SU-E-I-13: Evaluation of CT Number with Changing Scan Parameters for Nucletron Simulix Evolution CBCT.

PURPOSE: To quantify the variation in CT number generated by the Simulix Evolution CBCT with changes in scan length and phantom thickness. METHODS: Three phantoms were used in this study: CIRS Model 610 AAPM CT Phantom, Gammex 467 Tissue Characterization Phantom, and Catphan 600 phantom. The AAPM Phantom was used to assess the variation of HU with phantom thickness. Scans were acquired with two field size settings (full- and half-beam) with and without a 3.5 cm thick ring. The Catphan and Gammex phantoms were used to assess the Simulix's capability of producing a consistent CT-to-ED conversion table with different scan lengths, ranging from 1 cm (very thin) to 20 cm (clinical use). The data were also compared to data acquired with our in-house CT Sim (GE HiLite LightSpeed 16 slice). RESULTS: The AAPM phantom scans with and without the ring yielded an average difference in HU of 145 HU (full-beam) and 74 HU (half-beam) for each of five inserts. The HU for Cortical Bone (SB3) [largest Gammex electron density insert; 1.69] ranged from 923 to 1170 HU for the 4 cm and 1 cm scan lengths, respectively. The HU for Teflon [largest Catphan electron density insert; 1.867] ranged from 657 to 951 for the 20 cm and 1 cm scan lengths, respectively. The HU for air in Catphan ranged from -749 to -905, and the HU for LDPE [electron density 0.944] ranged from -82 to -42, for the 20 cm and 1 cm scan lengths, respectively. CONCLUSIONS: Results show a large variability in the calculated CT number with differences in phantom thickness, as evidenced by the results with the AAPM phantom. In addition, there appears to be a dependence on scan length, attributed to increased scatter contribution. Further tests will be done to evaluate the appropriateness of the use of the Simulix CBCT unit for heterogeneity corrected external beam treatment planning. The author has received no funding during the course of this research.

SU-E-J-75: Practical Annual Quality Assurance Program for a Real Time Optical Tracking System.

PURPOSE: To verify the accuracy of a real time optical surface tracking system using a practical annual quality assurance program with an anthropomorphic breast phantom. METHODS: An anthropomorphic breast phantom was used to determine the accuracy of registration, shift, source to surface distance (SSD), and live monitoring features of the AlignRT system. Both the two and three camera AlignRT systems were tested. Registration and shift accuracy were evaluated by comparing software output with known phantom displacements and rotations at 0, 90, and 270 couch angles. These tests were performed in static capture and live monitoring modes. SSD accuracy was determined by comparing AlignRT results with expected values from the treatment planning system and optical distance indicator at 0, 90, and 270 degree couch rotations. RESULTS: For the two camera system, registration accuracy at 0, 90, and 270 degree couch angles was within 1.0mm in the lateral, longitudinal, and vertical directions and 0.04 degree. For shifts of +/-10mm and +/-5 degree, static mode shift accuracy was within 1.6mm and 0.23 degree. Registration accuracy of the shifted phantom in live monitoring mode was within 1.7mm and 0.2 degree. For the three camera system, registration accuracy at 0, 90, and 270 degree couch angles was within 1.1 mm in the lateral, longitudinal, and vertical directions and 0.2 degree. For shifts of +/-10mm and +/-5 degree, static mode shift accuracy was within 0.9mm and 0.2 degree. Registration accuracy of the shifted phantom in live monitoring mode was within 1.1mm and 0.2 degree. SSD determination was accurate to within 2mm for both systems and modes. CONCLUSIONS: The AlignRT system accuracy can be easily verified for annual quality assurance using a realistic clinical setup to within 2mm/0.5 degree. System accuracy was comparable for both two and three camera systems in static and live monitoring modes.

SU-E-T-576: Investigation of Combining Modulated Electron Beams with Intensity Modulated Photons for Radiation Therapy of Breast Cases.

