develop methods to parameterize photon percentage depth doses(PDD), profiles, and total scatter output factors(Scp).
    Medical Physics 06/2015; 42(6):3473. DOI:10.1118/1.4924960 · 2.64 Impact Factor
  • C Park · H Zhang · Y Chen · Q Fan · D Kahler · J Li · C Liu · B Lu
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    ABSTRACT: Recently, compressed sensing (CS) based iterative reconstruction (IR) method is receiving attentions to reconstruct high quality cone beam computed tomography (CBCT) images using sparsely sampled or noisy projections. The aim of this study is to develop a novel baseline algorithm called Mask Guided Image Reconstruction (MGIR), which can provide superior image quality for both low-dose 3DCBCT and 4DCBCT under single mathematical framework. In MGIR, the unknown CBCT volume was mathematically modeled as a combination of two regions where anatomical structures are 1) within the priori-defined mask and 2) outside the mask. Then we update each part of images alternatively thorough solving minimization problems based on CS type IR. For low-dose 3DCBCT, the former region is defined as the anatomically complex region where it is focused to preserve edge information while latter region is defined as contrast uniform, and hence aggressively updated to remove noise/artifact. In 4DCBCT, the regions are separated as the common static part and moving part. Then, static volume and moving volumes were updated with global and phase sorted projection respectively, to optimize the image quality of both moving and static part simultaneously. Examination of MGIR algorithm showed that high quality of both low-dose 3DCBCT and 4DCBCT images can be reconstructed without compromising the image resolution and imaging dose or scanning time respectively. For low-dose 3DCBCT, a clinical viable and high resolution head-and-neck image can be obtained while cutting the dose by 83%. In 4DCBCT, excellent quality 4DCBCT images could be reconstructed while requiring no more projection data and imaging dose than a typical clinical 3DCBCT scan. The results shown that the image quality of MGIR was superior compared to other published CS based IR algorithms for both 4DCBCT and low-dose 3DCBCT. This makes our MGIR algorithm potentially useful in various on-line clinical applications. Provisional Patent: UF#15476; WGS Ref. No. U1198.70067US00.
    Medical Physics 06/2015; 42(6):3696. DOI:10.1118/1.4926096 · 2.64 Impact Factor
  • Bo Lu · Yunmei Chen · Justin C Park · Qiyong Fan · Darren Kahler · Chihray Liu
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    ABSTRACT: Purpose: Accurately localizing lung tumor localization is essential for high-precision radiation therapy techniques such as stereotactic body radiation therapy (SBRT). Since direct monitoring of tumor motion is not always achievable due to the limitation of imaging modalities for treatment guidance, placement of fiducial markers on the patient’s body surface to act as a surrogate for tumor position prediction is a practical alternative for tracking lung tumor motion during SBRT treatments. In this work, the authors propose an innovative and robust model to solve the multimarker position optimization problem. The model is able to overcome the major drawbacks of the sparse optimization approach (SOA) model.
    Medical Physics 01/2015; 42(1):244. DOI:10.1118/1.4903888 · 2.64 Impact Factor
  • C Liu · G Yan · R Helmig · S Lebron · D Kahler
    Medical Physics 06/2014; 41(6):165-166. DOI:10.1118/1.4888099 · 2.64 Impact Factor
  • AI Saito · JG Li · C Liu · KR Olivier · D Kahler · K Karasawa · JF Dempsey
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    ABSTRACT: Objective: To investigate the effect of the 5-bulk-density dose calculation method without considering small non-bone high-density regions in patients with thoracic cancers. Methods: A heterogeneity-corrected computed tomography plan and two types of 5-bulk-density treatment plans were generated for patients with lung or oesophageal tumours without any obvious findings of emphysema. In the first 5-bulk-density plan, the bone was contoured using an auto-contouring tool; in the second plan, the bone was contoured manually ignoring small non-bone high-density regions. Treatment plans were made with a commercial treatment-planning system and an adaptive convolution dose-calculation algorithm. The population's average density was applied, and the heterogeneity-corrected plan was compared with all of the 5-bulk-density regions for each case. Dose-volume histograms and dose-difference distributions were examined for all cases. Results: A total of 53 patients (57 tumours) were enrolled. Both the auto- and manually contoured plans had average bone and tissue densities of 1.12 g/cm3 and 1.02 g/cm3, respectively. When the manually contoured plan was compared with the heterogeneity-corrected plan, dose-volume histograms of the normal tissue and planning target volume agreed to within 2% of the dose. Conclusions: For bulk-tissue-density heterogeneous dose calculation, clinically acceptable dosimetric accuracy was achieved for auto-contoured bone cases without considering small non-bone high-density regions. This means that the current method could be applied with magnetic resonance imaging in the treatment planning system for dose calculation where no electron density information exists.
