Treatment Planning System Predictions Research Articles

Cherenkov imaging combined with scintillation dosimetry provides real-time positional and dose monitoring for radiotherapy patients with cardiac implanted electronic devices

Background and purposeCardiac implanted electronic devices (CIED) require dose monitoring during each fraction of radiotherapy, which can be time consuming and may have delayed read-out times. This study explores the potential of Cherenkov imaging combined with scintillation dosimetry as an alternative verification system. Methods and materialsTime-gated, complementary metal–oxide–semiconductor (iCMOS) cameras were used to collect video images of anthropomorphic phantoms and patients undergoing radiation treatment near chest wall cardiac devices. Scintillator discs and optically stimulated luminescence dosimeters (OSLDs) were used for dose measurement. Accuracy of spatial delivery was assessed by overlaying predicted surface dose outlines derived from the treatment planning system (TPS) with the Cherenkov images. Dose measurements from OSLDs and scintillators were compared. ResultsIn phantom studies, Cherenkov images visibly indicated when dose was delivered to the CIED as compared to non-overlapping dose deliveries. Comparison with dose overlays revealed congruence at the planned position and non-congruence when the phantom was shifted from the initial position. Absolute doses derived from scintillator discs aligned well with the OSLD measurements and TPS predictions for three different positions, measuring within 10 % for in-field positions and within 5 % for out-of-field positions. For two patients with CIEDs imaged over 18 fractions, Cherenkov imaging confirmed positional accuracy for all fractions, and dose measured by scintillator discs deviated by <0.015 Gy from the OSLD measurements. ConclusionsCherenkov imaging combined with scintillation dosimetry presents an alternative methodology for CIED monitoring with the added benefit of instantly detecting deviations, enabling timely corrective actions or proper patient triage.

Implementation of novel measurement-based patient-specific QA for pencil beam scanning proton FLASH radiotherapy.

Several studies have shown pencil beam scanning (PBS) proton therapy is a feasible and safe modality to deliver conformal and ultra-high dose rate (UHDR) FLASH radiation therapy. However, it would be challenging and burdensome to conduct the quality assurance (QA) of the dose rate along with conventional patient-specific QA (psQA). To demonstrate a novel measurement-based psQA program for UHDR PBS proton transmission FLASH radiotherapy (FLASH-RT) using a high spatiotemporal resolution 2D strip ionization chamber array (SICA). The SICA is a newly designed open-air strip-segmented parallel plate ionization chamber, which is capable of measuring spot position and profile through 2mm-spacing-strip electrodes at a 20kHz sampling rate (50μs per event) and has been characterized to exhibit excellent dose and dose rate linearity under UHDR conditions. A SICA-based delivery log was collected for each irradiation containing the measured position, size, dwell time, and delivered MU for each planned spot. Such spot-level information was compared with the corresponding quantities in the treatment planning system (TPS). The dose and dose rate distributions were reconstructed on patient CT using the measured SICA log and compared to the planned values in volume histograms and 3D gamma analysis. Furthermore, the 2D dose and dose rate measurements were compared with the TPS calculations of the same depth. In addition, simulations using different machine-delivery uncertainties were performed, and QA tolerances were deduced. A transmission proton plan of 250MeV for a lung lesion was planned and measured in a dedicated ProBeam research beamline (Varian Medical System) with a nozzle beam current between 100 to 215nA. The worst gamma passing rates for dose and dose rate of the 2D SICA measurements (four fields) compared to TPS prediction (3%/3mm criterion) were 96.6% and 98.8%, respectively, whereas the SICA-log reconstructed 3D dose distribution achieved a gamma passing rate of 99.1% (2%/2mm criterion) compared to TPS. The deviations between SICA measured log, and TPS were within 0.3ms for spot dwell time with a mean difference of 0.069±0.11 s, within 0.2mm for spot position with a mean difference of -0.016±0.03mm in the x-direction, and -0.036±0.059mm in the y-direction, and within 3% for delivered spot MUs. Volume histogram metric of dose (D95) and dose rate (V40Gy/s ) showed minimal differences, within less than 1%. This work is the first to describe and validate an all-in-one measurement-based psQA framework that can fulfill the goals of validating the dose rate accuracy in addition to dosimetric accuracy for proton PBS transmission FLASH-RT. The successful implementation of this novel QA program can provide future clinical practice with more confidence in the FLASH application.

