ArcCHECK Device Research Articles

Unbiased evaluation of predicted gamma passing rate by an event-mixing technique.

Predicting models of the gamma passing rate (GPR) have been studied to substitute the measurement-based gamma analysis. Since these studies used data from different radiotherapy systems comprising TPS, linear accelerator, and detector array, it has been difficult to compare the performances of the predicting models among institutions with different radiotherapy systems. We aimed to develop unbiased scoring methods to evaluate the performance of the models predicting the GPR, by introducing both best and worst limits for the performance of the GPR prediction. Two hundred head-and-neck VMAT plans were used to develop a framework. The GPRs were measured using the ArcCHECK device. The predicted GPR [p] was generated using a deep learning-based model [pDL ]. The predicting model was evaluated using four metrics: standard deviation (SD) [σ], Pearson's correlation coefficient (CC) [r], mean squared error (MSE) [s], and mean absolute error (MAE) [a]. The best limit [ , , , and ] was estimated by measuring the SD of measured GPR [m] by shifting the device along the longitudinal direction to measure different sampling points. Mimicked best and worst p's [pbest and pworst ] were generated from pDL . The worst limit was defined such that m and p have no correlation [CC ∼ 0]. The worst limit [σMix , rMix , sMix , and aMix ] was generated using the event-mixing (EM) technique originally introduced in high-energy physics experiments. The range of σ, r, s, and a was defined to be , , , and . The achievement score (AS) independently based on σ, r, s, and a were calculated for pDL , pbest and pworst . The probability that p fails the gamma analysis (alert frequency; AF) was estimated as a function of values within the [ , σMix ] range for the 3%/2mm data with a 95% criterion. SDs of the best limit were well reproduced by . The EM technique successfully generated the pairs with no correlation. The AS using four metrics showed good agreement. This agreement indicates successful definitions of both best and worst limits, consistent definitions of the AS, and successful generations of mixed events. The AF for the DL-based model with the 3%/2mm tolerance was 31.5% and 63.0% with CL's 99% and 99.9%, respectively. We developed the AS to evaluate the predicting model of the GPR in an unbiased manner by excluding the effects of the precision of the radiotherapy system and the spreading of the GPR. The best and worst limits of the GPR prediction were successfully generated using the measured precision of the GPR and the EM technique, respectively. The AS and are expected to enable objective evaluation of the predicting model and setting exact achievement goal of precision for the predicted GPR.

Decay Series Dose Delivery Validation of a Biology-Guided Radiotherapy (BgRT) Methodology

In this work, we compared the accuracy between the planned dose estimated by a commercial BgRT treatment planning system to the measured delivered dose by the BgRT treatment delivery device. The dose delivered was measured with the Sun Nuclear ArcCHECK device containing a customized phantom filled with fluorodeoxyglucose F18 (FDG) with a fixed target to background activity concentration ratio. The BgRT approach was evaluated for its capacity to accurately deliver a planned dose to a phantom target of an increasingly limited PET signal due to radioactive decay. The Sun Nuclear ArcCHECK is a helical detector assembly of 1,386 diodes around a 15 cm diameter cavity designed for quality assurance of linear accelerator (LINAC) driven therapies. In this work, the ArcCHECK was loaded with a cylindrical phantom containing a 22 mm diameter homogenous ball and a C-shape insert. The ball target was designated as an organ at risk (OAR) with appropriate dose constraints applied, and the C-shaped insert was designated as the target. The C-shaped target was used to simulate a tumor with a necrotic core. A cylindrical planning/gross tumor volume was placed around the C-shape, providing a test case of delivery to a partially PET-avid target. The target and OAR were filled with 58.46 kBq/mL of FDG and the background with 7.30 kBq/mL, giving an approximate target to background ratio of 8:1. A kVCT localization scan, a short PET pre-treatment scan, and a LINAC treatment sequence (1000 cGy per fraction) were performed each run with four runs performed over a duration of ∼3.5 hours (1.9 half-lives). The ArcCHECK measured the delivered dose during each LINAC treatment sequence and compared it to the plan predicted dose. The relative dose (RD) and absolute dose (AD) gamma values were then calculated for each run using Sun Nuclear's proprietary software with a gamma pass rate criterion of 3mm/3%. The PET scans for runs 1-4 were completed with background activity concentrations of 5.49 kBq/ml, 3.95 kBq/ml, 2.79 kBq/ml, and 2.02 kBq/ml, respectively. The scans further reported 17.48 kBq/ml, 12.26 kBq/ml, 7.93 kBq/ml, and 6.95 kBq/ml as the mean activity concentrations for the cylindrical gross tumor volume of the planned treatment. After treatment delivery, the resulting RD gamma values were 98%, 94%, 93%, and 95% and AD gamma values were 98%, 93%, 92%, and 94% for runs 1-4. Results from this study demonstrated treatment delivery stability with consistent repeatability in the 8:1 target to background contrast condition even with diminishing PET signal from the phantom target as the activity decayed. This work shows that BgRT is capable of delivering to a cylindrical target volume that is very different from the PET avid C-shaped that was used for the plan and delivery.

