Dosimetric Errors Research Articles

Real-Time Visual Tracking of the HDR Source during Skin Therapy Enabled by Scintillation Markers

The most frequent cause of high dose rate (HDR) brachytherapy medical events is incorrect measurement and/or entering of catheter length in the treatment planning system. Another common cause of medical events is the incorrect connection of catheters, transfer tubes and afterloader channels at the treatment. These errors displace the HDR source from its planned position up to a distance of 30 cm away from the treatment target [1], Resulting in dosimetric errors that under-dose, or completely miss, the treatment target and increase dose to healthy tissue. For the treatment of complex superficial skin lesions, the probability of these errors is increased due to the complex shape of the surface applicator and numerous catheters required for these treatments. Current quality assurance methods aim to prevent these errors from occurring prior to treatment by relying on manual checks and verbal verification between physicians, therapists and physicists. Despite these efforts, there remain a lack of methods to verify the HDR source position during treatment to ensure that radiation is being delivered correctly and prevent misadministration. Here, we developed a method for visual real-time tracking of the source to overcome the inadequacies of the present QA processes.

Characterization of the plastic scintillation detector Exradin W2 for small field dosimetry.

Small field dosimetry has been an active area of research for over a decade. It is now known that large dosimetric errors can be introduced if proper detectors or correction factors are not used. The International Atomic Energy Agency (IAEA) through the technical report series No. 483 provides guidelines for small field dosimetry procedures as well as correction factors for most detectors available in the market. The plastic scintillator detector (PSD) Exradin W1 has been found to have a correction factor close to unity; however, it is not designed for beam scanning. To overcome this limitation, the new PSD Exradin W2 has been developed to be used as a scanning as well as a relative dosimeter. Characterization of this detector in small field dosimetry is presented in this study. A 6MV beam from a Varian-Edge linac was used to collect data for the characterization of a W2 detector. Cerenkov light ratio (CLR) is corrected through a separate new electrometer system that comes with the W2 detector. The parameters investigated include the dose and dose rate linearity, beam profiles, percent depth dose (PDD), field output factors, and temperature response. The results were compared with Gafchromic film (EBT-3 film) for beam profiles. The field output factor and temperature response were compared to the Exradin W1 detector. The dose linearity measured with 600MU/min dose rate showed minimal variations (<0.5%) even for small MU, and similar results were seen for dose rate linearity. The comparison of field output factors between the W2 and W1 showed small differences for various depth and field sizes. The temperature response showed small variation when the temperature was varied from to . The slope was and for the W2 and the W1 detector, respectively. The differences in profiles are 0.5% in umbra and penumbra region for field size when compared to the EBT-3 film profile. The W2 scintillator detector showed similar dosimetric and temperature properties to the W1 scintillator detector. The main advantage of the W2 detector among other plastic scintillators is the beam scanning capabilities that, combined with its correction factor of 1.0, make it an ideal detector for commissioning of SRS and SBRT techniques.

Plan-Class Specific Reference Quality Assurance for Volumetric Modulated Arc Therapy

Background: There have been much efforts to develop the proper and realistic machine Quality Assurance (QA) reflecting on real Volumetric Modulated Arc Therapy (VMAT) plan. In this work we propose and test a special VMAT plan of plan-class specific (pcsr) QA, as a machine QA so that it might be a good solution to supplement weak point of present machine QA to make it more realistic for VMAT treatment. Materials and Methods: We divided human body into 5 treatment sites: brain, head and neck, chest, abdomen, and pelvis. One plan for each treatment site was selected from real VMAT cases and contours were mapped into the computational human phantom where the same plan as real VMAT plan was created and called plan-class specific reference (pcsr) QA plan. We delivered this pcsr QA plan on a daily basis over the full research period and tracked how much MLC movement and dosimetric error occurred in regular delivery. Several real patients under treatments were also tracked to test the usefulness of pcsr QA through comparisons between them. We used dynalog file viewer (DFV) and Dynalog file to analyze position and speed of individual MLC leaf. The gamma pass rate from portal dosimetry for different gamma criteria was analyzed to evaluate analyze dosimetric accuracy. Results and Discussion: The maxRMS of MLC position error for all plans were all within the tolerance limit of < 0.35 cm and the positional variation of maxPEs for both pcsr and real plans were observed very stable over the research session. Daily variations of maxRMS of MLC speed error and gamma pass rate for real VMAT plans were observed very comparable to those in their pcsr plans in good acceptable fluctuation. Conclusion: We believe that the newly proposed pcsr QA would be useful and helpful to predict the mid-term quality of real VMAT treatment delivery.