    Hong Kong Journal of Radiology 03/2014; 17(1):16-22. DOI:10.12809/hkjr1413180
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    ABSTRACT: Purpose: Our previous study [B. Lu et al., "A patient alignment solution for lung SBRT setups based on a deformable registration technique," Med. Phys. 39(12), 7379-7389 (2012)] proposed a deformable-registration-based patient setup strategy called the centroid-to-centroid (CTC) method, which can perform an accurate alignment of internal-target-volume (ITV) centroids between averaged four-dimensional computed tomography and cone-beam computed tomography (CBCT) images. Scenarios with variations between CBCT and simulation CT caused by irregular breathing and/or tumor change were not specifically considered in the patient study [B. Lu et al., "A patient alignment solution for lung SBRT setups based on a deformable registration technique," Med. Phys. 39(12), 7379-7389 (2012)] due to the lack of both a sufficiently large patient data sample and a method of tumor tracking. The aim of this study is to thoroughly investigate and compare the impacts of breathing pattern and tumor change on both the CTC and the translation-only (T-only) gray-value mode strategies by employing a four-dimensional (4D) lung phantom. Methods: A sophisticated anthropomorphic 4D phantom (CIRS Dynamic Thorax Phantom model 008) was employed to simulate all desired respiratory variations. The variation scenarios were classified into four groups: inspiration to expiration ratio (IE ratio) change, tumor trajectory change, tumor position change, tumor size change, and the combination of these changes. For each category the authors designed several scenarios to demonstrate the effects of different levels of breathing variation on both of the T-only and the CTC methods. Each scenario utilized 4DCT and CBCT scans. The ITV centroid alignment discrepancies for CTC and T-only were evaluated. The dose-volume-histograms (DVHs) of ITVs for two extreme cases were analyzed. Results: Except for some extreme cases in the combined group, the accuracy of the CTC registration was about 2 mm for all cases for both the single and the combined scenarios. The performance of the CTC method was insensitive to region-of-registration (ROR) size selections, as suggested by the comparable accuracy between 1 and 2 cm expansions of the ROR selections for the method. The T-only method was suitable for some single scenarios, such as trajectory variation, position variation, and size variation. However, for combined scenarios and/or a large variation in the IE ratio, the T-only method failed to produce reasonable registration results (within 3 mm). The discrepancy was close to, or even greater than, 1 cm. In addition, unlike the CTC method, the T-only method was sensitive to the ROR size selection. The DVH analysis suggested that a large ITV to PTV margin should be considered if a breathing pattern variation is observed. Conclusions: The phantom study demonstrated that the CTC method was reliable for scenarios in which breathing pattern variation was involved. The T-only gray value method worked for some scenarios, but not for scenarios that involved an IE ratio variation. For scenarios involving position variation, the T-only method worked only with a careful selection of the ROR, whereas the CTC method was independent of ROR size as long as the ITVs were included in the ROR. One indication of the dose consequence analysis was that a large ITV to PTV margin should be considered if a breathing pattern variation is observed.