Evaluating the effect of high-density measurement mode on patient-specific quality assurance for head and neck cancer with ArcCHECK.

The high-density measurement (HDm) mode of the ArcCHECK device can achieve a twofold resolution enhancement compared to the standard measurement (Sm) mode. The aim of this study was to evaluate the effect of HDm on the gamma passing rate (GPR) for the patient-specific quality assurance (PSQA) in head and neck cancer. We retrospectively evaluated 30 patients who underwent volumetric modulated arc therapy (VMAT) for head and neck cancer. Absolute gamma analysis was performed on Sm and HDm data. We also investigated correlations between the modulation complexity score for VMAT (MCSv) and differences in the GPR between the two measurement modes. The global GPR of Sm and HDm was 81.0% ± 8.4% and 82.6% ± 7.6% for the 2%/2 mm criterion, 94.0% ± 4.1% and 94.9% ± 3.6% for the 3%/2 mm criterion, and 96.6% ± 2.4% and 97.0% ± 2.4% for the 3%/3 mm criterion, respectively. HDm slightly improved GPR (p < 0.01) for the 2%/2 mm criterion. Differences in GPR between Sm and HDm for the 2%/2 mm, 3%/2 mm, and 3%/3 mm criteria were 1.6% ± 3.0%, 0.8% ± 2.0%, and 0.4% ± 1.2%, respectively. No correlation was identified between the MCSv and the difference in GPR between Sm and HDm. Despite an improvement in GPR with HDm, the difference in GPR between Sm and HDm was approximately 2% even when the tighter criteria were used. Moreover, the change in the GPR between Sm and HDm did not depend on plan complexity. Thus, the effect of HDm on GPR is limited for the PSQA in VMAT for head and neck cancer.

Impact of respiratory motion on lung dose during total marrow irradiation.

We evaluated the impact of respiratory motion on the lung dose during linac-based intensity-modulated total marrow irradiation (IMTMI) using two different approaches: (1) measurement of doses within the lungs of an anthropomorphic phantom using thermoluminescent detectors (TLDs) and (2) treatment delivery measurements using ArcCHECK where gamma passing rates (GPRs) and the mean lung doses were calculated and compared with and without motion. In the first approach, respiratory motions were simulated using a programmable motion platform by using typical published peak-to-peak motion amplitudes of 5, 8, and 12mm in the craniocaudal (CC) direction, denoted here as M1, M2, and M3, respectively, with 2mm in both anteroposterior (AP) and lateral (LAT) directions. TLDs were placed in five selected locations in the lungs of a RANDO phantom. Average TLD measurements obtained with motion were normalized to those obtained with static phantom delivery. The mean dose ratios were 1.01 (0.98-1.03), 1.04 (1.01-1.09), and 1.08 (1.04-1.12) for respiratory motions M1, M2, and M3, respectively. To determine the impact of directional respiratory motion, we repeated the experiment with 5-, 8-, and 12-mm motion in the CC direction only. The differences in average TLD doses were less than 1% when compared with the M1, M2, and M3 motions indicating a minimal impact from CC motion on lung dose during IMTMI. In the second experimental approach, we evaluated extreme respiratory motion 15 mm excursion in only the CC direction. We placed an ArcCHECK device on a commercial motion platform and delivered the clinical IMTMI plans of five patients. We compared, with and without motion, the dose volume histograms (DVHs) and mean lung dose calculated with the ArcCHECK-3DVH tool as well as GPR with 3%, 5%, and 10% dose agreements and a 3-mm constant distance to agreement (DTA). GPR differed by 11.1 ± 2.1%, 3.8 ± 1.5%, and 0.1 ± 0.2% with dose agreement criteria of 3%, 5%, and 10%, respectively. This indicates that respiratory motion impacts dose distribution in small and isolated parts of the lungs. More importantly, the impact of respiratory motion on the mean lung dose, a critical indicator for toxicity in IMTMI, was not statistically significant (p > 0.05) based on the Student's t-test. We conclude that most patients treated with IMTMI will have negligible dose uncertainty due to respiratory motion. This is particularly reassuring as lung toxicity is the main concern for future IMTMI dose escalation studies.

IMRT QA and gamma comparisons: The impact of detector geometry, spatial sampling, and delivery technique on gamma comparison sensitivity.