Volumetric modulated arc therapy planning based on virtual monochromatic images: Effect of inaccurate CT numbers on dose distributions

PurposeThough virtual monochromatic images (VMIs) at low energy levels can improve image quality, the measured Hounsfield unit (HU) values can be inaccurate. We assessed the dosimetric error due to inaccurate HU estimation in volumetric modulated arc therapy (VMAT) planning. MethodsBased on the VMIs at 50 keV (VMI50keV), 77 keV (VMI77keV) and single-energy CT (SECT) image for a phantom with different sizes, lookup tables (LUTL and LUTS) were created. Using an anthropomorphic phantom (head and spine regions), VMAT plans were generated based on VMI50keV, VMI77keV and SECT using the corresponding LUTL, and then, the doses were re-calculated using LUTS. For clinical cases, 30 VMAT plans (prostate, brain, and spine cases) were generated based on VMI50keV and VMI77keV. ResultsIn the anthropomorphic phantom study, the difference in the dosimetric parameters for planning target volume (PTV) in the VMAT plan based on the VMI77keV was smallest (within 0.1 Gy) among three types of treatment planning approach. In clinical cases, in general, the differences of the 3-dimensional gamma passing rate and dosimetric parameters in the treatment plans based on the VMI50keV were larger than those in the VMI77keV. Especially for brain cases, the difference for PTV was more prominent when AXB was used (the maximum difference was 0.5 Gy) than AAA. ConclusionsThe dosimetric error due to the inaccurate HU estimation was larger in the VMIs at low energy levels. This may be clinically insignificant, but should be avoided in the VMAT treatment planning.

Optical-Radiation-Calorimeter Refinement by Virtual-Sensitivity Analysis.

Digital holographic interferometry (DHI) radiation dosimetry has been proposed as an experimental metrology technique for measuring absorbed radiation doses to water with high spatial resolution via noninvasive optical calorimetry. The process involves digitally recording consecutive interference patterns resulting from variations in the refractive index as a function of the radiation-absorbed dose. Experiments conducted on prototype optical systems revealed the approach to be feasible but strongly dependent on environmental-influence quantities and setup configuration. A virtual dosimeter reflecting the prototype was created in a commercial optical modelling package. A number of virtual phantoms were developed to characterize the performance of the dosimeter under ideal conditions and with simulated disruptions in environmental-influence quantities, such as atmospheric and temperature perturbations as well as mechanical vibrations. Investigations into the error response revealed that slow drifts in atmospheric parameters and heat expansion caused the measured dose to vary between measurements, while atmospheric fluctuations and vibration contributed to system noise, significantly lowering the spatial resolution of the detector system. The impact of these effects was found to be largely mitigated with equalisation of the dosimeter’s reference and object path lengths, and by miniaturising the detector. Equalising path lengths resulted in a reduction of 97.5% and 96.9% in dosimetric error introduced by heat expansion and atmospheric drift, respectively, while miniaturisation of the dosimeter was found to reduce its sensitivity to vibrations and atmospheric turbulence by up to 41.7% and 54.5%, respectively. This work represents a novel approach to optical-detector refinement in which metrics from medical imaging were adapted into software and applied to a a virtual-detector system. This methodology was found to be well-suited for the optimization of a digital holographic interferometer.

Comparison of gamma index based on dosimetric error and clinically relevant dose\u2013volume index based on three-dimensional dose prediction in breast intensity-modulated radiation therapy