PURPOSE: Modulated electron radiation therapy (MERT) can offer significant advantages for breast treatments over conventional radiotherapy in terms of sparing distal critical structures. While intensity modulated radiation therapy (IMRT) has the advantage of achieving better dose homogeneity inside the target combining both MERT and IMRT will be the ideal scenario. The Aim of the present study is to investigate the possibility of further improving breast radiation therapy using combined MERT/IMRT treatment technique. METHODS: Accurate modeling of a prototype motorized electron multileaf collimator was verified in a separate study. In this work treatment planning was performed by an in house Monte Carlo based inverse planning system. Dose deposition coefficients were calculated using MCPLAN and utilizing real patients CTs. Optimization is then conducted based on an equivalent uniform dose objective function. MERT and IMRT plans were created for different patients. RESULTS: The clinical beneficial outcome for MERT either alone or combined with IMRT was investigated based on isodose distributions and dose volume histograms. It is shown that MERT can give similar dose distributions as IMRT in some cases. For some cases, MERT could be advantageous whenever more skin dose was required. In some cases MERT can be identified as the best option. It was found that MERT compared to IMRT could introduce hot spots inside the target. However this was resolved in combined MERT/IMRT treatment. Dose uniformity can be restored with a reduction in the maximum lung and heart received dose. CONCLUSION: MERT can improve treatment plan quality for many breast patients. In some cases better results can be obtained with a combined MERT/IMRT treatment, where a homogeneous dose in the target can be achieved with an improvement in the DVH of critical structures. This work has been supported by a UICC American Cancer Society Beginning Investigators Fellowship funded by the American Cancer Society.

SU-E-T-439: First Experience of Three Dimensional Conformai Radiotherapy (3DCRT) Planning with Helical Tomotherapy.

PURPOSE: A three-dimensional conformal radiotherapy (3DCRT) has been recently introduced to helical tomotherapy, allowing the user to plan and treat patients that do not require sophisticated IMRT planning and delivery. This study aims to test treatment planning on this modality and evaluate its performance by comparing to conventional LINAC-based 3DCRT planning. METHODS: Four clinical cases (whole brain, extremity, lung, and partial breast irradiation) were retrospectively selected from a Pinnacle planning system (Philips Medical System, Fitchburg, WI) and planned on Tomotherapy (Accuray Inc., Sunnyvale, CA). Computed tomography (CT) images together with contours of target and critical structures were exported from Pinnacle to the Tomotherapy planning station. The same prescription and fractionation scheme was adopted. The pitch factor for all clinical cases was set to 0.287. A 2.5 cm jaw was employed except in the lung case the field size was set to 1.0 cm for better dose conformity. The dose grid size was chosen to be half of that of the planning CT images. On Pinnacle 100% prescription dose was delivered to the treatment isocenter while onTomotherapy it was stipulated that at least 95% of the target volume received the prescribed dose. Comparison between two planning strategies was performed, in terms of dose volume histograms (DVH), dosimetric and radiobiological parameters, for plan quality assessment. RESULTS: Comparison of DVHs reveals that up to 25% healthy tissue sparing in volume can be accomplished with Tomotherapy 3DCRT while the same target coverage is ensured. Dosimetric and radiobiological indices between Tomotherapy and Pinnacle planning agree to within 3.0%. Additional beam modifiers and non-coplanar beams associated with LINAC-based 3DCRT are not needed on Tomotherapy, making it more favorable. CONCLUSIONS: Tomotherapy 3DCRT has similar dosimetric performance when compared to conventional LINAC-based 3DCRT while it is substantially easier to use.

SU-E-T-416: Performance Test Comparing Three Pre-Treatment Isocenter Localization Techniques for Single Fraction Cranial Stereotactic Radiosurgery.

PURPOSE: To develop a performance test comparing three pre-treatment isocenter localization techniques when using head-frame vs. immobilization mask for cranial Stereotactic Radiosurgery (SRS). This study will compare pre-treatment positioning techniques using laser alignment vs x-ray verification using ExacTrac or On Board Imaging. METHODS: A RANDO anthropomorphic head phantom was fitted with an in-house polystyrene insert to allow EDR2 film measurements in two orthogonal planes. A pin hole was pricked on each film to serve as a target during treatment planning. for each trial (three total), a CT scan was acquired of the phantom equipped with either an immobilization mask or invasive head frame. Treatment planning employed iPlan Image v4.1 and iPlan Dose v4.1.1. Positioning of the phantom equipped with the head-frame was performed by aligning vault lasers to coincide with cross-hair labels on a target positioner box. Setups utilizing an immobilization mask were verified by x-ray verification using ExacTrac and On Board Imaging, and if alignment were not within tolerance, then shifts were made using a 6D robotic couch. Gantry star-shot irradiation was performed using a 5mm cone to evaluate the differences between radiation isocenter and the target. The mean and standard deviations were calculated for differences in the x-, y-, and z-coordinate axes. RESULTS: Positional accuracy using ExacTrac for mask based SRS resulted in 1.10±0.86, 0.67±0.83, and 0.59±0.48mm for cross plane, inline, and vertical measurements, respectively. Differences for frame based SRS were 0.93±0.43, 0.76±0.18, and 0.34±0.12mm for cross plane, inline, and vertical measurements, respectively. Results for mask based SRS using OBI will soon follow. CONCLUSIONS: Although the frame-based SRS techniquegenerated smaller standard deviations, the mean difference from target to radiation isocenter for both techniques fall within the statistical uncertainty of one another. Planning margins must take into account target size when treating small lesions for both techniques. Project funded by CARTI.