To separately quantify sensitivity differences in patient-specific quality assurance comparisons analyzed with the gamma comparison for different measurement geometries, spatial samplings, and delivery techniques [intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT)]. Error-free calculations for 20 IMRT and 20 VMAT cases were compared to calculations with known induced errors of varying magnitudes, using gamma comparisons. Five error types (MU scaling, three different MLC errors, and collimator errors) were induced in plan calculations on three different detector geometries - ArcCHECK, MapCHECK, and Delta 4. To study detector geometry sensitivity effects alone, gamma comparisons were made with 1mm error-free calculations compared to 1mm error-induced calculations for each device. Effects of spatial sampling were studied by making the same gamma comparisons, but down-sampling the error-induced calculations to the real spatial sampling of each device. Additionally, 1mm vs 1mm comparisons between the IMRT and VMAT cases were compared to investigate sensitivity differences between IMRT and VMAT using IMRT and VMAT cohorts with similar ranges of plan complexity and average aperture size. For each case, induced error type, and device, five different gamma criteria were studied to ensure sensitivity differences between devices, spatial sampling scenarios, and delivery technique were not gamma criterion specific, resulting in over 36,000 gamma comparisons. For IMRT cases, Delta4 and MapCHECK devices had similar error sensitivities for lagging leaf, bank shift, and MU errors, while the ArcCHECK had considerably lower sensitivity than the planar-type devices. For collimator errors and perturbational leaf errors the ArcCHECK had higher error sensitivity than planar-type devices. This behavior was independent of gamma parameters (percent dose difference, distance-to-agreement, and low dose threshold), though use of local normalization resulted in error sensitivites that were markedly similar between all three devices. Differences between detector geometries were less pronounced for VMAT deliveries. Error sensitivity for a given gamma criterion when comparing IMRT and VMAT deliveries on the same devices showed that VMAT plans were more sensitive to some specific error types and less sensitive to others, when compared to IMRT plans. For the ArcCHECK device, the sensitivity of IMRT and VMAT cases was quite similar, whereas this was not the case for the planar-type devices. When comparing error sensitivity between 1mm vs 1mm calculations and 1mm vs the real spatial sampling for each device, results showed that increased spatial sampling did not systematically increase error sensitivity. Noticeable differences in error sensitivity were observed for different detector geometries, but differences were dependent on induced error type, and a particular device geometry did not offer universal improvements in error sensitivity across studied error types. This study demonstrates that the sensitivity of the gamma comparison does not largely hinge on detector spatial sampling. VMAT deliveries were generally less sensitive to errors when compared to IMRT plans for the planar-type devices, while similar sensitivities were observed between delivery techniques for the ArcCHECK device. Results of this work suggest that a universal gamma criterion is inappropriate for IMRT QA and that the percent pixels passing is an insufficient metric for evaluating quality assurance checks in the clinic.

Assessment of the Sun Nuclear ArcCHECK to detect errors in 6MV FFF VMAT delivery of brain SABR using ROC analysis.

Institutions use a range of different detector systems for patient‐specific quality assurance (QA) measurements conducted to assure that the dose delivered by a patient’s radiotherapy treatment plan matches the calculated dose distribution. However, the ability of different detectors to detect errors from different sources is often unreported. This study contains a systematic evaluation of Sun Nuclear’s ArcCHECK in terms of the detectability of potential machine‐related treatment errors. The five investigated sources of error were multileaf collimator (MLC) leaf positions, gantry angle, collimator angle, jaw positions, and dose output. The study encompassed the clinical treatment plans of 29 brain cancer patients who received stereotactic ablative radiotherapy (SABR). Six error magnitudes were investigated per source of error. In addition, the Eclipse AAA beam model dosimetric leaf gap (DLG) parameter was varied with four error magnitudes. Error detectability was determined based on the area under the receiver operating characteristic (ROC) curve (AUC). Detectability of DLG errors was good or excellent (AUC >0.8) at an error magnitude of at least ±0.4 mm, while MLC leaf position and gantry angle errors reached good or excellent detectability at error magnitudes of at least 1.0 mm and 0.6°, respectively. Ideal thresholds, that is, gamma passing rates, to maximize sensitivity and specificity ranged from 79.1% to 98.7%. The detectability of collimator angle, jaw position, and dose output errors was poor for all investigated error magnitudes, with an AUC between 0.5 and 0.6. The ArcCHECK device’s ability to detect errors from treatment machine‐related sources was evaluated, and ideal gamma passing rate thresholds were determined for each source of error. The ArcCHECK was able to detect errors in DLG value, MLC leaf positions, and gantry angle. The ArcCHECK was unable to detect the studied errors in collimator angle, jaw positions, and dose output.

