Pre-treatment Quality Assurance Research Articles

Impact of delivery characteristics on dose delivery accuracy of volumetric modulated arc therapy for different treatment sites.

This study aimed to investigate the impact of delivery characteristics on the dose delivery accuracy of volumetric modulated arc therapy (VMAT) for different treatment sites. The pretreatment quality assurance (QA) results of 344 VMAT patients diagnosed with gynecological (GYN), head and neck (H&N), rectal or prostate cancer were randomly chosen in this study. Ten metrics reflecting VMAT delivery characteristics were extracted from the QA plans. Compared with GYN and rectal plans, H&N and prostate plans had higher aperture complexity and monitor units (MU), and smaller aperture area. Prostate plans had the smallest aperture area and lowest leaf speed compared with other plans (P < 0.001). No differences in gantry speed were found among the four sites. The gamma passing rates (GPRs) of GYN, rectal and H&N plans were inversely associated with union aperture area (UAA) and leaf speed (Pearson’s r: −0.39 to −0.68). GPRs of prostate plans were inversely correlated with aperture complexity, MU and small aperture score (SAS) (absolute Pearson’s r: 0.34 to 0.49). Significant differences in GPR between high SAS and low SAS subgroups were found only when leaf speed was <0.42 cm s–1 (P < 0.001). No association of GPR with gantry speed was found in four sites. Leaf speed was more strongly associated with UAA. Aperture complexity and MU were more strongly associated with SAS. VMAT plans from different sites have distinct delivery characteristics. Affecting dose delivery accuracy, leaf speed is the key factor for GYN, rectal and H&N plans, while aperture complexity, MU and small apertures have a higher influence on prostate plans.

Clinical implementation of respiratory-gated spot-scanning proton therapy: An efficiency analysis of active motion management.

PurposeThe aim of this work is to describe the clinical implementation of respiratory‐gated spot‐scanning proton therapy (SSPT) for the treatment of thoracic and abdominal moving targets. The experience of our institution is summarized, from initial acceptance and commissioning tests to the development of standard clinical operating procedures for simulation, motion assessment, motion mitigation, treatment planning, and gated SSPT treatment delivery.Materials and methodsA custom respiratory gating interface incorporating the Real‐Time Position Management System (RPM, Varian Medical Systems, Inc., Palo Alto, CA, USA) was developed in‐house for our synchrotron‐based delivery system. To assess gating performance, a motion phantom and radiochromic films were used to compare gated vs nongated delivery. Site‐specific treatment planning protocols and conservative motion cutoffs were developed, allowing for free‐breathing (FB), breath‐holding (BH), or phase‐gating (Ph‐G). Room usage efficiency of BH and Ph‐G treatments was retrospectively evaluated using beam delivery data retrieved from our record and verify system and DICOM files from patient‐specific quality assurance (QA) procedures.ResultsMore than 70 patients were treated using active motion management between the launch of our motion mitigation program in October 2015 and the end date of data collection of this study in January 2018. During acceptance procedures, we found that overall system latency is clinically‐suitable for Ph‐G. Regarding room usage efficiency, the average number of energy layers delivered per minute was <10 for Ph‐G, 10‐15 for BH and ≥15 for FB, making Ph‐G the slowest treatment modality. When comparing to continuous delivery measured during pretreatment QA procedures, the median values of BH treatment time were extended from 6.6 to 9.3 min (+48%). Ph‐G treatments were extended from 7.3 to 13.0 min (+82%).ConclusionsActive motion management has been crucial to the overall success of our SSPT program. Nevertheless, our conservative approach has come with an efficiency cost that is more noticeable in Ph‐G treatments and should be considered in decision‐making.

Assessing the feasibility of single target radiosurgery quality assurance with portal dosimetry.

