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Purpose To evaluate the performance of a commercial plastic scintillator detector (PSD) for small-field stereotactic patient-specific quality assurance (QA) measurements using flattening-filter-free beam. Methods A total of ten spherical targets (volume range: [0.03cc-2cc]) were planned with two techniques: (1) Dynamic Conformal Arc (DCA-10 plans), and (2) Volumetric Modulated Arc Therapy (VMAT-10 plans). All plans were generated using Varian Eclipse treatment planning system, and AcurosXB v.13 algorithm in 1.0 mm grid size. Additionally, 14 previously-treated cranial and spine SRS plans were evaluated (6 DCA, 8 VMAT, volume range: [0.04cc-119.02cc]). Plan modulation was quantified via two metrics: MU per prescription dose (MU/Rx) and Average Leaf Pair Opening (ALPO). QA was performed on the Varian Edge linear accelerator equipped with HDMLC. Three detectors were used: (1) PinPoint ion chamber (PTW; active volume 0.015cc), (2) Exradin W1 PSD (Standard Imaging; active volume 0.002cc), and (3) Gafchromic EBT3 film (Ashland). PinPoint chamber and PSD were positioned perpendicular to beam axis in a Lucy phantom (Standard Imaging); films were placed horizontally capturing the coronal plane. Results PSD, film and PinPoint chamber measured average differences of 1.00±1.54%, 1.30±1.69%, and -0.66±2.36% respectively, compared to AcurosXB dose calculation. As the target volume decreased, PinPoint chamber measured lower doses (maximum -5.07% at 0.07cc target), while PSD and film measured higher doses (2.87% and 2.54% at 0.03cc target) than AcurosXB. Film agreed with the benchmark detector PSD by an average difference of 0.31±1.20%, but suffered from larger uncertainty; PinPoint chamber underestimated dose by more than 4% for targets smaller than 0.2cc. Taking PSD as the measurement standard, DCA plans achieved good QA results across all volumes studied, with an average difference of -0.07±0.89%; for VMAT plans, PSD measured consistently higher dose (1.95±1.36%) than AcurosXB. Correlation study revealed that plan modulation quantified by both MU/Rx and ALPO correlated significantly with QA results. Conclusion Among all three detectors, PSD demonstrated superior performances in plans with small fields and heavy modulation. High consistency and low uncertainty made PSD a suitable detector for clinical routine SRS QA. PinPoint chamber should be avoided for targets smaller than 0.2cc; film dosimetry can be utilized with careful evaluation of its uncertainty bracket. Compared to PSD measurements, AcurosXB calculation demonstrated high accuracy for non-modulated small fields. The positive correlation between plan modulation and QA discrepancy calls for our attention for clinical SRS plans with high modulation. This article is protected by copyright. All rights reserved.

Purpose: To use end-to-end dosimetric measurements with Gafchromic film to evaluate the effects of focal spot size parameter for small-field dose calculations using AAA for SRS lesions. Methods: A total of 13 plans, corresponding to 7 patients previously treated with cranial SRS, were analyzed in this study (target volume range:[0.67cc,13.9cc]). The plans included DCA delivery (4 plans total) and VMAT delivery (9 plans total). All plans were mapped to a solid water phantom (15 cm thickness; isocenter and film plane at 7.5 depth). Dose calculation was performed with AAA v.11 (1.0mm grid size); three focal spot size settings were tested: 0mm, 0.5mm, and 1.5mm. For each plan, three calculated doses (corresponding to each focal spot size setting) were compared to measured film dose using quantitative methods [Gamma Analysis(1%,1mm,10% threshold criteria)] and qualitative methods (visual dose profile comparison). Film calibration and analysis were performed using in-house calibration methods and software package. Results: Gamma(1%,1mm) analysis passing rate results [mean(st.dev){%}] were as follows. For DCA plans: 98.74(0.54)-[0mm Focal Spot Size]; 98.24(1.26)-[0.5mm Focal Spot Size]; 95.42(2.29)-[1.5mm Focal Spot Size]. For VMAT plans: 98.75(0.54)-[0mm Focal Spot Size]; 98.89(0.73)-[0.5mm Focal Spot Size]; 97.43(1.30)-[1.5mm Focal Spot Size]. The majority of failing points (Gamma value>1.0) were found to be within the high dose region for all Focal Spot Size calculation models. Visual inspection of the dose profile, showed that the 1.5mm Focal Spot size calculation exhibited blurring in the high dose region (defined as >85% of the peak dose), resulting in a more gradual shoulder of the dose profile relative to measurements. Conclusion: The dose calculation accuracy of DCA and VMAT plans is paramount for SRS treatment planning. Our results indicate similar behavior of the AAA model with focal spot sizes of 0mm and 0.5mm, while 1.5mm focal spot size tends to result in blurring of the high dose region. Henry Ford Health System has research agreements with Varian and Philips.