    Medical Physics 10/2013; 40(10):101704. DOI:10.1118/1.4820365 · 2.64 Impact Factor
  • P Kapur · D Kahler · C Liu · R Kapoor
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    ABSTRACT: Purpose: The goal of this study is to evaluate the impact of VMAT planning parameters on plan quality and delivery efficiency on linear accelerators. Methods: VMAT technology provide a number of variable on treatment delivery such as MLC motion, dose rate (DR) and gantry rotation speed. Three parameters‐Leaf Speed (LS), Minimum DR and Control points (CP) were chosen to evaluate their impact on the VMAT plan quality and delivery efficiency. Five prostate cases were selected to generate several VMAT plans on Pinnacle TPS (v9.2) with Elekta LINAC (Synergy‐S). Leaf Speed: Pinnacle provides options for maximum LS while optimizing a treatment plan. We chose 3 LS for a comparative study 2cm/sec (maximum LS allowed at LINAC), 1.5cm/sec & 1cm/sec. Minimum DR: Due to varying DR we evaluated the minimum DR that can be used to perform and deliver a treatment plan on the LINAC.Control points: TPS provides 3 options for choosing the number of CP 91, 121 & 181 i.e. CP generated with gantry spacing 4°, 3°, 2° in a single arc. Results: Leaf Speed: Due to LS impact of treatment delivery time, it is recommended to stay below the maximum LS of LINAC to avoid beam cut off. The optimum LS was found as 1.5cm/sec.Minimum DR: Beam cut off issue occurred on LINAC when the DR fell below 30MU/sec. So it becomes important to choose a higher value of DR while optimizing a plan in TPS.Control points: The number of CP used to generate a treatment plan has a minimal impact on plan quality and huge impact on plan delivery because it increases the optimization and delivery time. Conclusion: Each parameter in the above study has different impact on the plan quality & delivery efficiency. Hence it is important to evaluate all the parameters before clinical VMAT implementation.
    Medical Physics 06/2013; 40(6):292. DOI:10.1118/1.4814813 · 2.64 Impact Factor
  • K. Mittauer · B. Lu · G. Yan · D. Kahler · C. Liu
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    ABSTRACT: Purpose: To model the delivery time for step and shoot IMRT through developing a method for the user to characterize their machine. To investigate the impact of IMRT parameters on delivery time and recommend planning strategies in light of delivery time and plan quality. Methods: A two‐step delivery time model was derived, and verified against 33 delivered IMRT plans using an Elekta LINAC. Eleven head and neck patients with preexisting IMRT plans were selected for this retrospective study. The Pinnacle TPS was used to compute new plans by varying modulation parameters. 41 plans per patient (451 plans total) were generated with the following variations: 12 minimum segment area parameter (MSAP) plans, 9 minimum MU parameter (MMUP) plans, and 20 minimum MU and segment area parameter (MMUSAP) plans. Plans were evaluated based on DVHs (plan quality) and delivery time. Results: The delivery time model accuracy was 1.8%, 10 s. Delivery efficiency improves above 5 MU for MMUP, but no improvement was found with MSAP. MMUP and MSAP had the greatest influence on the number of control points and the plan MU, respectively. MLC speed and gun delay were more costly than the plan MU. However, the plan MU could affect the delivery times for dose rate dependent plans. Threshold values for plan quality were 5 cm2 for MSAP and 5 MU for MMUP. Conclusion: The proposed formalism provides a valid method to assess delivery time and can be characterized for the users machine. For dose rate dependent cases or future deliveries with an enhanced MLC speed, treatment efficiency could be improved with MSAP and/or MMUP. But for most current clinical cases, only MMUP can improve delivery efficiency. We, however, do not recommend increasing these parameters for the sake of delivery efficiency at a cost to plan quality.
    Medical Physics 06/2013; 40(6). DOI:10.1118/1.4815035 · 2.64 Impact Factor
  • S Lebron · J Li · G Yan · D Kahler · C Liu
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    ABSTRACT: Purpose: In radiation therapy, obtaining accurate data for photon beam dosimetric quantities such as fractionated depth doses (FDDs), profiles and output factors (OFs) is important since it affects the accuracy of dose calculation for patient treatment. The purpose of this study is to develop methods to parameterize all photon dosimetry parameters for a specific linear accelerator model and to create an expert data set in order to (1) minimize the interpersonal measurement uncertainties, (2) automate the quality assurance program, (3) minimize data collection during machine commissioning and (4) implement analytical de‐convolution method to remove the detector volume averaging effect. Methods: OFs, FDDs and profiles for different field sizes and depths were measured in an Elekta linear accelerator using a cylindrical 3D scanner (SunNuclear). All data were smoothed for the analysis and profile data were also centered and symmetrized. The OFs were analyzed using a polynomial equation. The FDDs were analyzed using two Methods: one consisted of a ratio of two exponential functions to parameterize the whole curve and the other dividing the curve into two parts characterized by Gaussian and polynomial functions. For profile modeling, half side of the profile was divided into four regions described by four different Gaussian equations. This set of equations was used to model the photon beam dosimetry for various field sizes and depths. Results: The differences between measured and modeled data were less than: 0.002% for OFs, 0.1% and 3.4% for FDDs for the first and second method respectively and 0.1% for the profiles. The calculated data generated an R2 =1 for the OFs, R2 >0.993 for the FDDs, and R2 >0.99 for the profiles. Conclusion: This novel analytically calculated model proved to be accurate in calculating the FDDs, profiles and OFs for different field sizes and depths.
