Kilovoltage Cone-beam CT Research Articles

Comparison of visual and semi-automated kilovoltage cone beam CT image QA analysis.

Established kilovoltage cone-beam computed tomography (kV-CBCT) image quality assurance (QA) guidelines often rely on recommendations provided by the American Association of Physicists in Medicine (AAPM) task group (TG) reports with metrics that use visual analysis. This can result in measurement variations by different users, especially in visually subjective analyzes such as low contrast resolution. Consequently, there is a growing interest in more automated means of image QA analysis that can offer increased consistency, accuracy, and convenience. This work compares visual QA to semi-automated software QA analysis to establish the performance and viability of a semi-automated method. In this study, a commercial product (RIT Radia. Radiological Imaging Technology, Colorado Springs, CO) was used to evaluate 68 months of kV-CBCT images of a Catphan® 504 phantom obtained from a Varian TrueBeam® linear accelerator. Six key metrics were examined: high contrast resolution, low contrast resolution, Hounsfield unit constancy, uniformity and noise, and spatial linearity. The results of this method were then compared to those recorded visually using Bland-Altman, and/or paired sample t-test. Comparison of all modules showed a non-random, statistically significant difference between visual and semi-automated methods except for LDPE and Teflon in the Hounsfield unit constancy analysis, which falls outside the paired sample t-test's 5% significance level. A small high contrast resolution bias indicates the two analysis methods are largely equivalent, while a large low contrast resolution bias indicates greater semi-automated target detection. Wide limits of agreement in the uniformity module suggests variability due to multiple visual observers. Spatial linearity results measured differences of less than 0.17%. Semi-automated QA analysis offered greater stability over visual analysis. Additionally, semi-automated QA results satisfied or exceeded visual QA passing criteria and allowed for fast and consistent image quality analysis.

Geometric accuracy in patient positioning for stereotactic radiotherapy of intracranial tumors.

This study determines and compares the geometric setup errors between stereoscopic x-ray and kilo-voltage cone beam CT (CBCT) in phantom tests on a linear accelerator (linac) for image-guided (IG) stereotactic radiotherapy of intracranial tumors. Additionally, dose-volume metrics in the target volumes of the setup errors of CBCT were evaluated. A Winston-Lutz- and an anthropomorphic phantom were used. The mean deviation and root mean square error (RMSE) of CBCT and stereoscopic x-ray were compared. Dose-volume metrics of the planning target volume (PTV) and gross target volume (GTV) for CBCT were calculated. The RMSEs in the tests with the Winston-Lutz-Phantom were 0.3mm, 1.1mm and 0.3mm for CBCT and 0.1mm, 0,1 mm and<0.1mm for stereoscopic x-ray in the translational dimensions (right-left, anterior-posterior and superior-inferior). The RMSEs in the tests with the anthropomorphic phantom were 0.3mm, 0.2mm and 0.1mm for CBCT and 0.1mm, 0,1 mm and<0.1mm for stereoscopic x-ray. The effects on dose-volume metrics of the setup errors of CBCT on the GTV were within 1 % for all considered dose values. The effects on the PTV were within 5 % for all considered dose values. Both IG systems provide high accuracy patient positioning within a submillimeter range. The phantom tests exposed a slightly higher accuracy of stereoscopic x-ray than CBCT. The comparison with other studies with a similar purpose emphasizes the importance of individual IG installation quality assurance.

Fast Isocenter Determination Using 3D Polymer Gel Dosimetry with Kilovoltage Cone-Beam CT Reading and the PolyGeVero-CT Software Package for Linac Quality Assurance in Radiotherapy.

This work presents an approach to the fast determination of a medical accelerator irradiation isocenter as a quality assurance (QA) procedure in radiotherapy. The isocenter determination tool is the tissue equivalent high-resolution 3D polymer gel dosimeter (PABIGnx) in a dedicated container combined with kilovoltage imaging systems and the polyGeVero-CT software package (v. 1.2, GeVero Co., Poland). Two accelerators were employed: Halcyon and TrueBeam (Varian, USA), both equipped with cone beam computed tomography (CBCT) and iterative reconstruction CBCT (iCBCT) algorithms. The scope of this work includes: (i) the examination of factors influencing image quality (reconstruction algorithms and modes), radiation field parameters (dose and multi-leaf collimator (MLC) gaps), fiducial markers, signal averaging for reconstruction algorithms and the scanning time interval between consecutive scans, (ii) the examination of factors influencing the isocenter determination, image processing (signal averaging, background subtraction, image filtering) and (iii) an isocenter determination report using a 2D and 3D approach. An optimized protocol and isocenter determination conditions were found. The time and effort required to determine the isocenter are discussed.