SU-E-T-397: Interplay Effect of Gated Lung Stereotactic Body Radiotherapy with RapidArc Delivery.

PURPOSE: To investigate the dosimetric effect of intrafraction tumor motion during gated RapidArc Stereotactic Body Radiotherapy (SBRT) delivery. METHOD: The realtime tumor motion data were retrieved from 6 lung patients. Each of them received 3 fractions of stereotactic radiotherapy treatments with Cyberknife Synchrony. Phase gating through an external surrogate was simulated with a gating window of 5 mm. The resulting residual tumor motion curves during gating (beam-on) were retrieved. RapidArc SBRT was planned on the platform of Varian Truebeam at 6 MV with 1400 MU/min. Planning target volume (PTV) was defined as physician-contoured clinical target volume (CTV) surrounded by an isotropic 5 mm margin. Each patient was prescribed with 60Gy/3 fractions. The RA plan typically consists of 2 arcs; each contains 90-120 control points. An algorithm was developed to reconstruct the delivered dose with tumor motion. The MLC segment is assumed to move relatively to a static tumor. Each MLC control point, mainly the leaf position were modified according to the probability density function of tumor motion. The newly created MLC control points were written back to the treatment file in the dicom format which was subsequently imported to treatment planning system (Varian Eclipse) for dose recalculation. RESULTS: The magnitude of dose deviation with motion is consistent with the excursion of the residual tumor movement. Overall CTV coverage of the study group is barely affected owing to the 5 mm margin. The fractional PTV dose coverage dropped by 4% at most and that from all fractions by 3%. An examination in the point dose shows an increase of 4% in the maximum dose and decrease of 10% for the minimum dose. CONCLUSION: With effective gating, interplay effect does not change the target coverage much during gated RapidArc SBRT. However it increases the dose nonuniformity inside target.

SU-E-T-437: Interfractional Dosimetric Verification of Lung Patients Treated by Passive Double Scattering Proton Radiotherapy.

PURPOSE: Proton radiotherapy, with the ability to confine the dose at desired depth, can potentially benefit lung tumor patients by significantly sparing the healthy lung tissue. However, the superior proton dose distribution could be affected by tumor shrinkage due to the quick response and by motion especially related to the respiration. Thus the treatment should be frequently verified and be adjusted accordingly if necessary to achieve the initial treatment goal. MATERIAL AND METHODS: A cohort of 20 patients were selected from lung patients treated with passive proton radiotherapy. All those patients were evaluated via 4D-CT scans and found to have tumor motion less than 1 cm. The internal target volumes (ITV) were derived based on the full inspiration and expiration phases. The average of the 4D-CT scan, full inspiration and expiration phases were used for the initial treatment planning. The planning objective was 95% of the prescription dose to at least 95% volume of the ITV. Bi-weekly verification 4D-CT scans were performed to assess the robustness of the initial treatment plan and no replanning was required for target dose variations less then 3%. RESULTS: Compared with the initial treatment plan, the standard deviations of target coverage on inspiration, expiration, and average verification CT scans are within 3% for all the patients, with the maximum difference up to 7%. No statistically significant differences were found among the initial and verification plans (p>0.1). The percentage deviations of OAR sparing were highly variable, e.g., up to 40% for mean lung dose, 100% for mean heart dose, 50% for max cord dose, particularly for OARs receiving small amount of doses. However, the absolute dose deviations are all with OAR's tolerance. CONCLUSION: Overall, the passive double scattering proton modality allows for robust proton treatment planning and delivery to treat the lung tumors with limited motion.

SU-E-T-441: A Feasibility Study to Replace Electron Cutouts with a Motorized Electron Multileaf Collimator.