P219] Analysis of the dose-rate dependence of the ArcCHECK device

Purpose The ArcCHECK device (Sun Nuclear) is a diode array designed to verify IMRT and VMAT treatments. During its commissioning, a dose-rate dependence higher than the specified in the manual (±1% over the range 50–1400 cGy/min) was observed. This may be relevant in VMAT verifications, as the dose-rate varies. Therefore, some tests were performed to study the dose-rate dependence of the ArcCHECK device and to estimate its relevance in VMAT verifications. Methods The ArcCHECK device was irradiated using beams of 6 MV provided by an Elekta Infinity LINAC. The measures were corrected with the measurements of a cc13 ionization chamber (Iba). To determine the dose-rate dependence, the device was irradiated with a sequence of beams of a field size of 5 × 25 cm varying its gantry angle by 4° until the whole device is covered, receiving all the detectors the same dose. Dose-rates over a range of 50–600 MU/min were employed. Then, the measurements of each diode were fitted with the dose-rates by an adjustment function. To study the influence of having different dose-rates in the same treatment, the device was divided in two sectors, one of 280° irradiated with a dose-rate of 74 MU/min and another of 80° with 597 MU/min. The response was compared with the value predicted by the adjustment function. Results It was found a higher response of the system as the dose-rate increased, having a difference of 2.4 ± 0.7% (k = 2) between the responses for 50 and 600 MU/min. The measurements was adjusted by a function in the form: D = a · dD dt b + dD dt c D is the response of the device, dD dt the dose-rate and a, b and c parameters. The best fit is achieved with these values: a = 97.36 ( 96 ) , b = 0.7 ( 2 ) , and c = 0.9996 ( 2 ) , with χ 2 / v = 0.25 . The measurement in the second test was 0.7 ± 0.4% (k = 2) lower than if the irradiation were entirely at 600MU/min, indicating that is necessary to correct the values in VMAT verifications. Conclusions The dose-rate dependence of the ArcCHECK device is higher than the specified and the measures must be corrected when performing VMAT verifications. Further tests using real VMAT treatments shall be done to do a better estimation of the influence of this dependence.

P254] Local gamma index analysis - new approach

Purpose The gamma index analysis is a well-known approach in radiotherapy. The weakest point of widely accepted gamma index method seems to be choosing global approach. Choosing one normalization value for all points in the plan can hide potential errors, especially in its high-gradient, medium and low dose areas. In this study, the new metrics based on the local gamma analysis were introduced. Methods All analyses were performed for the TG 119 IMRT tests. The suite structures were copied to the CT scan of I’mRT Phantom (Scanditronix). Plans were prepared according to AAPM TG 119 report. All measurements were performed using Cliniac 2300C/D (Varian). The recommended point dose and plan verification by EPID were performed to confirm compliance of plans with requirements. Additional measurments with the use of ArcCHECK (SunNuclear) device and dedicated software were caried out. To perform proposed local gamma analysis the planned dose file created by TPS and the measured dose file generated by SNC software were used. Three forms of metrics were used: gamma index metric by the definition, metric proposed by Van Dyk and dose difference metric. The reference tolerance value of distance-to-agreement was set globally to 3 mm. We propose two local approaches to gamma analysis: a physics-based local approach which explicitly accounts for the physics underlying the radiation therapy process (dose value measured in local point is a random variable drawn from a Poisson distribution characterized by an expected value equal to planned dose D and standard deviation proportional to D0.5) and a clustering-based approach, which is based on clustering of the dose values read by the detectors. In the latter case two clustering methods were implemented: K-means clustering of detector readings, and Gaussian mixture clustering of detector readings. Results The planning objectives for TG 119 IMRT tests were generally met. Using proposed local approaches we were able to successfully carry out local gamma analysis of plans. We compared advantages and disadvantages of methods based on standard global and proposed local approach. Conclusions Proposed solutions have the potential to improve QA practice referring to the agreement between calculated and measured dose distributions.

Application of semiconductor matrix in synchronization test in helical tomotherapy

Objective To investigate how to use the ArcCHECK to complete synchronization test in helical tomotherapy (HT). Methods According to the synchronization test suggested by AAPM TG148, three aspects should be tested.(1) Gantry angle consistency: ArcCHECK device in the long axis was perpendicular to the rotation plane using virtual isocenter positioning. A delivery sequence was defined with a slice width of 2.5 cm, a pitch of 0.1, and a minimum of 40 rotations. The control sinogram was set to open the middle two leaves (No. 32 and 33) at projections centered at 0°, 120°, and 240°.(2) Couch speed uniformity: The ArcCHECK was fixed on the treatment couch by positioning the long axis in the movement direction of couch. An irradiation was done with a static gantry in the 0° position, the collimation set to 1 cm, and all MLC leaves open. A couch travel distance of 20 cm was programmed using a clinically common couch speed (0.3-0.5 mm/s for 2.5 cm treatment slice width) in the course of irradiation.(3) Synchronization of couch translation and gantry rotation: In this test, a rotational irradiation was used with the nominal 1.0 cm beam and a pitch of 1 for 13 rotations. The control sonogram was set to open all the leaves for a half rotation on the second, seventh, and twelfth rotations. The initial collected data were used as the baseline data of parameters. The subsequent measurements were compared with the baseline data using the point to point absolute dose pass rates of DD (dose difference, 1%). Results The gantry angle consistency could check the correct initial angles at the beginning of the treatment and the ability to reproduce the projection every rotation. There were six parallel and straight high-dose regions in ArcCHECK software. In the couch speed uniformity test, the relative dose in the long axis in the high-dose regions was changed less than 2%. In the synchronization test of couch translation and gantry rotation, three parallel high-dose regions with the same separation spacing were present in the ArcCHECK software. The synchronization tests were done twenty-eight times in the second, seventh, and twelfth irradiation rotations and the average pass rates were 93.2%±1.5%, 93.7%±1.1%, and 93.5%±1.3%, respectively. Conclusions The ArcCHECK device is a suitable alternative method for performing the synchronization test in HT due to its lower workload and costs compared with the film. Key words: Helical tomotherapy; Synchronicity test; Semiconductor matrix