Concurrent kilovoltage CBCT imaging and megavoltage beam delivery: suppression of cross-scatter with 2D antiscatter grids and grid-based scatter sampling

Objective. The concept of using kilovoltage (kV) and megavoltage (MV) beams concurrently has potential applications in cone beam computed tomography (CBCT) guided radiation therapy, such as single breath hold scans, metal artifact reduction, and simultaneous imaging during MV treatment delivery. However, MV cross-scatter generated during MV beam delivery degrades CBCT image quality. To address this, a 2D antiscatter grid and a cross-scatter correction method were investigated in the context of high dose MV treatment delivery. Approach. A 3D printed, tungsten 2D antiscatter grid prototype was utilized in kV CBCT scans to reduce MV cross-scatter fluence during concurrent MV beam delivery. Remaining cross-scatter in projections was corrected by using the 2D grid itself as a cross-scatter intensity sampling device, referred to as grid-based scatter sampling (GSS). To test this approach, kV CBCT acquisitions were performed while delivering 6 and 10 MV beams, mimicking high dose rate treatment delivery scenarios. kV and MV beam deliveries were not synchronized to eliminate MV beam delivery interruption. MV cross-scatter suppression performance of the proposed approach was evaluated in projections and CBCT images of phantoms. Main results. 2D grid reduced the intensity of MV cross-scatter in kV projections by a factor of 3 on the average, when compared to conventional antiscatter grid. Remaining cross scatter as measured by the GSS method was within 7% of measured reference intensity values, and subsequently corrected. CBCT image quality was improved substantially during concurrent kV–MV beam delivery. Median Hounsfield Unit (HU) inaccuracy was up to 182 HU without our methods, and it was reduced to a median 6.5 HU with our 2D grid and scatter correction approach. Our methods provided a factor of 2–6 improvement in contrast-to-noise ratio. Significance. This investigation demonstrates the utility of 2D antiscatter grids and grid-based scatter sampling in suppressing MV cross-scatter. Our approach successfully minimized the effects of MV cross-scatter in concurrent kV CBCT imaging and high dose MV treatment delivery scenarios. Hence, robust MV cross-scatter suppression is potentially feasible without MV beam delivery interruption or compromising kV image acquisition rate.

The Usefulness of Adaptative Radiotherapy in Prostate Cancer: How, When, and Who?

The aim of this study was to develop a deformable image registration (DIR)-based offline ART protocol capable of identifying significant dosimetric changes in the first treatment fractions to determine when adaptive replanning is needed. A total of 240 images (24 planning CT (pCT) and 216 kilovoltage cone-beam CT (CBCT)) were prospectively acquired from 24 patients with prostate adenocarcinoma during the first three weeks of their treatment (76 Gy in 38 fractions). This set of images was used to plan a hypofractionated virtual treatment (57.3 Gy in 15 fractions); correlation with the DIR of pCT and each CBCT allowed to translate planned doses to each CBCT, and finally mapped back to the pCT to compare with those actually administered. In 37.5% of patients, doses administered in 50% of the rectum (D50) would have exceeded the dose limitation to 50% of the rectum (R50). We first observed a significant variation of the planned rectal volume in the CBCTs of fractions 1, 3, and 5. Then, we found a significant relationship between the D50 accumulated in fractions 1, 3, and 5 and the lack of compliance with the R50. Finally, we found that a D50 variation rate [100 × (administered D50 − planned D50/planned D50)] > 1% in fraction three can reliably identify variations in administered doses that will lead to exceeding rectal dose constraint.

Empirical performance of polycrystalline silicon pixel circuit components for monolithic, large-area photon counting arrays

Photon counting detectors (PCDs) offer several advantages for medical x-ray imaging compared to conventional imagers. However, current PCDs, whose circuits are fabricated using crystalline silicon semiconductor material, are not well-suited for large-area imaging applications such as breast CT and kilovoltage cone-beam CT (kV CBCT) in radiotherapy. To address this challenge, prototype PCDs based on polycrystalline silicon (a semiconductor better suited for manufacture of large-area devices) were created and, in this paper, an empirical determination of the maximum count rate of individual pixel circuit components (amplifier, comparator, clock generator and counter) corresponding to those prototypes is reported. For each circuit component, test input pulses (which were generated so as to approximate those in a complete pixel circuit) were used to obtain output response waveforms from which count rate was determined. The maximum count rate (in units of counts per second, cps) for the amplifier was determined to be 20.8 kcps while that of the comparator, clock generator and counter components were determined to be 1.2 Mcps, 98 kcps and 4.9 Mcps, respectively. The comparator and counter components provide count rates beyond that required for breast CT (∼108 kcps at a pixel pitch of 330 μm) while those components as well as the clock generator exceed the rate required for kV CBCT in radiotherapy (∼72 kcps at a pitch of 400 μm). It is anticipated that new polycrystalline silicon circuit designs for the amplifier and clock generator could provide count rates sufficient for both applications.