    Medical Physics 06/2013; 40(6):206. DOI:10.1118/1.4814449 · 2.64 Impact Factor
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    ABSTRACT: Purpose: To improve planning and delivery efficiency of head and neck IMRT without compromising planning quality through the evaluation of inverse planning parameters. Methods: Eleven head and neck patients with pre-existing IMRT treatment plans were selected for this retrospective study. The Pinnacle treatment planning system (TPS) was used to compute new treatment plans for each patient by varying the individual or the combined parameters of dose∕fluence grid resolution, minimum MU per segment, and minimum segment area. Forty-five plans per patient were generated with the following variations: 4 dose∕fluence grid resolution plans, 12 minimum segment area plans, 9 minimum MU plans, and 20 combined minimum segment area∕minimum MU plans. Each plan was evaluated and compared to others based on dose volume histograms (DVHs) (i.e., plan quality), planning time, and delivery time. To evaluate delivery efficiency, a model was developed that estimated the delivery time of a treatment plan, and validated through measurements on an Elekta Synergy linear accelerator. Results: The uncertainty (i.e., variation) of the dose-volume index due to dose calculation grid variation was as high as 8.2% (5.5 Gy in absolute dose) for planning target volumes (PTVs) and 13.3% (2.1 Gy in absolute dose) for planning at risk volumes (PRVs). Comparison results of dose distributions indicated that smaller volumes were more susceptible to uncertainties. The grid resolution of a 4 mm dose grid with a 2 mm fluence grid was recommended, since it can reduce the final dose calculation time by 63% compared to the accepted standard (2 mm dose grid with a 2 mm fluence grid resolution) while maintaining a similar level of dose-volume index variation. Threshold values that maintained adequate plan quality (DVH results of the PTVs and PRVs remained satisfied for their dose objectives) were 5 cm(2) for minimum segment area and 5 MU for minimum MU. As the minimum MU parameter was increased, the number of segments and delivery time were decreased. Increasing the minimum segment area parameter decreased the plan MU, but had less of an effect on the number of segments and delivery time. Our delivery time model predicted delivery time to within 1.8%. Conclusions: Increasing the dose grid while maintaining a small fluence grid allows for improved planning efficiency without compromising plan quality. Delivery efficiency can be improved by increasing the minimum MU, but not the minimum segment area. However, increasing the respective minimum MU and∕or the minimum segment area to any value greater than 5 MU and 5 cm(2) is not recommended because it degrades plan quality.
