Delta4 System Research Articles

Description and evaluation of a new volumetric-modulated arc therapy plan complexity metric

This study describes a new plan complexity metric for volumetric-modulated arc therapy (VMAT) and evaluates the relationship of this metric with the VMAT dosimetric accuracy. The new modulation complexity score for VMAT (NMCSv) that is based on the aperture shape and multi-leaf collimator (MLC) leaf travel is described. Its performance is evaluated through correlation and receiver operating characteristic (ROC) analyses with patient-specific gamma passing rates using 2 3-dimensional diode arrays. For comparison, the following metrics are evaluated using the same correlation analyses: average field width, average leaf travel, modulation complexity score, and leaf travel modulation complexity score. Spearman's rank correlation analysis is performed to examine any relationships between the complexity metrics and the patient-specific gamma passing rates. ROC curves are used to assess the performance of the plan metrics using a gamma passing rate of 3%/3 mm criterion with a 95% tolerance level. In both the diode arrays, the gamma passing rates (3%/3 mm and 2%/2 mm) for patient-specific dosimetric verification of VMAT plans are moderately or weakly correlated to all the complexity metrics. NMCSv demonstrates the highest correlation with the passing rates (r = 0.652, p < 0.001 for Delta4 and r = 0.499, p < 0.001 for ArcCheck) and the highest area under the curve value (0.809, p < 0.01 for Delta4 and 0.734, p < 0.01 for ArcCheck). While using the Delta4 system, NMCSv exhibits an excellent classification performance with area under the curves of 0.926 (sensitivity: 0.913; specificity: 0.860; p < 0.01) and 0.918 (sensitivity: 0.943; specificity: 0.720; p < 0.01) for rectal and cervical cancer plans, respectively. NMCSv as a novel potential clinical plan complexity metric is moderately correlated with the gamma passing rate. It demonstrates the best performance with respect to distinguishing the dosimetric accuracy of VMAT plans among the evaluated metrics. The classification performance of complexity metrics can be affected by various dosimetry verification devices and treatment sites.

Verification of IMRT and RapidArc Localized Prostate Cancer Treatment Plans Using EPID and Delta4 in vivo Dosimetry Methods

Background: Quality assurance for intensity modulated radiation therapy (IMRT) depends on the type of dosimetry system and its evaluation procedure. We can check both the intensity and distribution of each field as the required pretreatment verification with dosimetry systems. Method: Treatment verification for different plans (IMRT and RapidArc) applied on localized prostate cancer patients was done with electronic portal imager device (EPID), Delta4). The EPID used was Varian aS1000 mounted on Varian (TrueBeam) Linac with gamma criteria set to ΔD=3% and Δd=3mm. RapidArc plans were designed by arcs (179.0o CCW to 181.0o and 181.0o CW to 179.0o) under the same gamma criteria of (ΔD=3%, DTA=3mm and γ-index ≤1) while the threshold dose was 20%. Results: Evaluation analysis is passed for IMRT prostate plans with the area gamma ˂1.0 which equaled 99.1% (99.1% of the pixels had gamma˂1) within a tolerance of 95.0%, area gamma ˃0.8 (was equal to 2.1%) / area gamma ˃1.2 (was equal to 0.3%) and the average dose difference was 0.42CU. Delta4 dosimetry system was assessed with RapidArc plan; the agreement between the measured and planned doses was ±1% and gamma analysis resulted in 100% data points with the same agreement conditions. Conclusion: Portal dosimetry provided a good verification of the treatment unit ability to deliver doses according to plan. For an IMRT field comprised of several subfields, it could give rise to much more errors. RapidArc plans were verified using Delta4 system, which generated excellent dosimetry results. Periodic calibration was recommended for Delta4 dosimetry system; radiation damage affected sensitivity by ˃1% every 1kGy.

Predictive gamma passing rate for three-dimensional dose verification with finite detector elements via improved dose uncertainty potential accumulation model.