Feasibility evaluation of kilovoltage cone-beam computed tomography dose calculation following scatter correction: investigations of phantom and representative tumor sites.

BackgroundTo study the feasibility of kilovoltage cone-beam computed tomography (KV-CBCT) dose calculation following scatter correction.MethodsCIRS 062 and Catphan 504 phantoms were used in this study, and 40 randomly selected subjects representing a variety of cases (ten head cancer cases, ten chest cancer cases, ten abdominal cancer cases and ten pelvic cavity cancer cases) were enrolled. We developed in-house software called the cone-beam CT imaging toolkit (CITK) to improve the quality of CBCT images. We first aligned each planning computed tomography (pCT) image with the corresponding CBCT image using rigid registration after scatter correction. Hounsfield unit-relative electron density (HU-RED) calibration was applied to the CBCT images. The pCT plan was then recalculated on CBCT images. Finally, the dosimetric differences between the two plans were evaluated. The dosimetric parameters included the D98, D2, Dmean, conformity index (CI), homogeneity index (HI) and other organ at risk (OAR) dose parameters of the planning target volume (PTV). The dose distribution index (DDI) and the gamma index were also assessed. Paired Student’s t-tests or Wilcoxon rank tests were used to evaluate differences. P<0.05 was considered significant.ResultsIn the phantom and patient cases, the average dosimetric difference was less than 1% in the PTV and OARs. There was no significant difference in the CI or HI between the two plans. The gamma pass rate of 2%/2 mm was greater than 95% in both plans. There was a significant difference in the DDI between the two plans in the chest group but not in the other groups.ConclusionsThe results suggest that CBCT has high accuracy in dose calculation via scatter correction and HU-RED calibration.

The effects of set-up errors on dose distribution in radiotherapy treatment for lung cancer

Background: Our aim was to analyze the effects of set-up errors on dose distribution in radiotherapy treatment for lung cancer by using kilovoltage cone-beam CT (CBCT). Materials and Methods: In this study, we used a Varian IX linear accelerator system to perform CBCT scans of 30 lung cancer patients before radiotherapy. Subsequently, the image was matched with the planned CT, and the left and right (LR), top and bottom (SI), and front and back (AP) directions were set incorrectly. And in the CMS planning system, the center of the plan has been moved to the center of the actual scan. Finally, the dose distribution before the bed-moving is simulated. We want to explore the impact of the planned target volume setting error (PTV), the total tumor volume (GTV), and radiation of normal tissues. Results: The set-up errors of the LR, SI and AP directions were (-0.20±2.84), (-1.09±5.40), and (-2.61±2.08) mm, respectively. The 5mm error accounted for 97.8%, 73% and 92.6% in the three directions. Statistically significant differences were found in the distribution of 95%PTV dose, the average dose of PTV, 95% GTV dose and the average dose of GTV without bed-moving, compared with the original plan. Conclusions: In clinical lung cancer radiotherapy, the commonly used setting error is usually less than 5mm, most of which are along the AP direction. In this study, we found that the setting error is related to the patient's inherent characteristics and can significantly change the radiation treatment dose in the target area.

FastCAT: Fast cone beam CT (CBCT) simulation.

To develop fastCAT, a fast cone-beam computed tomography (CBCT) simulator. fastCAT uses pre-calculated Monte Carlo (MC) CBCT phantom-specific scatter and detector response functions to reduce simulation time for megavoltage (MV) and kilovoltage (kV) CBCT imaging. Pre-calculated x-ray beam energy spectra, detector optical spread functions and energy deposition, and phantom scatter kernels are combined with GPU raytracing to produce CBCT volumes. MV x-ray beam spectra are simulated with EGSnrc for 2.5- and 6MeV electron beams incident on a variety of target materials and kV x-ray beam spectra are calculated analytically for an x-ray tube with a tungsten anode. Detectors were modeled in Geant4 extended by Topas and included optical transport in the scintillators. Two MV detectors were modeled-a standard Varian AS1200 GOS detector and a novel CWO high detective quantum efficiency detector. A kV CsI detector was also modeled. Energy-dependent scatter kernels were created in Topas for two 16cm diameter phantoms: A Catphan 515 contrast phantom and an anthropomorphic head phantom. The Catphan phantom contained inserts of 1-5mm in diameter of six different tissue types: brain, deflated lung, compact and cortical bone, adipose, and B-100. fastCAT simulations retain high fidelity to measurements and MC simulations: MTF curves were within 3.5% and 1.2% of measured values for the CWO and GOS detectors, respectively. HU values and CNR in a fastCAT Catphan 515 simulation were seen to be within 95% confidence intervals of an equivalent MC simulation for all of the tissues with root mean squared errors less than 16 HU and 1.6 in HU values and CNR comparisons, respectively. The anthropomorphic head phantom CWO detector CBCT image resulted in much higher tissue contrast and lower noise compared to the GOS detector CBCT image. A fastCAT simulation of the Catphan 515 module with an image size of 1024 1024 10 voxels took 61s on a GPU while the equivalent Topas MC was estimated to take more than 0.3CPU years. We present an open source fast CBCT simulation with high fidelity to MC simulations. The fastCAT python package can be found at https://github.com/jerichooconnell/fastCAT.git.