    Medical Physics 06/2013; 40(6):061704. DOI:10.1118/1.4803460 · 2.64 Impact Factor
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    ABSTRACT: Purpose: In this work, the authors propose a novel registration strategy for translation-only correction scenarios of lung stereotactic body radiation therapy setups, which can achieve optimal dose coverage for tumors as well as preserve the consistency of registrations with minimal human interference. Methods: The proposed solution (centroid-to-centroidor CTC solution) uses the average four-dimensional CT (A4DCT) as the reference CT. The cone-beam CT (CBCT) is deformed to acquire a new centroid for the internal target volume (ITV) on the CBCT. The registration is then accomplished by simply aligning the centroids of the ITVs between the A4DCT and the CBCT. Sixty-seven cases using 64 patients (each case is associated with separate isocenters) have been investigated with the CTC method and compared with the conventional gray-value (G) mode and bone (B) mode registration methods. Dosimetric effects among the tree methods were demonstrated by 18 selected cases. The uncertainty of the CTC method has also been studied. Results: The registration results demonstrate the superiority of the CTC method over the other two methods. The differences in the D99 and D95 ITV dose coverage between the CTC method and the original plan is small (within 5%) for all of the selected cases except for one for which the tumor presented significant growth during the period between the CT scan and the treatment. Meanwhile, the dose coverage differences between the original plan and the registration results using either the B or G method are significant, as tumor positions varied dramatically, relative to the rib cage, from their positions on the original CT. The largest differences between the D99 and D95 dose coverage of the ITV using the B or G method versus the original plan are as high as 50%. The D20 differences between any of the methods versus the original plan are all less than 2%. Conclusions: The CTC method can generate optimal dose coverage to tumors with much better consistency compared with either the G or B method, and it is especially useful when the tumor position varies greatly from its position on the original CT, relative to the rib cage.
    Medical Physics 12/2012; 39(12):7379-89. DOI:10.1118/1.4766875 · 2.64 Impact Factor
  • A Gopal · S Lee · K Mittauer · D Kahler · B Lu · S Samant
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    ABSTRACT: Purpose: To evaluate the precision of 4D cone-beam CT as an image guidance tool for stereotactic body radiotherapy using intensity modulated arc or fixed gantry radiotherapy in sites with significant intrafraction tumor motionMethods: 4D cone-beam CT (4D-CBCT) is a recent innovation that has the potential to significantly improve the precision of highly conformai radiotherapy treatments. The performance of a commercial Elekta Synergy X-ray Volume Imager (XVI) 4D-CBCT system was quantitatively analyzed using a motor driven respiration phantom for a series of uniform and irregular breathing patterns. The quality of image guidance was assessed based on the precision of the 4D-CBCT registration obtained from the XVI system with respect to the known motion of the respiration phantom (Quasar ModusQA). The quality of the registration was evaluated with various scan acquisition settings for field size, projection arc, gantry speed, etc., to develop an optimized image guidance protocol for 4D treatment sites. Results: The results indicated that the 4D-CBCT registration shifts were in good overall agreement with the actual phantom motion. After rigid registration corrections using static regions of the phantom to eliminate systematic shifts, the respiration amplitudes were accurately reflected to within 1-3 mm for most breathing patterns, which is well within the expected uncertainty of registration algorithms as well as the amplitude and phase variability of the motor control. However, position differences in individual phase comparisons were found to be as high as 8-10 mm. We also observed that the larger field settings (m(2)0) for the 4D-CBCT acquisition rendered comparable accuracy to smaller vendor recommended fields (S20). Conclusions: The accuracy of 4D-CBCT based patient positioning for treatment sites with significant respiratory motion was verified along with its potential to facilitate greater precision for dose convergence in hyperfractionated radiotherapy as well as to evaluate the feasibility of respiration gated radiotherapy.
    Medical Physics 06/2012; 39(6):3605. DOI:10.1118/1.4734635 · 2.64 Impact Factor
