Tool For Patient-specific Quality Assurance Research Articles

Clinical Efficacy of an Electronic Portal Imaging Device versus a Physical Phantom Tool for Patient-Specific Quality Assurance.

Pre-treatment patient-specific quality assurance (QA) is critical to prevent radiation accidents. The electronic portal imaging device (EPID) is a dose measurement tool with good resolution and a low volume-averaging effect. EPIbeam—an EPID-based portal dosimetry software—has been newly installed in three institutions in Korea. This study evaluated the efficacy of the EPID-based patient-specific QA tool versus the PTW729 detector (a previously used QA tool) based on gamma criteria and planning target volume (PTV). A significant difference was confirmed through the R statistical analysis software. The average gamma passing rates of PTW729 and EPIbeam were 98.73% and 99.60% on 3 mm/3% (local), 96.66% and 97.91% on 2 mm/2% (local), and 88.41% and 74.87% on 1 mm/1% (local), respectively. The p-values between them were 0.015 (3 mm/3%, local), 0.084 (2 mm/2%, local), and less than 0.01 (1 mm/1%, local). Further, the average gamma passing rates of PTW 729 and EPIbeam according to PTV size were 99.55% and 99.91% (PTV < 150 cm3) and 97.91% and 99.28% (PTV > 150 cm3), respectively. The p-values between them were 0.087 (PTV < 150 cm3) and 0.036 (PTV > 150 cm3). These results confirm that EPIbeam can be an effective patient-specific QA tool.

Gamma Index Analysis as a Patient-Specific Quality Assurance Tool for High-Precision Radiotherapy: A Clinical Perspective of Single Institute Experience.

Purpose Patient-specific quality assurance (QA) by gamma (γ) analysis is an important component of high-precision radiotherapy. It is important to standardize institute-specific protocol. In this study, we describe our institutional experience of patient-specific QA for high-precision radiotherapy from a clinical perspective. Methods The planning data of 56 patients treated with intensity-modulated radiotherapy (IMRT)/volumetric modulated arc therapy (VMAT) were included. γ index analysis was done using Octavius 4D IMRT QA phantom(PTW, Freiburg, Germany) using 3 mm/3% criteria. Local, global, and volumetric gammas were calculated and compared. The relationship of γ index in the transverse, coronal, and sagittal direction and anatomical region of treatment was explored. Results Global three-dimensional (3D) γ indices in the coronal, sagittal, and transverse axes were 96.73 ± 2.35, 95.66 ± 3.01, and 93.36 ± 4.87 (p < 0.05). The average local two-dimensional (2D) γ index was 78.23 ± 5.44 and the global γ index was 92.41 ± 2.41 (p < 0.005). The average local 3D γ index was 84.99 ± 4.24 and the global 3D γ index was 95.25 ± 1.72 (p < 0.005, paired t-test). The average local volumetric γ index was 84.29 ± 4.73 and the global volumetric γ index was 95.96 ± 2.08 (p < 0.005). 3D global gamma index was significantly different in different anatomical regions (p < 0.05). Conclusion Our study shows that γ index analysis is a useful parameter for routine clinical IMRT QA. The choice of type of γ index depends on the context of use and degree of stringency in measurement. Average 2D and 3D global γ were different in anatomical regions. The average 3D γ index was significantly different in axes. No difference was observed with techniques of IMRT/VMAT. Localization of failed points in CT anatomy can be advantageous for clinical decision-making.

Evaluation of a High-Resolution Large-Area Two-Dimensional Detector Array for Pre-Treatment Verification of Multiple-Target SRS Plans Using a Single Isocenter