Purpose: Investigate use of standardized non-coplanar arcs to improve plan quality in lung Stereotactic Body Radiation Therapy(SBRT) VMAT planning. Methods: VMAT planning was performed for 9 patients previously treated with SBRT for peripheral lung tumors (tumor size:12.7cc to 32.5cc). For each patient, 7 VMAT plans (couch rotation values:0,5,10,15,20,25,and 30 deg) were generated; the coplanar plans were pushed to meet the RTOG0915 constraints and each non-coplanar plans utilized the same optimization constraints. The following plan dose metrics were used (taken from RTOG 0915): D-2cm: the maximum dose at 2 cm from the PTV, conformality index (CI), gradient index (GI), lung volume receiving 5 Gy (V5) and lung volume receiving 20 Gy (V20). The couch collision clearance was checked for each plan through a dry run using the couch position from the patient’s treatment. Results: Of the 9 cases, one coplanar plan failed to meet two protocol guidelines (both gradient index and D-2cm parameter), and an additional plan failed the D-2cm parameter. When introducing at least 5 degree couch rotation, all plans met the protocol guidelines. The largest feasible couch angle available was 15 to 20 degrees due to gantry collision issues. Non-coplanar plans resulted in the average (standard deviation) reduction of the following metrics: GI by 7.3% (3.7%); lung V20 by 11.1% (3.2%); D-2cm by 12.7% (3.9%). The CI was unchanged (−0.3%±0.6%), and lung V5 increased (3.8%±8.2%). Conclusion: The use of couch rotations as little as 5 degrees allows for plan quality that will meet RTOG0915 constraints while reducing D-2cm, GI, and lung V20. Using default couch rotations while planning SBRT cases will allow for more efficient planning with the stated goal of meeting RTOG0915 criteria for all clinical cases. Gantry clearance checks in the treatment room may be necessary to ensure safe treatments for larger couch rotation values.

Purpose: In image‐guidedradiotherapy(IGRT), cone‐beam CT(CBCT) scan parameters do not take into account patient size information, which can result in unnecessary dose. So‐called “smart scans” in radiology attempt to customize exposure parameters to reduce dose without sacrificing image quality. This work presents the feasibility of tube current modulation as a dose reduction technique in CBCT‐based IGRT. Methods: A CATPHAN® phantom with water‐equivalent annulus was used to evaluate noise. A CIRS phantom was used to evaluate dose characteristics. Gies et. al.[Med Phys, 26:1999], tells us that noise for the central pixel is minimal for tube current modulated by the square root of the given angular attenuation. This formulation is used to calculate modulation factors for each 10 degree imaging arc. Modulated image dataset was generated by compositing projection data from scans performed with each modulated current setting. Noise characterization was performed by analyzing the standard deviation of the uniformity section of the CATPHAN. Imagingdose was measured using an ionization chamber located in the phantom. For the modulated scan, each ten degree segment was measured separately and then accumulated to get a composite dose reading.Results: Central axis dose reduction of 31% was attained using this modulation scheme. Visual inspection of a modulated CBCT slices shows appreciable image quality relative to the conventional CBCT. The noise characteristics of the modulated scan were about 20 HU worse than a conventional scan, likely due to the presence of scatter in the enlarged CATPHAN phantom because theoretical predictions are based on primary beam attenuation alone Conclusions: Initial results demonstrate that the use of a modulated scanning technique, developed by taking into account patient anatomic variation, may be a feasible method to reduce dose while preserving image quality in CBCT‐based IGRT.