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    ABSTRACT: The purpose of this work is to investigate the impact of small rotational errors on the magnitudes and distributions of spatial dose variations for intracranial stereotactic radiotherapy (SRT) treatment setups, and to assess the feasibility of using the original dose map overlaid with rotated contours (ODMORC) method as a fast, online evaluation tool to estimate dose changes (using DVHs) to clinical target volumes (CTVs) and organs-at-risks (OARs) caused by small rotational setup errors. Fifteen intracranial SRT cases treated with either three-dimensional conformal radiation therapy (3DCRT) or intensity-modulated radiation therapy (IMRT) techniques were chosen for the study. Selected cases have a variety of anatomical dimensions and pathologies. Angles of ±3° and ±5° in all directions were selected to simulate the rotational errors. Dose variations in different regions of the brain, CTVs, and OARs were evaluated to illustrate the various spatial effects of dose differences before and after rotations. DVHs accounting for rotations that were recomputed by the treatment planning system (TPS) and those generated by the ODMORC method were compared. A framework of a fast algorithm for multicontour rotation implemented by ODMORC is introduced as well. The average values of relative dose variations between original dose and recomputed dose accounting for rotations were greater than 4.0% and 10.0% in absolute mean and in standard deviation, respectively, at the skull and adjacent regions for all cases. They were less than 1.0% and 2.5% in absolute mean and in standard deviation, respectively, for dose points 3 mm away from the skull. The results indicated that spatial dose to any part of the brain organs or tumors separated from the skull or head surface would be relatively stable before and after rotations. Statistical data of CTVs and OARs indicate the lens and cochleas have the large dose variations before and after rotations, whereas the remaining ROIs have insignificant dose differences. DVH comparisons suggest that the ODMORC method is able to estimate the DVH of CTVs fairly accurately (within 1.5% of relative dose differences for evaluation volumes). The results also show that most of the OARs including the brain stem, spinal cord, chiasm, hippocampuses, optic nerves, and retinas, which were relatively distal from the skull and surface, had good agreement (within 2.0% of relative dose differences for 0.1 cc of the volumes ) between the ODMORC method and the recomputation, whereas OARs more proximate to the bone-tissue interface or surface, such as the lenses and cochlea, had larger dose variations (greater than 5.0%) for some cases due to the incapability of the ODMORC to account for scatter contribution variations proximate to interfaces and intrinsic dose calculation uncertainties for ROIs with small volumes. The ODMORC method can be implemented as an online evaluation system for rotation-induced dose changes of CTVs and most OARs and for other related dose consequence analyses.
    Medical Physics 11/2011; 38(11):6203-15. DOI:10.1118/1.3656954 · 2.64 Impact Factor
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    ABSTRACT: The purpose of this study was to investigate the feasibility of using a single QA device for comprehensive, efficient daily QA of a linear accelerator (Linac) and three image-guided stereotactic positioning systems (IGSPSs). The Sun Nuclear Daily QA 3 (DQA3) device was used to perform daily dosimetry and mechanical accuracy tests for an Elekta Linac, as well as daily image geometric and isocenter coincidence accuracy tests for three IGSPSs: the AlignRT surface imaging system; the frameless SonArray optical tracking System (FSA) and the Elekta kV CBCT. The DQA3 can also be used for couch positioning, repositioning, and rotational tests during the monthly QA. Based on phantom imaging, the Linac coordinate system determined using AlignRT was within 0.7 mm/0.6° of that of the CBCT system. The difference is attributable to the different calibration methods that are utilized for these two systems. The laser alignment was within 0.5 mm of the isocenter location determined with the three IGSPSs. The ODI constancy was ± 0.5 mm. For gantry and table angles of 0°, the mean isocenter displacement vectors determined using the three systems were within 0.7 mm and 0.6° of one another. Isocenter rotational offsets measured with the systems were all ≤ 0.5°. For photon and electron beams tested over a period of eight months, the output was verified to remain within 2%, energy variations were within 2%, and the symmetry and flatness were within 1%. The field size and light-radiation coincidence were within 1mm ± 1 mm. For dosimetry reproducibility, the standard deviation was within 0.2% for all tests and all energies, except for photon energy variation which was 0.6%. The total measurement time for all tasks took less than 15 minutes per QA session compared to 40 minutes with our previous procedure, which utilized an individual QA device for each IGSPS. The DQA3 can be used for accurate and efficient Linac and IGSPS daily QA. It shortens QA device setup time, eliminates errors introduced by changing phantoms to perform different tests, and streamlines the task of performing dosimetric checks.
