Treatment Planning System Model Research Articles

Commissioning and validation of a single photon beam model in RayStation for multiple matched Elekta Linacs.

A single treatment planning system (TPS) model for matched linacs provides flexible clinical workflows from patient treatment to intensity-modulated radiation therapy (IMRT) quality assurance (QA) measurement. Since general guidelines for building a single TPS model and its validation for matched linacs are not well established, we present our RayStation photon TPS modeling strategy for matched Elekta VersaHD linacs. The four linacs installed from 2013 to 2020 were matched in terms of Percent Depth Dose (PDD), profile, output factor and wedge factors for 6-MV, 10-MV, 15-MV, and 6-MV-FFF, and maintained following TG-142 recommendations until RayStation commissioning. The RayStation single model was built to represent all four linacs within the tolerance limits recommended by MPPG-5.a. The comprehensive validation tests were performed for one linac following MPPG-5.a and TG-119 guidelines, and spot checks for the other three. Our TPS modeling/validation method was evaluated by re-analyzing the previous 103 patient-specific IMRT/volumetric modulated arc therapy (VMAT) QA measurements with the calculated planar doses by the single model in comparison with the analysis results using four individual Pinnacle TPS models. For all energies, our single model PDDs were within 1% agreement of the four-linac commissioning measurements. The MPPG-5.a validation tests from 5.1 through 7.5 and all TG-119 measurements passed within the recommended tolerance limits. The IMRT QA results (mean±standard deviation) for RayStation single model versus Pinnacle individual models were 98.9%±1.3% and 98.0%±1.4% for 6-MV, 99.9%±0.1% and 99.1%±1.9% for 10-MV, and 98.2%±1.3% and 97.9%±1.8% for 6-MV-FFF, respectively. We successfully built and validated a single photon beam model in RayStation for four Elekta Linacs. The proposed new validation methods were proven to be both efficient and effective.

Commissioning and validation of a novel commercial TPS for ocular proton therapy.

Until today, the majority of ocular proton treatments worldwide were planned with the EYEPLAN treatment planning system (TPS). Recently, the commercial, computed tomography (CT)-based TPS for ocular proton therapy RayOcular was released, which follows the general concepts of model-based treatment planning approach in conjunction with a pencil-beam-type dose algorithm (PBA). To validate RayOcular with respect to two main features: accurate geometrical representation of the eye model and accuracy of its dose calculation algorithm in combination with an Ion Beam Applications (IBA) eye treatment delivery system. Different 3D-printed eye-ball-phantoms were fabricated to test the geometrical representation of the corresponding CT-based model, both in orthogonal 2D images for X-ray image overlay and in fundus view overlaid with a funduscopy. For the latter, the phantom was equipped with a lens matching refraction of the human eye. Funduscopy was acquired in a Zeiss Claus 500 camera. Tantalum clips and fiducials attached to the phantoms were localized in the TPS model, and residual deviations to the actual position in X-ray images for various orientations of the phantom were determined, after the nominal eye orientation was corrected in RayOcular to obtain a best overall fit. In the fundus view, deviations between known and displayed distances were measured. Dose calculation accuracy of the PBA on a 0.2mm grid was investigated by comparing between measured lateral and depth-dose profiles in water for various combinations of range, modulation, and field-size. Ultimately, the modeling of dose distributions behind wedges was tested. A 1D gamma-test was applied, and the lateral and distal penumbra were further compared. Average residuals between model clips and visible clips/fiducials in orthogonal X-ray images were within 0.3mm, including different orientations of the phantom. The differences between measured distances on the registered funduscopy image in the RayOcular fundus view and the known ground-truth were within 1mm up to 10.5mm distance from the posterior pole. No clear benefit projection of either polar mode or camera mode could be identified, the latter mimicking camera properties. Measured dose distributions were reproduced with gamma-test pass-rates of >95% with 2%/0.3mm for depth and lateral profiles in the middle of spread-out Bragg-peaks. Distal falloff and lateral penumbra were within 0.2mm for fields without a wedge. For shallow depths, the agreement was worse, reaching pass-rates down to 80% with 5%/0.3mm when comparing lateral profiles in air. This is caused by low-energy protons from a scatter source in the IBA system not modeled by RayOcular. Dose distributions modified by wedges were reproduced, matching the wedge-induced broadening of the lateral penumbra to within 0.4mm for the investigated cases and showing the excess dose within the field due to wedge scatter. RayOcular was validated for its use with an IBA single scattering delivery nozzle. Geometric modeling of the eye and representation of 2D projections fulfill clinical requirements. The PBA dose calculation reproduces measured distributions and allows explicit handling of wedges, overcoming approximations of simpler dose calculation algorithms used in other systems.