Evaluation of patient organ doses from kilovoltage cone-beam CT imaging in radiation therapy.

Currently, CBCT system is an indispensable component of radiation therapy units. Because of that, it is important in treatment planning and diagnosis. CBCT is also an crucial tool for patient positioning and verification in image-guided radiation therapy (IGRT). Therefore, it is critical to investigate the patient organ doses arising from CBCT imaging. The purpose of this study is to evaluate patient organ doses and effective dose to patients from three different protocols of Elekta Synergy XVI system for kV CBCT imaging examinations in image guided radiation therapy. Organ dose measurements were done with thermoluminescent dosimeters in Alderson RA NDO male phantom for head & neck (H&N), chest and pelvis protocols of the Elekta Synergy XVI kV CBCT system. From the measured organ dose, effective dose to patients were calculated according to the International Commission on Radiological Protection 103 report recommendations. For H&N, chest and pelvis scans, the organ doses were in the range of 0.03-3.43 mGy, 6.04-22.94 mGy and 2.5-25.28 mGy, respectively. The calculated effective doses were 0.25 mSv, 5.56 mSv and 4.72 mSv, respectively. The obtained results were consistent with the most published studies in the literature. Although the doses to patient organs from the kV CBCT system were relatively low when compared with the prescribed treatment dose, the amount of delivered dose should be monitored and recorded carefully in order to avoid secondary cancer risk, especially in pediatric examinations.

Contribution of Imaging to Organs at Risk Dose during Lung Stereotactic Body Radiation Therapy.

Background: The use of imaging is indispensable in modern radiation therapy, both for simulation and treatment delivery. For safe and sure utilization, dose delivery from imaging must be evaluated.Objective: This study aims to investigate the dose to organ at risk (OAR) delivered by imaging during lung stereotactic body radiation therapy (SBRT) and to evaluate its contribution to the treatment total dose.Material and Methods: In this retrospectively study, imaging total dose to organs at risk (OARs) (spinal cord, esophagus, lungs, and heart) and effective dose were retrospectively evaluated from 100 consecutive patients of a single institution who had lung SBRT. For each patient, dose was estimated using Monte-Carlo convolution for helical computed tomography (helical CT), Four-Dimensional CT (4D-CT), and kilovoltage Cone-Beam CT (kV-CBCT). Helical CT and kV-CBCT dose were evaluated for the entire thorax acquisition, while 4D-CT dose was analyzed on upper lobe (UL) or lower lobe (LL) acquisition. Treatment dose was extracted from treatment planning system and compared to imaging total dose. Results: Imaging total dose maximum values were 117 mGy to the spinal cord, 127 mGy to the esophagus, 176 mGy to the lungs and 193 mGy to the heart. The maximum effective dose was 19.65 mSv for helical CT, 10.62 mSv for kV-CBCT, 25.95 mSv and 38.45 mSv for 4D-CT in UL and LL regions, respectively. Depending on OAR, treatment total dose was higher from 1.7 to 8.2 times than imaging total dose. Imaging total dose contributed only to 0.3% of treatment total dose. Conclusion: Imaging dose delivered with 4D-CT to the OARs is higher than those of others modalities. The heart received the highest imaging dose for both UL and LL. Total imaging dose is negligible since it contributed only to 0.3% of treatment total dose.

Data-Driven Margin Determination for Online Adaptive Radiotherapy Using Batch Automated Planning