We aim to develop a method to predict the gamma passing rate (GPR) of a three-dimensional (3D) dose distribution measured by the Delta4 detector system using the dose uncertainty potential (DUP) accumulation model. Sixty head-and-neck intensity-modulated radiation therapy (IMRT) treatment plans were created in the XiO treatment planning system. All plans were created using nine step-and-shoot beams of the ONCOR linear accelerator. Verification plans were created and measured by the Delta4 system. The planar DUP (pDUP) manifesting on a field edge was generated from the segmental aperture shape with a Gaussian folding on the beam's-eye view. The DUP at each voxel ( ) was calculated by projecting the pDUP on the Delta4 phantom with its attenuation considered. The learning model (LM), an average GPR as a function of the DUP, was approximated by an exponential function to compensate for the low statistics of the learning data due to a finite number of the detectors. The coefficient was optimized to ensure that the difference between the measured and predicted GPRs ( ) was minimized. The standard deviation (SD) of the was evaluated for the optimized LM. It was confirmed that the coefficient was larger for tighter tolerance. This result corresponds to the expectation that the attenuation of the will be large for tighter tolerance. The and were observed to be proportional for all tolerances investigated. The SD of was 2.3, 4.1, and 6.7% for tolerances of 3%/3mm, 3%/2mm, 2%/2mm, respectively. The DUP-based predicting method of the GPR was extended to 3D by introducing DUP attenuation and an optimized analytical LM to compensate for the low statistics of the learning data due to a finite number of detector elements. The precision of the predicted GPR is expected to be improved by improving the LM and by involving other metrics.

P170] Influence of detector selection for VMAT patient QA on a gamma analysis result

Purpose The aim of this study was to investigate how the selection of detector for VMAT patient QA influences on a gamma analysis result. Methods Twenty RapidArc (10 prostate, 10 head&neck) plans were made in an Eclipse (ver. 10.0, AAA algorithm) treatment planning system (Varian Medical Systems) were measured with Delta 4PT, Delta 4+ (ScandiDos) and MatriXX MultiCube (IBA Dosimetry) detectors. Most prostate plans consisted of 2 full arc fields while some of the head&neck plans had only 2 half arc fields. The measurements were performed on a Varian Novalis Tx linear accelerator with use of a 6 MV energy. With one device we have measured the entire number of plans on a single day in order to avoid the error of the daily dose output variation and the daily phantom setup error. Before the start of the RA plan measurements we performed a warm up and measured the daily correction factor for the linear accelerator output as well as the temperature correction. Based on the measurements we have applied a phantom position correction to eliminate the error caused by alignment of the phantom to laser system. Gamma evaluations using the same global settings (DTA 3 mm, dose difference 3%, gamma 95% Results In total 41 fields were measured with one device. The biggest difference between the results of gamma analysis for different detectors was detected in a dose difference parameter. The Delta 4+ and Delta 4PT had an average difference between the dose difference parameter 2%, while the average differences regarding to MatriXX MultiCube were 7.5% and 9.5%. Conclusions Based on the results we can conclude that both Delta4 systems are more suitable for VMAT patient QA than the MatriXX MultiCube. The MatriXX is designed for verification where the device is perpendicular to radiation field. Nevertheless the decision of the appropriate equipment to be used for the patient QA is upon the medical physicist.

Patient-related QA for helical TomoTherapy with Delta4: analysis of the results

Abstract The biplanar diode arrays Delta4PT and Delta4+ has been used in our hospital since the introduction of the TomoTherapy in 2013 to ensure a good agreement between the calculated and the measured dose distributions in patient-related QA with helical TomoTherapy. The aim of this presentation is to evaluate the quality of the measurement procedure with the Delta4 phantoms Delta4PT and (since January 2016) Delta4+. This includes the influence of a cross calibration with a treatment plan with low modulation. Two analyses were performed: (i) All treatment plans in a period of three months (n=86) were not only calculated and measured with Delta4PT or Delta4+ but also with an ionization chamber (Exradin A1SL) in the homogeneous “cheese phantom”. (ii) All data measured from January 2016 to April 2017 (Delta4+, n=132) were analyzed regarding median dose deviation, Gamma analysis and others. The comparison with chamber measurements shows that all measurements with Delta4 and almost all with the ionization chambers (79 of 86) yield a deviation of measured vs. planned dose in the PTV of less than 2.5%, but with a lower variation of the Delta4 measurements. However, a strong correlation between both was not observed. The separate analysis of the measurements with the newer Delta4+ (since January 2016) shows a mean dose deviation in the PTVs of only 0.14% with a standard deviation (S.D.) of 0.69%. Before every measurement a cross calibration has been performed. Without this cross calibration, the deviation would be 0.96% with an increased standard deviation of 0.93%. It is concluded that the Delta4 systems are well suited for patient-related QA for helical TomoTherapy treatment plans. The comparison with chamber measurements shows a plausible accordance between both systems whereas the variation of single measurements is quite different. With the help of a daily cross calibration the variability of the Delta4 results is further decreased and the results show higher accuracy and reliability. According to our experience, a daily cross calibration is mandatory for a reliable patient-related QA.