Plan quality in radiotherapy treatment planning - Review of the factors and challenges.

A high-quality treatment plan aims to best achieve the clinical prescription, balancing high target dose to maximise tumour control against sufficiently low organ-at-risk dose for acceptably low toxicity. Treatment planning (TP) includes multiple steps from simulation/imaging and segmentation to technical plan production and reporting. Consistent quality across this process requires close collaboration and communication between clinical and technical experts, to clearly understand clinical requirements and priorities and also practical uncertainties, limitations and compromises. TP quality depends on many aspects, starting from commissioning and quality management of the treatment planning system (TPS), including its measured input data and detailed understanding of TPS models and limitations. It requires rigorous quality assurance of the whole planning process and it links to plan deliverability, assessable by measurement-based verification. This review highlights some factors influencing plan quality, for consideration for optimal plan construction and hence optimal outcomes for each patient. It also indicates some challenges, sources of difference and current developments. The topics considered include: the evolution of TP techniques; dose prescription issues; tools and methods to evaluate plan quality; and some aspects of practical TP. The understanding of what constitutes a high-quality treatment plan continues to evolve with new techniques, delivery methods and related evidence-based science. This review summarises the current position, noting developments in the concept and the need for further robust tools to help achieve it.

Proton Radiosurgery: A Clinical Transition From Passive Scattering to Pencil Beam Scanning

Our institution developed proton stereotactic radiosurgery (PSRS) techniques and treated patients since 1961. A recent upgrade from passive scattering (PS) delivery to pencil beam scanning (PBS) required validation of this modality for small fields. We describe the adaptation of an existing clinical PSRS service to PBS. We outline potential advantages and disadvantages the modality has to offer.Patient alignment is determined from 2D/3D x-ray corrections of surgically implanted fiducials. End-to-end tests using a skull phantom were performed to validate a change of treatment planning system (TPS). PSRS PS fields from prior years established applicable parameters for dose, range, modulation, field size and aspect ratio for PBS validation. 18 fields were created to verify agreement with the TPS. A diamond detector was used to measure SOBP doses and film to measure profiles. A QA phantom enabled simultaneous measurement with film and diamond detector. X-rays were used to verify detector alignment prior to measurements. A hybrid 2D/3D γ-analysis was used to assess lateral profiles. Plans were generated for previously treated patients and used to perform end-to-end tests. These included 6 AVM, 3 pituitaries and 1 cavernous sinus lesion. Clinical directives were obtained from the original plans.End-to-end phantom tests conformed sub-mm alignment. Distal and proximal depth agreement were 0.10 ± 0.66% and 0.74 ± 0.48 mm. Isocenter and shallow depth dose agreement was -1.7 ± 2.9% and 2.5 ± 5.1%, respectively. Global 2%/1mm/10% γ-analysis resulted in pass rates of 97.2 ± 4.8%. 24 clinical fields were delivered. Dose agreement was -2.8 ± 2.7%. Global 3%/1mm/10% γ- analysis PASS rates were 99.5 ± 1.4%.We have revalidated SRS appropriate fiducial-based alignment and defined a small-field QA process using a diamond detector & film to demonstrate PSRS performed with PBS delivery is accurate. Proton delivery systems and TPS modeling can differ significantly. Therefore, validation and plan quality results should not be generalized to all systems. Proton range uncertainties play an important role in determining the appropriateness of PSRS. PBS does not offer distal benefits for small targets compared to passive scattering using range compensators. Larger SRS targets with irregular shapes may benefit from intensity variations and variable modulation offered by PBS. VMAT generally provides slightly superior target coverage for small pituitary lesion, but target coverage for larger pituitary lesions not abutting the optic structures and prescribed to lower doses are more comparable for VMAT and PBS while lowering integral dose. Our validation effort enabled the transition of an active PSRS program from PS to PBS.M. Bussiere: None. J. Daartz: None. J. Verburg: Employee; Massachusetts General Hospital. Research Grant; National Cancer Institute. N. Depauw: None. H.M. Kooy: None. J.S. Loeffler: Employee; Massachusetts General Hospital. Advisory Board; Mevion. P.H. Chapman: None. H.A. Shih: Employee; Dartmouth Hitchcock. Research Grant; AbbVie, NIH. Honoraria; UpToDate. Consultant; Cleveland Clinic. Speaker's Bureau; prIME Oncology. advisory; The Radiosurgery Society. director of clinical operations; Massachusetts General Hospital. clinical operational leader; Massachusetts General Hospital.

Development and evaluation of a GEANT4-based Monte Carlo Model of a 0.35 T MR-guided radiation therapy (MRgRT) linear accelerator.