31. Impact of Pinnacle model calculation on the dynamic arc-therapy dose delivered by a linac, and the need to cross-check control tools

Introduction Volumetric modulated arc-therapy (VMAT) is a fast treatment technique allowing the delivery of good quality radiotherapy. TPS are used for treatment planning and need to perform precise calculations. It is thus important to affine and validate the models created before starting treatments. Our study demonstrates the need to do quality assurance adapted to the model. It is necessary to cross-check the measurements from at least two detectors before proceeding to pretreatment controls. Methods Photon beams of 6MV, delivered by an Elekta Synergy linac equipped with an Agility multi-leaf collimator, were modeled in Pinnacle TPS V9.8. Two models were created and compared: M1 and M2. M1 focuses on border and out-of-field parameters while M2 focuses on output factors agreement between models and measurements. Both models were evaluated using several tests found in the literature used as quality assurance. On the other hand, 20 treatment cases were controlled with the ArcCHECK device, the PTW TM31010 ionization chamber as well as EBT3 gafchromic films. Results M1 exhibits good agreement with measurements when performing tests from the literature, while M2 performs poorly in the “strip pattern” test. However, M1 calculation/measurement ratio goes up to 4.0% when comparing the dose in atypical fields, while M2 ratio stays below 1.3%. ArcCHECK gamma-test pass rates are similar between both models with 98.2% and 97.6% for M1 and M2, respectively (3%, 3 mm, local). Single point calculations differences with ionization chamber measurements are 2.5% and 1.0% in average, and reach a maximum of 3.6% and 2.0% for the M1 and M2 models, respectively. Gafchromic films analysis confirm the significant differences found with the ionization chamber for the M1 model and exhibit much better agreement for the M2 model. Conclusions The results demonstrate the importance to cross-check control tools. The significant gaps found with the M1 model were neither detected with quality assurance tests, neither with ArcCHECK measurements. It was necessary to implement home-made ionization chamber and gafchromic films test. This defect is preponderant with respect to the dose delivered on the target volume but is not prevalent in the zones of low doses where the diodes of the ArcCHECK are located.

Evaluation and Performance of ArcCheck and Film using Gamma Criteria in Pre-treatment Quality Assurance of Stereotactic Ablative Radiotherapy.

Aim:The aim of this study is to assess the use of ArcCHECK (AC) as an alternative method to replace film dosimetry for pre-treatment quality assurance (QA) of three-dimensional conformal radiation therapy, intensity-modulated radiation therapy (IMRT), and volumetric-modulated arc therapy (VMAT) stereotactic ablative radiotherapy (SABR) treatment plans.Materials and Methods:Twenty-five patients with a varied diagnosis of lung, spine, sacrum, sternum, ribs, scapula, and femur undergoing SABR were selected for this study. Pre-treatment QA was performed for all the patients using ionization chamber and film dosimetry. Measurements were also carried out on an AC phantom. The planned and measured doses from the AC device and EBT3 films were compared using four different gamma criteria: 2%/2 mm, 3%/2 mm, 3%/1 mm, and 3%/3 mm.Results:The mean gamma passing rates at 3%/3 mm for all non-spine SABR cases were 98.79 ± 0.96 and 99.27 ± 1.03 with AC and films, respectively. The mean passing rates at 3%/2 mm for AC and films were 98.76 ± 0.42 and 99.43 ± 0.27 respectively for spine VMAT SABR, and 87.15 ± 2.45 and 99.79 ± 0.14 respectively for spine IMRT SABR. In the case of spine tumors, the gamma criterion was tightened due to the proximity of spinal cord to the planning target volume. Our results show that AC provides good results for all VMAT SABR plans.Conclusion:The AC results at 3%/3 mm were in good agreement with film dosimetry for most cases. We observed a significant reduction in QA time on using AC for SABR QA. This study showed that AC results are comparable to film dosimetry for all studied sites except for spine IMRT SABR.

Initial clinical experience with ArcCHECK for IMRT/VMAT QA.