PURPOSE: Fabrication of electron beam cutouts not only is a time consuming process but also involves the handling of cerrobend which is a toxic material. Hospital workers involved in cutout construction can actually be exposed to toxic fumes that are usually generated during the process. The aim of this work is to study the feasibility of replacing electron cutouts with our prototype motorized electron multileaf collimator (eMLC). METHODS: Electron beams collimated by an eMLC have very similar penumbra to those collimated by applicators and cutouts as we already demonstrated in a previous study. However undulation of the isodose curves is expected due to the finite size of the eMLC. This may be a problem when the field edge is close to critical structure. Thus ten different breast cases that were previously treated with an electron boost were selected from our database. An inhouse Monte Carlo based treatment planning system were used for dose calculation using the patients CTs. For each patient two plans were generated one with electron beams collimated using the applicator/cutout combination and the other plan with beams collimated only by the eMLC. Treatment plan quality was compared for each patient based on dose distribution and dose volume histogram. In order to determine the optimal position of the leaves, the impact of the different leaf positioning strategies were investigated. RESULTS: Results have shown that target coverage and critical structure sparing can be effectively achieved by electron beams collimated by eMLC. Preliminary results have shown that the out-of-field strategy is most conservative and would be the recommended method to define the actual leaf position for the eMLC defined field. CONCLUSION: The eMLC represents an effective time saving and pollution free device that can completely eliminate the need for patient specific cutouts. This work has been supported by a UICC American Cancer Society Beginning Investigators Fellowship funded by the American Cancer Society.

SU-E-T-648: Comparison of VMAT Vs Arc Treatment Plans for Patients Undergoing SBRT of Early-Stage Non-Small Cell Lung Cancer (NSCLC).

PURPOSE: To evaluate the dosimetric implications of using VMAT (Volume Modulated Arc Therapy) treatment planning techniques compared to traditional Arc therapy methods for patients undergoing SBRT (Stereotactic Body Radiation Therapy) for early-stage Non-Small Cell Lung Cancer (NSCLC). METHODS: Ten NSCLC cancer patients are planned with both VMAT and Arc techniques. The SBRT treatment plans comparison was quantified by several Dose-Volume Histogram (DVH) indicators including mean, maximum and minimum doses for GTV, ITV, PTV, OAR (Organs At Risk), and V95 (volume receiving at least 95% of the prescribed dose) for PTV. RESULTS: On average VMAT plans require for treatment delivery 16.6 ± 20.2 % more monitor units (MU) than the traditional Arc plans. The average PTV minimum, maximum and mean doses as a percentage of prescribed dose are 94.5 ± 3.9 %, 114.1 ± 3.3 % and 106.6 ± 1.6 % for VMAT vs 91.6 ± 4.4 %, 119.5 ± 5.3 % and 109.5 ± 2.5 % for the Arc technique. The V95 PTV coverage for VMAT plans range from 99.4 % to 100 % with a mean of 99.7 %, compared with a range of 96.8 % to 100 % with a mean of 99 % for the Arc plans. The maximum dose received by the lungs, spinal cord and chest wall show on average significant increases for Arc plans as opposed to VMAT plans (5.7 ± 6 % increase for lungs, 4.4 ± 9.2 % for cord and 2.4 ± 6.3 % for chest wall). The average mean doses and minimum doses for the OAR are similar for both techniques. CONCLUSIONS: The comparison of VMAT vs Arc plans for SBRT of NSCLC patients is subject to many variables, including GTV and PTV volume sizes, shape and their proximity relative to the OAR.

SU-E-J-24: Image-Guidance Using Cone-Beam CT for Stereotactic Body Radiotherapy (SBRT) of Lung Cancer Patients: Bony Alignment or Soft Tissue Alignment?

PURPOSE: To assess the reliability of soft tissue alignment by comparing pre- and post-treatment cone-beam CT (CBCT) for image guidance in stereotactic body radiotherapy (SBRT) of lung cancers. METHODS: Our lung SBRT procedures require all patients undergo 4D CT scan in order to obtain patient-specific target motion information through reconstructed 4D data using the maximum-intensity projection (MIP) algorithm. The internal target volume (ITV) was outlined directly from the MIP images and a 3-5 mm margin expansion was then applied to the ITV to create the PTV. Conformal treatment planning was performed on the helical images, to which the MIP images were fused. Prior to each treatment, CBCT was used for image guidance by comparing with the simulation CT and for patient relocalization based on the bony anatomy. Any displacement of the patient bony structure would be considered as setup errors and would be corrected by couch shifts. Theoretically, as the PTV definition included target internal motion, no further shifts other than setup corrections should be made. However, it is our practice to have treating physicians further check target localization within the PTV. Whenever the shifts based on the soft-tissue alignment (that is, target alignment) exceeded a certain value (e.g. 5 mm), a post-treatment CBCT was carried out to ensure that the tissue alignment is reliable by comparing between pre- and post-treatment CBCT. RESULTS: Pre- and post-CBCT has been performed for 7 patients so far who had shifts beyond 5 mm despite bony alignment. For all patients, post CBCT confirmed that the visualized target position was kept in the same position as before treatment after adjusting for soft-tissue alignment. CONCLUSIONS: For the patient population studied, it is shown that soft-tissue alignment is necessary and reliable in the lung SBRT for individual cases.