PurposeThe aim of this work was to develop and benchmark a magnetic resonance (MR)‐guided linear accelerator head model using the GEANT4 Monte Carlo (MC) code. The validated model was compared to the treatment planning system (TPS) and was also used to quantify the electron return effect (ERE) at a lung–water interface.MethodsThe average energy, including the spread in the energy distribution, and the radial intensity distribution of the incident electron beam were iteratively optimized in order to match the simulated beam profiles and percent depth dose (PDD) data to measured data. The GEANT4 MC model was then compared to the TPS model using several photon beam tests including oblique beams, an off‐axis aperture, and heterogeneous phantoms. The benchmarked MC model was utilized to compute output factors (OFs) with the 0.35 T magnetic field turned on and off. The ERE was quantified at a lung–water interface by simulating PDD curves with and without the magnetic field for 6.6 × 6.6 cm2 and 2.5 × 2.5 cm2 field sizes. A 2%/2 mm gamma criterion was used to compare the MC model with the TPS data throughout this study.ResultsThe final incident electron beam parameters were 6.0 MeV average energy with a 1.5 MeV full width at half maximum (FWHM) Gaussian energy spread and a 1.0 mm FWHM Gaussian radial intensity distribution. The MC‐simulated OFs were found to be in agreement with the TPS‐calculated and measured OFs, and no statistical difference was observed between the 0.35 T and 0.0 T OFs. Good agreement was observed between the TPS‐calculated and MC‐simulated data for the photon beam tests with gamma pass rates ranging from 96% to 100%. An increase of 4.3% in the ERE was observed for the 6.6 × 6.6 cm2 field size relative to the 2.5 × 2.5 cm2 field size. The ratio of the 0.35 T PDD to the 0.0 T PDD was found to be up to 1.098 near lung–water interfaces for the 6.6 × 6.6 cm2 field size using the MC model.ConclusionsA vendor‐independent Monte Carlo model has been developed and benchmarked for a 0.35 T/6 MV MR‐linac. Good agreement was obtained between the GEANT4 and TPS models except near heterogeneity interfaces.

Dose calculation accuracy in proximity of a pacemaker: A multicenter study with threecommercial treatment planning systems.

This study compares Treatment Planning System (TPS) out of field dose calculation on a pacemaker (PMK) during external beam radiotherapy treatment. We consider four TPSs (Elekta-Monaco, Oncentra- Masterplan and two Philips-Pinnacle3) commissioned for two linacs (Elekta Sinergy and Varian Clinac) delivering two test beams (a highly modulated one and a square field) and two clinical breast plans. To calculate and measure dose to a PMK we built a Real Water3 phantom with a PMK embedded in it. Measures are performed with thermo-luminescent dosimeters and Mosfet dosimeters. We evaluate differences between TPS calculated values for the dose to the PMK (both point dose and dose-volume histogram parameters) when the PMK is positioned in the first 10cm outside the radiation fields. TPS calculation accuracy is evaluated comparing such values with measures. Differences in TPS calculations are on average 3.5cGyGy-1 for the modulated beam, and always lower than 2cGyGy-1 for the square beam. TPS dose calculation depends mostly on the TPS algorithm and model rather than the linac commissioned. TPSs considered show different degrees of calculation accuracy. In the first 4cm to the field edge three out of four TPSs are in good agreement with measurements in the square beam, but only one keeps the agreement in the modulated beam: the others show over and underestimations up to +20% -40%. The same accuracy is found considering a homogeneous phantom. Our results confirm what reported in previous studies and highlight the impact of TPS commissioning.

Monte Carlo calculations of radiotherapy dose in "homogeneous" anatomy.

Given the substantial literature on the use of Monte Carlo (MC) simulations to verify treatment planning system (TPS) calculations of radiotherapy dose in heterogeneous regions, such as head and neck and lung, this study investigated the potential value of running MC simulations of radiotherapy treatments of nominally homogeneous pelvic anatomy. A pre-existing in-house MC job submission and analysis system, built around BEAMnrc and DOSXYZnrc, was used to evaluate the dosimetric accuracy of a sample of 12 pelvic volumetric arc therapy (VMAT) treatments, planned using the Varian Eclipse TPS, where dose was calculated with both the Analytical Anisotropic Algorithm (AAA) and the Acuros (AXB) algorithm. In-house TADA (Treatment And Dose Assessor) software was used to evaluate treatment plan complexity, in terms of the small aperture score (SAS), modulation index (MI) and a novel exposed leaf score (ELS/ELA). Results showed that the TPS generally achieved closer agreement with the MC dose distribution when treatments were planned for smaller (single-organ) targets rather than larger targets that included nodes or metastases. Analysis of these MC results with reference to the complexity metrics indicated that while AXB was useful for reducing dosimetric uncertainties associated with density heterogeneity, the residual TPS dose calculation uncertainties resulted from treatment plan complexity and TPS model simplicity. The results of this study demonstrate the value of using MC methods to recalculate and check the dose calculations provided by commercial radiotherapy TPSs, even when the treated anatomy is assumed to be comparatively homogeneous, such as in the pelvic region.