Many devices designed for the purpose of performing patient‐specific IMRT/VMAT QA are commercially available. In this work we report our experience and initial clinical results with the ArcCHECK. The ArcCHECK consists of a cylindrical array of diode detectors measuring entry and exit doses. The measured result is a cumulative dose displayed as a 2D matrix. The detector array requires both an absolute dose calibration, and a calibration of the detector response, relative to each other. In addition to the calibrations suggested by the manufacturer, various tests were performed in order to assess its stability and performance prior to clinical introduction. Tests of uniformity, linearity, and repetition rate dependence of the detector response were conducted and described in this work. Following initial testing, the ArcCHECK device was introduced in the clinic for routine patient‐specific IMRT QA. The clinical results from one year of use were collected and analyzed. The gamma pass rates at the 3%/3 mm criterion were reported for 3,116 cases that included both IMRT and VMAT treatment plans delivered on 18 linear accelerators. The gamma pass rates were categorized based on the treatment site, treatment technique, type of MLCs, operator, ArcCHECK device, and LINAC model. We recorded the percent of failures at the clinically acceptable threshold of 90%. In addition, we calculated the threshold that encompasses two standard deviations (2 SD) (95%) of QAs (T95) for each category investigated. The commissioning measurements demonstrated that the device performed as expected. The uniformity of the detector response to a constant field arc delivery showed a 1% standard deviation from the mean. The variation in dose with changing repetition rate was within 1 cGy of the mean, while the measured dose showed a linear relation with delivered MUs. Our initial patient QA results showed that, at the clinically selected passing criterion, 4.5% of cases failed. On average T95 was 91%, ranging from 73% for gynecological sites to 96.5% for central nervous system sites. There are statistically significant differences in passing rates between IMRT and VMAT, high‐definition (HD) and non‐HD MLCs, and different LINAC models (p‐values <<0.001). An additional investigation into the failing QAs and a comparison with ion‐chamber measurements reveals that the differences observed in the passing rates between the different studied factors can be largely explained by the field size dependence of the device. Based on our initial experience with the ArcCHECK, our passing rates are, on average, consistent with values reported in the AAPM TG‐119. However, the significant variations between QAs that were observed based on the size of the treatment fields may need to be corrected to improve the specificity and sensitivity of the device.PACS number(s): 87.55.Qr, 87.56.Fc

SU‐F‐T‐558: ArcCheck for Patient Specific QA in Stereotactic AblativeRadiotherapy

Purpose:Stereotactic Ablative Radiotherapy (SABR) is one of the most preferred treatment techniques for early stage lung cancer. This technique has been extended to other treatment sites like Spine, Liver, Scapula, Sternum etc., This has resulted in increased physics QA time on machine. In this study, we've tested the feasibility of using ArcCheck as an alternative method to replace film dosimetry.Methods:Twelve patients with varied diagnosis of Lung, Liver, scapula, sternum and Spine undergoing SABR were selected for this study. Pre‐treatment QA was performed for all the patients which include ionization chamber and film dosimetry. The required gamma criteria for each SABR plan to pass QA and proceed to treatment is 95% (3%,1mm). In addition to this routine process, the treatment plans were exported on to an ArcCheck phantom. The planned and measured dose from the ArcCheck device were compared using four different gamma criteria: 2%,2 mm, 3%,2 mm, 3%,1 mm and 3%, 3 mm. In addition to this, we've also introduced errors to gantry, collimator and couch angle to assess sensitivity of the ArcCheck with potential delivery errors.Results:The ArcCheck mean passing rates for all twelve cases were 76.1%±9.7% for gamma criteria 3%,1 mm, 89.5%±5.3% for 2%,2 mm, 92.6%±4.2% for 3%,2 mm, and 97.6%±2.4% for 3%,3 mm gamma criteria. When SABR spine cases are excluded, we observe ArcCheck passing rates higher than 95% for all the studied cases with 3%, 3mm, and ArcCheck results in acceptable agreement with the film gamma results.Conclusion:Our ArcCheck results at 3%, 3 mm were found to correlate well with our non‐SABR spine routine patient specific QA results (3%,1 mm). We observed significant reduction in QA time on using ArcCheck for SABR QA. This study shows that ArcCheck could replace film dosimetry for all sites except SABR spine.

SU‐F‐T‐282: Quality Assurance for IMRT/VMAT QA Devices: Issues Affecting the Timing for ArcCHECK Recalibration

Purpose:To discuss several factors surrounding the decision on when to recalibrate the ArcCHECK device as well as present a simple and efficient monthly check to evaluate ArcCHECK calibrations.Methods:ArcCheck (Sun Nuclear) calibrations were evaluated monthly by measuring a 25×25cm2 field with 100 MU. Since ArcCHECK measurements are run on an almost nightly basis, such additional square field measurements are obtained with minimal additional effort. An in‐house MATLAB script compares two radial (y‐direction) profiles from the top/center of the new measurement relative to a baseline measurement acquired at the last device calibration. The program automatically generates PDF profile and percent difference comparisons for inspection. Recalibration is based on inspection of measurement profile shapes and percent differences from the baseline measurement.Results:The method presented here shows the utility of a simple monthly check for evaluating ArcCHECK calibrations, and in addition shows the importance of recalibrating after Linac beam steering. Our device required recalibration approximately every 8–10 months. However, for ease of scheduling, we propose a bi‐annual recalibration interval. Clinics with a lighter/heavier IMRT/VMAT QA case load may require different recalibration intervals, which are easily determined using the single‐field method presented. Analysis of additional square fields is also easily incorporated, if desired. We further illustrate the importance of array recalibration given that diode irradiation is not uniform over the entire device, with central diodes receiving more than 900 Gy over the course of 10 months and peripheral diodes receiving as little as 50 Gy (in our experience). Finally, we show that timely device recalibration decreases spread in clinical IMRT/VMAT QA gamma passing rates.Conclusion:Quality assurance for ArcCHECK array calibrations is important to ensure quality IMRT/VMAT QA comparisons. For many clinics, calibrations should be performed at least annually, if not more frequently, based on use of the proposed monthly check.