Purpose: To describe and evaluate a novel algorithm for generating synthetic CT images from MR-SIM data for dose calculations in MR-only treatment planning. Methods: A voxel-based weighted summation method was implemented to generate synthetic CT (synCT) images. MR data were acquired using Philips 1.0T Panorama high-field open MR-SIM. Retrospective patient data from seven prostate patients and one brain patient (three lesions) enrolled in an IRB-approved study were used. 3D T1-weighted fast field echo and 3D T2-weighted turbo spin echo sequences were utilized for all patients. A 3D balanced turbo field echo sequence using spectral presaturation with inversion recovery was acquired for prostate patients, but 3D ultra-short echo time (UTE)-DIXON was instead acquired for the brain patient to amplify bone signal for semi-automatic bone segmentation. Weight optimization was performed using a training subset of patients. HU value differences between planning CT and synCTs were analyzed using mean absolute error (MAE). Original patient CT-based treatment plans were mapped onto synCTs, dose was recalculated using original leaf motion and MU values, and DRRs were generated. Dosevolume metrics and gamma analysis were used for dosimetric evaluation. Results: Average whole-body MAE of synCTs across all patients was 75+12 HU. In prostate cancer patients, average HU difference between planning and synCTs was 0.9±1.0% for soft tissue structures and 4.3±2.5% for bony structures. DRRs were generated from synCTs and qualitatively showed good geometric agreement with planning CT-generated DRRs. D99, mean dose, and maximum dose to CTV calculated using the synCT remained within 1.2% of planning CT-based dose calculations. All gamma analysis evaluated at 2%/2mm dose difference/distance to agreement) pass rates were greater than 95% with an average of 99.9±0.1% for prostate patients and 98.4±2.2% for three brain lesions. Conclusion: SynCTs were generated with clinically acceptable accuracy comparable to planning CTs, enabling dose computations for MR-only simulation. Research supported in part by a grant from Philips HealthCare (Best, Netherlands).

Purpose: An important question rarely discussed in the CT vs. MRI simulation debate is whether MRI reference images provide an adequate image‐set to use with daily localization such as cone‐beam CT (CBCT). This study compares clinical couch shifts based on daily CBCT images to shifts measured from MR images as the reference dataset for prostate IMRT treatment. Methods: Eight patients undergoing a pilot study had MR imaging along with CT simulation with the intent of evaluating a MR‐simulation process. Patients had T1, T2 and bTFE (balanced Turbo Field Echo) sequences. The remainder of the treatment planning process continued using traditional procedures using CT. Therapists used only the CT scan as a reference for localization. Retrospectively, an observer measured shifts between daily CBCT images and MR reference images. Results: The differences in shift positions for the cohort between therapists and the observer are −0.16cm ± 0.25cm (AP), 0.04cm ± 0.19cm (SI), and — 0.01cm ± 0.14cm (LR). The mean group error for the therapists and the observer were less than 2 mm in all directions. Based on these shifts, the calculated margins for the therapists would be 0.87cm (AP), 0.65cm (SI), and 0.71cm (LR) and for the observer would be 1.1cm (AP), 0.66cm (SI), and 0.70cm (LR). For SI and LR directions both sets of margins are very close to one another. An outlier impacted the AP margin difference by 2.3mm and should be investigated further. This initial analysis suggests that each modality can be considered clinically sufficient for daily localization. Conclusion: The results of this study suggest that MR reference image‐sets can be used for daily image‐guided localization of prostate cancers with at least the same accuracy as current methods. MR simulation provides substantial soft‐tissue contrast and can improve tissue targeting in radiation oncology, as a Result further investigation is warranted.

Purpose: To establish a centralized database platform for radiotherapyquality assurance (QA). This can be an interactive platform where physicists can download digital benchmark cases from, perform radiotherapy planning and QA measurements with various planning and delivery systems, and upload the results. Data retrieval and analysis functions can then be developed for the quality assessment of the radiation plan and measurement in the context of prior data with similar planning, delivery and measurement technologies. Data mining of the system can in turn generate quantitative guidance for clinical trial QA requirements that have the optimum combination of stringent mandates and realistic accommodations. Methods: Built upon prior experience of Task Group(TG) 119 from IMRT workgroup of AAPM, and digital data phantoms from EORTC, these groups collaborate with clinical trial work group of AAPM and advanced technology consortium (ATC) to establish a virtual QA environment within National Biomedical Imaging Archive (NBIA). Digital phantoms from TG 119 and EORTC dosimetric evaluation with DICOM image, structures, plan and dose were uploaded to the virtual QA environment for the established community to download and perform evaluation of basic dosimetric parameters including, structure volumes, geometric location of structure center, designated points on dose volume histogram (DVH). Nine different treatment planning or related systems with various versions are included in these evaluations. Results: Volume variations among versions of these nine systems are 2.7%, 4.2%, 14.7%, 4.7% for prostate PTV, prostate, rectum and bladder respectively. DVH reading differences are 0.1%, 2.4% and 1.5% for prostate D95, rectum D30 and bladder D30. DVH reading differences for cube 10 are 1%, 86% and 100% for V0, V20 and V30 respectively. Conclusions: A virtual QA environment has been established within NBIA for radiation oncology inter‐system and inter‐institution QA. Initial testing in reading geometric and dosimetric parameters identified significant variations among systems.