TrueBeam Low Dose Rate Investigation for Pulsed Reduced Dose Rate IMRT

The capability of the TrueBeam treatment system to deliver step and shoot IMRT plans at low dose rates was evaluated. Beam characteristics during low dose rates (5 to 100 MU/min) were evaluated for consistency using a planar ion chamber array. MU constancy, linearity, and beam profile symmetry were all found to be equivalent within 0.5%. The response of the Scandi Dos Delta 4 system was also evaluated at low dose rates of using static open beams compared to ion chamber measurements, and step and shoot IMRT plans comparing 5 - 20 MU/min and 100 MU/min dose rates, with a maximum observed absolute dose difference of 0.8% and equivalence margin of 0.2%. The Gamma Index and measurement reproducibility were also found to be equivalent.

27. Intra cranial hypo-fractionated radiotherapy end-to-end QA

Introduction Intra cranial hypo-fractionated radiotherapy concerns small targets which need a complex delivery to optimize the dose distribution. Current techniques combine multiple arcs and intensity modulated beams. Such very precise irradiations need an efficient dosimetric validation. Many detectors enable the comparison between the calculation and the irradiation. They are variably efficient to get a real proof of a good match. We investigated dosimetric validations from 2D detector to 3D anthropomorphic configurations. Material and methods A patient head Dicom CT image files were sent to Rt-Safe company. They 3D-printed the head with a plastic material and filled the brain cavity with an MRI dosimetric gel. The head phantom was placed in the same patient position using the original immobilization devices. The patient plan was imported in the head phantom CT-data to be calculated and then to deliver the irradiation. An MRI acquisition of the head phantom was obtained with a specific configuration. The MRI Dicom file was used to provide a 3D measured dose distribution. The same plan was used with the portal dosimetry PDIP/Varian and the Delta4/Scandidos phantom. The three dosimetric systems provided profiles comparisons and gamma index evaluations. Results The measurements and analyses concern a very small target which PTV diameter is 1cm. The profiles comparisons show a perfect matching with the Delta4 system. The PDIP measurement is slightly compromised with the continuous detector position face to the beam. The gel dosimetry contains uncertainties linked to the complex process and particularly to the MRI acquisition. Delta4 and MRI-gel display respectively a percentage of gamma index passing 3%/3 mm tolerances, of 100% and 99.2%. Table 1 displays the results with different parameters in %, mm for a global Gamma index. Download : Download high-res image (79KB) Download : Download full-size image Conclusion The way to validate and secure an irradiation depends on its complexity. The PDIP is well adapted to a routine check. The delta 4 gives a high level of validation but cannot be irradiated with non-coplanar beams or arcs. Concerning the highest dose per session treatments it is relevant to rely on an end-to-end QA process based on an anthropomorphic phantom close to the real treatment conditions. The 3D-printed head and the gel dosimetry reproduce the exact patient conditions and give a fully satisfactory validation.

Comparison of three commercial dosimetric systems in detecting clinically significant VMAT delivery errors

AimTo study the sensitivity of three commercial dosimetric systems, Delta4, Multicube and Octavius4D, in detecting Volumetric Modulated Arc Therapy (VMAT) delivery errors. MethodsFourteen prostate and head and neck (H&N) VMAT plans were considered for this study. Three types of errors were introduced into the original plans: gantry angle independent and dependent MLC errors, and gantry angle dependent dose errors. The dose matrix measured by each detector system for the no-error and error introduced delivery were compared with the reference Treatment Planning System (TPS) calculated dose matrix for no-error plans using gamma (γ) analysis with 2%/2mm tolerance criteria. The ability of the detector system in identifying the minimum error in each scenario was assessed by analysing the gamma pass rates of no error delivery and error delivery using a Wilcoxon signed-rank test. The relative sensitivity of the system was assessed by determining the slope of the gamma pass line for studied error magnitude in each error scenario. ResultsIn the gantry angle independent and dependent MLC error scenario the Delta4, Multicube and Octavius4D systems detected a minimum 2mm error. In the gantry angle dependent dose error scenario all studied systems detected a minimum 3% and 2% error in prostate and H&N plans respectively. In the studied detector systems Multicube showed relatively less sensitivity to the errors in the majority of error scenarios. ConclusionThe studied systems identified the same magnitude of minimum errors in all considered error scenarios.