BackgroundMeasurement-guided dose reconstruction has lately attracted significant attention because it can predict the delivered patient dose distribution. Although the treatment planning system (TPS) uses sophisticated algorithm to calculate the dose distribution, the calculation accuracy depends on the particular TPS used. This study aimed to investigate the relationship between the gamma passing rate (GPR) and the clinically relevant dose–volume index based on the predicted 3D patient dose distribution derived from two TPSs (XiO, RayStation).MethodsTwenty-one breast intensity-modulated radiation therapy plans were inversely optimized using XiO. With the same plans, both TPSs calculated the planned dose distribution. We conducted per-beam measurements on the coronal plane using a 2D array detector and analyzed the difference in 2D GPRs between the measured and planned doses by commercial software. Using in-house software, we calculated the predicted 3D patient dose distribution and derived the predicted 3D GPR, the predicted per-organ 3D GPR, and the predicted clinically relevant dose–volume indices [dose–volume histogram metrics and the value of the tumor-control probability/normal tissue complication probability of the planning target volume and organs at risk]. The results derived from XiO were compared with those from RayStation.ResultsWhile the mean 2D GPRs derived from both TPSs were 98.1% (XiO) and 100% (RayStation), the mean predicted 3D GPRs of ipsilateral lung (73.3% [XiO] and 85.9% [RayStation]; p < 0.001) had no correlation with 2D GPRs under the 3% global/3 mm criterion. Besides, this significant difference in terms of referenced TPS between XiO and RayStation could be explained by the fact that the error of predicted V5Gy of ipsilateral lung derived from XiO (29.6%) was significantly larger than that derived from RayStation (− 0.2%; p < 0.001).ConclusionsGPR is useful as a patient quality assurance to detect dosimetric errors; however, it does not necessarily contain detailed information on errors. Using the predicted clinically relevant dose–volume indices, the clinical interpretation of dosimetric errors can be obtained. We conclude that a clinically relevant dose–volume index based on the predicted 3D patient dose distribution could add to the clinical and biological considerations in the GPR, if we can guarantee the dose calculation accuracy of referenced TPS.

Fraction-specific post-treatment quality assurance for active breath-hold radiation therapy

AbstractPurposeThe purpose of this study is to evaluate variation in the treatment hold pattern and quantify its dosimetric impact in breath-hold radiotherapy, using fraction-specific post-treatment quality assurance.Material and MethodsA patient with lung mets treated using intensity-modulated radiation therapy (IMRT) with active breath coordinator (ABC) was recruited for the study. Treatment beam hold conditions were recorded for all the 25 fractions. The linearity and reproducibility of the dosimetric system were measured. Variation in the dose output of unmodulated open beam with beam hold was studied. Patient-specific quality assurance (PSQA) was performed with and without beam hold, and the results were compared to quantify the dosimetric impact of beam hold.ResultsThere was a considerable amount of variation observed in the number of beam hold for the given field and the monitor unit at which the beam held. Linearity and reproducibility of the dosimetric system were found within the acceptable limits. The average difference over the 25 measurements was 0·044% (0·557 to −0·318%) with standard deviation of 0·248.ConclusionPatient comfort with the ABC system and responsiveness to the therapist communication help to maintain consistent breathing pattern, in turn consistent treatment delivery pattern. However, the magnitude of dosimetric error is much less than the acceptable limits recommended by IROC. The dosimetric error induced by the beam hold is over and above the dose difference observed in conventional PSQA.

Optimizing beam models for dosimetric accuracy over a wide range of treatments

This work presents a systematic approach for testing a dose calculation algorithm over a variety of conditions designed to span the possible range of clinical treatment plans. Using this method, a TrueBeam STx machine with high definition multi-leaf collimators (MLCs) was commissioned in the RayStation treatment planning system (TPS). The initial model parameters values were determined by comparing TPS calculations with standard measured depth dose and profile curves. The MLC leaf offset calibration was determined by comparing measured and calculated field edges utilizing a wide range of MLC retracted and over-travel positions. The radial fluence was adjusted using profiles through both the center and corners of the largest field size, and through measurements of small fields that were located at highly off-axis positions. The flattening filter source was adjusted to improve the TPS agreement for the output of MLC-defined fields with much larger jaw openings. The MLC leaf transmission and leaf end parameters were adjusted to optimize the TPS agreement for highly modulated intensity-modulated radiotherapy (IMRT) plans. The final model was validated for simple open fields, multiple field configurations, the TG 119 C-shape target test, and a battery of clinical IMRT and volumetric-modulated arc therapy (VMAT) plans. The commissioning process detected potential dosimetric errors of over 10% and resulted in a final model that provided in general 3% dosimetric accuracy. This study demonstrates the importance of using a variety of conditions to adjust a beam model and provides an effective framework for achieving high dosimetric accuracy.