To fully realize the potential benefits of adaptive radiotherapy (ART), target margins must be reduced. The purpose of this study was to develop a data-driven methodology to determine minimum clinical target volume (CTV) to planning target volume (PTV) margin expansions in daily online ART scenarios. To empirically determine the minimum allowable CTV-to-PTV margins for various adaptive scenarios, we seek to measure the frequency at which the minimum dose (Dmin) to the daily CTV falls below 100% prescription dose in both the non-adaptive and varied margin adaptive scenarios. By measuring across many treatment observations in a disease site, an adaptive strategy that holds fixed this frequency ensures equivalent outcomes to existing non-adaptive scenarios. The intended adaptive system can create auto-segmented targets (CTVauto) and organs-at-risk (OARs) for previously captured on-treatment images. Each daily CTVauto is edited to establish ground truth (CTVtrue) for dosimetric evaluation. Without manual intervention, batch automated planning can generate adapted plans with varied spatial margins for both CTVauto-to-PTVauto (fully automated adaptive scenario) and CTVtrue-to-PTVtrue (daily edited CTV scenario).To demonstrate this framework in prostate radiotherapy, we examined volumetric modulated arc therapy delivered on a linear accelerator with kilovoltage cone-beam CT and adaptive contours from the Ethos platform. Batch automated treatment planning was conducted with a clinically-validated knowledge-based planning routine. Every fraction from a single prostate case was analyzed for the two adaptive scenarios and uniform margins ranging from 2-5mm. The volumes of bladder and rectum receiving prescription dose (V100%) relative to the non-adaptive scenario were measured to quantify the normal tissue benefit of each margin reduction. In a 27-fraction test case with institutional margins (3mm post, 5mm ant/right/left, 7mm sup/inf), CTVtrue Dmin <100% was observed in 22% of fractions. ART with unedited auto-segmentation allowed a 4mm uniform PTV margin with only 11% of fractions where CTVtrue Dmin<100%, with a 45±3% and 16±5% reduction in bladder and rectum V100%, respectively. If ART includes CTVauto correction, a 3mm uniform PTV margin resulted in 0% of fractions where CTVtrue Dmin<100% with a 63±2% and 45±4% reduction in bladder and rectum V100%, respectively. This framework allows for an intuitive methodology to empirically determine margins for daily ART, tuned to auto-segmentation and auto-planning performance in intended disease sites.Tabled 1Abstract 2836; TableNo adaptation (% CTVtrue Dmin<100%)Adapted to CTVauto+margin (% CTVtrue Dmin<100%)Adapted to CTVtrue+margin (% CTVtrue Dmin<100%)V100% bladderV100% rectum3mm P, 5mm A/R/L, 7mm S/I22%----5mm symmetric-4%0%-22% ± 2%+14% ± 5%4mm symmetric-11%0%-45% ± 3%-16% ± 5%3mm symmetric-30%0%-63% ± 2%-45% ± 4%2mm symmetric-67%41%-80% ± 8%-70% ± 4% Open table in a new tab

Prostate bed and organ-at-risk deformation: Prospective volumetric and dosimetric data from a phase II trial of stereotactic body radiotherapy after radical prostatectomy

PurposeStereotactic body radiotherapy (SBRT) in the post-prostatectomy setting is investigational. A major concern is the deformable prostate bed clinical target volume (CTV) and the closely juxtaposed organs-at-risk (OARs). We report a volumetric and dosimetric analysis of kilovoltage cone-beam CT (CBCT) data from the first 18 patients enrolled on a phase II trial of post-prostatectomy SBRT. With instructions on bladder filling and rectal preparation, we hypothesized acceptable CTV coverage while minimal overdosing to OARs could be achieved. MethodsAll patients received 5 fractions of 6–6.8 Gy to the prostate bed. CBCT were taken prior to and halfway through each fraction. CTV and OARs were contoured for each CBCT. Changes in inter- and intra-fraction volume and dose were calculated. Relative changes in CTV V95%, bladder V32.5 Gy, and rectal V32.5 Gy and V27.5 Gy were evaluated. ResultsInterfraction CTV volume remained stable, with median change +5.69% (IQR −1.73% to +9.84%). CTV V95% exhibited median change −0.74% (IQR −9.15% to −0.07%). Volumetric and dosimetric changes were minor from interfraction rotation and intrafraction motion. CTV V95% was ≥93% in 13 of 18 (72%) patients; in the remaining five, median change was −14.09% (IQR −16.64% to −13.56%). Interfraction CTV volume change was significantly larger among patients with CTV V95% <93% (+25.04% vs. +2.85%, p = 0.002). ConclusionsWith specific bladder and rectum filling protocols, CTV underdosing and overdosing to bladder and rectum are avoided in majority of patients. Changes in CTV shape may account for the underdosing that may be observed.

Count rate capabilities of polycrystalline silicon photon counting detectors for CBCT applications—a theoretical study