SU-F-T-586: Pre-Treatment QA of InCise2 MLC Plans On a Cyberknife-M6 Using the Delta4 System in SBRT

PURPOSE Performing pre-treatment quality assurance (QA) with the Delta4 system (ScandiDos Inc., Madison, WI) is well established for linac-based radiotherapy. This is not true when using a Cyberknife (Accuray Inc., Sunnyvale, CA) where, typically film-based QA is applied. The goal of this work was to test the feasibility to use the Delta4 system for pre-treatment QA for stereotactic body radiation therapy (SBRT) using a Cyberknife-M6 equipped with the InCise2 multileaf collimator (MLC). METHODS In order to perform measurements without accelerator pulse signal, the Tomotherapy option within the Delta4 software was used. Absolute calibration of the Delta4 phantom was performed using a 10×10 cm(2) field shaped by the InCise2 MLC of the Cyberknife-M6. Five fiducials were attached to the Delta4 phantom in order to be able to track the phantom before and during measurements. For eight SBRT treatment plans (two liver, two prostate, one lung, three bone metastases) additional verification plans were recalculated on the Delta4 phantom using MultiPlan. Dicom data was exported from MultiPlan and was adapted in order to be compatible with the Delta4 software. The measured and calculated dose distributions were compared using the gamma analysis of the Delta4 system. RESULTS All eight SBRT plans were successfully measured with the aid of the Delta4 system. In the mean, 98.0±1.9%, 95.8±4.1% and 88.40±11.4% of measured dose points passed the gamma analysis using a global dose deviation criterion of 3% (100% corresponds to the dose maximum) and a distance-to-agreement criterion of 3 mm, 2 mm and 1 mm, respectively, and a threshold of 20%. CONCLUSION Pre-treatment QA of SBRT plans using the Delta4 system on a Cyberknife-M6 is feasible. Measured dose distributions of SBRT plans showed clinically acceptable agreement with the corresponding calculated dose distributions.

Beam perturbation characteristics of a 2D transmission silicon diode array, Magic Plate.

The main objective of this study is to demonstrate the performance characteristics of the Magic Plate (MP) system when operated upstream of the patient in transmission mode (MPTM). The MPTM is an essential component of a real‐time QA system designed for operation during radiotherapy treatment. Of particular interest is a quantitative study into the influence of the MP on the radiation beam quality at several field sizes and linear accelerator potential differences. The impact is measured through beam perturbation effects such as changes in the skin dose and/or percentage depth dose (PDD) (both in and out of field). The MP was placed in the block tray of a Varian linac head operated at 6, 10 and 18 MV beam energy. To optimize the MPTM operational setup, two conditions were investigated and each setup was compared to the case where no MP is positioned in place (i.e., open field): (i) MPTM alone and (ii) MPTM with a thin passive contamination electron filter. The in‐field and out‐of‐field surface doses of a solid water phantom were investigated for both setups using a Markus plane parallel (Model N23343) and Attix parallel‐plate, MRI model 449 ionization chambers. In addition, the effect on the 2D dose distribution measured by the Delta4 QA system was also investigated. The transmission factor for both of these MPTM setups in the central axis was also investigated using a Farmer ionization chamber (Model 2571A) and an Attix ionization chamber. Measurements were performed for different irradiation field sizes of 5×5 cm2 and 10×10 cm2. The change in the surface dose relative to dmax was measured to be less than 0.5% for the 6 MV, 10 MV, and 18 MV energy beams. Transmission factors measured for both set ups (i & ii above) with 6 MV, 10 MV, and 18 MV at a depth of dmax and a depth of 10 cm were all within 1.6% of open field. The impact of both the bare MPTM and the MPTM with 1 mm buildup on 3D dose distribution in comparison to the open field investigated using the Delta4 system and both the MPTM versions passed standard clinical gamma analysis criteria. Two MPTM operational setups were studied and presented in this article. The results indicate that both versions may be suitable for the new real‐time megavoltage photon treatment delivery QA system under development. However, the bare MPTM appears to be slightly better suited of the two MP versions, as it minimally perturbs the radiation field and does not lead to any significant increase in skin dose to the patient.PACS number(s): 87.50.up, 87.53.Bn, 87.55.N, 87.55.Qr, 87.56.Fc.

Image-guided volumetric arc radiotherapy of pancreatic cancer with simultaneous integrated boost: Optimization strategies and dosimetric results