Commissioning of Helium Ion Therapy and the First Patient Treatment With Active Beam Delivery

Helium ions offer intermediate physical and biological properties to the clinically used protons and carbon ions. This work presents the commissioning of the first clinical treatment planning system (TPS) for helium ion therapy with active beam delivery to prepare the first patients' treatment at the Heidelberg Ion-Beam Therapy Center (HIT). Through collaboration between RaySearch Laboratories and HIT, absorbed and relative biological effectiveness (RBE)-weighted calculation methods were integrated for helium ion beam therapy with raster-scanned delivery in the TPS RayStation. At HIT, a modified microdosimetric kinetic biological model was chosen as reference biological model. TPS absorbed dose predictions were compared against measurements with several devices, using phantoms of different complexities, from homogeneous to heterogeneous anthropomorphic phantoms. RBE and RBE-weighted dose predictions of the TPS were verified against calculations with an independent RBE-weighted dose engine. The patient-specific quality assurance of the first treatment at HIT using helium ion beam with raster-scanned delivery is presented considering standard patient-specific measurements in a water phantom and 2 independent dose calculations with a Monte Carlo or an analytical-based engine. TPS predictions were consistent with dosimetric measurements and independent dose engines computations for absorbed and RBE-weighted doses. The mean difference between dose measurements to the TPS calculation was 0.2% for spread-out Bragg peaks in water. Verification of the first patient treatment TPS predictions against independent engines for both absorbed and RBE-weighted doses presents differences within 2% in the target and with a maximum deviation of 3.5% in the investigated critical regions of interest. Helium ion beam therapy has been successfully commissioned and introduced into clinical use. Through comprehensive validation of the absorbed and RBE-weighted dose predictions of the RayStation TPS, the first clinical TPS for helium ion therapy using raster-scanned delivery was employed to plan the first helium patient treatment at HIT.

Validating a double Gaussian source model for small proton fields in a commercial Monte-Carlo dose calculation engine

PurposeThe primary fluence of a proton pencil beam exiting the accelerator is enveloped by a region of secondaries, commonly called “spray”. Although small in magnitude, this spray may affect dose distributions in pencil beam scanning mode e.g., in the calculation of the small field output, if not modelled properly in a treatment planning system (TPS). The purpose of this study was to dosimetrically benchmark the Monte Carlo (MC) dose engine of the RayStation TPS (v.10A) in small proton fields and systematically compare single Gaussian (SG) and double Gaussian (DG) modeling of initial proton fluence providing a more accurate representation of the nozzle spray. MethodsThe initial proton fluence distribution for SG/DG beam modeling was deduced from two-dimensional measurements in air with a scintillation screen with electronic readout. The DG model was either based on direct fits of the two Gaussians to the measured profiles, or by an iterative optimization procedure, which uses the measured profiles to mimic in-air scan-field factor (SF) measurements. To validate the DG beam models SFs, i.e. relative doses to a 10 × 10 cm2 field, were measured in water for three different initial proton energies (100MeV, 160MeV, 226.7MeV) and square field sizes from 1×1cm2 to 10×10cm2 using a small field ionization chamber (IBA CC01) and an IBA ProteusPlus system (universal nozzle). Furthermore, the dose to the center of spherical target volumes (diameters: 1cm to 10cm) was determined using the same small volume ionization chamber (IC). A comprehensive uncertainty analysis was performed, including estimates of influence factors typical for small field dosimetry deduced from a simple two-dimensional analytical model of the relative fluence distribution. Measurements were compared to the predictions of the RayStation TPS. ResultsSFs deviated by more than 2% from TPS predictions in all fields <4×4cm2 with a maximum deviation of 5.8% for SG modeling. In contrast, deviations were smaller than 2% for all field-sizes and proton energies when using the directly fitted DG model. The optimized DG model performed similarly except for slightly larger deviations in the 1×1cm2 scan-fields. The uncertainty estimates showed a significant impact of pencil beam size variations (±5%) resulting in up to 5.0% standard uncertainty. The point doses within spherical irradiation volumes deviated from calculations by up to 3.3% for the SG model and 2.0% for the DG model. ConclusionProperly representing nozzle spray in RayStation’s MC-based dose engine using a DG beam model was found to reduce the deviation to measurements in small spherical targets to below 2%. A thorough uncertainty analysis shows a similar magnitude for the combined standard uncertainty of such measurements.

Distributive quality assurance and delivery of stereotactic ablative radiotherapy treatments amongst beam matched linear accelerators: A feasibility study.

