Compensator thickness verification using an a-Si EPID

Electronic portal imaging devices (EPIDs) are being increasingly employed to make therapy verification and dose measurements in the clinic. In this work, we investigate the use of an amorphous silicon (a-Si) EPID to verify the accuracy of compensator fabrication and mounting. Compensator thickness estimates on a two-dimensional grid were calculated from the primary component of transmission obtained by subtracting a modeled scatter component from the total transmission measured with the EPID. The primary component was related to the thickness via an exponential relation that includes beam hardening. Implementation of the method involved determination of: (i) a calibration curve relating EPID pixel values to energy fluence for open and attenuated fields, which was found to be linear for open fields but to have a small quadratic component for attenuated beams; (ii) EPID scatter factors to account for field size effects, which exhibited a small dependence on compensator thickness and field size; (iii) the attenuation coefficient of the steel shot compensator material, which varied slightly with off-axis distance and field size, and (iv) an analytical model to predict scatter from the compensator, which was calculated to be <4% at the standard EPID imaging distance of 140 cm. Thickness distributions were then measured for several types of attenuators including flat, test, and clinical compensators. Although uncertainties associated with compensator manufacturing were non-negligible and made assessment of thickness measurement uncertainty difficult, we estimate the latter to be approximately 0.5 mm for steel shot compensators of thickness <4 cm.     

射野大小对全脑调强放疗计划EPID验证结果的影响

目的:利用电子射野影像系统（EPID）对全脑调强放疗计划进行γ测试,寻找计划设计对测试结果的影响,以此分析如何优化全脑调强计划以及推测EPID在剂量验证方面的局限性。方法:选取67例全脑放疗患者,对其放疗计划用加速器自带的EPID进行计划验证,对于容积旋转调强放疗（VMAT）计划统计并分析X方向射野大小与γ（3 mm/3%）通过率的关系,对于调强放疗（IMRT）对比分析大野调强和分野调强计划γ（3 mm/3%）通过率的差异。结果:VMAT计划验证结果发现X方向小于15 cm的射野γ（3 mm/3%）通过率普遍优于大于等于15 cm的射野,利用SPSS软件进行t检验,发现结果具有统计学意义（t=-3.828, P<0.05）;IMRT验证结果发现,X方向大于等于15 cm的射野会包含两个子野,合野验证时其交叠部分γ（3 mm/3%）通过率较差,而采用分野验证时,由于无交叠则通过率普遍较好。结论:全脑放疗VMAT计划将X方向射野控制在15 cm以内可以提升多叶准直器调节能力,并提高EPID验证的γ（3 mm/3%）通过率;EPID原件对低剂量区的响应偏差会导致全脑IMRT大野调强计划两子野交叠处γ（3 mm/3%）通过率较差,改用分野验证可以显著消除这种影响。     

Dosimetric IMRT verification with a flat-panel EPID

A convolution-based calibration procedure has been developed to use an amorphous silicon flat-panel electronic portal imaging device (EPID) for accurate dosimetric verification of intensity-modulated radiotherapy (IMRT) treatments. Raw EPID images were deconvolved to accurate, high-resolution 2-D distributions of primary fluence using a scatter kernel composed of two elements: a Monte Carlo generated kernel describing dose deposition in the EPID phosphor, and an empirically derived kernel describing optical photon spreading. Relative fluence profiles measured with the EPID are in very good agreement with those measured with a diamond detector, and exhibit excellent spatial resolution required for IMRT verification. For dosimetric verification, the EPID-measured primary fluences are convolved with a Monte Carlo kernel describing dose deposition in a solid water phantom, and cross-calibrated with ion chamber measurements. Dose distributions measured using the EPID agree to within 2.1% with those measured with film for open fields of 2 x 2 cm2 and 10 x 10 cm2. Predictions of the EPID phantom scattering factors (SPE) based on our scatter kernels are within 1% of the SPE measured for open field sizes of up to 16 x 16 cm2. Pretreatment verifications of step-and-shoot IMRT treatments using the EPID are in good agreement with those performed with film, with a mean percent difference of 0.2 +/- 1.0% for three IMRT treatments (24 fields).     