PurposeTo assess the feasibility of using portal dosimetry (PD) for pre‐treatment quality assurance of single target, flattening filter free (FFF), volumetric arc therapy intracranial radiosurgery plans.MethodsA PD algorithm was created for a 10X FFF beam on a Varian Edge linear accelerator (Varian Inc, Palo Alto, CA, USA). Treatment plans that were previously evaluated with Gafchromic EBT‐XD (Ashland, Bridgewater, NJ, USA) film were measured via PD and analyzed with the ARIA Portal Dosimetry workspace. Absolute dose evaluation for film and PD was done by computing the mean dose in the region receiving greater than or equal to 90% of the max dose and comparing to the mean dose in the same region calculated by the treatment planning system (TPS). Gamma analysis with 10% threshold and 3%/2 mm passing criteria was performed on film and portal images.ResultsThirty‐six PD verification plans were delivered and analyzed. The average PD to TPS dose was 0.989 ± 0.01 while film to TPS dose was 1.026 ± 0.01. All PD plans passed the gamma analysis with 100% of points having gamma <1. Overall, PD to TPS dose agreement was found to be target size dependent. As target size decreases, PD to TPS dose ratio decreased from 1.004 for targets with diameters between 15–31 mm and 0.978 for targets with diameters less than 15 mm.ConclusionThe agreement of PD to TPS mean dose in the high dose region was found to be dependent on target size. Film measurements did not exhibit size dependence. All PD plans passed the 3%/2 mm gamma analysis, but caution should be used when using PD to assess overall dosimetric accuracy of the treatment plan for small targets.

A comprehensive phantom with multi-detector inserts for pre-treatment quality assurance in stereotactic ablative radiotherapy

Stereotactic ablative radiotherapy (SABR) is an advanced high precision radiotherapy procedure where a high dose of radiation is delivered in a few fractions. Pre-treatment quality assurance (QA) is conducted prior to treatment to ensure the planned dose is delivered precisely. Several types of detectors have been explored for the use of pre-treatment QA in SABR. This study discusses the design of an in-house phantom with inserts designed to accommodate multiple detectors such as ion chambers, films, alanine, optically stimulated luminescence (OSLs), thermoluminescence dosimeter (TLDs), diodes, diamond detectors, and presage dosimeter. The study also presents some of the measurement results performed for selective treatment sites. The inserts were designed using Perspex and 3D printed materials. The Presage dosimeter provided a three-dimensional view of the delivered dose for SABR plans and matched closer to the planned dose. Most of the detectors/dosimeters explored in this study demonstrated measured results within 3% of the planned dose.

Simple Electronic Portal Imager-Based Pretreatment Quality Assurance using Acuros XB: A Feasibility Study.

Objective:This study demonstrates a novel electronic portal imaging device (EPID)-based forward dosimetry approach for pretreatment quality assurance aided by a treatment planning system (TPS).Materials and Methods:Dynamic multileaf collimator intensity-modulated radiation therapy (IMRT) plans were delivered in EPID and fluence was captured on a beam-by-beam basis (FEPID). An open field having dimensions equal to those of the largest IMRT field was used in the TPS to obtain the transmitted fluence. This represented the patient-specific heterogeneity correction (Fhet). To obtain the resultant heterogeneity-corrected fluence, EPID measured fluence was corrected for the TPS generated heterogeneity (FResultant = FEPID × Fhet). Next, the calculated fluence per beam basis was collected from TPS (FTPS). Finally, FResultant and FTPS were compared using a 3% percentage dose difference (% DD)-3 mm distance to agreement [DTA] gamma analysis in an isocentric plane (two-dimensional [2D]) and multiple planes (quasi three-dimensional [3D]) orthogonal to the beam axis.Results:The 2D heterogeneity-corrected dose reconstruction revealed a mean γ passing of the pelvis, thorax, and head-and-neck cases of 96.3% ±2.0%, 96.3% ±1.8%, and 96.1% ±2.2%, respectively. Quasi-3D γ passing for the pelvis, thorax, and head-and-neck cases were 97.5% ±1.4%, 96.3% ±2.4%, and 97.5% ±1.0%, respectively.Conclusion:EPID dosimetry produced an inferior result due to no heterogeneity corrections for sites such as the lung and esophagus. Incorporating TPS-based heterogeneity correction improved the EPID dosimetry result for those sites with large heterogeneity.