    Journal of Applied Clinical Medical Physics 01/2011; 12(3):3535. · 1.17 Impact Factor
  • J. Peng · M. Ashenafi · D. Kahler · D. McDonald · K. Vanek · N. Koch · C. Liu
    Medical Physics 01/2011; 38(6):3499-. DOI:10.1118/1.3612011 · 2.64 Impact Factor
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    ABSTRACT: A quality assessment of intracranial stereotactic radiotherapy was performed using cone beam computed tomography (CBCT). Setup errors were analyzed for two groups of patients: (1) those who were positioned using a frameless SonArray (FSA) system and immobilized with a bite plate and thermoplastic (TP) mask (the bFSA group); and (2) those who were positioned by room laser and immobilized using a TP mask (the mLAS group). A quality assurance phantom was used to study the system differences between FSA and CBCT. The quality assessment was performed using an Elekta Synergy imager (XVI) (Elekta Oncology Systems, Norcross, GA) and an On-Board Imager (OBI) (Varian Medical Systems, Palo Alto, CA) for 25 patients. For the first three fractions, and weekly thereafter, the FSA system was used for patient positioning, after which CBCT was performed to obtain setup errors. (1) Phantom tests: The mean differences in the isocenter displacements for the two systems was 1.2 ± 0.7 mm. No significant variances were seen between the XVI and OBI units (p~0.208). (2)Patient tests: The mean of the displacements between FSA and CBCT were independent of the CBCT system used; mean setup errors for the bFSA group were smaller (1.2 mm) than those of the mLAS group (3.2 mm) (p < 0.005). For the mLAS patients, the 90th percentile and the maximum rotational displacements were 3° and 5°, respectively. A 4-mm drift in setup accuracy occurred over the treatment course for 1 bFSA patient. System differences of less than 1 mm between CBCT and FSA were seen. Error regression was observed for the bFSA patients, using CBCT (up to 4 mm) during the treatment course. For the mLAS group, daily CBCT imaging was needed to obtain acceptable setup accuracies.
    International journal of radiation oncology, biology, physics 12/2010; 78(5):1586-93. DOI:10.1016/j.ijrobp.2010.02.011 · 4.26 Impact Factor
  • J. L. Peng · J. G. Li · D. Kahler · R. Amdur · C. Liu
    Fuel and Energy Abstracts 11/2010; 78(3). DOI:10.1016/j.ijrobp.2010.07.1613
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    ABSTRACT: The aim of this work was to characterize a multi-axis ion chamber array (IC PROFILER; Sun Nuclear Corporation, Melbourne, FL, USA) that has the potential to simplify the acquisition of LINAC beam data. The IC PROFILER (or panel) measurement response was characterized with respect to radiation beam properties, including dose, dose per pulse, pulse rate frequency (PRF), and energy. Panel properties were also studied, including detector-calibration stability, power-on time, backscatter dependence, and the panel's agreement with water tank measurements [profiles, fractional depth dose (FDD), and output factors]. The panel's relative deviation was typically within (+/-) 1% of an independent (or nominal) response for all properties that were tested. Notable results were (a) a detectable relative field shape change of approximately 1% with linear accelerator PRF changes; (b) a large range in backscatter thickness had a minimal effect on the measured dose distribution (typically less than 1%); (c) the error spread in profile comparison between the panel and scanning water tank (Blue Phantom, CC13; IBA Schwarzenbruck, DE) was approximately (+/-) 0.75%. The ability of the panel to accurately reproduce water tank profiles, FDDs, and output factors is an indication of its abilities as a dosimetry system. The benefits of using the panel versus a scanning water tank are less setup time and less error susceptibility. The same measurements (including device setup and breakdown) for both systems took 180 min with the water tank versus 30 min with the panel. The time-savings increase as the measurement load is increased.