Three-dimensional IMRT verification with a flat-panel EPID

A three-dimensional (3D) intensity-modulated radiotherapy (IMRT) pretreatment verification procedure has been developed based on the measurement of two-dimensional (2D) primary fluence profiles using an amorphous silicon flat-panel electronic portal imaging device (EPID). As described in our previous work, fluence profiles are extracted from EPID images by deconvolution with kernels that represent signal spread in the EPID due to radiation and optical scattering. The deconvolution kernels are derived using Monte Carlo simulations of dose deposition in the EPID and empirical fitting methods, for both 6 and 15 MV photon energies. In our new 3D verification technique, 2D fluence modulation profiles for each IMRT field in a treatment are used as input to a treatment planning system (TPS), which then generates 3D doses. Verification is accomplished by comparing this new EPID-based 3D dose distribution to the planned dose distribution calculated by the TPS. Thermoluminescent dosimeter (TLD) point dose measurements for an IMRT treatment of an anthropomorphic phantom were in good agreement with the EPID-based 3D doses; in contrast, the planned dose under-predicts the TLD measurement in a high-gradient region by approximately 16%. Similarly, large discrepancies between EPID-based and TPS doses were also evident in dose profiles of small fields incident on a water phantom. These results suggest that our 3D EPID-based method is effective in quantifying relevant uncertainties in the dose calculations of our TPS for IMRT treatments. For three clinical head and neck cancer IMRT treatment plans, our TPS was found to underestimate the mean EPID-based doses in the critical structures of the spinal cord and the parotids by approximately 4 Gy (11%-14%). According to radiobiological modeling calculations that were performed, such underestimates can potentially lead to clinically significant underpredictions of normal tissue complication rates.     

Dose verification for respiratory-gated volumetric modulated arc therapy

A novel commercial medical linac system (TrueBeam?, Varian Medical Systems, Palo Alto, CA) allows respiratory-gated volumetric modulated arc therapy (VMAT), a new modality for treating moving tumors with high precision and improved accuracy by allowing for regular motion associated with a patient's breathing during VMAT delivery. The purpose of this work is to adapt a previously-developed dose reconstruction technique to evaluate the fidelity of VMAT treatment during gated delivery under clinic-relevant periodic motion related to patient breathing. A Varian TrueBeam system was used in this study. VMAT plans were created for three patients with lung or pancreas tumors. Conventional 6 and 15 MV beams with flattening filter and high-dose-rate 10 MV beams with no flattening filter were used in these plans. Each patient plan was delivered to a phantom first without gating and then with gating for three simulated respiratory periods (3, 4.5 and 6 s). Using the adapted log-file-based dose reconstruction procedure supplemented with ion chamber array (Seven29?, PTW, Freiburg, Germany) measurements, the delivered dose was used to evaluate the fidelity of gated VMAT delivery. Comparison of Seven29 measurements with and without gating showed good agreement with gamma-index passing rates above 99% for 1%/1 mm dose accuracy/distance-to-agreement criteria. With original plans as reference, gamma-index passing rates were 100% for the reconstituted plans (1%/1 mm criteria) and 93.5-100% for gated Seven29 measurements (3%/3 mm criteria). In the presence of leaf error deliberately introduced into the gated delivery of a pancreas patient plan, both dose reconstruction and Seven29 measurement consistently indicated substantial dosimetric differences from the original plan. In summary, a dose reconstruction procedure was demonstrated for evaluating the accuracy of respiratory-gated VMAT delivery. This technique showed that under clinical operation, the TrueBeam system faithfully realized treatment plans with gated delivery. This methodology affords a useful tool for machine- and patient-specific quality assurance of the newly available respiratory-gated VMAT.     