Modeling of flattening filter free photon beams with analytical and Monte Carlo TPS

Recent conventional medical linear particle accelerators (linacs) come with the capacity to deliver flattening filter free (FFF) beams, allowing higher dose rates. Intensity modulated radiation therapy (IMRT) and small-fields stereotactic techniques no longer seem to require flattened beams. To implement such beams, it is important to correctly measure and model the dose distribution they deliver. The objective of this work was to model the recent VERSA-HD (Elekta®) FFF beams using RayStation® TPS, as done externally on Monaco TPS. There exists no publication on the modeling of this machine in FFF mode. Also, no published work relates to the use of RayStation® to model unflattened beams. Flattened X6 and unflattened 6FFF and 10FFF beams have been modeled. Lateral and depth dose profiles as well as output factors were measured using ionization chambers and diodes detectors. Models were created in RayStation® and Monaco® treatment planning systems (TPS), offering analytically- and Monte Carlo-algorithms, respectively. Surface dose, out-of-field dose and collimator transmission were also measured to improve models. All steps and parameters used during the modeling process are presented, along with measured and calculated dose profiles. Agreement between measurements and models was assessed by one-dimensional gamma analysis. Stereotactic and VMAT (volumetric modulated arc therapy) treatment plans were controlled by measuring isocenter absolute dose and dose distribution with the ArcCheck device. The impacts of the model parameters on each region of the depth and lateral dose profiles were explained. FFF beams models exhibited different energy spectra, smaller source sizes and lower electron contamination compared to flattened beams, in both TPS. Monaco® and RayStation® exhibit good agreement with depth and lateral dose profile measurements, 1% and 1 mm criteria being respected in the high dose regions. Stereotactic and VMAT treatment plans measured led to good agreement with calculation.

Poster — Thur Eve — 17: In‐phantom and Fluence‐based Measurements for Quality Assurance of Volumetric‐driven Adaptation of Arc Therapy

During volumetric modulated arc therapy (VMAT) of head and neck cancer, some patients lose weight which may result in anatomical deviations from the initial plan. If these deviations are substantial a new treatment plan can be designed for the remainder of treatment (i.e., adaptive planning). Since the adaptive treatment process is resource intensive, one possible approach to streamlining the quality assurance (QA) process is to use the electronic portal imaging device (EPID) to measure the integrated fluence for the adapted plans instead of the currently‐used ArcCHECK device (Sun Nuclear). Although ArcCHECK is recognized as the clinical standard for patient‐specific VMAT plan QA, it has limited length (20 cm) for most head and neck field apertures and has coarser detector spacing than the EPID (10 mm vs. 0.39 mm). In this work we compared measurement of the integrated fluence using the EPID with corresponding measurements from the ArcCHECK device. In the past year nine patients required an adapted plan. Each of the plans (the original and adapted) is composed of two arcs. Routine clinical QA was performed using the ArcCHECK device, and the same plans were delivered to the EPID (individual arcs) in integrated mode. The dose difference between the initial plan and adapted plan was compared for ArcCHECK and EPID. In most cases, it was found that the EPID is more sensitive in detecting plan differences. Therefore, we conclude that EPID provides a viable alternative for QA of the adapted head and neck plans and should be further explored.

SU‐F‐BRE‐13: Replacing Pre‐Treatment Phantom QA with 3D In‐Vivo Portal Dosimetry for IMRT Breast Cancer

Purpose:Pre‐treatment QA of individual treatment plans requires costly linac time and physics effort. Starting with IMRT breast treatments, we aim to replace pre‐treatment QA with in‐vivo portal dosimetry.Methods:Our IMRT breast cancer plans are routinely measured using the ArcCheck device (SunNuclear). 2D‐Gamma analysis is performed with 3%/3mm criteria and the percentage of points with gamma&lt;1 (nG1) is calculated within the 50% isodose surface. Following AAPM recommendations, plans with nG1&lt;90% are approved; others need further inspection and might be rejected. For this study, we used invivo portal dosimetry (IPD) to measure the 3D back‐projected dose of the first three fractions for IMRT breast plans. Patient setup was online corrected before for all measured fractions. To reduce patient related uncertainties, the three IPD results were averaged and 3D‐gamma analysis was applied with abovementioned criteria . For a subset of patients, phantom portal dosimetry (PPD) was also performed on a slab phantom.Results:Forty consecutive breast patients with plans that fitted the EPID were analysed. The average difference between planned and IPD dose in the reference point was −0.7+/−1.6% (1SD). Variation in nG1 between the 3 invivo fractions was about 6% (1SD). The average nG1 for IPD was 89+/−6%, worse than ArcCheck (95+/−3%). This can be explained by patient related factors such as changes in anatomy and/or model deficiencies due to e.g. inhomogeneities. For the 20 cases with PPD, mean nG1 was equal to ArcCheck values, which indicates that the two systems are equally accurate. These data therefore suggest that proper criteria for 3D invivo verification of breast treatments should be nG1&gt;80% instead of nG1&gt;90%, which, for our breast cases, would result in 5% (2/40) further inspections.Conclusion:First‐fraction in‐vivo portal dosimetry using new gamma‐evaluation criteria will replace phantom measurements in our institution, saving resources and yielding 3D dosimetry of the actual patient treatment.