    Medical Physics 11/2010; 37(11):6101-11. DOI:10.1118/1.3505452 · 2.64 Impact Factor
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    ABSTRACT: The AlignRT3C system is an image-guided stereotactic positioning system (IGSPS) that provides real-time target localization. This study involves the first use of this system with three camera pods. The authors have evaluated its localization accuracy and tracking ability using a cone-beam computed tomography (CBCT) system and an optical tracking system in a clinical setting. A modified Rando head-and-neck phantom and five patients receiving intracranial stereotactic radiotherapy (SRT) were used to evaluate the calibration, registration, and position-tracking accuracies of the AlignRT3C system and to study surface reconstruction uncertainties, including the effects due to interfractional and intrafractional motion, skin tone, room light level, camera temperature, and image registration region of interest selection. System accuracy was validated through comparison with the Elekta kV CBCT system (XVI) and the Varian frameless SonArray (FSA) optical tracking system. Surface-image data sets were acquired with the AlignRT3C daily for the evaluation of pretreatment and interfractional and intrafractional motion for each patient. Results for two different reference image sets, planning CT surface contours (CTS) and previously recorded AlignRT3C optical surface images (ARTS), are reported. The system origin displacements for the AlignRT3C and XVI systems agreed to within 1.3 mm and 0.7 degrees. Similar results were seen for AlignRT3C vs FSA. For the phantom displacements having couch angles of 0 degrees, those that utilized ART_S references resulted in a mean difference of 0.9 mm/0.4 degrees with respect to XVI and 0.3 mm/0.2 degrees with respect to FSA. For phantom displacements of more than +/- 10 mm and +/- 3 degrees, the maximum discrepancies between AlignRT and the XVI and FSA systems were 3.0 and 0.4 mm, respectively. For couch angles up to +/- 90 degrees, the mean (max.) difference between the AlignRT3C and FSA was 1.2 (2.3) mm/0.7 degrees (1.2 degrees). For all tests, the mean registration errors obtained using the CT_S references were approximately 1.3 mm/1.0 degrees larger than those obtained using the ART_S references. For the patient study, the mean differences in the pretreatment displacements were 0.3 mm/0.2 degrees between the AlignRT3C and XVI systems and 1.3 mm/1 degrees between the FSA and XVI systems. For noncoplanar treatments, interfractional motion displacements obtained using the ART_S and CT_S references resulted in 90th percentile differences within 2.1 mm/0.8 degrees and 3.3 mm/0.3 degrees, respectively, compared to the FSA system. Intrafractional displacements that were tracked for a maximum of 14 min were within 1 mm/1 degrees of those obtained with the FSA system. Uncertainties introduced by the bite-tray were as high as 3 mm/2 degrees for one patient. The combination of gantry, aSi detector panel, and x-ray tube blockage effects during the CBCT acquisition resulted in a registration error of approximately 3 mm. No skin-tone or surface deformation effects were seen with the limited patient sample. AlignRT3C can be used as a nonionizing IGSPS with accuracy comparable to current image/marker-based systems. IGSPS and CBCT can be combined for high-precision positioning without the need for patient-attached localization devices.
    Medical Physics 10/2010; 37(10):5421-33. DOI:10.1118/1.3483783 · 2.64 Impact Factor
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    ABSTRACT: The aim of this work was to simulate the effect of dose distribution changes on detector array calibrations and to explore compensatory methods that are used during calibration measurements. The array calibration technique that was investigated is known as wide field (WF) calibration. Using this method, a linear array [y-axis (65 detectors) of the IC PROFILER (Sun Nuclear Corporation, Melbourne, FL)] is calibrated with three measurements (alpha, theta, and lamda); each measurement uses the same radiation field, which is larger than the array. For measurement configuration theta, the array is rotated by 180 degrees from its position in a; for lamda, the array is shifted by one detector from its position in theta. The relative detector sensitivities are then determined through ratios of detector readings at the same field locations (using theta and lamda). This method results in error propagation that is proportional to the number of detectors in the array. During the procedure, the calibration protocol operates under three postulates, which state that (a) the beam shape does not change between measurements; (b) the relative sensitivities of the detectors do not change; and (c) the scatter to the array does not change as the array is moved. The WF calibration's sensitivity to a postulate (a) violation was quantified by applying a sine shaped perturbation (of up to 0.1%) to a, theta, or lamda, and then determining the change relative to a baseline calibration. Postulate (a) violations were minimized by using a continuous beam and mechanized array movement during theta and lamda. A continuously on beam demonstrated more stable beam symmetry as compared to cycling the beam on and off between measurements. Additional side-scatter was also used to satisfy postulate (c). Simulated symmetry perturbations of 0.1% to theta or lamda resulted in calibration errors of up to 2%; alpha was relatively immune to perturbation (<0.1% error). Wide field calibration error on a linear accelerator with similar symmetry variations was +/- 1.6%. Using a continuous beam during theta and lamda with additional side-scatter reduced the calibration error from +/- 1.6% to +/- 0.48%. This work increased the reproducibility of WF calibrations by limiting the effect of measurement perturbations primarily due to linear accelerator symmetry variations. The same technique would work for any array using WF calibration.
    Medical Physics 07/2010; 37(7):3501-9. DOI:10.1118/1.3442028 · 2.64 Impact Factor