A respiratory-gated treatment system for proton therapy

Proton therapy offers the potential for excellent dose conformity and reduction in integral dose. The superior dose distribution is, however, much more sensitive to changes in radiological depths along the beam path than for photon fields. Respiratory motion can cause such changes for treatments sites like lung, liver, and mediastinum and thus affect the proton dose distribution significantly. We have implemented and commissioned a respiratory-gated system for range-modulated treatment fields. The gating system was designed to ensure that each gate always contains complete modulation cycles so that for any beam segment the delivered dose has the planned depth-dose distribution. Measurements have been made to estimate the time delays for the various components of the system. The total delay between the actual motion and the beam on/off control is in the range of 65-195 ms. Time-resolved dose measurements and film tests were also conducted to examine the overall gating effect.     

Clinical experience with EPID dosimetry for prostate IMRT pre-treatment dose verification

The aim of this study was to demonstrate how dosimetry with an amorphous silicon electronic portal imaging device (a-Si EPID) replaced film and ionization chamber measurements for routine pre-treatment dosimetry in our clinic. Furthermore, we described how EPID dosimetry was used to solve a clinical problem. IMRT prostate plans were delivered to a homogeneous slab phantom. EPID transit images were acquired for each segment. A previously developed in-house back-projection algorithm was used to reconstruct the dose distribution in the phantom mid-plane (intersecting the isocenter). Segment dose images were summed to obtain an EPID mid-plane dose image for each field. Fields were compared using profiles and in two dimensions with the y evaluation (criteria: 3%/3 mm). To quantify results, the average gamma (gamma avg), maximum gamma (gamma max), and the percentage of points with gamma < 1(P gamma < 1) were calculated within the 20% isodose line of each field. For 10 patient plans, all fields were measured with EPID and film at gantry set to 0 degrees. The film was located in the phantom coronal mid-plane (10 cm depth), and compared with the back-projected EPID mid-plane absolute dose. EPID and film measurements agreed well for all 50 fields, with (gamma avg) =0.16, (gamma max)=1.00, and (P gamma < 1)= 100%. Based on these results, film measurements were discontinued for verification of prostate IMRT plans. For 20 patient plans, the dose distribution was re-calculated with the phantom CT scan and delivered to the phantom with the original gantry angles. The planned isocenter dose (plan(iso)) was verified with the EPID (EPID(iso)) and an ionization chamber (IC(iso)). The average ratio, (EPID(iso)/IC(iso)), was 1.00 (0.01 SD). Both measurements were systematically lower than planned, with (EPID(iso)/plan(iso)) and (IC(iso)/plan(iso))=0.99 (0.01 SD). EPID mid-plane dose images for each field were also compared with the corresponding plane derived from the three dimensional (3D) dose grid calculated with the phantom CT scan. Comparisons of 100 fields yielded (gamma avg)=0.39, gamma max=2.52, and (P gamma < 1)=98.7%. Seven plans revealed under-dosage in individual fields ranging from 5% to 16%, occurring at small regions of overlapping segments or along the junction of abutting segments (tongue-and-groove side). Test fields were designed to simulate errors and gave similar results. The agreement was improved after adjusting an incorrectly set tongue-and-groove width parameter in the treatment planning system (TPS), reducing (gamma max) from 2.19 to 0.80 for the test field. Mid-plane dose distributions determined with the EPID were consistent with film measurements in a slab phantom for all IMRT fields. Isocenter doses of the total plan measured with an EPID and an ionization chamber also agreed. The EPID can therefore replace these dosimetry devices for field-by-field and isocenter IMRT pre-treatment verification. Systematic errors were detected using EPID dosimetry, resulting in the adjustment of a TPS parameter and alteration of two clinical patient plans. One set of EPID measurements (i.e., one open and transit image acquired for each segment of the plan) is sufficient to check each IMRT plan field-by-field and at the isocenter, making it a useful, efficient, and accurate dosimetric tool.     

An iterative EPID calibration procedure for dosimetric verification that considers the EPID scattering factor.