The signal-to-noise properties of active matrix, flat-panel imagers (AMFPIs) limit the imaging performance of this x-ray imaging technology under conditions of low dose per image frame. This limitation can affect cone-beam computed tomography (CBCT) procedures where an AMFPI is used to acquire hundreds of image frames to form a single volumetric data set. An approach for overcoming this limitation is to replace the energy-integrating pixel circuits of AMFPI arrays with photon counting pixel circuits which examine the energy of each x-ray interaction and count those events whose signals exceed user-defined energy thresholds. A promising material for fabricating the circuits of such photon-counting detectors (PCDs) is polycrystalline silicon (poly-Si)—a semiconductor that facilitates economic manufacture of large area, monolithic arrays of the size presently provided by AMFPIs as well as provides good radiation damage resistance. In this paper, results are reported from a theoretical investigation of the potential for poly-Si PCDs to satisfy the count rate needs, while maintaining good energy resolution, of two CBCT applications—CBCT used for breast imaging and kilo-voltage CBCT used for providing localization information in image guided radiotherapy (referred to as BCT and kV-CBCT, respectively). The study focused on the performance of the critical first component of a PCD pixel circuit, the amplifier, under conditions relevant to the two applications. The study determined that, compared to the average input fluxes associated with BCT and kV-CBCT, a promising amplifier design employing poly-Si thin-film transistors can provide count rates two and four times in excess of those levels, respectively, assuming a dead time loss of 10%. In addition, calculational estimates based on foreseeable poly-Si circuit densities suggest that it should be possible to include sufficient circuitry to support 2 and 3 energy thresholds per pixel, respectively. Finally, prospects for further improvements are discussed.

Assessment of the use of different imaging and delivery techniques for cranial treatments on the Halcyon linac

PurposeIn this work, we investigated the effect on the workflow and setup accuracy of using surface guided radiation therapy (SGRT) for patient setup, megavoltage cone beam CT (MVCBCT) or kilovoltage cone beam CT (kVCBCT) for imaging and fixed IMRT or volumetric‐modulated arc therapy (VMAT) for treatment delivery with the Halcyon linac.MethodsWe performed a retrospective investigation of 272 treatment fractions, using three different workflows. The first and second workflows used MVCBCT and fixed IMRT for imaging and treatment delivery, and the second one also used SGRT for patient setup. The third workflow used SGRT for setup, kVCBCT for imaging and VMAT for delivery. Workflows were evaluated by comparing the number of fractions requiring repeated imaging acquisitions and the time required for setup, imaging and treatment delivery. Setup position accuracy was assessed by comparing the daily kV‐ or MV‐ CBCT with the planning CT and measuring the residual rotational errors for pitch, yaw and roll angles.ResultsWithout the use of SGRT, the imaging fields were delivered more than once on 11.1% of the fractions, while re‐imaging was necessary in 5.5% of the fractions using SGRT. The total treatment time, including setup, imaging, and delivery, for the three workflows was 531 ± 157 s, 503 ± 130 s and 457 ± 91 s, respectively. A statistically significant difference was observed when comparing the third workflow with the first two. The total residual rotational errors were 1.96 ± 1.29°, 1.28 ± 0.67° and 1.22 ± 0.76° and statistically significant differences were observed when comparing workflows with and without SGRT.ConclusionsThe use of SGRT allowed for a reduction of re‐imaging during patient setup and improved patient position accuracy by reducing residual rotational errors. A reduction in treatment time using kVCBCT with SGRT was observed. The most efficient workflow was the one including kVCBCT and SGRT for setup and VMAT for delivery.

Prostate Bed and Organ-at-Risk Deformation: Prospective Volumetric and Dosimetric Data from a Phase II Trial of Stereotactic Body Radiotherapy after Radical Prostatectomy

Stereotactic body radiotherapy (SBRT) for localized prostate cancer has emerged as an attractive treatment currently being studied in phase III trials. SBRT in post-prostatectomy setting, however, has been minimally investigated. A major concern for this setting is the highly deformable nature of adjacent organs-at-risk (OARs) and prostate bed clinical target volume (CTV). We report a volumetric and dosimetric analysis of kilovoltage cone-beam CT (CBCT) data from 18 patients enrolled on a phase II clinical trial of post-prostatectomy SBRT. We hypothesized that with instructions on bladder filling and rectal preparation, we could achieve acceptable CTV coverage and avoid overdosing OARs. Eighteen patients receiving 5 fractions of 6-6.8 Gy to prostate bed were included. Patients received instructions on bladder filling and rectal preparation, specifically to urinate 60 minutes and drink 16-24 oz of water prior to treatment, as well as take enemas the night and morning before treatment. CBCT prior to each fraction was taken for anatomic verification and image guidance, and a second CBCT halfway through treatment for intrafractional motion analysis. CBCTs were imported into a deformable registration algorithm. Prostate bed CTV and OARs were contoured for each image. Percent changes in inter and intrafractional volume and mean dose change were calculated for all structures; V95 was also calculated for each CTV. Interfractional volumetric and dosimetric changes are summarized in Table 1. CTV volume was relatively constant (median change +5.7% [IQR -1.7% to + 9.8%]) as was CTV V95 (median change -0.74% [IQR -9.2% to -0.1%]). Rectal volume showed greatest change between treatment sessions and treatment plan, with median change of +21.0% [IQR +5.1% to 34.6%]. Intrafractional volumetric and dosimetric changes were overall nonsignificant. Among 11 patients with at least 3 months follow-up, there was one instance of acute grade 2 gastrointestinal toxicity by CTCAE 4.03, and no instances of grade 2 genitourinary toxicity. So far, one patient had biochemical failure which may be due to an out of field relapse. All others have had PSA response. Dosimetric and volumetric data from this prospective trial indicate CTV coverage is well maintained without overdosing OARs despite the highly deformable anatomy involved with post-prostatectomy SBRT. Acute toxicity is favorable, though longer follow-up and more patients are needed to evaluate late toxicity and efficacy.Abstract 1253; Table 1Interfractional percent change in volume and dose of critical treatment structuresMeanMedianQ1Q3CTV volume2.95%5.69%-1.73%9.84%CTV mean dose-1.63%-0.40%-2.43%-0.17%Bladder volume0.85%2.17%-28.19%10.55%Bladder mean dose0.30%-1.33%-11.77%6.55%Rectum volume25.38%20.97%5.10%34.64%Rectum mean dose-6.98%-8.53%-15.34%-2.86%Rectal wall volume23.76%23.14%3.46%40.13%Rectal wall mean dose-11.72%-12.79%-19.58%-6.76%CTV V95-5.05%-0.74%-9.15%-0.07% Open table in a new tab