Impact of the MLC leaf‐tip model in a commercial TPS: Dose calculation limitations and IROC‐H phantom failures

PurposeTreatment planning system (TPS) dose calculation is sensitive to multileaf collimator (MLC) modeling, especially when treating with intensity‐modulated radiation therapy (IMRT) or VMAT. This study investigates the dosimetric impact of the MLC leaf‐tip model in a commercial TPS (RayStation v.6.1). The detectability of modeling errors was assessed through both measurements with an anthropomorphic head‐and‐neck phantom and patient‐specific IMRT QA using a 3D diode array.Methods and MaterialsAn Agility MLC (Elekta Inc.) was commissioned in RayStation. Nine IMRT and VMAT plans were optimized to treat the head‐and‐neck phantom from the Imaging and Radiation Oncology Core Houston branch (IROC‐H). Dose distributions for each plan were re‐calculated on 27 beam models, varying leaf‐tip width (2.0, 4.5, and 6.5 mm) and leaf‐tip offset (−2.0 to +2.0 mm) values. Doses were compared to phantom TLD measurements. Patient‐specific IMRT QA was performed, and receiver‐operating characteristic (ROC) analysis was performed to determine the detectability of modeling errors.ResultsDose calculations were very sensitive to leaf‐tip offset values. Offsets of ±1.0 mm resulted in dose differences up to 10% and 15% in the PTV and spinal cord TLDs respectively. Offsets of ±2.0 mm caused dose deviations up to 50% in the spinal cord TLD. Patient‐specific IMRT QA could not reliably detect these deviations, with an ROC area under the curve (AUC) value of 0.537 for a ±1.0 mm change in leaf‐tip offset, corresponding to >7% dose deviation. Leaf‐tip width had a modest dosimetric impact with <2% and 5.6% differences in the PTV and spinal cord TLDs respectively.ConclusionsSmall changes in the MLC leaf‐tip offset in this TPS model can cause large changes in the calculated dose for IMRT and VMAT plans that are difficult to identify through either dose curves or standard patient‐specific IMRT QA. These results may, in part, explain the reported high failure rate of IROC‐H phantom tests.

Acceptance and verification of the Halcyon-Eclipse linear accelerator-treatment planning system without 3D water scanning system.

We tested whether an ionization chamber array (ICA) and a one‐dimensional water scanner (1DS) could be used instead of a three‐dimensional water scanning system (3DWS) for acceptance testing and commissioning verification of the Varian Halcyon–Eclipse Treatment Planning System (TPS). The Halcyon linear accelerator has a single 6‐MV flattening‐filter‐free beam and a nonadjustable beam model for the TPS. Beam data were measured with a 1DS, ICA, ionization chambers, and electrometer. Acceptance testing and commissioning were done simultaneously by comparing the measured data with TPS‐calculated percent‐depth‐dose (PDD) and profiles. The ICA was used to measure profiles of various field sizes (10‐, 20‐, and 28 cm2) at depths of dmax (1.3 cm), 5‐, 10‐, and 20 cm. The 1DS was used for output factors (OFs) and PDDs. OFs were measured with 1DS for various fields (2–28 cm2) at a source‐to‐surface distance of 90 cm. All measured data were compared with TPS‐calculations. Profiles, off‐axis ratios (OAR), PDDs and OFs were also measured with a 3DWS as a secondary check. Profiles between the ICA and TPS (ICA and 3DWS) at various depths across the fields indicated that the maximum discrepancies in high‐dose and low‐dose tail were within 2% and 3%, respectively, and the maximum distance‐to‐agreement in the penumbra region was <3 mm. The largest OAR differences between ICA and TPS (ICA and 3DWS) values were 0.23% (−0.25%) for a 28 × 28 cm2 field, and the largest point‐by‐point PDD differences between 1DS and TPS (1DS and 3DWS) were −0.41% ± 0.12% (−0.32% ± 0.17%) across the fields. Both OAR and PDD showed the beam energy is well matched to the TPS model. The average ratios of 1DS‐measured OFs to the TPS (1DS to 3DWS) values were 1.000 ± 0.002 (0.999 ± 0.003). The Halcyon–Eclipse system can be accepted and commissioned without the need for a 3DWS.

Modeling AeroForm tissue expander for postmastectomy radiation therapy.