Development of Novel Immobilization Adapter for Head and Neck Radiotherapy with Low-attenuation Material

The dosimetric error due to immobilization devices has been highlighted by the AAPM Task Group 176. We developed a novel low-radiation-absorbent immobilization adaptor (HMA), which can be used with a Styrofoam headrest for head and neck region in radiotherapy. The purpose of this study was to investigate the impact of the HMA on the dose distribution and compare with a commercially released plastic adapter. Computed tomography (CT) simulation and dose calculation on a treatment planning system (TPS) were performed by the use of HMA and the plastic adapter with a cylindrical phantom. Both the adapters were placed on the phantom upside and the attenuation rate was measured. Gantry angles were changed at every 1°interval from 0°to 50°for measurements. The measured dose was normalized by the value of 90°. The treatment equipment was TrueBeam (Varian medical systems); X-ray energies were set on 4, 6 and 10 MV, respectively. The measured attenuation rates were also compared with calculation results of TPS. The highest differences on attenuation rate of both the adapters were observed at a gantry angle of 32.0°; the differences were 3.0% at 4 MV, 2.7% at 6 MV and 3.0% at 10 MV, respectively, and lower absorption was HMA. TPS calculation results of monitor unit for the HMA were within 1.0% in each energy. The HMA was able to provide absorption dose and calculation errors lower than a commercially released adapter. It can also provide more accurate dose delivery for radiotherapy in head and neck because of the low absorption characteristics.

Commissioning and early experience with a new generation low-energy linear accelerator in zahedan

Introduction: Since commissioning beam data are treated as a reference and ultimately used by treatment planning systems, it is vitally important that the collected data are of the highest quality to avoid dosimetric and patient treatment errors that may subsequently lead to a poor radiation outcome. Beam data commissioning should be performed with appropriate knowledge and proper tools and should be independent of the person collecting the data. low-energy linear accelerator was introduced in the clinical area by ELEKTA Medical Systems. This machine was first installed in Sistan and Baluchestan state in 2016. The aim of the present contribution was to report experience about its commissioning and first year results from clinical operation. Materials and Methods: In this research, we used water phantom model DoseviewTM 3D version1.2 and A19 and A28 ion chambers. Commissioning data, beam characteristics and the modeling into the treatment planning system were summarized. . Finally, since the system is capable of delivering Threaetment dose with 6 Mev of energy , a summary of the tests performed for such modality to assess itsperformance in preclinicalsettings and during clinical usage was included. Results: The absorbed dose according to the TRS398 protocol in the absolute dosimetry was D = 0.010042Gy / Mu. In a relative dosimetry, PDD was calculated at different depths and fields, results at 10 cm depth for field 10 × 10 was equal to 0.661 and for the field 30 × 30 at the same depth of 10 cm was equal to 0.690. Also Measurements were made to determine the flatness and symmetry in different fields at a depth of 10cm. For example, the flatness in field 10×10 at depth of 10cm was 3.7 and its Asymmetry value was -0.7%. Conclusion: The results of the commissioning tests and of the first period of clinical operation, resulted meeting specifications and having good margins respect to tolerances. Linac was put into operation for all deliverytechniques; in particular, as shown by the pre-treatment quality assurance results, it enabledaccurate and safedelivery of 3D planning.

18 Dosimetric study of the couch top influence in VMAT

Introduction During these last years, treatment planning system (TPS) algorithms that compute the absorbed dose have improved. A high precision between delivered absorbed dose and prescribed dose is required, limiting the uncertainties up to 5% between delivered dose and prescribed dose according to the ICRU recommendations [1] . One of the parameters contributing to these uncertainties is the couch top, which attenuates the beam from 2% to 15% when it passes through [2] . Last generation’s carbon fiber couch tops act as a bolus material, increasing skin dose while decreasing depth-dose. Therefore, the consideration of the couch top by the TPS is important for avoiding bad tumor control, overdose of organs at risk and skin radiotoxicity effects [2] . Methods The reference technique for the dose distribution calculations is based on Monte Carlo method (MC) [3] . Among the most well-known codes, BEAMnrc/EGSnrc allows to model the accelerator treatment head and to use CT images (DICOM format) in order to simulate the dose distribution to voxelized phantoms. Furthermore, the CT data have to be converted to densities and to the corresponding materials, taking into account the couch top [4] . A software developed by the team using a conversion ramp between Hounsfield Units (HU) and densities/materials, specific to the couch top, will be used to create the voxelized phantoms. Results MC method takes into consideration not only voxel densities, but also materials, if a wrong material assignment is made, it could lead to dosimetric errors in the dose distribution calculations. Different treatment sites will be considered in Volumetric Modulated Arc Therapy (VMAT). Phantoms with the couch top will be created and dose distributions will be compared to TPS dose distributions. Conclusions The results of this study contributed to evaluate different methods of taking into account the couch top, using MC.