SU-E-T-201: Safety-Focused Customization of Treatment Plan Documentation.

PURPOSE: Plan report documentation contains numerous details about the treatment plan, but critical information for patient safety is often presented without special emphasis. This can make it difficult to detect errors from treatment planning and data transfer during the initial chart review. The objective of this work is to improve safety measures in radiation therapy practice by customizing the treatment plan report to emphasize safety-critical information. METHODS: Commands within the template file from a commercial planning system (Eclipse, Varian Medical Systems) that automatically generates the treatment plan report were reviewed and modified. Safety-critical plan parameters were identified from published risks known to be inherent in the treatment planning process. Risks having medium to high potential impact on patient safety included incorrect patient identifiers, erroneous use of the treatment prescription, and incorrect transfer of beam parameters or consideration of accessories. Specific examples of critical information in the treatment plan report that can be overlooked during a chart review included prescribed dose per fraction and number of fractions, wedge and open field monitor units, presence of beam accessories, and table shifts for patient setup. RESULTS: Critical information was streamlined and concentrated. Patient and plan identification, dose prescription details, and patient positioning couch shift instructions were placed on the first page. Plan information to verify the correct data transfer to the record and verify system was re-organized in an easy to review tabular format and placed in the second page of the customized printout. Placeholders were introduced to indicate both the presence and absence of beam modifiers. Font sizes and spacing were adjusted for clarity, and departmental standards and terminology were introduced to streamline data communication among staff members. CONCLUSIONS: Plan reporting documentation has been customized to concentrate and emphasize safety-critical information, which should allow for a more efficient, robust chart review process.

SU-E-T-554: PTV to Skin Proximity for Head and Neck IMRT Treatment Planning.

PURPOSE: The goal of this work was to evaluate measured vs. calculated surface dose as a function of PTV-to-skin proximity and calculation matrix oxel size, determine effects on plan quality, and provide parameters and levels of uncertainty for clinical use. METHODS: A right-sided CTV with the lateral border 5mm from the surface was delineated on the CT data of a head and neck phantom. A 5mm PTV was generated except laterally where distances of 0-5mm were used. A 7-field IMRT plan was generated using the Eclipse TPS. Optimization was performed where 95% of the PTV receives the prescription dose using a matrix size of 2mm3 . Dose calculations were repeated for grid sizes of 1, 3 and 5mm3 . For each plan nine point dose values were obtained just inside the phantom surface, corresponding to a 2cm2 grid near the central target region. Nine ultra-thin TLDs were placed on the phantom surface corresponding to the grid. Measured and calculated dose values were compared. Conformality, homogeneity and target coverage were compared. RESULTS: Surface dose is over-estimated by the TPS by 21 and 8% for 5 and 3mm3 voxels, respectively and accurately predicted for 2mm3 voxels. A voxel size of 1mm3 results in underestimation of 13%. Conformality improves with increasing PTV to skin distance and a CI of unity results for grid sizes of 1-3mm3 between 4 and 4.5mm. Hot spot decreases as the PTV moves away from the surface and falls below 110% at 4mm. Underdosage worsens as the PTV approaches the skin. CONCLUSIONS: For decreasing PTV-to-skin distance with this TPS, isodose conformality decreases, 'hot spot' increases, and target coverage degrades. Surface dose is accurately predicted for a 2mm3 voxel size, while choosing a finer or coarser grid results in underestimation or overestimation, respectively. All of the above appear to hold for VMAT.

SU-E-T-540: Evaluation of Forced-Density Corrected Dose Calculation for Lung Cancer Treatment.