Commissioning of portal dosimetry using a novel method for flattening filter-free photon beam in a nontrue beam linear accelerator

The aim of this study is to commission and validate the portal dosimetry (PD) system using an indirect method for flattening filter free (FFF) photon beam of the upgraded c-series linear accelerator. Varian Medical System clinacs with amorphous-silicon portal imager panel (aSi-1000) do not have PD for FFF beams. Recently, our c-series linear accelerator was upgraded to deliver 6MV FFF (6MVFFF) photon beam with the highest dose rate of 1400 monitor unit (MU)/min. The study, therefore, focuses on the commissioning and validation of PD for the 6MVFFF beam. An indirect method was implemented to predict the portal dose for FFF beam in Eclipse as the treatment planning system does not have direct prediction algorithm for FFF beam (version. 11). Dosimetrical characteristics of aSi-electronic portal imaging device (EPID) were evaluated for 6MVFFF beam and validation of PD for 6MVFFF beam was performed for open fields along with pretreatment quality assurance of intensity-modulated radiation therapy (IMRT), volumetric-modulated arc therapy (VMAT), and stereotactic radiosurgery (SRS) techniques for 30 patients planned with 6MVFFF beam. ASi-EPID saturates between 100 and 130 cm source to detector distance (SDD) for 6MVFFF beam and resolved at more than 140 cm SDD. The squared correlation coefficient (R2) for MU linearity was found to be 1 (R2 = 1), and instantaneous dose response linearity at different SDD's was found to be 0.999 (R2 = 0.999) for the 6MVFFF beam. Maximum gamma area index (GAI) for 3% dose difference and 3 mm distance-to-agreement criteria for IMRT, VMAT, and SRS/stereotactic radiotherapy plans was 97.9% ± 0.3%, 96.3% ± 0.5%, and 98.2% ± 0.2%, respectively. The results reveal that this novel method can be used to commission portal dosimetry for 6MVFFF beam as it is a convenient, faster, and accurate method.

Electronic Portal Imaging Device-Based Three-Dimensional Volumetric Dosimetry for Intensity-modulated Radiotherapy Pretreatment Quality Assurance.

Aim:This study aimed at evaluating the efficacy of treatment planning system (TPS)-based heterogeneity correction for two- and three-dimensional (2D and 3D) electronic portal imaging device (EPID)-based pretreatment dose verification. An experiment was conducted on the EPID back-projection technique and intensity-modulated radiotherapy (IMRT).Materials and Methods:Treatment plans were delivered in EPID without a patient to obtain the fluence pattern (FEPID). A heterogeneity correction plane (Fhet) for an open beam of 30 cm × 30 cm was extracted from the TPS. The heterogeneity-corrected measured fluence is developed by matrix element multiplication (FResultant = FEPID × Fhet). Further planes were summed to develop a 3D dose distribution and exported to the TPS. Dose verifications for 2D and 3D were carried out with the corresponding TPS values using 2D gamma analysis (ɣ) and dose volume histogram (DVH) comparison, respectively. Totally, 33 patients (17 head–neck and 16 thorax cases) were evaluated in this study.Results:The head–neck and thorax plans show a 3-mm-distance to agreement (DTA) 3% DD gamma passing of 96.3% ± 2.0% and 95.4% ± 1.8% points, respectively, between FTPS and FResultant. The comparison of the uncorrected measured fluence (FEPID) with FTPS reveals a gamma passing of 82.2% ± 7.3% and 80.4% ± 8.6% for head–neck and thorax cases, respectively. A total of 87 out of the 102 head–neck and thorax beams exhibit a planner gamma passing of 97.6% ± 2.1%. In the 3D-DVH comparison of thorax and head–neck cases, D5% for planning target volume were −0.5% ± 2.2% and −2.1% ± 3.5%, respectively; D95% varies as 1.0% ± 2.7% and 1.4% ± 1.1% between TPS calculated and heterogeneity-corrected-EPID-based dose reconstruction.Conclusion:The novel TPS-based heterogeneity correction can improve the 2D and 3D EPID-based back projection technique. Structures with large heterogeneities can also be handled using the proposed technique.