PurposeTo introduce volumetric modulated arc therapy treatments (VMAT) with simultaneous integrated boost (SIB) for pancreatic cancer and describe dosimetric results on a large patient series. Methods and materials45 patients with pancreatic malignancies were treated with 18 MV single-arc VMAT. Image guidance was performed with daily online kilo-volt cone-beam computed tomography (CBCT). The conformity index (CI) and homogeneity index (HI) to the target volumes, PTV45Gy and PTV54Gy, and dose–volume indices to OARs from the QUANTEC task group were reported. The risk of clinical nephritis was evaluated using normal tissue complication probability (NTCP). Treatments were verified in-phantom with the Delta4 system. ResultsAverage CI was 1.06 with 95% confidence intervals (95% CI) of 0.97–1.22 for PTV45Gy and 1.17 (0.66–1.61) for PTV54Gy. HI of PTV54Gy was 1.06 (1.04–1.10). OAR constraints were achieved in all patients, except for kidneys V12Gy of 48 (35.4–72.3)%. NTCP of the kidneys was 0.98 (0.6–1.7)%. Kidneys V12Gy and V20Gy were inversely related to PTV54Gy CI and maximum dose. All in-phantom tests had gamma pass rates exceeding 95% with global 3% dose difference and 3 mm distance to agreement. Patient shifts measured with CBCT had 95% CI of −0.8, +0.8 in the RL, −0.7, +0.8 in the SI, and −0.8, +0.7 cm in the AP directions. ConclusionsDosimetric results of VMAT were excellent on PTVs and organs at risk. The kidneys represent the dose-limiting organ at risk for this technique. NTCP indicates that this technique is safe from radiation-induced side effects to the kidneys.

The quantitative method to evaluate the plan complexity of volumetric-modulated arc therapy

Objective To study a new quantitative method–arc-beam modulation complexity score (AMCS) to evaluate the plan complexity of volumetric-modulated arc therapy (VMAT), and the effect of the six metrics on VMAT dosimetric verification. Methods The six metrics, which contain the average of beam area (ABA), the average of leaf travel (ALT), the average of beam width (ABW), the modulation complexity score (MCS), the leaf travel and modulation complexity score (LTMCS) and AMCS, was calculated to quantify the plan complexity of VMAT for 127 selected patients. Delta4 system was used for verifying the VMAT plans and the γ pass rate was calculated. The relativity between the six metrics and the pass rates or between the 6 parameters were analyzed with Pearson method. Results The average of ABA, ALT, ABW, MCS, LTMCS and AMCS were (73.5±24.1) cm2, (69.6±9.8) cm, (3.63±1.03) cm, 0.33±0.05, 0.14±0.04 and 0.16±0.05, respectively. A negative correlation was observed for ALT and the pass rate, and the other metrics had a positive correlation with it (P=0.000). The correlation between AMCS and the pass rate was the strongest with Pearson’s (r=0.637) in the six metrics. The percentage of arcs with a pass rate lower the 95% was 60.7 when AMCS is lower than the average, while it was 9.9% when AMCS is higher than the average. Both ABA and ABW had no significant correlation with ALT (P=0.720, 0.073), and the other metrics had significant correlations (P=0.000-0.003). Conclusions AMCS is a suitable quantitative metric to evaluate the plan complexity of VMAT, and could be used to forecast the result of dosimetric verification of VMAT plans. It could be also use to compare the multiple VMAT plans complexity and help us to choose the best plan. Key words: Volumetric-modulated arc therapy; γ passing rate; Arc-beam modulation complexity score

SU‐E‐J‐202: Performance of the Patient QA Systems in a Hybrid MRI‐Linac

Purpose:The first clinical MR‐linac will soon become operational, therefore patient plan QA procedures and equipment have to become MRcompatible. Reference dosimetry is affected by the magnetic field, however, relative dosimetry using patient QA systems haven't been investigated extensively. The purpose of this study was to examine the performance characteristics of the MR‐compatible ArcCHECK and Delta4 systems in a transverse 1.5 T magnetic field.Methods:Recently MR‐compatible versions of both ArcCheck (Sun Nuclear) and Delta4 (Scandidos) have been developed. To examine the performance characteristics within the magnetic field, the reproducibility, dose linearity, dose rate dependence, field size and angular dependence were evaluated on the MR‐linac (8 MV FFF beam, SAD of 142 cm) and a conventional linac (Elekta, 6MV). To allow comparison of the measurements with and without magnetic field, the measurement setup for the conventional linac is adapted to mimic the setup at the MR‐linac if possible (e.g. SAD, dose rate). The results from the MR‐linac were benchmarked to the results from the conventional linac as being the clinical reference.Results:At the moment of writing, measurements for the Delta4 are still running. Therefore, only the results of the ArcCheck are presented in this abstract. No significant difference was observed in the reproducibility of the ArcCheck (&lt;0.06%) between both linacs. The maximum dose response difference when measuring the dose linearity was less than 0.4% and the varying dose rate resulted in maximal dose differences of 1.0% for both linacs. Response variation for varying field sizes was &lt;2.6% at the conventional linac, and &lt;1.0% at the MR‐linac. Angular response was similar for both linacs.Conclusion:The reproducibility, dose linearity, dose rate dependence, field size and angular dependence of the MR compatible ArcCheck were not influenced by the presence of a transverse 1.5 T magnetic field. The results for the Delta4 are about to follow.Equipment is generously provided by Sun Nuclear Corporation (Melbourne, USA) and Scandidos (Uppsala, Sweden)