Registration accuracy with the low dose kilovoltage cone-beam CT: A phantom study.

Objective:The aim of this study was to investigate low-dose kilovoltage cone-beam CT (kV-CBCT) for image-guided radiotherapy, with a particular focus on the accuracy of image registration with low-dose protocols.Methods:Imaging doses were measured with a NOMEX semiconductor detector positioned at the front of head, thorax, and pelvis human body phantoms while kV-CBCT scans were acquired at different tube currents. Aspects of image quality (spatial resolution, noise, uniformity, contrast, geometric distortion, and Hounsfield unit sensitivity) and image registration accuracy using bone and soft tissue were evaluated.Results:With preset and the lowest tube currents, the imaging doses were 0.16 and 0.08 mGy, 5.29 and 2.80 mGy, and 18.23 and 2.69 mGy for head, thorax, and pelvis, respectively. Noise was the only quality aspect directly dependent on tube current, being increased by 1.5 times with a tube current half that of the preset in head and thorax, and by 2.2 times with a tube current 1/8 of the preset in the pelvis. Accurate auto-bone matching was performed within 1 mm at the lowest tube current. The auto-soft tissue matching could not be performed with the lowest tube current; however, manual-soft tissue matching could still be performed within 2 mm or less.Conclusion:Noise was the only image quality aspect dependent on the imaging dose. Auto-bone and manual-soft tissue matching could still be performed at the lowest imaging dose.Advances in knowledge:When optimizing kV-CBCT imaging dose, the impact on bone and soft tissue image registration accuracy should be evaluated.

Dosimetric comparison of fractionated radiosurgery plans using frameless Gamma Knife ICON and CyberKnife systems with linear accelerator-based radiosurgery plans for multiple large brain metastases.

For patients with multiple large brain metastases with at least 1 target volume larger than 10 cm3, multifractionated stereotactic radiosurgery (MF-SRS) has commonly been delivered with a linear accelerator (LINAC). Recent advances of Gamma Knife (GK) units with kilovolt cone-beam CT and CyberKnife (CK) units with multileaf collimators also make them attractive choices. The purpose of this study was to compare the dosimetry of MF-SRS plans deliverable on GK, CK, and LINAC and to discuss related clinical issues. Ten patients with 2 or more large brain metastases who had been treated with MF-SRS on LINAC were identified. The median planning target volume was 18.31 cm3 (mean 21.31 cm3, range 3.42-49.97 cm3), and the median prescribed dose was 27.0 Gy (mean 26.7 Gy, range 21-30 Gy), administered in 3 to 5 fractions. Clinical LINAC treatment plans were generated using inverse planning with intensity modulation on a Pinnacle treatment planning system (version 9.10) for the Varian TrueBeam STx system. GK and CK planning were retrospectively performed using Leksell GammaPlan version 10.1 and Accuray Precision version 1.1.0.0 for the CK M6 system. Tumor coverage, Paddick conformity index (CI), gradient index (GI), and normal brain tissue receiving 4, 12, and 20 Gy were used to compare plan quality. Net beam-on time and approximate planning time were also collected for all cases. Plans from all 3 modalities satisfied clinical requirements in target coverage and normal tissue sparing. The mean CI was comparable (0.79, 0.78, and 0.76) for the GK, CK, and LINAC plans. The mean GI was 3.1 for both the GK and the CK plans, whereas the mean GI of the LINAC plans was 4.1. The lower GI of the GK and CK plans would have resulted in significantly lower normal brain volumes receiving a medium or high dose. On average, GK and CK plans spared the normal brain volume receiving at least 12 Gy and 20 Gy by approximately 20% in comparison with the LINAC plans. However, the mean beam-on time of GK (∼ 64 minutes assuming a dose rate of 2.5 Gy/minute) plans was significantly longer than that of CK (∼ 31 minutes) or LINAC (∼ 4 minutes) plans. All 3 modalities are capable of treating multiple large brain lesions with MF-SRS. GK has the most flexible workflow and excellent dosimetry, but could be limited by the treatment time. CK has dosimetry comparable to that of GK with a consistent treatment time of approximately 30 minutes. LINAC has a much shorter treatment time, but residual rotational error could be a concern.