203. Patient specific pre-treatment Quality Assurance in multiple lesions lung SBRT VMAT plans

Purpose SBRT of lung multiple lesions is a frequent clinical request. VMAT is preferentially used because it is possible to treat multiple lesions with single arc and single isocenter in a relatively short time. This treatment technique frequently make use of MLC interdigitation and the peripheral part of the treatment field, challenging conditions for both TPS calculation and LINAC delivery. The aim of this study is to evaluate the reliability of this irradiation technique in our department through different methods of patient specific pre-treatment Quality Assurance (QA). Methods VMAT plans have been calculated with the Monaco TPS for an Elekta Synergy Linac mounting the Agility MLC (80 leaf pair, 5 mm width, interdigitation). PTW Octavius 4D system and Octavius detector have been used to measure 3D dose distributions. Each plan has also been recalculated on the Tomotherapy Cheese phantom in order to measure absorbed dose at the center of each lesion with an Exradin A1SL ionization chamber. EBT3 gafchromic films have also been exposed in the Cheese phantom in coronal, sagittal or transversal planes to measure absorbed dose distributions. Results Even with good gamma passing rate (>90%, 3%, 3 mm, local), measurements with PTW Octavius 4D are sometime affected by poor spatial resolution and/or limited acquisition volume, and occasional failures in the high dose region have also been found. On the contrary, very good agreement ( Conclusions By using different independent methods of pre-treatment QA we came to the conclusion that lung SBRT VMAT plans of multiple lesion with single isocenter can be accurately delivered with the technology in use in our department.

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.

190. Correlation between modulation degree and dosimetric accuracy of prostate VMAT treatment planning

Purpose To evaluate the impact of modulation degree (MD) on patient specific pre-treatment quality assurance for prostate VMAT treatment planning. Methods Ten patients with similar PTV and rectum size were included in the study. The prescribed fractional dose to PTV was 2,5 Gy daily. VMAT plans were calculated using a MonteCarlo-based TPS. For each patient we optimized 4 different plans using the same constraint cost-functions for PTV and organ-at-risk (OARs), the same plan parameters (arc number, maximum number of control points for arc) but changing the minimum distance between opposing MLC leafs (MDW = 0.5, 0.8, 1, 1.5 cm). Such parameter influences the MD and the plan quality. MD was calculated from the DICOM-RT files using the mean segment area value and total Monitor Unit. To evaluate the plan quality we took into account the PTV coverage and rectum dose. Gamma index analysis was performed to compare measured and calculated dose. Plans were accepted if 95% of the points satisfied the 2% dose-difference and distance-to-agreement 2 mm (γ2%,2 mm). All plans were delivered by a 6 MV Linac and the patient specific pre-treatment quality assurance verification was performed using a cylindrical diode array-phantom. Descriptive statistic was performed to evaluate the correlation between MD and gamma passing rate measured. Results We observed a negative linear correlation (Fig. 1) between MD values and γ2%,2 mm (correlation coefficient r = 0.7, p = 0.0004). The plans with MD 4 yielded a γ2%,2 mm inferior than 95% in 100% of cases (Fisher-exact-test p = 0.002). 93% of plans with MDW > 0.8 cm had a MD Conclusions The gamma index value was significantly related to MD 4. Rival plans with a MD 0.8 cm allows to obtain a desirable MD value. The calculation of MD for prostate VMAT treatment planning is important to predict the patient-QA in term of gamma index analysis.

136. Which is the optimum cylindrical phantom grid size calculation to evaluate the accuracy of gamma index for times optimizing in the pre-treatment quality assurance procedure of VMAT and IMRT tecnique?

PurposeIMRT and VMAT are the state-of-the-art irradiation techniques for the delivery of highly conformal radiation fields to PTV. The use of QA tools for the verification of the planned dose distribution prior to the treatment of the patient has become a standard procedure in clinical routine. Aim of this study is to evaluate the optimal grid size calculation on cylindrical phantoms to determine the gamma index (γ) levels for the routinely pre-treatment dosimetric QA. Methods and materialsThe measurements were made on a Elekta Precise linear accelerator and compared with the created plans by the Philips Pinnacle3 TPS. The pre-treatment evaluations were made with the PTW_OCTAVIUS-4D phantom with the 729 cubical ion chambers arranged in the OCTAVIUS_Detector 2D_array of 27 cm × 27 cm. The beam incidence is always perpendicular to the surface of the detector panel and the data were collected and evaluated by means of the PTW_VeriSoft software package. On about 100 patients who underwent to radiotherapy with VMAT, were evaluated the γ between the dose deliveries and the dose calculations on the cylindrical phantom with cubic step size of 4 mm, 3 mm and 2 mm. For each phantom grid size the γ value with [3 mm;3%], [2 mm;2%] and [1 mm;1%] as distance and dose difference criteria respectively were determined. ResultsFor each fixed γ[d(mm);D(%)] criteria the γ global ratios to the 3 mm voxel phantom calculation, followed a Gaussian distribution. That mean that there isn’t a correlation between the used phantom grid size and γ accordance. The agreement in γ differences were found within 2%(2SD), 3%(2SD) and 5%(2SD) for γ[3 mm;3%], γ[2 mm;2%] and γ[1 mm;1%] respectively. ConclusionsDue to the computer limits and the large amount of calculation, the TPS algorithms are often time consuming, strongly depending on the set grid size calculation. For a busy Center a voxel grid size of 3–4 mm is enough appropriate.