Evaluation of Dynamic Tumor-tracking Intensity-modulated Radiotherapy for Locally Advanced Pancreatic Cancer

Intensity-modulated radiotherapy (IMRT) is now regarded as an important treatment option for patients with locally advanced pancreatic cancer (LAPC). To reduce the underlying tumor motions and dosimetric errors during IMRT as well as the burden of respiratory management for patients, we started to apply a new treatment platform of the dynamic tumor dynamic tumor-tracking intensity-modulated radiotherapy (DTT-IMRT) using the gimbaled linac, which can swing IMRT toward the real-time tumor position under patients’ voluntary breathing. Between June 2013 and March 2015, ten patients were treated, and the tumor-tracking accuracy and the practical benefits were evaluated. The mean PTV size in DTT-IMRT was 18% smaller than a conventional ITV-based PTV. The root-mean-squared errors between the predicted and the detected tumor positions were 1.3, 1.2, and 1.5 mm in left-right, anterior-posterior, and cranio-caudal directions, respectively. The mean in-room time was 24.5 min. This high-accuracy of tumor-tracking with reasonable treatment time are promising and beneficial to patients with LAPC.

Dosimetric and Radiobiological Evaluation of Delivered Dose Distribution with Intra-Fractional Motion in Prostate Stereotactic Radiation Therapy Treatments Using VMAT and Helical Tomotherapy

The implementation of motion-management techniques during intensity modulated radiation therapy (IMRT) is still under challenging issue. Moreover, the dose-per-fraction regime and beam-delivery methods affect the dose distribution owing to the interplay effect. Hypofractionation may be a uniquely appropriate treatment for prostate cancer because of a lower α/β ratio. Several prospective studies support the safety of moderate hypofractionation; however, extreme fractionation may lead to greater toxicity. It has been reported that respiratory motion during IMRT delivery of stereotactic body radiation therapy (SBRT) can lead to dosimetric errors in the target of up to ±20%. Therefore, intra-fractional prostate motion may occur unpredictably during IMRT and lead to under-treatment of the target. In this study, we evaluated the dosimetric and radiobiological effects of the SBRT dose distributions with patient-specific intra-fractional motion in prostate cancer for volumetric modulated arc therapy (VMAT) and helical tomotherapy (HT) using a commercially available six-dimensional (6D) motion system. Dose distributions with patient-specific prostate intra-fractional motion were evaluated using the 6D-motion simulator system, which was coupled to a biplanar diode array detector phantom. These bidirectional prostate motions were acquired from magnetic resonance (MR)-cine images. Then, these motions were continuously generated for approximately 4 min, and dose measurements were repeated 5 times to measure the uncertainty of prostate motion during irradiation. We tested two typical motion patterns: large (over 2.0 mm) and small (under 1.0 mm). The SBRT plan was created in VMAT and HT with a prescription of 36.25 Gy/5 fractions to D95% of the PTV. We compared dose distributions with static irradiations using the 3 dimensional gamma (3D-γ) analysis. Moreover, the equivalent uniform dose (EUD)-based tumor control probability (TCP) and normal tissue complication probability (NTCP) of the rectum were compared with 76 Gy/38 fractions, which is the standard prostate fractionation plan. In the results of the dosimetric evaluation, the pass rate of the γ-analysis (criterion of 2 mm, 3%) was degraded by 9.5% and 22.7% in VMAT and HT plans in the large motion patterns and 3.0% and 8.7% in the small motion patterns, respectively. However, degradation of the D99 for CTV was only 0.7% and 1.8% in VMAT and HT plans, respectively, even in the large motion patterns. Moreover, these errors were negligible in small motion patterns. For the radiobiological evaluation, no significant differences in TCP and NTCP degradations were found in VMAT and HT SBRT plans compared with the standard fractionation plan. SBRT dose distributions with intra-fractional motion over 2.0 mm in prostate cancer were more degraded in HT than those observed in VMAT; however, these dosimetric errors might not be clinically significant degradation for TCP and/or NTCP.