SU‐E‐T‐341: DVH‐Based Comparison of True 3D Measurements to a Delta4 System

Purpose:Delta4 dosimetric software can be used to calculate DVH‐based metrics for patient‐specific quality assurance from measurements made by a Delta4 QA device. This study investigates the effectiveness of a novel transform method that transposes measurements made with a full‐density 3D dosimeter onto patient anatomy, enabling the calculation of DVHs. This allows for DVH comparisons from the transformed dose distribution, which are based on true 3D measurements, to those from the Delta4 system, which are based on semi‐3D measurements and interpolation.Methods:A double‐arc VMAT treatment for a head‐and‐neck case was delivered to a 1kg PRESAGE 3D dosimeter inserted into a polyurethane head phantom. The dosimeter was readout using an in‐house optical‐CT scanner to gather full‐density 3D dosimetric data. The transform method is achieved by multiplication of the measured doses with a “transformation matrix” which accounts for heterogeneities and differences in geometry between the patient and the phantom. The transformation matrix is a voxel‐by‐voxel division of the patient planned dose by the phantom planned dose, both calculated in the treatment planning system (Eclipse). The transformed distribution was then overlaid on the patient CT data, enabling the calculation of DVHs. The same VMAT treatment was delivered to the Delta4 phantom and DVH data was calculated using its associated software.Results:The transformed dose distribution showed good agreement with calculated patient values, determined by similarity in dose profiles between the two distributions and a 3D gamma index passing rate of 94.87% for 3%/3mm criteria. For every structure contained within the dosimeter volume, the transformed DVHs demonstrated better agreement than the Delta4 DVHs, when compared to the values calculated in the treatment planning system.Conclusion:The coupled technique of full‐density 3D dose measurements and the presented transform method enables clinical patient‐specific quality assurance data that is more accurate than the semi‐3D Delta4 system.This work was supported by NIH R01CA100835.

A novel technique for VMAT QA with EPID in cine mode on a Varian TrueBeam linac

Volumetric modulated arc therapy (VMAT) is a relatively new treatment modality for dynamic photon radiation therapy. Pre-treatment quality assurance (QA) is necessary and many efforts have been made to apply electronic portal imaging device (EPID)-based IMRT QA methods to VMAT. It is important to verify the gantry rotation speed during delivery as this is a new variable that is also modulated in VMAT. In this paper, we present a new technique to perform VMAT QA using an EPID. The method utilizes EPID cine mode and was tested on Varian TrueBeam in research mode. The cine images were acquired during delivery and converted to dose matrices after profile correction and dose calibration. A sub-arc corresponding to each cine image was extracted from the original plan and its portal image prediction was calculated. Several analyses were performed including 3D γ analysis (2D images + gantry angle axis), 2D γ analysis, and other statistical analyses. The method was applied to 21 VMAT photon plans of 3 photon energies. The accuracy of the cine image information was investigated. Furthermore, this method's sensitivity to machine delivery errors was studied. The pass rate (92.8 ± 1.4%) for 3D γ analysis was comparable to those from Delta4 system (99.9 ± 0.1%) under similar criteria (3%, 3 mm, 5% threshold and 2° angle to agreement) at 6 MV. The recorded gantry angle and start/stop MUs were found to have sufficient accuracy for clinical QA. Machine delivery errors can be detected through combined analyses of 3D γ, gantry angle, and percentage dose difference. In summary, we have developed and validated a QA technique that can simultaneously verify the gantry angle and delivered MLC fluence for VMAT treatment.This technique is efficient and its accuracy is comparable to other QA methods.

Evaluation of the sensitivity of two 3D diode array dosimetry systems to setup error for quality assurance (QA) of volumetric‐modulated arc therapy (VMAT)