Analysis of different procedures for absolute dosimetry with EBT3 radiochromic film

Purpose: Absolute dose comparison between different methodologies of multichannel dosimetry in pretreatment quality assurance (QA), including highly modulated techniques in a wide range of doses (180 cGy to 1800 cGy). Methods: Several parametric forms for film dosimetry calibration were studied and compared. Then, different algorithms for radiochromic film dosimetry were implemented in Python code. With the objective of comparing these procedures, several IMRT and VMAT plans were irradiated and studied attending to the gamma criterion. A denoising protocol was developed, removing completely Gaussian noise dependency. In addition, spatial resolution dependency was also removed. Results: The algorithms that account for perturbations gave good results in the gamma analysis. The results could not be associated with noise or resolution. The red channel is inadequate for film dosimetry. Conclusions: The studied methodologies offered good agreement with the absolute gamma analysis, even with the 2%, 2 mm criteria. The proposed modifications reduced noise and resolution dependencies and improved the results.

ArcCHECK® with Monaco® – Tricks of the trade

Dynamic deliveries have become increasingly common and these techniques demand great diligence during pre-treatment Quality Assurance (QA). The ArcCHECK® is an efficient QA tool but requires careful commissioning prior to use and have specific issues to be addressed when used in combination with a Monte Carlo based TPS. We determined the best fit for the RED to be used with various methods. The variation of bulk RED was studied as a function of (a) make and model of the ArcCHECK (b) Beam energy (c) Field size and (d) static Vs arc fields. Effects of array calibration and the reference diode calibration dose were determined. The effect of Dose to Medium (Dm) Vs Dose to Water (Dw) was studied and its effect on point dose measurement determined. No significant difference was found in the RED between different ArcCHECKs, field size and type of delivery but beam energy. Gamma results are comparable if the dose calculation method (Dw or Dm) is maintained. Appropriate stopping power ratio correction must be applied for point dose measurement if Dm is used. The ArcCHECK when used with a MC TPS needs special considerations during commissioning compared to non-MC TPS.

P233] Investigation of the response of cylindrical ionization chamber in non-reference conditions when using high-energy photon beams

Purpose To investigate the response of CC13 cylindrical ionization chamber in non-reference conditions with focus on the penumbra region and the depth of maximum dose when using high-energy photon beams. This chamber is used for beam data measurements and also for pre-treatment quality assurance procedures for dynamic treatments. Methods Measurements and Monte Carlo calculations are performed, employing a Varian iX Linac (6 MV photon beam), at the depths of 15 mm and 100 mm for the field sizes of 20 × 20 mm 2 ; 20×100 mm 2 and 100×100 mm 2 . The Monte Carlo model is developed using the egs-chamber in the EGSnrc package. Conversion factors are calculated relating the absorbed dose to air in the air cavity of the chamber to the absorbed dose to water through using: (a) water volume equal to the air cavity of the chamber, (b) 1×1×1 mm 3 water voxel representing point dose. Corresponding correction factors, C w/chamber and C w,point/chamber , are determined to quantify the variations in the conversion factors. Results Agreement within 3.5%/0.5 mm is obtained between the measurements and calculations. The correction factors for a chamber position at the central axis of the beam and at the center of the field penumbra are between 0.98 and 1.02 for all field sizes and depths examined. The largest corrections are seen for off-axis positions 3 mm beyond the penumbra center where C w,point/chamber varies between 0.54 and 0.71 for the depth of 100 mm and 0.45 and 0.58 for the depth of 15 mm indicating an overestimation of the absorbed dose to water when using the CC13 chamber. C w,point/chamber factors up to 1.27 are obtained in regions close to the penumbra inside the 20×20 mm 2 and 20×100 mm 2 fields indicating an underestimation of the absorbed dose to water. The maximum variation of the C w/chamber factors is between 0.70 and 1.06 for the 20×20 mm 2 field at the depth of 15 mm. Conclusions Uncorrected chamber response of the CC13 chamber, or similar, results in under or overestimation of the absorbed dose to water at positions inside or outside of the beam close to the penumbra for small as well as large field sizes.