Comparative study of algorithms for synthetic CT generation from MRI: Consequences for MRI‐guided radiation planning in the pelvic region

Magnetic resonance imaging (MRI)-guided radiation therapy (RT) treatment planning is limited by the fact that the electron density distribution required for dose calculation is not readily provided by MR imaging. We compare a selection of novel synthetic CT generation algorithms recently reported in the literature, including segmentation-based, atlas-based and machine learning techniques, using the same cohort of patients and quantitative evaluation metrics. Six MRI-guided synthetic CT generation algorithms were evaluated: one segmentation technique into a single tissue class (water-only), four atlas-based techniques, namely, median value of atlas images (ALMedian), atlas-based local weighted voting (ALWV), bone enhanced atlas-based local weighted voting (ALWV-Bone), iterative atlas-based local weighted voting (ALWV-Iter), and a machine learning technique using deep convolution neural network (DCNN). Organ auto-contouring from MR images was evaluated for bladder, rectum, bones, and body boundary. Overall, DCNN exhibited higher segmentation accuracy resulting in Dice indices (DSC) of 0.93±0.17, 0.90±0.04, and 0.93±0.02 for bladder, rectum, and bones, respectively. On the other hand, ALMedian showed the lowest accuracy with DSC of 0.82±0.20, 0.81±0.08, and 0.88±0.04, respectively. DCNN reached the best performance in terms of accurate derivation of synthetic CT values within each organ, with a mean absolute error within the body contour of 32.7±7.9HU, followed by the advanced atlas-based methods (ALWV: 40.5±8.2HU, ALWV-Iter: 42.4±8.1HU, ALWV-Bone: 44.0±8.9HU). ALMedian led to the highest error (52.1±11.1HU). Considering the dosimetric evaluation results, ALWV-Iter, ALWV, DCNN and ALWV-Bone led to similar mean dose estimation within each organ at risk and target volume with less than 1% dose discrepancy. However, the two-dimensional gamma analysis demonstrated higher pass rates for ALWV-Bone, DCNN, ALMedian and ALWV-Iter at 1%/1mm criterion with 94.99±5.15%, 94.59±5.65%, 93.68±5.53% and 93.10±5.99% success, respectively, while ALWV and water-only resulted in 86.91±13.50% and 80.77±12.10%, respectively. Overall, machine learning and advanced atlas-based methods exhibited promising performance by achieving reliable organ segmentation and synthetic CT generation. DCNN appears to have slightly better performance by achieving accurate automated organ segmentation and relatively small dosimetric errors (followed closely by advanced atlas-based methods, which in some cases achieved similar performance). However, the DCNN approach showed higher vulnerability to anatomical variation, where a greater number of outliers was observed with this method. Considering the dosimetric results obtained from the evaluated methods, the challenge of electron density estimation from MR images can be resolved with a clinically tolerable error.

Daily Based Quality Assurance of Volumetric Modulated Arc Therapy for the Full Session of Treatment

Volumetric modulated arc therapy (VMAT) with highly complex advanced delivering technique, is widely practiced in radiation therapy and requires a quality assurance for treatment plan at least once a treatment session. In this study we collected six VMAT patients with different treatment sites. The VMAT plans and their quality assurance (QA) plans were performed on a daily basis before treatment for their full period of treatment. We tried to evaluate the accuracy of their delivering through analyses of first mechanical error of multileaf collimator (MLC) movement and second dosimetric error of beam delivery. We used a MATLAB program and portal dosimetry to estimate a maximum root mean square (RMS) error from dynalog files for mechanical analysis and a gamma index from portal dosimetry for dosimetric analysis, respectively. We introduced newly in this study a representative VMAT QA to be used to perform a routine VMAT QA. The daily variation of maximum error RMSs for all treatment sites were all below maximum 0.3503 ± 0.0002 mm for treatment plans and QA plans and they all showed the gamma pass rate of more than 98%. Mean of inter difference between treatment and QA plans stayed below 0.04 mm, showing that QA deliveries agree well with treatment deliveries. The Linac used kept a good performance within the tolerances in American Association of Physicists in Medicine (AAPM) TG142 guideline for the period of research. We suggest that the representative VMAT QA can be a good surrogate for real VMAT plan and “whole time” plan QA for a patient selected randomly at least once a year be performed to confirm the long-term stability of Linac performance.