The purpose of this study is to evaluate the sensitivities of 3D diode arrays to setup error for patient‐specific quality assurance (QA) of volumetric‐modulated arc therapy (VMAT). Translational setup errors of ±1,±2, and ±3 mm in the RL, SI, and AP directions and rotational setup errors of ±1° and ±2° in the pitch, roll, and yaw directions were set up in two phantom systems, ArcCHECK and Delta4, with VMAT plans for 11 patients. Cone‐beam computed tomography (CBCT) followed by automatic correction using a HexaPOD 6D treatment couch ensured the position accuracy. Dose distributions of the two phantoms were compared in order to evaluate the agreement between calculated and measured values by using γ analysis with 3%/3 mm, 3%/2 mm, and 2%/2 mm criteria. To determine the impact on setup error for VMAT QA, we evaluated the sensitivity of results acquired by both 3D diode array systems to setup errors in translation and rotation. For the VMAT QA of all patients, the pass rate with the 3%/3 mm criteria exceeded 95% using either phantom. For setup errors of 3 mm and 2°, respectively, the pass rates with the 3%/3 mm criteria decreased by a maximum of 14.0% and 23.5% using ArcCHECK, and 14.4% and 5.0% using Delta4. Both systems are sensitive to setup error, and do not have mechanisms to account for setup errors in the software. The sensitivity of both VMAT QA systems was strongly dependent on the patient‐specific plan. The sensitivity of ArcCHECK to the rotational error was higher than that of Delta4. In order to achieve less than 3% mean pass rate reduction of VMAT plan QA with the 3%/3 mm criteria, a setup accuracy of 2 mm/1° and 2 mm/2° is required for ArcCheck and Delta4 devices, respectively. The cumulative effect of the combined 2 mm translational and 1° rotational errors caused 3.8% and 2.4% mean pass rates reduction with 3%/3 mm criteria, respectively, for ArcCHECK and Delta4 systems. For QA of VMAT plans for nasopharyngeal cancer (NPC) using the ArcCHECK system, the setup should be more accurate.PACS numbers: 87.55.ne, 87.55.Qr, 87.55.km

SU‐E‐T‐401: On the Use of Onboard Portal Dosimetry for Patient‐Specific QA of RapidArc Plans

Purpose: The goal of this project is to conduct a systematic investigation on the utility of an onboard portal dosimetry (PD) system for patient‐specific QA of RapidArc plans. This abstract reports the preliminary results of this ongoing investigation. Methods: The PD system of Varian Medical Systems has been used in many centers as a convenient and viable tool for patient‐specific QA of fixed‐beam IMRT plans. Because the onboard portal imager rotates with the x‐ray source, portal dose obtained at each pixel from a complete RapidArc delivery is a sum of portal doses delivered at different gantry angles of the arc. As a Result, the sensitivity of PD in detecting deviations of photon fluence delivered at different gantry angles is inherently low. We investigated the potential sensitivity loss and its implications for patient‐specific QA by 1) dividing the arc into multiple smaller arc‐segments, 2) analyzing the dose concordance as a function of arc‐segments, and 3) comparing the concordance of PD QA results with those obtained from 3D dose verification using the Delta4 system. Results: Preliminary studies were conducted using RapidArc plans for prostate and head &amp; neck cases. For each case, three verification plans (using single arc (300°), five 60° arc‐segments and ten 30° arc‐segments) were examined. All PD QA plans passed the QA criteria based on the gamma index. Detailed comparison of portal‐°dose profiles indicated general concordance between the measured and predicted, with increased local variations for plans with smaller arc segments. The QA results using PD for all patients were concordant with that obtained by Delta4 based on 3D dose comparison. Conclusion: Although the preliminary results shown concordant QA results between PD and Delta4, this does not constitute a sufficient test of PD for RapidArc QA. Further investigation is ongoing and updated results will be reported in the meeting.

Sensitivity test of a 3D diode array system for MLC and GANTRY errors in IMRT delivery

Introduction Delta4 device is designed for three-dimensional dose verification and shows excellent reproducibility, linearity and doserate independence. The purpose of this work was to verify the sensitivity of the Delta4 (ScandiDos) for small MLC and gantry errors in intensity modulated radiotherapy (IMRT). Materials and methods Ten IMRT plans (5 prostate and 5 head and neck) were used. Systematic MLC errors of (–0.2), (–0.5), (–0.7), (–1), (–2), (–3), (–5) and (–10) mm, were introduced for one leaf. Gantry errors of 0.2°, 0.5°, 0.7°, 1°, 2°, 2.5°, 3°, 4° and 5° were introduced for the same plans. The Delta4 system was used to acquire and analyze the data. All the measurements were acquired consecutively for each patient. To eliminate the repeatability errors, standard deviations (SD) of the maximum gamma (Gmax) were calculated for 10 successively beams with no offsets. Gmax values were noted for both global gamma (normalization dose) and local gamma (local detector dose) with pass/fail criteria of 2%/2 mm. Gmax was evaluated for each modified field when compared to the measured field with no introduced error (0 mm or 0°). Because we had a single leaf with a small offset we evaluated only the Gmax and not the mean gamma or the percentage of area gamma Results For local gamma evaluation (LGE), the maximum SD for Gmax was of 0.06. For global gamma evaluation (GGE), the maximum SD was of 0.02. The MLC and gantry offsets had a noticeable impact on the plan evaluation. For all the ten plans modifications in Gmax were observed starting with 0.2 mm (from 0.2 to 4.75 for LGE and from 0.13 to 2.45 for GGE) for MLC plans and with 0.2° for the gantry errors (from 0.54 to 9.23 for LGE and from 0.37 to 4.71 for GGE). Conclusion The Delta4 system seems to be a very sensitive instrument for the quality assurance of IMRT plans. In our study it detected submilimetric and subdegree errors for MLC and gantry, respectively. In clinical use these small errors may not be detected and could be hidden by the residual error between planned and delivered dose.