A comprehensive EPID-based 3D validation technique for TrueBeam-delivered VMAT plans

Purpose: To develop and validate a pre-treatment EPI dosimetry method on Varian TrueBeam linacs using continuous imaging, with reconstruction in a 3D cylindrical phantom geometry.Methods: Delivery of VMAT plans with continuous imaging is currently possible only in Research Mode on TrueBeam linacs, with images acquired in a proprietary format. An earlier technique was adapted to take advantage of technical improvements in EPID delivery, and was tested under various acquisition conditions. The dosimetry of VMAT plans was evaluated at isocentre and within patient volumes that had been transferred to the virtual phantom.Results: Approximately 60 portal image projections per arc were found to be adequate for 3D reconstruction in phantom volumes of 28cm diameter. Twelve prostate, CNS and Head & Neck deliveries were evaluated in Research mode relative to the corresponding Eclipse (v.10) treatment plans, and to measurements on an ArcCheck device in Treatment mode. Mean dose differences at isocentre were within 2% for the three-way comparison, and in PTV volumes were within 1% (s.d. 1%). However, some discrepancies were observed in ArcCheck results that may be related to the small dimensions of certain VMAT apertures.Conclusions: EPI dosimetry with 3D dose reconstruction is an accurate, comprehensive and efficient pre-treatment validation technique for VMAT delivery. Although currently limited to a research mode on TrueBeam, it has the potential to be implemented for clinical use.

Detecting VMAT delivery errors: A study on the sensitivity of the ArcCHECK-3D electronic dosimeter

The sensitivity of the ArcCHECK 3D dosimeter in detecting VMAT delivery errors has been investigated. Dose and leaf positional errors of different magnitudes were introduced to whole arc and individual control points (CPs) of a simple open arc VMAT plan. The error introduced and error free plans were delivered and measured using the ArcCHECK device. The measured doses were compared against the treatment planning system calculated doses using gamma (γ) criteria with 2%/2mm and 3%/3mm tolerance levels. ArcCHECK effectively detected the dose errors resulting from MLC leaf positioning errors in limited CPs and Whole arc. For errors introduced to MU, ArcCHECK effectively detected the MU delivery errors in whole arc but not the MU errors introduced to CPs in integrated dose comparison.

SU‐E‐T‐433: Evaluation of the ArcCheck Device for Patient‐Specific Quality Assurance in Step‐And‐Shoot IMRT for Single‐Fraction Spinal SBRT

Purpose: Current standard for patient specific spine SBRT IMRT QA in our institution is a combination of a point dose measurement in solid water under delivery of the beams at actual treatment angles, and 2D planar verification using an ion chamber (IC) array with all beams delivered at standard gantry angle of zero. The very small active area of ArcChecks diodes (∼0.6mm2) has the potential to improve measurement accuracy in high dose gradient regions present in spinal SBRT. Given high clinical workload we seek to improve ease of workflow and decrease time necessary for patient‐specific QA, by obtaining 2D verification and point dose measurement at actual treatment angles in one single measurement. Methods: The results of 10 patient specific QA measurements, as performed with single‐point and 2D IC array were compared to results obtained with ArcCheck. ArcCheck and 2D IC array pass rates are evaluated for 3%/3mm gamma criteria, single‐point measurement based on percent deviation from QA plan dose. The constancy of the device was assessed by re‐measuring the same treatment plan over the course of three weeks, and evaluating pass rate variationResults: The gamma pass rates are generally higher for ArcCheck (average 99.3%) than for Matrixx (average 93.1%). Average absolute deviation for the single‐point IC measurement was 3.4%. Maximum variation in pass rate between any two ArcCheck measurements within the three‐week evaluation period was 0.5%. Since ArcCheck allows for the use of an insert with an ion chamber in place, simultaneous redundant point measurement is possible with no time addedConclusion: Based on our results patient‐specific QA for spine SBRT treatments will be performed using ArcCheck with pass/fail criteria 98%. High sensitivity towards geometrical shifts necessitates careful initial alignment of the device.