Detection of the focal spot motion relative to the collimator axis of a linear accelerator under gantry rotation

The potential for delivering high precision radiotherapy using linear accelerators (linacs) has been improved with the development of digital x-ray electronic portal imaging devices (EPID) for acquiring kilovoltage (kV) cone-beam CT and megavoltage (MV) images for patient positioning. EPIDs have also opened the possibilities of developing novel quality assurance and insight into radiotherapy equipment performance.The aim of this work was to measure the offset of the focal spot position (FSP) of a linac under gantry rotation relative to the collimator axis using an EPID. The focal spot was assumed to be a point source of MV x-ray generation.A special phantom was designed for measurement of FSP as a function of gantry angle on clinical linacs. The phantom was designed for attachment to the gantry head and supporting two tungsten-carbide ball-bearings at two different distances from the focal spot.The methodology was demonstrated on a series of images acquired of the phantom on three Elekta linacs in clinical use with 6 MV flattening-filter-free (FFF) beams. The gantry and collimator were rotated 360° in steps of 30°. For each position an image of the phantom was acquired using the EPID. Each series consisted of 169 EPID images. The images were analysed using in-house developed software. Analyses of the EPID images acquired with 6 MV FFF beams showed that the focal spot motion amplitudes relative to the collimator axis during gantry rotation in the longitudinal and lateral directions were less than 0.10 mm and 0.50 mm, respectively, for an optimized 6 MV FFF FSP calibrated linac.In a treatment planning system (TPS) the focal spot is assumed to be located on the rotation axis of the collimator at all gantry angles. This work introduces a method for quantifying the actual variation from this assumption in practice.

68. Scatter ambient equivalent dose from a Cone Beam Computed Tomography for radiation protection purpose

Purpose Cone Beam Computed Tomography (CBCT) is a growing modality of complementary radiodiagnostic. It is applied in a wide range of clinical applications like dentistry, interventional radiology in an operating room and radiotherapy. When a new CBCT is to be installed, a preventive structural shielding design is needed in order to evaluate and limit the dose in the adjacent rooms under the national regulation thresholds. Scatter dose at a specific point is an essential input for a radiation protection project. Unfortunately we did not find in literature any systematic data set for scatter dose from a CBCT; values ranging from 2 to 40 μSv/scan at 1 m distance from the centre of the scattering media are reported in the European Commission Document “Radiation Protection N. 172” [1] and relative to dentistry applications. In the report ICRP 129 [2] the dose to the eye of an operator was reported ranging from 28 to 79 μSv for CBCT hepatic arterial embolisation and biliary tube placement procedures. The purpose of this work was to systematic measure the scatter ambient dose equivalent from a CBCT installed on a linac. Methods Primary kilovoltage beam was generated by a CBCT mounted on a VARIAN TRUEBEAM linear accelerator. The target is composed by tungsten (95%) and rhodium (5%); the permanent source filtration was 2.7 mm Alluminium with an additional filtration made of 0.89 mm Titanium. The primary beam irradiated an anthropomorphic CIRS Atom 701-D phantom (adult male phantom at thorax level) during a single complete gantry rotation. Scatter ambient equivalent dose was measured for 4 energy levels (80 K V p , 100 K V p , 125 K V p and 140 K V p ) and for 3 collimations (approximately 7 cm, 16 cm and 23 cm corresponding to a small, medium and large collimation) along the longitudinal axis (axis around which the X-ray source was rotating). Measurements were performed at a height from the floor equal to the isocentric level and repeated for different angles between the longitudinal axis and the line formed by the center of the irradiated region in the phantom and the measurement point. The used survey meter was the ATOMTEX AT1121. Results The scatter radiation (H*(10) in μSv/mA*min at 1 m distance from the center of the irradiated region in the phantom to the measurement point) was not isotropic at different angles for the same irradiation conditions. For radiation protection purpose only the maximum measured values are reported in the following table. Conclusions The reported H*(10) values give an idea of the scattered radiation from a kilovoltage CBCT installed on a VARIAN TRUEBEAM linac. A new CBCT installation with similar spectral features (e.g. same target and filtration) and outside a radiotherapy environment may benefit from these measurements for radiation protection purposes.