Purpose: Needle applicators have shown displacement between fractions during HDR prostate brachytherapy, which will degrade plan quality. Typically, displacement is determined by analyzing catheters on a verification CT (vCT) or port films obtained before treatment relative to fiducials or bony landmarks. We present a procedure to verify applicator positions to achieve planned dose distributions by registering vCT directly to planning CTs (pCT). Method and Materials: Patients received a vCT prior to each fraction. This was imported to BrachyVision (Varian Medical Systems) and fused to the pCT by rigid‐body registration based on matching urethra and three prostate‐implanted gold fiducials. Then, applicators in the vCT were compared to applicators in the pCT in a reconstructed plane through each catheter and any difference larger than 3mm was manually adjusted by a radiation oncologist before treatment. To assess treatment quality, the prostate volume was copied from pCT to vCT and the planned dwell positions/time was applied to the vCT applicators. Results: Two consecutive patients were treated using our CT registration needle verification method. Both patients required applicator adjustments in the first two fractions. Patient one had an average 7mm cranial‐to‐caudal applicator displacement, corresponding to 23% and 12% fractional drop in V100prostate respectively; Patient two had displacement in both cranial‐to‐caudal and caudal‐to‐cranial directions up to 9mm, but due to minimum loading of affected applicators, no significant decrease in V100prostate was seen. Neither patient required applicator adjustment at the Srd/last fraction. Despite differences in applicator displacement, post‐adjustment, the achieved V100prostate at each fraction deviated from planned value by less than 4% for both patients. The distances between fiducials in vCTs were consistent with those in pCTs within 3mm, confirming that fiducials are appropriate landmarks for registration. Conclusion: We have successfully treated two patients using this method. This technique ensures the treatment quality closely matches plan quality.

Purpose: Patient specific IMRT QA has been an integral part of the IMRT process in most institutions since the advent of IMRT. There has been the question whether we need to continue with this practice for every patient now that we have data for multiple similar cases. However, there have always been singular cases that fail IMRT QA criteria. This study looks to correlate treatment planning parameters and other metrics with the IMRT QA passing rate hoping to predict cases which may still require IMRT QA measurements. Methods and Materials: 15 head and neck treatment plans (101 total treatment beams) were delivered and analyzed using a 2‐dimensional diode array (MapCheck®). Each treatment beam was analyzed using a gamma analysis with evaluation criteria of 10 percent threshold, 3 percent dose deviation and 3 millimeters distance‐to‐agreement as this is the standard for our institution. Parameters used to correlate with passing rate were: monitor units delivered, dose at isocenter, delivery efficiency (MU/cGy), number of segments, average leaf pair opening, and a 2‐dimensional modulation index developed from the original Webb equation. Results: Univariate analysis showed no factors that correlated with passing rate. The R‐squared values ranged from 0.0014 to 0.0565. MU number did have a p value of 0.016, but no other indicator of plan complexity predicted passing rate. Conclusions: This work demonstrates our attempt to show a relationship between IMRT QA passing rates and treatment plan parameters. We are unable to correlate any of these metrics with patient QA pass rate as specified. Treatment delivery uncertainties (e.g. flatness and symmetry) may play as pivot a role in QA passing rate as the treatment planning parameters used. Further investigation will involve combination of these uncertainties.