PURPOSE: The study was aimed to evaluate the accuracy of lung cancer treatment dose calculations using a bulk electron density for forced-density correction, in situations where CT images are acquired in other institutions and the information of CT number to electron density (CT-to-ED) conversion is unavailable for conducting pixel density correction. METHODS: Eleven 3D SBRT lung cases were studied. Treatment plans were generated initially with pixel-density correction using a known CT-to-ED conversion, in a CMS XIO treatment planning system using superposition algorithm. The plans were re-calculated with contour-based density correction, i.e., forced-density correction: a density of 0.26 g/cm3 was assigned to lung structures, which was a population average taken from a literature, and unit density was assigned to other structures. Monitor units were kept the same in both plans. RESULTS: The doses calculated using forced-density correction were compared against those calculated using pixel-density correction. The absolute percentage differences of PTV D95, PTV mean dose, and V20, among the 11 cases, were 2.49±1.69%, 1.69±1.5%, and 1.88±2.36%, respectively. CONCLUSIONS: The results showed that the dose calculation using the bulk density and forced density correction generated dose distributions close to those calculated using pixel-density correction and actual CT-to-ED conversion. None.

SU-E-T-533: Evaluating Effective Source Position Corrections during Modeling with Pinnacle Version 9.2 Software for Flattening Free (FFF) Small Field Treatments.

PURPOSE: Advanced treatment delivery techniques require more robust treatment planning system (TPS) models for accurate dose planning and computation. As stereotactic body radiation therapy (SBRT) and volumetric-modulated arc therapy (VMAT) treatments become more prevalent, the ability of the TPS to accurately calculate dose delivered from small fields becomes of greater importance. An additional level of complexity in photon beam modeling is added with the inclusion of flattening filter free (FFF) treatment beams, due to steep in field dose gradients and non uniform penumbra regions. The purpose of this work is to examine the ability of ADAC/Pinnacle TPS to accurately model and calculate dose from small field FFF treatment beams. METHODS: Photon beam data was first modeled using Pinnacle v9.0. Flattened and FFF beam models were generated for 6MV and 10MV energies. Models were fine-tuned using a 5×5 cm2 field as the standard field in order to place more emphasis on the accuracy of the model for the small fields. An upgrade to Pinnacle v9.2 allowed for comparison between version 9.0 and version 9.2. RESULTS: Accurately modeling small fields using Pinnacle v9.0 proved difficult, particularly in the lower penumbra region. An asymmetry appeared in the small field models for both the flattened and FFF models. The Pinnacle v9.2 upgrade eliminated the small field asymmetries, and allowed for more accurate penumbra region calculations. Preliminary results have shown that Pinnacle v9.2 is capable of developing accurate beam models when an emphasis is placed on the small fields. In addition, patient specific dosimetric information for version 9.0 and version 9.2 was also calculated and examined for prostate, head & neck, and lung treatments. CONCLUSIONS: Pinnacle v9.2 showed improved accuracy in small field FFF dose modeling over version 9.0. Comparison of patient dosimetric information reflected the improvement in the small field modeling.

TU-F-BRCD-01: Tolerance Levels and Methodologies for IMRT Verification QA.

Intensity modulated radiation therapy (IMRT) is a technology intensive treatment modality involving the delivery of highly conformal dose distributions to patients. IMRT is becoming a standard of care for many disease sites and approximately 30%-60% of cancer patients in the United States receive IMRT treatments. Given the complexity of the IMRT treatment planning and delivery processes, a number of AAPM reports and guidance documents addressed the technical aspects of IMRT, including the need for comprehensive acceptance testing, commissioning, and QA programs for IMRT planning and delivery equipment. The implementation of these verification programs is essential to ensure the accuracy of IMRT delivery. Despite the critical role of patient-specific IMRT verification QA to ensure the safe delivery of IMRT treatments to patients as planned, there is little systematic guidance on the type of methodologies, tools, and acceptable tolerance levels that are needed in clinical practice. Furthermore, there are limited discussion on the pros and cons of the different delivery methods for QA measurements, and no recommendations on how to assess the clinical relevance of failed IMRT plans. LEARNING OBJECTIVES: 1. To discuss commonly employed IMRT measurement methods and discuss the pros and cons of each method. 2. To review methodologies for absolute dose verification (single small-volume, 1D, 2D methods), and review dose-difference, DTA, and Gamma analysis techniques including the variability of vendors implementation 3. To review IMRT QA passing rates for given tolerances and action levels, and discuss the clinical relevance of failed IMRT QA.