The AeroForm chest wall tissue expander (TE) is a silicon shell containing a metallic CO2 reservoir, placed surgically after mastectomy. The patient uses a remote control to release compressed CO2 from the reservoir to inflate the expander. AeroForm poses challenges in a radiation therapy setting: The high density of the metallic reservoir causes imaging artifacts on the planning CT, which encumber structure definition and cause misrepresentation of density information, in turn affecting dose calculation. Additionally, convolution‐based dose calculation algorithms may not be well‐suited to calculate dose in and around high‐density materials. In this study, a model of the AeroForm TE was created in Eclipse treatment planning system (TPS). The TPS model was validated by comparing measured to calculated transmission through the AeroForm. Transmission was measured with various geometries using radiochromic film. Dose was calculated with both Varian’s Anisotropic Analytical Algorithm (AAA) and Acuros External Beam (AXB) algorithms. AAA and AXB were compared using dose profile and gamma analyses. While both algorithms modeled direct transmission well, AXB better modeled lateral scatter from the AeroForm TE. Clinical significance was evaluated using clinical data from four patients with AeroForm TEs. The AeroForm TPS model was applied, and RT plans were optimized using AAA, then re‐calculated with AXB. Structures of clinical significance were defined and dose volume histogram analysis was performed. Compared to AXB, AAA overestimates dose in the AeroForm device. Changes in clinically significant regions were patient‐ and plan‐specific. This study proposes a clinical procedure for modeling the AeroForm in a commercial TPS, and discusses the limitations of dose calculation in and around the device. An understanding of dose calculation accuracy in the vicinity of the AeroForm is critical for assessing individual plan quality, appropriateness of different planning techniques and dose calculation algorithms, and even the decision to use the AeroForm in a postmastectomy radiation therapy setting.

Commissioning and testing of treatment planning system model of an Edge accelerator with a high-definition multi-leaf collimator

Objective To investigate the commissioning and testing of the Eclipse model of an Edge accelerator with high-definition multi-leaf collimator (HD-MLC). Methods The percentage depth dose (PDD), profile, output factor measured by Razor and CC13 were statistically compared with the standard data. Penumbra, transmission factor (TF), leakage, concave-convex groove, accuracy of movement and dosimetry leaf gap (DLG) were measured with EBT3, electronic portal image device (EPID) and PTW SRS1000&SRS1500. The optimal DLG/TF was acquired when the γ pass rate of test cases was the highest. The point dose of regular fields, intensity-modulated radiation therapy (IMRT) and volume-modulated radiation therapy (VMAT) was verified with FC65-G. The planar dose of these case was verified with Octavius 4D and EBT3. Results The measured PDD data were consistent with the standard data. The measured penumbra of 3 cm and 4 cm square fields was smaller, whereas that of 6 cm square field was larger than the standard values. The left and right edge, field size, center of the field were distributed within the range of -1.0-0.4 mm、0.2-1.7 mm, -0.3-1.9 mm and -0.1-0.8 mm, respectively. The mean penumbra of the left and right MLC in different positions were (2.5±0.042) mm and (2.7±0.005) mm. The leakage of MLC was 0.009-0.016. The measured DLG/TF was 0.1861 cm/0.0116 and the optimal DLG/TF was 0.015 cm/0.014. The differences of point dose of all the test cases except the one which was in the low-dose area were within ±3%. Local and global γ pass rates of all IMRT were 79.81%-100% and 96.3%-100% (3%/3 mm), 71.3%-98.9% and 94.3%-99.8% for VMAT cases. Conclusions This method can accurately test and commission the Eclipse treatment planning model of Edge Linac equipped with HD-MLC. Key words: Commission of treatment planning model; Small-field measurement; HD-MLC feature parameter testing; Edge Linac

191. Commissioning of a high definition multileaf collimator with flattening filter free photon beams

PurposeApex is a micro multileaf collimator mounted on the Elekta Versa HD. It has 56 leaf pairs with leaf thickness of 2.46 mm at isocenter and maximum field size of 12 × 14 cm2. Apex has been commissioned on treatment planning system (TPS) Pinnacle 9.10 for high dose rate stereotactic treatment using volumetric modulated arc therapy (VMAT) technique in conventional (6/10 MV) and flattening-filter free energies (6FFF/10FFF). MethodsAll measurements were done according the request of the TPS [1]: Percentage Dose Depth, Profiles, Output factor, Absolute dose with PTW MP3-M water tank phantom using a synthetic diamond detector (PTW microdiamond 60019) and ionization chamber (PTW Farmer 30013, PTW Semiflex 31010) [2] for field dimension showed in Fig. 1; the validation of TPS model, was carried out by comparing calculated and measured doses acquired in water phantom for different source surface distance (SSD), detector depth and field configuration. Leaf leakage and transmission test was evaluated with PTW Octavius SRS 100. A comparison with multileaf Agility has been done. ResultsComparison between Agility and Apex showed an average reduction of 25%. Discrepancies between calculated and measured dose were in tolerance, 2% or 3%, according the complexity of the geometry. Mean intra leaf transmission were 0.31% and 0.40% (6/10 MV FFF). Mean inter leaf leakage were 0.32% for 6 MV FFF and 0.39% for 10 MV FFF. ConclusionsThe high resolution of multileaf gives dosimetric advantages in penumbra of profiles and less dose outside the filed, thanks to smaller leaves double focused (Table 1). Unfortunately, there is few literatures on this specific topic so our experience could be useful to other center starting a similar commissioning. The time needed to mount the micromultileaf and manual fine tuning takes several minutes. Must be taking account several electronic troubles of this technologies with high dose FFF delivered during a commissioning.