Purpose: To commission CMS Monaco IMRTtreatment planning system for clinical applications using a standard test suite. Methods and Materials: A system test including IMRT planning and plan delivery was performed for CMS Monaco IMRTtreatment planning system, version 1.0.2. Four cases from TG‐119 were studied: one head and neck case, one prostate case, and two C shape cases. IMRT plans were generated with the dose goals set by TG‐119 and were compared with the plans generated using CMS XiO treatment planning system. Analysis metrics were for dose coverage, number of segments, total MU, MU efficiency, number of segments per beam, and delivery time. The IMRT plans were delivered on an Elekta Synergy linear accelerator with step‐and‐shoot technique. This linac has 4‐mm MLC.IMRT QA was performed with field by field review using a diode array device Mapcheck. Results: Monaco plans showed similar target dose coverage as XiO plans and improved organ sparing in some cases. The ratios (Monaco/XiO) of number of total segments, total MU, number of segments per beam, and MU efficiency (Dose/MU) were 0.21–0.76, 0.56–0.77, 0.21–0.50, and 1.31–1.86, respectively. The delivery time of a Monaco plan was shorter than the XiO plan having the same number of beams by approximately 33%. IMRT QA pass rates of the Monaco plans were 93.8%–97.6% for the criteria of 3‐mm, 3%, and 10% threshold, which in three cases were higher than the XiO plans (93.1%–96.6%). Conclusion: The IMRT plans from the Monaco treatment planning system showed advantages over XiO plans. These advantages include smaller number of segments, smaller MU, smaller number of segments per beam, and higher MU efficiency. Additionally, the delivery time was shorter. The QA pass rates were similar to or higher than the XiO plans.

Purpose: With the increased presence of volumetric radiotherapy cone beam CT (RT‐CBCT) systems, the importance of high grade image quality is crucial to achieve daily image‐guided adaptive radiotherapy (IGART) capabilities. We subjected three commercially available RT‐CBCT systems to a battery of standard diagnostic image quality tests acquired under clinical conditions. This study reports on the evaluation of image qualities for RT‐CBCT's and a diagnostic CT.Methods and Materials: RT‐CBCT scans were performed on Elekta XVI, Nucletron Simulix, and Varian OBI Advanced Imaging using clinical pelvis scan settings on a CATPHAN Model 600. They were then compared to results from a GE Lightspeed CT scanner. The phantom contained modules allowing measurement of low contrast resolution, slice thickness, Hounsfield Unit (HU) sensitivity, spatial resolution, and image uniformity. Results: No CBCT system was able to detect any targets in the low contrast module. Six targets could be seen on diagnostic CT's. All RT‐CBCT systems had greater than 33% variations in slice thickness reconstruction. The HU sensitometry test showed absolute differences between accepted HU values and measured values for XVI and Simulix systems to be on average 6 times and 3 times the error seen in a diagnostic CT respectively. HU sensitivity for OBI is within 15% of a diagnostic CT. Both OBI and Simulix had spatial resolution twice that of XVI but were 3 lp/cm worse than a diagnostic CT. Measurements of image uniformity showed that all three RT‐CBCT systems have a standard deviation of HU values on the order of 10 times that of a diagnostic CT.Conclusions: The image qualities of three evaluated RT‐CBCT systems are relatively comparable, yet still inferior to those from helical CT. It is important to note that dosimetric settings can have a clear impact on image quality, and dose measurements were not done for this work.

Purpose: To evaluate the feasibility of respiratory gating with respect to Active Breathing Control (ABC) for motion reduction through quantitative comparison. Methods and Materials: 4D scans from a GE(Light speed)scanner with RPM were used for the feasibility study. Ten phase reconstructions were generated from the 4D projections and target volumes on all 10 phases were created by radiation oncologists. For free breathing treatments, target volumes from all 10 phases were added to generate the ITV used for clinical target. In order to simulate the gated treatment, the three phases of exhalation which best matched the gated delivery time period were combined. The target thus obtained was then compared with those obtained from the standard scan with the ABC device in place. Targets were contoured for the ABC scan as well. A volumetric comparison of the targets was performed for the above three methods. Results: We tested the feasibility of the method on one of our patient cases for whom we utilized the ABC device for breathing motion control. The target volume for the 10 phase target was 25.49cc. The target volume from the simulated gated phases was 12.09cc and the target volume with the ABC was 11.19cc. Both the gated technique and the ABC control are able to reduce the target volume significantly, with about the same amount of reduction (52.57% and 56.1%). Conclusions: This study established quantitatively that gated delivery technique can mimic the volumetric outcome of ABC delivery for motion control. Both are able to reduce the target volume significantly. We plan to further this study with more data collection.