PurposeBeam matching occurs on all linacs to some degree and when two are more are matched to each other, patients are able to be transferred between machines. Quality assurance of plans can also be performed “distributively” on any of the matched linacs. The degree to which machines are matched and how this translates to like delivery of plans has been the focus of a number of studies. This concept has not yet been explored for stereotactic techniques which require a higher degree of accuracy. This study proposes beam matching criteria which allows for the distributive delivery and quality assurance of stereotactic body radiotherapy (SBRT) plans.MethodTwo clinically relevant and complex volumetric modulated arc therapy (VMAT) SBRT spine and lung plans were chosen as benchmarking cases. These were delivered on nine previously beam matched linacs with quality assurance performed through ArcCheck and film exposure in the sagittal plane. Measured doses were compared to their treatment planning system predictions through gamma analysis at a range of criteria.ResultsDespite differences in beam match parameters and variations in small fields, all nine linacs produced accurate deliveries with a tight deviation in the population sample. Pass rates were well above suggested tolerances at the recommended gamma criterion. Film was able to detect dose errors to a greater degree than ArcCheck.ConclusionDistributive quality assurance and delivery of stereotactic ablative radiotherapy treatments amongst beam matched linacs is certainly feasible provided the linacs are matched to a strict protocol like that suggested in this study and regular quality assurance is performed on the matched fleet. Distributive quality assurance and delivery of SBRT provides the possibility of efficiency gains for physicists as well as treatment staff.

Automation of routine elements for spot-scanning proton patient-specific quality assurance.

At our institution, all proton patient plans undergo patient-specific quality assurance (PSQA) prior to treatment delivery. For intensity-modulated proton beam therapy, quality assurance is complex and time consuming, and it may involve multiple measurements per field. We reviewed our PSQA workflow and identified the steps that could be automated and developed solutions to improve efficiency. We used the treatment planning system's (TPS) capability to support C# scripts to develop an Eclipse scripting application programming interface (ESAPI) script and automate the preparation of the verification phantom plan for measurements. A local area network (LAN) connection between our measurement equipment and shared database was established to facilitate equipment control, measurement data transfer, and storage. To improve the analysis of the measurement data, a Python script was developed to automatically perform a 2D-3D γ-index analysis comparing measurements in the plane of a two-dimensional detector array with TPS predictions in a water phantom for each acquired measurement. Device connection via LAN granted immediate access to the plan and measurement information for downstream analysis using an online software suite. Automated scripts applied to verification plans reduced time from preparation steps by at least 50%; time reduction from automating γ-index analysis was even more pronounced, dropping by a factor of 10. On average, we observed an overall time savings of 55% in completion of the PSQA per patient plan. The automation of the routine tasks in the PSQA workflow significantly reduced the time required per patient, reduced user fatigue, and frees up system users from routine and repetitive workflow steps allowing increased focus on evaluating key quality metrics.

A model-based 3D patient-specific pre-treatment QA method for VMAT using the EPID

This study reports the development and validation of a model-based, 3D patient dose reconstruction method for pre-treatment quality assurance using EPID images. The method is also investigated for sensitivity to potential MLC delivery errors. Each cine-mode EPID image acquired during plan delivery was processed using a previously developed back-projection dose reconstruction model providing a 3D dose estimate on the CT simulation data. Validation was carried out using 24 SBRT-VMAT patient plans by comparing: (1) ion chamber point dose measurements in a solid water phantom, (2) the treatment planning system (TPS) predicted 3D dose to the EPID reconstructed 3D dose in a solid water phantom, and (3) the TPS predicted 3D dose to the EPID and our forward predicted reconstructed 3D dose in the patient (CT data). AAA and AcurosXB were used for TPS predictions. Dose distributions were compared using 3%/3 mm (95% tolerance) and 2%/2 mm (90% tolerance) γ-tests in the planning target volume (PTV) and 20% dose volumes. The average percentage point dose differences between the ion chamber and the EPID, AcurosXB, and AAA were 0.73 ± 1.25%, 0.38 ± 0.96% and 1.06 ± 1.34% respectively. For the patient (CT) dose comparisons, seven (3%/3 mm) and nine (2%/2 mm) plans failed the EPID versus AAA. All plans passed the EPID versus Acuros XB and the EPID versus forward model γ-comparisons. Four types of MLC sensitive errors (opening, shifting, stuck, and retracting), of varying magnitude (0.2, 0.5, 1.0, 2.0 mm), were introduced into six different SBRT-VMAT plans. γ-comparisons of the erroneous EPID dose and original predicted dose were carried out using the same criteria as above. For all plans, the sensitivity testing using a 3%/3 mm γ-test in the PTV successfully determined MLC errors on the order of 1.0 mm, except for the single leaf retraction-type error. A 2%/2 mm criteria produced similar results with two more additional detected errors.