<h3>Purpose/Objective(s)</h3> Pre-treatment verification of multiple-target SRS plans using a single isocenter is challenging because of small target sizes and multiple off-isocenter targets. Targets located further away from the isocenter are more sensitive to rotational setup variations, increasing the risk of geometrical miss. It demands high-resolution detector for small target sizes and large detector area for measuring multiple targets at once. The purpose of this study is to evaluate a high-resolution large-area two-dimensional (2D) detector array for pre-treatment verification of multiple-target SRS plans using a single isocenter. <h3>Materials/Methods</h3> A 2D detector array was evaluated for this purpose. It is based on a silicon complementary metal-oxide-semiconductor platform, which enables a compact design, fast read-out, and high pixel density with each pixel comprising a radiation-sensitive photodiode, a capacitor and three transistors. Its spatial resolution is 0.4 mm, with 105,000 pixels across a large active area of 120 × 140 mm². Ten multiple-target SRS plans were selected such that each plan has at least 2 targets appear on a longitudinal plane cross the isocenter. All plans were created in a treatment planning system using 6 MV flattening filter free photon beam. The typical plan consisted of one or two coplanar arcs, and two to three non-coplanar arcs. All plans were delivered in patient geometry with the detector center aligned to the plan isocenter. The detector plane was rotated to the corresponding desired longitudinal plane to measure the composite dose in a single measurement. The smallest target measured was 0.12 cc. The largest center-to-center target separation measured was 12 cm. Six of the plans in which the target separations were relatively short were able to be cross verified with a patient-specific quality assurance (PSQA) tool, whose detector area is of 77 × 77 mm². The measured 2D dose distributions in true composite were compared with the calculated dose distributions. Gamma index analysis was performed using 3%/1 mm criteria with a 10% dose threshold. <h3>Results</h3> With the 2D detector array, the gamma passing rates are all greater than 95% except one, which is 93%. The average gamma-passing rate is 97.7%. For the 6 plans cross verified, the average gamma-passing rates are 98.2% and 96.1% with the 2D detector array and PSQA tool, respectively. <h3>Conclusion</h3> The 2D detector array was demonstrated to be suitable and offers an efficient way for pre-treatment verification of multiple-target SRS plans using a single isocenter. The advantages of this 2D array include: 1) High-resolution of 0.4 mm avoids dose interpolations for plans calculated with 0.5 -1.0 mm dose grid and potentially avoids "false positive" results due to more measured points; 2) Large active area of 120 × 140 mm² allows simultaneous verification of multiple targets in true composite in patient geometry.

Performance of a new commercial high-definition 3D patient specific quality assurance system for CyberKnife robotic radiotherapy and radiosurgery

Conventional two dimensional and low-definition measurement techniques for dosimetric verification of radiotherapy treatment deliveries are no longer adequate in the era of hypofractionation and extremely high dose gradients. New quality assurance (QA) tools with 3D capability and high definition are urgently needed. The purpose of this study was to evaluate the performance of one of the first such commercial systems as applied to CyberKnife (CK) radiotherapy/radiosurgery system.This study employed the recently developed commercial 3D patient-specific quality assurance (PSQA) system. Tissue-equivalent radiochromic polymer gel dosimeters, sealed in light protective thin-wall glass spheres (CrystalBalls), were mounted in a high-precision acrylic QA stand. Patient treatment plans were superimposed on the simulator CT scan of one selected CrystalBall and were irradiated in CK machine. CrystalBalls were then scanned using a laser CT scanner (OCTOPUS, MGS Research, Inc., DBA 3D Dosimetry, Madison, CT, USA). The simulator CT scan and the laser CT scan of the CrystalBall were registered in 6D using the VOLQA (MGS Research, Inc., DBA 3D Dosimetry, Madison, CT, USA) software. Planned and delivered dose distributions were compared by means of 3D gamma passing rates at user-selected acceptance criteria. Cumulative 3D dose-volume histograms and overlays of orthogonal profiles and isodoses both in 2D and in 3D were also generated, along with voxel statistics for dose difference and distance-to-agreement criteria. Twelve stereotactic radiosurgery/stereotactic radiotherapy/stereotactic body radiation therapy patients were selected randomly such that each set of two represented one of the six diagnoses: brain metastasis, trigeminal neuralgia, spine metastasis, prostate cancer, lung cancer and liver cancer. The validation of the 3D system was carried out by comparing the 2D gamma passing rates for all the patients using EBT3 films sandwiched between PMMA slabs and then analyzed with PTW VeriSoft software.The 3D gamma passing rate averaged over all patients studied was (90.5 ± 6.7) % at acceptance criteria of 2% (local) dose difference and 2 mm distance-to-agreement down to 20% isodose. For 3% (local) dose difference and 3 mm distance-to-agreement (also down to 20% cutoff dose), the average passing rate was (96.0 ± 3.1) %.The results of the study presented here form part of the information needed to assess suitability of a system and hence, the results suggest that the new high-definition 3D CrystalBall system can be a useful PSQA tool as a part of quality assurance for CK.