Application of detector array in treatment planning system modeling adjustment

Objective To investigate the feasibility of detector array in Monaco modeling for MLC parameters adjustment. Methods One parameter was fixed, and then the other parameter was changed. The γ pass rates of the test beams, namely 3ABUT, 7SegA, and FOUR L, were assessed to determine the values of leaf transmission and leaf offset. A total of 12 tumor cases from different anatomical sites were randomly selected. Two-dimensional dose verification (rack angle zero) of Step& Shot and dMLC plans as well as three-dimensional dose validation of VMAT plan were performed using Octavius 4D system. The γ pass rates were analyzed at a standard of 3%/3 mm. Meanwhile, the point dose verification for these three plans was analyzed to obtain the dose deviations. Results The values of leaf transmission and leaf offset were 0.0105 and-0.08 mm, respectively. The average γ pass rates (%) of Step& Shot, dMLC, and VMAT plans were 88.59±2.94, 87.81±3.28, and 87.45±2.24 before adjustment and 98.45±1.23, 98.9±1.01, and 96.03±1.66 after adjustment. In addition, the average dose deviations (%) according to the point dose verification were 0.85±0.75, 0.95±0.39, and 0.98±0.40 before adjustment and 0.97±0.57, 1.08±0.76, and 0.86±0.45 after adjustment. Conclusions Octavius detector 729 ionization chamber array is a feasible and reliable device in Monaco modeling for MLC parameters adjustment. Key words: Post modeling adjustment; Detector array; γpass rates; Leaf transmission; Leaf offset

Implementation of the validation testing in MPPG 5.a "Commissioning and QA of treatment planning dose calculations-megavoltage photon and electron beams".

The AAPM Medical Physics Practice Guideline (MPPG) 5.a provides concise guidance on the commissioning and QA of beam modeling and dose calculation in radiotherapy treatment planning systems. This work discusses the implementation of the validation testing recommended in MPPG 5.a at two institutions. The two institutions worked collaboratively to create a common set of treatment fields and analysis tools to deliver and analyze the validation tests. This included the development of a novel, open‐source software tool to compare scanning water tank measurements to 3D DICOM‐RT Dose distributions. Dose calculation algorithms in both Pinnacle and Eclipse were tested with MPPG 5.a to validate the modeling of Varian TrueBeam linear accelerators. The validation process resulted in more than 200 water tank scans and more than 50 point measurements per institution, each of which was compared to a dose calculation from the institution's treatment planning system (TPS). Overall, the validation testing recommended in MPPG 5.a took approximately 79 person‐hours for a machine with four photon and five electron energies for a single TPS. Of the 79 person‐hours, 26 person‐hours required time on the machine, and the remainder involved preparation and analysis. The basic photon, electron, and heterogeneity correction tests were evaluated with the tolerances in MPPG 5.a, and the tolerances were met for all tests. The MPPG 5.a evaluation criteria were used to assess the small field and IMRT/VMAT validation tests. Both institutions found the use of MPPG 5.a to be a valuable resource during the commissioning process. The validation testing in MPPG 5.a showed the strengths and limitations of the TPS models. In addition, the data collected during the validation testing is useful for routine QA of the TPS, validation of software upgrades, and commissioning of new algorithms.