There has been an increasing interest in the application of electronic portal imaging devices (EPIDs) to dosimetric verification, particularly for intensity modulated radiotherapy. Although not water equivalent, the phantom scatter factor of an EPID, Spe, is generally assumed to be that of a full phantom, Sp, a slab phantom, Sps, or a mini phantom. This assumption may introduce errors in absolute dosimetry using EPIDs. A calibration procedure that iteratively updates Spe and the calibration curve (pixel value to dose rate) is presented. The EPID (Varian Portal Vision) is irradiated using a 20 x 20 cm2 field with different beam intensities. The initial guess of dose rates in the EPID is calculated from ionization chamber measurements in air, multiplied by Sp or Sps. The calibration curve is obtained by fitting EPID readings from pixels near the beam central axis and dose rates in EPID to a quadratic equation. The Spe is obtained from EPID measurements in 10 X 10 cm2 and 20 x 20 cm2 field and from the calibration curve, and is in turn used to adjust the dose rate measurements and hence the calibration curve. The above procedure is repeated until it converges. The final calibration curve is used to convert portal dose to dose in the slab phantom, using the calibrated Spe, or assuming Spe = Sp or Spe=Sps . The converted doses are then compared with the dose measured using an ionization chamber. We also apply this procedure to off-axis points and study its dependence on the energy spectrum. The hypothesis testing results (on the 95% significance level) indicate that systematic errors are introduced when assuming Spe = Sp or Spe=Sps and the dose calculated using Spe is more consistent with ionization chamber measurements. Differences between Spe and Sps are as large as 2% for large field sizes. The measured relative dose profile at dmax using the EPID agrees well with the measured profile at dmax of the isocentric plane using film in a polystyrene phantom with full buildup and full backup, for open and wedged fields, and for a broad range of field sizes of interest. The dependence of the EPID response on the energy spectrum is removed once the calibration is performed under the same conditions as the actual measurements.     

Investigation of patient, tumour and treatment variables affecting residual motion for respiratory-gated radiotherapy

Respiratory gating can reduce the apparent respiratory motion during imaging and treatment; however, residual motion within the gating window remains. Respiratory training can improve respiratory reproducibility and, therefore, the efficacy of respiratory-gated radiotherapy. This study was conducted to determine whether residual motion during respiratory gating is affected by patient, tumour or treatment characteristics. The specific aims of this study were to: (1) identify significant characteristics affecting residual motion, (2) investigate time trends of residual motion over a period of days (inter-session) and (3) investigate time trends of residual motion within the same day (intra-session). Twenty-four lung cancer patients were enrolled in an Institutional Review Board (IRB)-approved protocol. For approximately five sessions, 331 four-minute, respiratory motion traces were acquired with free breathing, audio instructions and audio-visual biofeedback for each patient. The residual motion was quantified by the standard deviation of the displacement within the gating window. The generalized linear model was used to obtain coefficients for each variable within the model and to evaluate the clinical and statistical significance. The statistical significance was determined by a p-value<0.05, while effect sizes of 0.1 cm (one standard deviation) were considered clinically significant. This data analysis was applied to patient, tumour and treatment variables. Inter- and intra-session variations were also investigated. The only variable that was significant for both inhale- and exhale-based gating was disease type. In addition, visual-training displacement, breathing type and Karnofsky performance status (KPS) values were significant for inhale-based gating, and dose-per-fraction was significant for exhale-based gating. Temporal respiratory variations within and between sessions were observed for individual patients. However inter- and intra-session analyses did not show significant time trends on average for any of the variables considered. The lack of significant time trends within and between sessions indicates that on average (1) there is no significant learning period for breathing training, (2) the patients did not experience training-related fatigue and (3) the margin component to account for residual motion during gated radiotherapy appears to remain constant throughout the treatment.     

A system for three-dimensional dosimetric verification of treatment plans in intensity-modulated radiotherapy with heavy ions.