Portal Dosimetry as a patient specific Quality Assurance tool for Volumetric Arc Radiotherapy

Introduction: Volumetric Arc Radiotherapy (VMAT) is an advanced technique. Calculations of VMATplans are not so accurate even with State-of-Art dose calculation algorithms due to their complexity.Hence pre-treatment patient specific Quality Assurance (QA) of each VMAT plan is required. In thepresent study Electronic Portal Imaging Device (EPID) based portal dosimetry system was used forpre-treatment patient specific QA. Material and Methods: A total of 50 patients were chosen inthis study. Verification plans of each patient were calculated for portal dosimetry then executed onthe EPID system to measure the spatial distribution of radiation dose. Calculated and measured dosedistribution were compared to evaluate Gamma Index (GI) passing criteria of Dose Difference (DD)of 3% and Distance–to-Agreement (DTA) of 3mm, Area Gamma (γ% ≤1) &gt;95%, Average Gamma(gAve) &lt;0.5% and Maximum Gamma (gMax) &lt;3.5%. Results: The mean values of Area Gamma (γ%≤1) were observed to be varied from 99.14±0.23% to 99.87±0.18%. The Mean Values of AverageGamma (gAve) are found to vary from 0.19±0.05% to 0.15±0.04% and the mean values ofMaximum Gamma (gMax) found to be varied from 1.94±0.37% to 1.59±0.41%. All the plans werepassed the gamma index criteria with very good agreement. Thus the use of Portal Dosimetry forpre-treatment patient QA is found to be a very useful, quick, precise, efficient and effective pre-treatment patient specific QA tool for VMAT treatment. Conclusion: Portal Dosimetry can be utilizedfor routine use for patient specific quality assurance for Volumetric Arc Radiotherapy treatment.

Log file based Monte Carlo calculations for proton pencil beam scanning therapy

Patient specific quality assurance is crucial to guarantee safety in proton pencil beam scanning. In current clinical practice, this requires extensive, time consuming measurements. Additionally, these measurements do not consider the influence of density heterogeneities in the patient and are insensitive to delivery errors.In this work, we investigate the use of log file based Monte Carlo calculations for dose reconstructions in the patient CT, which takes the combined influence of calculational and delivery errors into account.For one example field, 87%/90% of the voxels agree within ±3% when taking either calculational or delivery uncertainties into account (analytical versus Monte Carlo calculation/Monte Carlo from planned versus Monte Carlo from log file). 78% agree when considering both uncertainties simultaneously (nominal field versus Monte Carlo from log files).We then show the application of the log file based Monte Carlo calculations as a patient specific quality assurance tool for a set of five patients (16 fields) treated for different indications. For all fields, absolute dose scaling factors based on the log file Monte Carlo agree within ±3% to the measurement based absolute dose scaling. Relative comparison shows that more than 90% of the voxels agree within ± 5% between the analytical calculated plan and the Monte Carlo based on log files.The log file based Monte Carlo approach is an end-to-end test incorporating all requirements of patient specific quality assurance. It has the potential to reduce the workload and therefore to increase the patient throughput, while simultaneously enabling more accurate dose verification directly in the patient geometry.

Development of a High-Resolution End-to-End Patient-Specific Quality Assurance Tool for Rotational Stereotactic Body Radiation Therapy Deliveries Using an Electronic Portal Imaging Device

Development of a High-Resolution End-to-End Patient-Specific Quality Assurance Tool for Rotational Stereotactic Body Radiation Therapy Deliveries Using an Electronic Portal Imaging Device

SU‐F‐T‐300: Impact of Electron Density Modeling of ArcCHECK Cylindricaldiode Array On 3DVH Patient Specific QA Software Tool Analysis