P173] Proposal of a comprehensive pre-treatment QA procedure in IMRT/VMAT techniques

Purpose The abstract aims to propose a comprehensive pre-treatment QA procedure, independent of TPS calculation and Linac control system, to detect errors related to dose calculation and/or dose delivery in state-of-art IMRT/VMAT techniques. Methods The proposed pre-treatment QA procedure consists of the subsequent application of two independent methodologies: one relevant to dose calculation QA and the other to dose delivery QA. The dose calculation QA relies on validating the analytical TPS dose distribution with a more accurate equipment-specific Monte-Carlo (MC) dose simulation [1] . A Geant4 (MC) model is developed to be used for 3D dose distribution simulations. The dose delivery QA relies on an in-house developed, plan-specific, 3D non-transit dosimetry approach based on EPID technology. The methodology reconstructs a 3D dose distribution in a virtual cylindrical water phantom based on calibrated and back-projected EPID images [2] . The accuracy of the reconstructed 3D dose distribution has been investigated in this work. The sensitivity to geometric and dosimetric errors between the original treatment plan and intentionally modified treatment plans has been assessed in terms of controlled translational shifts of the MLC, rotational shifts of the gantry and over-dosages. Quantification is based on gamma evaluation with acceptance criteria of (3 mm, 3%). The QA evaluation of the dose calculation is performed between the TPS dose distribution and the MC dose simulation. Afterwards, the dose delivery QA evaluation is performed between the TPS calculation and the reconstructed 3D dose distribution in the virtual water phantom, as sufficiently accurate in homogeneous (water) media. Results The initial adequacy of the two presented methodologies for the proposal of a comprehensive pre-treatment QA procedure is promising. An overview of the results under controlled changes and/or errors will be presented and the interpretation of detected errors in real clinical scenarios will be discussed. Conclusions The proposed comprehensive pre-treatment QA procedure provides independent validation methods with the potential to quantitatively support the decision making process in presence of errors related to dose calculation or dose delivery.

Composite QA for intensity-modulated radiation therapy using individual volume-based 3D gamma indices.

The aim of this study was to investigate the feasibility and sensitivity of using individual volume–based 3D gamma indices for composite dose–volume histogram (DVH)–based intensity-modulated radiation therapy (IMRT) quality assurance (QA). Composite IMRT QA for 15 cervical cancer patients was performed with ArcCHECK. The percentage dosimetric errors (%DEs) of DVH metrics when comparing treatment planning system and QA-reconstructed dose distribution, percentage gamma passing rates (%GPs) with different criteria for individual volumes and global gamma indices were evaluated, as well as their correlations. Receiver operating characteristic (ROC) curves were applied in order to study the sensitivities of the global and individual volume gamma indices. Most %DEs of the DVH metrics were within 3%. The γPTV and γrectum were <80% at 2%/2 mm; apart from these two individual volume indices, all other individual volume gamma indices and global indices had acceptable %GPs. For the criteria of 2%/2 mm, 3%/3 mm and 4%/4 mm, individual volume-based %GPs and global %GPs were correlated in 11, 1 and 12 out of 24 %DE metrics, and in 5, 4 and 5 out of 24 %DE metrics, respectively. Individual volume–based %GPs had a higher percentage of correlation with DVH metrics (%DEs) compared with global %GPs in composite IMRT QA. The areas under the curve (AUCs) of individual volume %GPs were higher than those of global %GPs. In conclusion, individual volume–based %GPs had a higher correlation with %DEs of metrics and a higher sensitivity presented by ROC analysis compared with global %GPs for composite IMRT QA. Thus, use of individual volume-based 3D gamma indices was found to be feasible and sensitive for composite IMRT QA.

Model-basedcardiacdoseestimation in radiationtreatment of leftbreastcancer.