The introduction of dynamic intensity modulation into radiotherapy using conventional photon beams or scanning particle beams requires additional and efficient methods of dose verification. Dose measurements in dynamically generated dose distributions with a single ionization chamber require a complete application of the treatment field for each single measurement. Therefore measurements are performed by simultaneous use of multiple ionization chambers. The measurement is performed by a computer controlled system and is comprised of the following steps: (a) automated positioning of the ionization chambers, (b) measurement at these points, (c) a comparison with the calculated dose from the treatment planning system, and (d) documentation of the measurement. The ionization chambers are read out by a multichannel electrometer and are densely packed into a mounting of polymethylmetacrylate, which is attached to the arm of a three-dimensional motor-driven water phantom. The measured and planned dose values are displayed numerically as well as graphically. The mean deviation between measured and planned doses as well as their standard deviation are calculated and displayed. Through printouts complete documentation of the measurement is obtained and a quick decision can be made whether the dose distribution is acceptable for the patient. The system is now routinely used for dose verification at the heavy ion therapy project at the Gesellschaft für Schwerionenforschung in Darmstadt. Up to now 242 measurements have been performed for heavy ion treatment of 30 patients. The system allows efficient verification and documentation of carbon ion fields and is in principle also applicable to intensity-modulated photon beams.     

The enhancement of radiotherapy verification images by an automated edge detection technique.

Adaptive histogram equalization techniques are known to be effective for the enhancement of contrast in portal images acquired during radiotherapy treatments. A significant drawback is the loss of definition on the edges of the treatment field. Analysis of this problem shows that it can be remedied by separating the treatment field from the background prior to the enhancement, and using only the pixels within the field boundary in the enhancement procedure. An edge extraction algorithm has been developed for delineating the treatment field in portal images, and consists of four modules that are applied to the original portal image in sequence. In the first step, edges are enhanced with a derivative of Gaussian operator that assures high response to the field edges relative to anatomical or other edges in the image. Pixels for which the response of the edge operator was the strongest are subsequently connected by an edge following algorithm to produce a raw contour of the field. In the last two steps the contour is refined by converting it into straight line segments and appending to the contour any parts of the field edge that might have been missed out during the initial edge following. The final contour encloses exclusively those pixels that belong to the treatment field, and the adaptive histogram equalization is applied selectively to this region. The combination of edge detection and selective enhancement was shown to produce images of superior contrast on the patient's anatomical features as well as accurate definition of treatment field edges.     

Investigation of an FFT-based correlation technique for verification of radiation treatment setup.

Electronic portal imaging devices that capture an image of a patient's treatment field electronically by means of a computer data acquisition system operating in real time are becoming available. This paper reports on a study of a field verification correlation algorithm that can compare each treatment portal image to an image of the correct treatment field positioning. The algorithm requires no human intervention or analysis of the images but rather uses fast Fourier transforms to produce a correlation distribution. The position and amplitude of the correlation distribution maximum were tested as objective measures of translational and rotational differences of subject positions between pairs of images. The concept was tested by using a prototype algorithm to obtain the correlation distributions for images of an Alderson Rando head phantom. Images of the phantom setup with various errors were compared with an image of the phantom in the initial, correct treatment position. Translations, in-plane and out-of-plane rotations, and combinations of translations and rotations were studied. The algorithm accurately measured translations. The value of the correlation distribution maximum was found to be a reasonable candidate for an alignment parameter for which tolerable error thresholds might be established.     

Introduction of a novel dose saving acquisition mode for the PortalVision aS500 EPID to facilitate on-line patient setup verification

In external beam radiotherapy, electronic portal imaging becomes more and more an indispensable tool for the verification of the patient setup. For the safe clinical introduction of high dose conformal radiotherapy like intensity modulated radiation therapy, on-line patient setup verification is a prerequisite to ensure that the planned dosimetric coverage of the tumor volume is actually realized in the patient. Since the direction of setup fields often deviates from the direction of the treatment beams, extra dose is delivered to the patient during the acquisition of these portal images which may reach clinical relevance. The aim of this work was to develop a new acquisition mode for the PortalVision aS500 electronic portal imaging device from Varian Medical Systems that allows one to take portal images with reduced dose while keeping good image quality. The new acquisition mode, called RadMode, selectively enables and disables beam pulses during image acquisition allowing one to stop wasting valuable dose during the initial acquisition of "reset frames." Images of excellent quality can be taken with 1 MU only. This low dose per image facilitates daily setup verification with considerably reduced extra dose.     