Should we perform the quality assurance for the stereotactic planning of prostate cancer in the treatment condition configuration? An evaluation with a cylindrical diode array phantom

Introduction The Delta4 system (ScandiDos) is dedicated for quality assurance (QA) of treatment delivery and is an independent-unit from the accelerator. The purpose of this work was to evaluate the impact of the irradiation geometry on the QA for prostate SBRT using the Delta4 device. Materials and methods The prostate and the tumor were delineated by a radiologist on CT/MRI registrations of 5 patients. As part of a protocol, a 9-coplanar fields IMRT plan was optimized following a schema of prostate SBRT (5 × 6.5 Gy) with a simultaneous integrated boost into the tumor (5 × 8Gy). All the beams were verified four times consecutively using Delta4, with the gantry at: (a) 0° (Plan B0), (b) 90° (B90), (c) 315° (B315) and (d) at their original angulations (242°, 270°, 297°, 328°, 0°, 38°, 76°, 101°, 127°), named BFbyF (field-by-field). The plans B90, B315 and BFbyF were compared with B0 using two evaluation indexes (global gamma index (GGI) and local gamma index (LGI)) as differences of: percentage of detectors with gamma Results For GGI, the average differences ± SD in ‘‘%G Conclusion Usually the IMRT plan verification is made using all beams with gantry at 0_. However, it is important to verify the irradiation plans in the exact situation as at the time of treatment. We have showed here that there is an important difference in the evaluation of a stereotactic IMRT plan as it depends of the irradiation condition. These differences could be explained by the impact of the MLC weight and the couch attenuation. Therefore, it can be recommended that the IMRT verification is made with a detector independent from the accelerator and in the exact configuration of the irradiation as at the time of the treatment (gantry angle and couch position).

SU-E-T-44: Phantom 3D Dose Calculation and Anatomy Based DVH Evaluation On VMAT Patient QA Using the Newest Version of Delta4 Dosimetry System

Purpose: To validate the two new patient QA methods on ScandiDos Delta4 system: (1) Phantom 3D dose calculation (2) Anatomy DVH evaluation, compared with conventional gamma analysis criteria. Methods: Phantom 3D dose calculation and anatomy DVHs evaluation have the capability to perform gamma analysis on critical organ contours and calculate DVHs from Delta4 measurement and calculation on either phantom or patient anatomy. The calculation speed, performance time and algorithm accuracy were investigated on these two novel methods. Elekta Synergy beam PDDs and output factors were characterized by importing plans from Pinnacle (v9.2). VMAT H&N QA plans were performed. The patient CT images and contours were transferred from Pinnacle to Delta4. Three critical structures (PTV, Brainstem and Cord) were analyzed using gamma analysis, phantom 3D dose calculation and anatomy DVH evaluation. Results: Conventional gamma pass rate is analyzed by combining the dose difference and distance to agreement on each detector. 3D dose calculation is implemented on the homogeneous phantom using ray-tracing algorithm. Patient anatomy dose is calculated using the fluence map and kernel-based pencil beam algorithm. The average calculation time is 1.1, 5.3 and 25.5 minutes for each method. The performance time is 22, 30, and 55 minutes for each patient QA, since anatomy evaluation needs CT images and anatomy structures. Regarding the algorithm accuracy, conventional gamma analysis and phantom based PTV 3D-dose are correlated. The passing rate difference are insignificant (P>0.2, t test). There are discrepancies when the phantom 3D-Dose gamma and DVH are compared with anatomy based evaluation. The passing rate difference are significant (P<0.005, t test). Conclusion: Conventional gamma analysis has weak correlation to critical DVH errors. Phantom 3D dose and anatomy DVH evaluation on Delta4 are sensitive and specific for VMAT patient QA. Further investigation on accuracy and potential errors of anatomy DVH will be performed.