To develop a mathematical model for cardiac dose estimation for patients who have been treated for left-sided breast cancer without CT data. After obtaining institutionalreview board approvals, 147 patients with left-sided breast cancer were selected that were treated supine with opposed tangents. The heart blocks from the tangential fields were removed and dose calculations were performed with 6 MV beams using an advanced algorithm. This study was performed with CT data with DRR to represent a radiographic simulator image of yesteryear treatments. The beam'seye-view images showing delineated breast, lung and heart were created to represent views on radiograph. The maximum heart distance (MHD) was recorded and irradiated heart area (AREA) was computed by combination of triangles and rectangles. Based on accurate 3D dose calculation, mean dose (Dmean) and V10 to V50 of heart were analyzed with respect to MHD and AREA for dosimetric parameters using linear and quadratic fit. The treatment parameters calculated by MHD and segments using 2D radiographs were within 2% of the actual dosimetric parameters computed from the 3D planning system. The MHD and AREA vs Dmean, V10, V20, V30 and V50 showed very good correlation with linear model (R2 >0.91); however the correlation was significantly better with quadratic model (R2 >0.92). The analysis of the dosimetric error with our linear and quadratic model is remarkable within <3% error for most cases. The proposed mathematical model for the cardiac dose estimation is accurate within ±3% using a radiograph without CT data. This provides avenues for patient pooling in future studies related to radiation dose and cardiac toxicity. These results will help in estimating cardiac dose analysis accurately from previous studies as well as in centers still using 2D planning. The evidence of cardiac risk following radiotherapy continues to be one of the important considerations for the management of left-sided breast cancer patients. One of the problem in the estimation of dose-risk effects is the reconstruction of heart dose for pre-CT treatments. In this study, a simple mathematical model is presented that could estimate cardiac dose within ±3% in left breast cancer treatment from 2D radiograph where CT data do not exist.

Assessment of dosimetric errors induced by deformable image registration methods in 4D pencil beam scanned proton treatment planning for liver tumours

PurposeRespiratory impacts in pencil beam scanned proton therapy (PBS-PT) are accounted by extensive 4D dose calculations, where deformable image registration (DIR) is necessary for estimating deformation vector fields (DVFs). We aim here to evaluate the dosimetric errors induced by different DIR algorithms in their resulting 4D dose calculations by using ground truth(GT)-DVFs from 4DMRI. Materials and methodsSix DIR methods: ANACONDA, Morfeus, B-splines, Demons, CT Deformable, and Total Variation, were respectively applied to nine 4DCT-MRI liver data sets. The derived DVFs were then used as input for 4D dose calculation. The DIR induced dosimetric error was assessed by individually comparing the resultant 4D dose distributions to those obtained with GT-DVFs. Both single-/three-field plans and single/rescanned strategies were investigated. ResultsDifferences in 4D dose distributions among different DIR algorithms, and compared to the results using GT-DVFs, were pronounced. Up to 40 % of clinically relevant dose calculation points showed dose differences of 10 % or more between the GT. Differences in V95(CTV) reached up to 11.34 ± 12.57 %. The dosimetric errors became in general less substantial when applying multiple-field plans or using rescanning. ConclusionIntrinsic geometric errors by DIR can influence the clinical evaluation of liver 4D PBS-PT plans. We recommend the use of an error bar for correctly interpreting individual 4D dose distributions.

Sensitivity study of an automated system for daily patient QA using EPID exit dose images.

The dosimetric consequences of errors in patient setup or beam delivery and anatomical changes are not readily known. A new product, PerFRACTION (Sun Nuclear Corporation), is designed to identify these errors by comparing the exit dose image measured on an electronic portal imaging device (EPID) from each field of each fraction to those from baseline fraction images. This work investigates the sensitivity of PerFRACTION to detect the deviation caused by these errors in a variety of realistic scenarios. Integrated EPID images were acquired in clinical mode and saved in ARIA. PerFRACTION automatically pulled the images into its database and performed the user‐defined comparison. We induced errors of 1 mm and greater in jaw, multileaf collimator (MLC), and couch position, 1° and greater in collimation rotation (patient yaw), 0.5–1.5% in machine output, rail position, and setup errors of 1–2 mm shifts and 0.5–1° roll rotation. The planning techniques included static, intensity modulated radiation therapy (IMRT) and VMAT fields. Rectangular solid water phantom or anthropomorphic head phantom were used in the beam path in the delivery of some fields. PerFRACTION detected position errors of the jaws, MLC, and couch with an accuracy of better than 0.4 mm, and 0.5° for collimator rotation error and detected the machine output error within 0.2%. The rail position error resulted in PerFRACTION detected dose deviations up to 8% and 3% in open field and VMAT field delivery, respectively. PerFRACTION detected induced errors in IMRT fields within 2.2% of the gamma passing rate using an independent conventional analysis. Using an anthropomorphic phantom, setup errors as small as 1 mm and 0.5° were detected. Our work demonstrates that PerFRACTION, using integrated EPID image, is sensitive enough to identify positional, angular, and dosimetric errors.