Purpose:3DVH software is a patient specific quality assurance tool which estimates the 3D dose to the patient specific geometry with the help of Planned Dose Perturbation algorithm. The purpose of this study is to evaluate the impact of HU value of ArcCHECK phantom entered in Eclipse TPS on 3D dose &amp; DVH QA analysis.Methods:Manufacturer of ArcCHECK phantom provides CT data set of phantom &amp; recommends considering it as a homogeneous phantom with electron density (1.19 gm/cc or 282 HU) close to PMMA. We performed this study on Eclipse TPS (V13, VMS) &amp; trueBEAM STx VMS Linac &amp; ArcCHECK phantom (SNC). Plans were generated for 6MV photon beam, 20cm×20cm field size at isocentre &amp; SPD (Source to phantom distance) of 86.7 cm to deliver 100cGy at isocentre. 3DVH software requires patients DICOM data generated by TPS &amp; plan delivered on ArcCHECK phantom. Plans were generated in TPS by assigning different HU values to phantom. We analyzed gamma index &amp; the dose profile for all plans along vertical down direction of beam's central axis for Entry, Exit &amp; Isocentre dose.Results:The global gamma passing rate (2% &amp; 2mm) for manufacturer recommended HU value 282 was 96.3%. Detector entry, Isocentre &amp; detector exit Doses were 1.9048 (1.9270), 1.00(1.0199) &amp; 0.5078(0.527) Gy for TPS (Measured) respectively.The global gamma passing rate for electron density 1.1302 gm/cc was 98.6%. Detector entry, Isocentre &amp; detector exit Doses were 1.8714 (1.8873), 1.00(0.9988) &amp; 0.5211(0.516) Gy for TPS (Measured) respectively.Conclusion:Electron density value assigned by manufacturer does not hold true for every user. Proper modeling of electron density of ArcCHECK in TPS is essential to avoid systematic error in dose calculation of patient specific QA.

GafChromic® EBT3 films for patient specific IMRT QA using a multichannel approach

PurposeTo evaluate EBT3 for pre-treatment patient specific quality assurance (QA). The method we propose combines the experience gained in our center with the guidelines of the protocol proposed by Lewis et al. in 2012. To compare the multichannel approach with the single channel dosimetry. MethodsGafchromic® EBT3 films were irradiated both at linac and TomoTherapy and calibration curves were obtained. A series of irradiations with simple fields (uniform dose distributions on regular shaped targets) was performed. In a second stage, films were exposed to full clinical plans at linac (step and shoot IMRT and VMAT). At TomoTherapy dose maps were obtained for a clinical plan in three different coronal planes. Films were digitized using an Epson 10000XL scanner and FilmQA™ Pro software was employed for the analysis. ResultsThe measured calibration curves suggest that, at least for the two beams taken into account (6 MV linac and TomoTherapy), a single calibration can be successfully adopted for each film lot. The application of the multichannel optimization method strongly improves the results in terms of gamma passing rates of the comparison between measured and calculated maps. ConclusionsUp to now EBT films, although attractive, were not preferred for routine patient specific QA due to their complex and time consuming processing and to the challenging work of characterization. The application of the mentioned protocol, together with some additional precautions, and the adoption of the multichannel optimization dosimetry, make this detector a handy and reliable tool for patient specific QA.

Derivative based sensitivity analysis of gamma index.

Originally developed as a tool for patient-specific quality assurance in advanced treatment delivery methods to compare between measured and calculated dose distributions, the gamma index (γ) concept was later extended to compare between any two dose distributions. It takes into effect both the dose difference (DD) and distance-to-agreement (DTA) measurements in the comparison. Its strength lies in its capability to give a quantitative value for the analysis, unlike other methods. For every point on the reference curve, if there is at least one point in the evaluated curve that satisfies the pass criteria (e.g., δDD = 1%, δDTA = 1 mm), the point is included in the quantitative score as “pass.” Gamma analysis does not account for the gradient of the evaluated curve - it looks at only the minimum gamma value, and if it is <1, then the point passes, no matter what the gradient of evaluated curve is. In this work, an attempt has been made to present a derivative-based method for the identification of dose gradient. A mathematically derived reference profile (RP) representing the penumbral region of 6 MV 10 cm × 10 cm field was generated from an error function. A general test profile (GTP) was created from this RP by introducing 1 mm distance error and 1% dose error at each point. This was considered as the first of the two evaluated curves. By its nature, this curve is a smooth curve and would satisfy the pass criteria for all points in it. The second evaluated profile was generated as a sawtooth test profile (STTP) which again would satisfy the pass criteria for every point on the RP. However, being a sawtooth curve, it is not a smooth one and would be obviously poor when compared with the smooth profile. Considering the smooth GTP as an acceptable profile when it passed the gamma pass criteria (1% DD and 1 mm DTA) against the RP, the first and second order derivatives of the DDs (δD’, δD”) between these two curves were derived and used as the boundary values for evaluating the STTP against the RP. Even though the STTP passed the simple gamma pass criteria, it was found failing at many locations when the derivatives were used as the boundary values. The proposed derivative-based method can identify a noisy curve and can prove to be a useful tool for improving the sensitivity of the gamma index.