2D Dose Reconstruction by Artificial Neural Network for Pretreatment Verification of IMRT Fields

The use of intensity-modulated radiation therapy (IMRT) is developing rapidly in clinical routines. Because of the high complexity and uniqueness of IMRT treatment plans, patient-specific pretreatment quality assurance is generally considered a necessary prerequisite for patient treatment. In this work, we proposed a modified methodology of electronic portal imaging device (EPID)–based dose validation for pretreatment verification of IMRT fields by applying artificial neural networks (ANNs). The ANN must be trained and validated before use for pretreatment dose verification. For this purpose, 20 EPID fluence maps of IMRT prostate anterior-posterior fields were used as an input for ANN (feed forward type) and a dose map of those fluence maps that were predicted by treatment planning system as an output for ANN. After the training and validation of the neural network, the analysis of 10 IMRT prostate anterior-posterior fields showed excellent agreement between ANN output and dose map predicted by the treatment planning system. The average overall fields pass rate was 96.0% ± 0.1% with 3 mm/3% criteria. The results indicated that the ANN can be used as a low-cost, fast, and powerful tool for pretreatment dose verification, based on an EPID fluence map.

Comparison of two different EPID-based solutions performing pretreatment quality assurance: 2D portal dosimetry versus 3D forward projection method

PurposeThe aim of this paper is to characterize two different EPID-based solutions for pre-treatment VMAT quality assurance, the 2D portal dosimetry and the 3D projection technique. Their ability to catch the main critical delivery errors was studied. MethodsMeasurements were performed with a linac accelerator equipped with EPID aSi1000, Portal Dose Image Prediction (PDIP), and PerFRACTION softwares. Their performances were studied simulating perturbations of a reference plan through systematic variations in dose values and micromultileaf collimator position. The performance of PDIP, based on 2D forward method, was evaluated calculating gamma passing rate (%GP) between no-error and error-simulated measurements. The impact of errors with PerFRACTION, based on 3D projection technique, was analyzed by calculating the difference between reference and perturbed DVH (%ΔD). Subsequently pre-treatment verification with PerFRACTION was done for 27 patients of different pathologies. ResultsThe sensitivity of PerFRACTION was slightly higher than sensitivity of PDIP, reaching a maximum of 0.9. Specificity was 1 for PerFRACTION and 0.6 for PDIP. The analysis of patients’ DVHs indicated that the mean %ΔD was (1.2 ± 1.9)% for D2%, (0.6 ± 1.7)% for D95% and (−0.0 ± 1.2)% for Dmean of PTV. Regarding OARs, we observed important discrepancies on DVH but that the higher dose variations were in low dose area (<10 Gy). ConclusionsThis study supports the introduction of the new 3D forward projection method for pretreatment QA raising the claim that the visualization of the delivered dose distribution on patient anatomy has major advantages over traditional portal dosimetry QA systems.

Real-time high spatial resolution dose verification in stereotactic motion adaptive arc radiotherapy.

PurposeRadiation treatments delivered with real‐time multileaf collimator (MLC) tracking currently lack fast pretreatment or real‐time quality assurance. The purpose of this study is to test a 2D silicon detector, MagicPlate‐512 (MP512), in a complex clinical environment involving real‐time reconfiguration of the MLC leaves during target tracking.Methods MP512 was placed in the center of a solid water phantom and mounted on a motion platform used to simulate three different patient motions. Electromagnetic target tracking was implemented using the Calypso system (Varian Medical Systems, Palo Alto, CA, USA) and an MLC tracking software. A two‐arc VMAT plan was delivered and 2D dose distributions were reconstructed by MP512, EBT3 film, and the Eclipse treatment planning system (TPS). Dose maps were compared using gamma analysis with 2%/2 mm and 3%/3 mm acceptance criteria. Dose profiles were generated in sup‐inf and lateral directions to show the agreement of MP512 to EBT3 and to highlight the efficacy of the MLC tracking system in mitigating the effect of the simulated patient motion.ResultsUsing a 3%/3 mm acceptance criterion for 2D gamma analysis, MP512 to EBT3 film agreement was 99% and MP512 to TPS agreement was 100%. For a 2%/2 mm criterion, the agreement was 95% and 98%, respectively. Full width at half maximum and 80%/20% penumbral width of the MP512 and EBT3 dose profiles agreed within 1 mm and 0.5 mm, respectively. Patient motion increased the measured dose profile penumbral width by nearly 2 mm (with respect to the no‐motion case); however, the MLC tracking strategy was able to mitigate 80% of this effect.Conclusions MP512 is capable of high spatial resolution 2D dose reconstruction during adaptive MLC tracking, including arc deliveries. It shows potential as an effective tool for 2D small field dosimetry and pretreatment quality assurance for MLC tracking modalities. These results provide confidence that detector‐based pretreatment dosimetry is clinically feasible despite fast real‐time MLC reconfigurations.