A megavoltage CT scanner for radiotherapy verification.

We have further developed a system for generating megavoltage CT images immediately prior to the administration of external beam radiotherapy. The detector is based on the scanner of Simpson (Simpson et al 1982)--the major differences being a significant reduction in dose required for image formation, faster image formation and greater convenience of use in the clinical setting. Attention has been paid to the problem of ring artefacts in the images. Specifically, a Fourier-space filter has been applied to the sinogram data. After suitable detector calibration, it has been shown that the device operates close to its theoretical specification of 3 mm spatial resolution and a few percent contrast resolution. Ring artefacts continue to be a major source of image degradation. A number of clinical images have been presented. The next stage of this work is to use the system to make clinical measurements of patient set-up inaccuracies building on our work making such measurements from digital portal images (Evans et al 1992).     

An integrated Monte Carlo dosimetric verification system for radiotherapy treatment planning

An integrated Monte Carlo (MC) dose calculation system, MCRTV (Monte Carlo for radiotherapy treatment plan verification), has been developed for clinical treatment plan verification, especially for routine quality assurance (QA) of intensity-modulated radiotherapy (IMRT) plans. The MCRTV system consists of the EGS4/PRESTA MC codes originally written for particle transport through the accelerator, the multileaf collimator (MLC), and the patient/phantom, which run on a 28-CPU Linux cluster, and the associated software developed for the clinical implementation. MCRTV has an interface with a commercial treatment planning system (TPS) (Eclipse, Varian Medical Systems, Palo Alto, CA, USA) and reads the information needed for MC computation transferred in DICOM-RT format. The key features of MCRTV have been presented in detail in this paper. The phase-space data of our 15 MV photon beam from a Varian Clinac 2300C/D have been developed and several benchmarks have been performed under homogeneous and several inhomogeneous conditions (including water, aluminium, lung and bone media). The MC results agreed with the ionization chamber measurements to within 1% and 2% for homogeneous and inhomogeneous conditions, respectively. The MC calculation for a clinical prostate IMRT treatment plan validated the implementation of the beams and the patient/phantom configuration in MCRTV.     

Transit dosimetry in dynamic IMRT with an a-Si EPID

Using an amorphous silicon (a-Si) EPID for transit dosimetry requires detailed characterization of its dosimetric response in a variety of conditions. In this study, a measurement-based model was developed to calibrate an a-Si EPID response to dose for transit dosimetry by comparison with a reference ionization chamber. The ionization chamber reference depth and the required additional buildup thickness for electronic portal imaging devices (EPID) transit dosimetry were determined. The combined effects of changes in radiation field size, phantom thickness, and the off-axis distance on EPID transit dosimetry were characterized. The effect of scattered radiation on out-of-field response was investigated for different field sizes and phantom thicknesses by evaluation of the differences in image profiles and in-water measured profiles. An algorithm was developed to automatically apply these corrections to EPID images based on the user-specified field size and phantom thickness. The average phantom thickness and an effective field size were used for IMRT fields, and images were acquired in cine mode in the presence of an anthropomorphic phantom. The effective field size was defined as the percentage of the jaw-defined field that was involved during the delivery. Nine head and neck dynamic IMRT fields were tested by comparison with a MatriXX two-dimensional array dosimeter using the Gamma (3 %, 3 mm) evaluation. A depth of 1.5 cm was selected as the ionization chamber reference depth. An additional 2.2 mm of copper buildup was added to the EPID. Comparison of EPID and MatriXX dose images for the tested fields showed that using a 10 % threshold, the average number of points with Gamma index <1 was 96.5 %. The agreement in the out-of field area was shown by selection of a 2 % threshold which on average resulted in 94.8 % of points with a Gamma index <1. The suggested method is less complicated than previously reported techniques and can be used for all a-Si EPIDs regardless of the manufacturer.