In-house spread sheet based monitor unit verification program for volumetric modulated arc therapy

Independent monitor unit verification calculation (MUVC) has been recommended by several authors for intensity modulated radiotherapy (IMRT) as a patient specific quality assurance tool. Aim of the present work is to develop an in-house excel spread sheet based MUVC program for volumetric modulated arc therapy (VMAT) using Clarkson's integration technique. Total scatter factor (Sc,p) and tissue maximum ratio (TMR) for circular fields obtained from Treatment planning system (TPS) were used for the calculation. Multileaf collimator (MLC) interleaf leakage, MLC round edge transmission and tongue and groove effect were accounted. MUVC calculation was performed for 58 patients both for patient anatomy and for homogenous cylindrical phantom. Radiological path lengths were used as water equivalent depths (WED) for calculations using patient anatomy. Monitor unit (MU) discrepancies between −2.60% and 0.28% with mean deviation of −0.92% ± 0.75% were obtained for homogenous cylindrical phantom calculations. MUVC for patient anatomy resulted in large variations between −19.02% and 0.67% for 14 plans where isocenter was at a region below −350 HU. But For 44 plans where the isocenter was at a region above −350 HU, variations between −3.44% and 0.48% were obtained with mean deviation of −1.73% ± 1.12%. For VMAT patient specific quality assurance, the independent MUVC algorithm can be used as an easy and quick auxiliary to measurement based verification for plans with isocenter at a region above −350 HU.

WE‐A‐108‐11: Patient Specific Quality Assurance Tool in Rectal Brachytherapy

Purpose: To design and evaluate a patient‐specific quality assurance program for HDR Brachytherapy of rectal cancer. Methods: A solid water phantom with a rectal applicator positioned in the center was designed. Dose distributions were measured with two sheets of EBT3 model GAFCHROMIC™ film placed at 2 cm above and 5 cm below the center of the applicator. Treatment plans of 12 patients were studied, with each plan recalculated for the phantom geometry. FilmQA software was used to compare calculated and measured doses using the Gamma function. The region of interest (ROI) for analysis was defined as doses in excess of 20%, 30% or 40% of the maximum film dose. The average percentage of passing pixels (APP) over the 12 patients was scored for analysis. Sensitivity to source positional errors was evaluated by introducing errors of 1, 3, and 5 mm in one channel and quantifying the resultant change in APP. Results: The patient‐averaged data showed that an APP greater than 95% was achieved for the 2 cm film using criteria of 2%/3 mm with ROI=30%. For the 5 cm film an APP greater than 95% was achieved with criteria of 3%/3 mm with ROI =40%. Introducing positional errors greater than 1 mm resulted in drastic fall off in APP for all chosen ROIs and all DD/DTA criteria. For film at 2 cm and 2%/3 mm criteria, 2 mm shift error resulted in as much as 50% decrease in APP. Conclusion: The patient plans in this study were used to establish the use of 2%/3mm for gamma analysis in the 2 cm film plane on an ROI that incorporates doses in excess of 30% of the max film dose. In the 5 cm film plane, 3%/3mm for doses in excess of 40% of the max film dose is appropriate. Natural Sciences and Engineering; Research Council of Canada Contract No. 386009.

MO‐D‐108‐01: Cloud‐Based Monte Carlo Patient‐Specific Quality Assurance (QA) Method for Volumetric‐Modulated Arc Therapy (VMAT): Clinical and Educational Impacts