EP-1762: Pre-treatment quality assurance for flattening filter free dynamic arcs : a detector comparison

EP-1762: Pre-treatment quality assurance for flattening filter free dynamic arcs : a detector comparison

How the detector resolution affects the clinical significance of SBRT pre-treatment quality assurance results

PurposeAim of this work was to study how the detector resolution can affect the clinical significance of SBRT pre-treatment volumetric modulated arc therapy (VMAT) verification results. MethodsThree detectors (PTW OCTAVIUS 4D 729, 1500 and 100 SRS) used in five configurations with different resolution were compared: 729, 729 merged, 1500, 1500 merged and 1000 SRS. Absolute local gamma passing rates of 3D pre-treatment quality assurance (QA) were evaluated for 150 dose distributions in 30 plans. Five different kinds of error were introduced in order to establish the detection sensitivity of the three devices. Percentage dosimetric differences were evaluated between planned dosevolume histogram (DVH) and patients’ predicted DVH calculated by PTW DVH 4D® software. Results:The mean gamma passing rates and the standard deviations were 92.4% ± 3.7%, 94.6% ± 1.8%, 95.3% ± 4.2%, 97.4% ± 2.5% and 97.6% ± 1.4 respectively for 729, 729 merged, 1500, 1500 merged and 1000 SRS with 2% local dose/2mm criterion. The same trend was found on the sensitivity analysis: using a tight gamma analysis criterion (2%L/1mm) only the 1000 SRS detected every kind of error, while 729 and 1500 merged detected three and four kinds of error respectively. Regarding dose metrics extracted from DVH curves, D50% was within the tolerance level in more than 90% of cases only for the 1000 SRS. ConclusionsThe detector resolution can significantly affect the clinical significance of SBRT pre-treatment verification results. The choice of a detector with resolution suitable to the investigated field size is of main importance to avoid getting false positive.

20. Pre-treatment quality assurance for flattening filter free dynamic arcs: A detector comparison

Introduction Pre-treatment quality assurance (QA) with flattening filter free (FFF) beams is challenging because of the high dose rate and the required spatial resolution as small field are usually used. Our current clinical practice is to perform pre-treatment QA with a diode array in a rotational phantom. This study investigates the replacement of this process by an electronic portal imager (EPI) measurement without a phantom in place in order to shorten the QA time on the machine. Methods Pre-treatment QA was performed with PTW Octavius 4D phantom equipped with SRS1000 diode array and with Varian aS1000 EPI. Diode array measurements were compared with the dose calculated in Octavius phantom with Varian Eclipse 13.7. EPI measurements was converted to water measurement and compared with water calculation performed in Dosisoft EPIBEAM (ThinkQA 1.0.2.2ALPHA). QA results were compared for Varian Truebeam 10 FFF dynamic arcs of 10 liver stereotactic radiotherapy plans and for plans with dose and MLC errors voluntarily introduced during beam delivery. Results Clinical plans covered a wide range of PTV size (average = 129.5 cm3, minimum = 9.8 cm3, maximum = 327.5 cm3) and dose prescription (average = 11.8 Gy, minimum = 6 Gy, maximum = 18 Gy) resulting in 1866 MU on average (minimum = 806 MU, maximum = 3072). Diode array and portal imager QA process gave both gamma values higher than 97% for 2%-2 mm criteria (10% threshold). Analysis of plans with errors is pending. Conclusions EPI QA process for FFF beams is achievable. It is much faster than diode array process as there is no need for a phantom setup and it provides similar results.