SU-F-T-574: MLC Based SRS Beam Commissioning - Minimum Target Size Investigation

Purpose: To implement a MLC accelerator based SRS program using small fields down to 1 cm × 1 cm and to determine the smallest target size safe for clinical treatment. Methods: Computerized beam scanning was performed in water using a diode detector and a linac-head attached transmission ion chamber to characterize the small field dosimetric aspects of a 6 MV photon beam (Trilogy-Varian Medical Systems, Inc.). The output factors, PDD and profiles of field sizes 1, 2, 3, 4, and 10 cm2 were measured and utilized to create a new treatment planning system (TPS) model (AAA ver 11021). Static MLC SRS treatment plans were created and delivered to a homogeneous phantom (Cube 20, CIRS, Inc.) for a 1.0 cm and 1.5 cm “PTV” target. A 12 field DMLC plan was created for a 2.1 cm target. Radiochromic film (EBT3, Ashland Inc.) was used to measure the planar dose in the axial, coronal and sagittal planes. A micro ion chamber (0.007 cc) was used to measure the dose at isocenter for each treatment delivery. Results: The new TPS model was validated by using a tolerance criteria of 2% dose and 2 mm distance to agreement. For fields ≤ 3 cm2, the max PDD, Profile and OF difference was 0.9%, 2%/2mm and 1.4% respectively. The measured radiochromic film planar dose distributions had gamma scores of 95.3% or higher using a 3%/2mm criteria. Ion chamber measurements for all 3 test plans effectively met our goal of delivering the dose accurately to within 5% when compared to the expected dose reported by the TPS (1 cm plan Δ= −5.2%, 1.5 cm plan Δ= −2.0%, 2 cm plan Δ= 1.5%). Conclusion: End to end testing confirmed that MLC defined SRS for target sizes ≥ 1.0 cm can be safely planned and delivered.

MO-FG-202-05: Identifying Treatment Planning System Errors in IROC-H Phantom Irradiations

Purpose: Treatment Planning System (TPS) errors can affect large numbers of cancer patients receiving radiation therapy. Using an independent recalculation system, the Imaging and Radiation Oncology Core-Houston (IROC-H) can identify institutions that have not sufficiently modelled their linear accelerators in their TPS model. Methods: Linear accelerator point measurement data from IROC-H's site visits was aggregated and analyzed from over 30 linear accelerator models. Dosimetrically similar models were combined to create “classes”. The class data was used to construct customized beam models in an independent treatment dose verification system (TVS). Approximately 200 head and neck phantom plans from 2012 to 2015 were recalculated using this TVS. Comparison of plan accuracy was evaluated by comparing the measured dose to the institution's TPS dose as well as the TVS dose. In cases where the TVS was more accurate than the institution by an average of >2%, the institution was identified as having a non-negligible TPS error. Results: Of the ∼200 recalculated plans, the average improvement using the TVS was ∼0.1%; i.e. the recalculation, on average, slightly outperformed the institution's TPS. Of all the recalculated phantoms, 20% were identified as having a non-negligible TPS error. Fourteen plans failed current IROC-H criteria; the average TVS improvement of the failing plans was ∼3% and 57% were found to have non-negligible TPS errors. Conclusion: IROC-H has developed an independent recalculation system to identify institutions that have considerable TPS errors. A large number of institutions were found to have non-negligible TPS errors. Even institutions that passed IROC-H criteria could be identified as having a TPS error. Resolution of such errors would improve dose delivery for a large number of IROC-H phantoms and ultimately, patients.

Technical Report: Reference photon dosimetry data for Varian accelerators based on IROC-Houston site visit data.

Accurate data regarding linear accelerator (Linac) radiation characteristics are important for treatment planning system modeling as well as regular quality assurance of the machine. The Imaging and Radiation Oncology Core-Houston (IROC-H) has measured the dosimetric characteristics of numerous machines through their on-site dosimetry review protocols. Photon data are presented and can be used as a secondary check of acquired values, as a means to verify commissioning a new machine, or in preparation for an IROC-H site visit. Photon data from IROC-H on-site reviews from 2000 to 2014 were compiled and analyzed. Specifically, data from approximately 500 Varian machines were analyzed. Each dataset consisted of point measurements of several dosimetric parameters at various locations in a water phantom to assess the percentage depth dose, jaw output factors, multileaf collimator small field output factors, off-axis factors, and wedge factors. The data were analyzed by energy and parameter, with similarly performing machine models being assimilated into classes. Common statistical metrics are presented for each machine class. Measurement data were compared against other reference data where applicable. Distributions of the parameter data were shown to be robust and derive from a student's t distribution. Based on statistical and clinical criteria, all machine models were able to be classified into two or three classes for each energy, except for 6 MV for which there were eight classes. Quantitative analysis of the measurements for 6, 10, 15, and 18 MV photon beams is presented for each parameter; supplementary material has also been made available which contains further statistical information. IROC-H has collected numerous data on Varian Linacs and the results of photon measurements from the past 15 years are presented. The data can be used as a comparison check of a physicist's acquired values. Acquired values that are well outside the expected distribution should be verified by the physicist to identify whether the measurements are valid. Comparison of values to this reference data provides a redundant check to help prevent gross dosimetric treatment errors.