Purpose: Patient‐specific QA for VMAT is labor‐intensive,especially in cases which tend to be associated with severe heterogeneities or small aperture beams. We developed a robust cloud‐based Monte Carlo method as an independent QA tool or backup for commercial dosimeters. It is also educationally innovative because it helps physicists to comprehend the underlying physics of complex VMAT procedures. Methods: The workflow of the cloud‐based voxel Monte Carlo method(cVMC++) developed here is the following: (1) After a VMAT plan is approved by the physician, a dose verification plan is created and delivered to the phantom using the Varian Trilogy™ or TrueBeam™ system. (2) Actual delivery parameters (i.e., dose fraction, gantry angle, and MLC leaf position) are extracted from dynamic log or trajectory files. (3) Based on the delivery parameters, a new phase space is constructed for each control point. The 3D dose distribution in the phantom containing the detector array (e.g., Delta 4 from ScandiDos) are then recomputed using cVMC++. (4) Comparison and Gamma index analyses are then conducted to evaluate the agreement between measured, recomputed, and planned dose distributions. To enhance the educational function, we are developing visualization and animation modules to display the intermediate processes such as leaf movement, dose distribution and DVH accumulation with each control point. The software has been wrapped as a web‐based service so that it can readily be used by physicists via a browser. To test the robustness of this system, we examined several representative VMAT treatments. Results: For all cases, agreement between measured, recomputed and planned doses was examined with the gamma criterion of &gt;90% of the points satisfying &lt;3% of relative dose difference and &lt;3 mm of distance to agreement. Conclusion: The proposed method offers a robust patient specific QA tool for VMAT treatments and provides a valuable educational tool for physicists. NIH (1R01 CA133474) and Department of Radiation Oncology of Stanford University Seed Grant

A study on rectal dose measurement in phantom and in vivo using Gafchromic EBT3 film in IMRT and CyberKnife treatments of carcinoma of prostate.

The objective of this study is to check the feasibility of in vivo rectal dose measurement in intensity-modulated radiotherapy (IMRT) and CyberKnife treatments for carcinoma prostate. An in-house pelvis phantom made with bee's wax was used in this study. Two cylindrical bone equivalent materials were used to simulate the femur. Target and other critical structures associated with carcinoma prostate were delineated on the treatment planning images by the radiation oncologist. IMRT treatment plan was generated in Oncentra Master Plan treatment planning system and CyberKnife treatment plan was generated in Multiplan treatment planning system. Dose measurements were carried out in phantom and in patient using Gafchromic EBT3 films. RIT software was used to analyze the dose measured by EBT3 films. The measured doses using EBT3 films were compared with the TPS-calculated dose along the anterior rectal wall at multiple points. From the in-phantom measurements, it is observed that the difference between calculated and measured dose was mostly within 5%, except for a few measurement points. The difference between calculated and measured dose in the in-patient measurements was higher than 5% in regions which were away from the target. Gafchromic EBT3 film is a suitable detector for in vivo rectal dose measurements as it offers the possibility of analyzing the dose at multiple points. In addition, the method of extending this in vivo rectal dose measurement technique as a tool for patient-specific quality assurance check is also analyzed.

SU-E-T-188: Patient Specific Quality Assurance for Volumetric Modulated Arc Therapy (RapidArc) Using COMPASS 3D Dosimetry System

Purpose: To implement the COMPASS (IBA, Inc) quality assurance (QA) system as a patient specific QA tool for Volumetric Modulated Arc Therapy. Methods: RapidArc is a treatment technique which produces conformal dose distribution by delivering the dose in a rotational fashion while simultaneously changing MLC position, dose rate as well as gantry speed. The COMPASS has the potential to calculate and display the delivered 3D dose distribution on a patient CT data by using beam modeling, dose map from detector measurements (I‐MatriXX Evolution) and dose map reconstruction using Collapsed Cone Convolution Algorithm. Dose maps for 10 Ten RapidArc plans were measured using I‐MatriXX Evolution with a gantry mount (SSD=76.2cm) along with gantry angle sensor. This device captures the RapidArc plan delivery in real‐time in a pre‐treatment QA context. The measurement data were read directly by the control software, which provides the ability to import patient plan data from the treatment planning system via DICOM export. The COMPASS software also provides the user a dose calculation engine, including a physics based head fluence model. The doses and dose‐volume histograms reconstructed from the fluence measurements were compared to the TPS calculated plans.Results: Maximum and minimum deviation of PTV Volume receiving 95% of the prescribed dose(V95%) was found to be −2.89% and 0.16% respectively and mean deviation −1.12%. Average deviation of maximum and mean doses of OAR's (organ at risk) were −0.5880±0.012% and − 0.6952±0.011% respectively. Average 3D mean gamma for 3mm and 3% criteria was found to be 0.067±0.0013 and Maximum absolute dose deviation at isocentre was 1.3573% Conclusions: Compass system can be used as an accurate and visually enhanced patient specific QA tool for VMAT plans to provide a complete clinical relevance of dose discrepancies for better patient treatment.