Purpose:Treatment planning system/delivery system combinations affect IMRT optimization. Delivery method (step and shoot vs. dynamic), leakage radiation variations (lack of secondary collimators on the Elekta SL‐S MLC head), hardware constraints and leaf edge shape all factor in when a radiation treatment planning system (RTPS) optimizes MLC segments. Different optimization algorithms within treatment planning systems influence IMRT planning results as well. In our study we evaluated delivery efficiencies from Elekta, Varian and Siemens machines coupling with two different treatment planning systems. Method and Materials: 16 IMRT plans were created for prostate and headneck (HN) sites (5 and 6 co‐planar beams respectively) using CMS Xio and Philips ADAC Pinnacle. Plans were optimized using the same dose and DVH objectives and modulated using clinical thresholds. The monitor units needed to deliver the weight point dose for each field were tabulated for each site and machine source data. Results: All plans have similar quality meeting the objectives. HN plans have an average MU efficiency of 5.02 +/− 1.09 MU/cGy. The plans created on the Pinnacle RTPS had a lower mean efficiency but was not statistically significant due to the high standard deviation. Prostate plans were more efficient than the HN plans with an average value of 2.19 +/− 0.47 MU/cGy. Pinnacle prostate plans had a mean MU efficiency of 1.69 +/− 0.18 MU/cGy compared to 2.51 +/− 0.22 MU/cGy from Xio. Conclusion: There is no statistical difference in MU efficiency for HN plans from different planning/delivery system combinations. Prostate plans have higher delivery MU efficiencies than those from HN plans, most likely due to less modulation. For the prostate plans there exist slight differences in MU efficiencies with unknown clinical significance between different planning/delivery systems.

Purpose: As IGRT strategies evolve from localization to adaptive planning, it becomes more important to acquire guidance images of higher quality. We utilized a CATPHAN to subject radiotherapyCBCTimages acquired under various conditions to the same rigorous tests and evaluation that diagnostic CTs undergo. This study reports on the evaluation of image qualities of diagnostic CT and radiotherapyCBCT and identifies the area of quality differences between the images.Method and Materials: RT‐CBCT scans were performed on Elekta XVI using pelvis scan settings on a CATPHAN Model 600. They were then compared to results from a GE Lightspeed16 multidetector CT scanner. The phantom contained modules allowing measurement of slice thickness, spatial resolution, low contrast resolution, Hounsfield Unit (HU) sensitivity and image uniformity. HU uniformity in the axial direction and geometric distortion were also measured. Results: For low contrast resolution tests, no targets could be clearly defined in the medium resolution reconstruction on RT‐CBCT. Only three spheres could be seen with the high resolution reconstruction (not used clinically) compared to six on diagnostic CTs. Calculated thicknesses for a 1.0 mm slice varied from 1.33 to 2.45 mm. The HU sensitometry test showed large variation from accepted HU values of the targets. These variations were not linear or affected by the reconstruction method. Spatial resolution for clinical settings (medium resolution) was constant at three lp/cm and improved to ten lp/cm with the high resolution acceptance testing reconstruction. This is comparable to diagnostic CT resolution. Axial direction HU uniformity varied in a stepwise fashion as the uniformity module moved away from the CBCT isocenter. No measurable geometric distortion was found as a function of distance from isocenter. Conclusion: A consolidated CBCT QA procedure is feasible for monitoring changes in image acquisition systems over time for image quality evaluation in adaptive therapy.

Purpose: Commissioning treatment planning systems for IMRT require more tests and verification of the beam model than those required for 3D conformal techniques. This study presents the dependence of IMRT QA pass/fail rate on the modeling of MLC leaf transmission in a MiniMLC linear accelerator. This particular modeling parameter doesn't affect the tests for 3D conformal situations per commissioning guideline. The results of both film QA and Mapcheck analysis will be presented. Method and Materials: Elekta Synergy S linac equipped with Beam Modulator collimating head was used (4mm leaves, no movable backup diaphragms) Beams were modeled in CMS XIO following the guidelines of TG53. IMRT plans were calculated for a phantom target (mock prostate) and the accuracy of the dose delivery was measured using both film and Mapcheck Gamma analysis. Then, the beam model in XIO planning system was replaced by the model with substantially lower MLC transmission factors, the plans were re‐optimized, and re‐delivered. Results: Original beam fitting (following linac commissioning) was internally verified by at least two independent measurements and checked against standard datasets (average passing rate of 97.8%). However, in our test plans, these fits showed relatively poor agreement between planned and delivered treatment as verified by both film and Mapcheck‐based dosimetry. Average passing rate using film dosimetry was 88% and using Mapcheck it was 71%. After beam re‐fitting using the decreased MLC Transmission factor, the agreement between planned and delivered dose increased to 98% and 87%, respectively. Conclusion: Commissioning of treatment planning systems for IMRT should include measurement verifications for IMRT plans of relatively complex modulation, in addition to standard tests from guidelines for 3D conformal planning.