Agreement Between Institutional Measurements and Treatment Planning System Calculations for Basic Dosimetric Parameters as Measured by the Imaging and Radiation Oncology Core-Houston

To compare radiation machine measurement data collected by the Imaging and Radiation Oncology Core at Houston (IROC-H) with institutional treatment planning system (TPS) values, to identify parameters with large differences in agreement; the findings will help institutions focus their efforts to improve the accuracy of their TPS models. Between 2000 and 2014, IROC-H visited more than 250 institutions and conducted independent measurements of machine dosimetric data points, including percentage depth dose, output factors, off-axis factors, multileaf collimator small fields, and wedge data. We compared these data with the institutional TPS values for the same points by energy, class, and parameter to identify differences and similarities using criteria involving both the medians and standard deviations for Varian linear accelerators. Distributions of differences between machine measurements and institutional TPS values were generated for basic dosimetric parameters. On average, intensity modulated radiation therapy-style and stereotactic body radiation therapy-style output factors and upper physical wedge output factors were the most problematic. Percentage depth dose, jaw output factors, and enhanced dynamic wedge output factors agreed best between the IROC-H measurements and the TPS values. Although small differences were shown between 2 common TPS systems, neither was superior to the other. Parameter agreement was constant over time from 2000 to2014. Differences in basic dosimetric parameters between machine measurements and TPS values vary widely depending on the parameter, although agreement does not seem to vary by TPS and has not changed over time. Intensity modulated radiation therapy-style output factors, stereotactic body radiation therapy-style output factors, and upper physical wedge output factors had the largest disagreement and should be carefully modeled to ensure accuracy.

SU-E-T-272: Direct Verification of a Treatment Planning System Megavoltage Linac Beam Photon Spectra Models, and Analysis of the Effects On Patient Plans

Purpose: For the photon dose calculation Philips Pinnacle Treatment Planning System (TPS) uses collapsed cone convolution algorithm, which relies on energy spectrum of the beam in computing the scatter component. The spectrum is modeled based on Linac’s standard commissioning data and typically is not independently verified. We explored a methodology of using transmission measurements in combination with regularization data processing to unfold Linac spectra. The measured spectra were compared to those modeled by the TPS, and the effect on patient plans was evaluated. Methods: Transmission measurements were conducted in narrow-beam geometry using a standard Farmer ionization chamber. Two attenuating materials and two build -up caps, having different atomic numbers, served to enhance discrimination between absorption of low and high-energy portions of the spectra, thus improving the accuracy of the results. The data was analyzed using a regularization technique implemented through spreadsheet-based calculations. Results: The unfolded spectra were found to deviate from the TPS beam models. The effect of such deviations on treatment planning was evaluated for patient plans through dose distribution calculations with either TPS modeled or measured energy spectra. The differences were reviewed through comparison of isodose distributions, and quantified based on maximum dose values for critical structures. While in most cases no drastic differences in the calculated doses were observed, plans with deviations of 4 to 8% in the maximum dose values for critical structures were discovered. The anatomical sites with large scatter contributions are the most vulnerable to inaccuracies in the modeled spectrum. Conclusion: An independent check of the TPS model spectrum is highly desirable and should be included as part of commissioning of a new Linac. The effect is particularly important for dose calculations in high heterogeneity regions. The developed approach makes acquisition of megavoltage Linac beam spectra achievable in a typical radiation oncology clinic.

SU‐E‐T‐186: Cloud‐Based Quality Assurance Application for Linear Accelerator Commissioning

Purpose:To identify anomalies and safety issues during data collection and modeling for treatment planning systemsMethods:A cloud‐based quality assurance system (AQUIRE ‐ Automated QUalIty REassurance) has been developed to allow the uploading and analysis of beam data aquired during the treatment planning system commissioning process. In addition to comparing and aggregating measured data, tools have also been developed to extract dose from the treatment planning system for end‐to‐end testing. A gamma index is perfomed on the data to give a dose difference and distance‐to‐agreement for validation that a beam model is generating plans consistent with the beam data collection.Results:Over 20 linear accelerators have been commissioning using this platform, and a variety of errors and potential saftey issues have been caught through the validation process. For example, the gamma index of 2% dose, 2mm DTA is quite sufficient to see curves not corrected for effective point of measurement. Also, data imported into the database is analyzed against an aggregate of similar linear accelerators to show data points that are outliers. The resulting curves in the database exhibit a very small standard deviation and imply that a preconfigured beam model based on aggregated linear accelerators will be sufficient in most cases.Conclusion:With the use of this new platform for beam data commissioning, errors in beam data collection and treatment planning system modeling are greatly reduced. With the reduction in errors during acquisition, the resulting beam models are quite similar, suggesting that a common beam model may be possible in the future. Development is ongoing to create routine quality assurance tools to compare back to the beam data acquired during commissioning.I am a medical physicist for Alzyen Medical Physics, and perform commissioning services