Study of a treatment planning system efficiency to estimate the absorbed dose by pacemakers in external radiotherapy

Introduction The number of patients with pacemakers receiving radiation therapy is increasing. It is well known that these devices are sensible to radiation and, according with the AAPM TG-34 guidelines, pacemakers should not be irradiated with primary radiation fields. Absorbed dose should be estimated before treatment and should not exceed 2 Gy. Purpose The purpose of this work was to assess the accuracy of Varian Eclipse Treatment Planning System (TPS) used to estimate the absorbed dose by pacemakers. Material and methods Two VMAT plans (faciocervical and oesophagus areas), previously used to treat patients with pacemakers, were simulated in a Rando phantom. Doses were computed on the Eclipse TPS. In both plans, the distance between the pacemakers and the border of the treatment fields was less than 15 cm. Gafchromic EBT3 films were used to measure dose. For each plan, a piece of film was placed between phantom slices at estimated pacemaker position. The plans were delivered in a Varian Trilogy linac. 2D calculated dose distributions were compared with the dose distributions of irradiated films using the software DoseLab. Calculated absorbed dose on the pacemaker was estimated by the dose-volume histogram (DVH). Results Irradiated films showed higher doses in the pacemaker than the TPS prediction: 5% in the oesophagus and 13% in the faciocervical plan. In the former case, the pacemaker is closer to the field border (8 cm vs 10.2 cm, respectively). Conclusions The TPS underestimates dose in the region where pacemakers are usually placed. Other methodologies should be implemented in the future to improve dose estimation. Disclosure The authors have no relevant financial or non-financial relationships to disclose.

TU‐AB‐BRA‐12: Quality Assurance of An Integrated Magnetic Resonance Image Guided Adaptive Radiotherapy Machine Using Cherenkov Imaging

Purpose:To investigate the viability of using Cherenkov imaging as a fast and robust method for quality assurance tests in the presence of a magnetic field, where other instruments can be limited.Methods:Water tank measurements were acquired from a clinically utilized adaptive magnetic resonance image guided radiation therapy (MR‐IGRT) machine with three multileaf‐collimator equipped 60Co sources. Cherenkov imaging used an intensified charge coupled device (ICCD) camera placed 3.5m from the treatment isocenter, looking down the bore of the 0.35T MRI into a water tank. Images were post‐processed to make quantitative comparison between Cherenkov light intensity with both film and treatment planning system predictions, in terms of percent depth dose curves as well as lateral beam profile measurements. A TG‐119 commissioning test plan (C4: C‐Shape) was imaged in real‐time at 6.33 frames per second to investigate the temporal and spatial resolution of the Cherenkov imaging technique.Results:A .33mm/pixel Cherenkov image resolution was achieved across 1024×1024 pixels in this setup. Analysis of the Cherenkov image of a 10.5×10.5cm treatment beam in the water tank successfully measured the beam width at the depth of maximum dose within 1.2% of the film measurement at the same point. The percent depth dose curve for the same beam was on average within 2% of ionization chamber measurements for corresponding depths between 3–100mm. Cherenkov video of the TG‐119 test plan provided qualitative agreement with the treatment planning system dose predictions, and a novel temporal verification of the treatment.Conclusions:Cherenkov imaging was successfully used to make QA measurements of percent depth dose curves and cross beam profiles of MRI‐IGRT radiotherapy machines after only several seconds of beam‐on time and data capture; both curves were extracted from the same data set. Video‐rate imaging of a dynamic treatment plan provided new information regarding temporal dose deposition.This study has been funded by NIH grants R21EB17559 and R01CA109558, as well as Norris Cotton Cancer Center Pilot funding.

SU-F-T-344: Commissioning Constant Dose Rate VMAT in the Raystation Treatment Planning System for a Varian Clinac IX

Purpose: To demonstrate the commissioning of constant dose rate volumetric modulated arc therapy (VMAT) in the Raystation treatment planning system for a Varian Clinac iX with Exact couch. Methods: Constant dose rate (CDR) VMAT is an option in the Raystation treatment planning system, enabling VMAT delivery on Varian linacs without a RapidArc upgrade. Raystation 4.7 was used to commission CDR-VMAT for a Varian Clinac iX. Raystation arc model parameters were selected to match machine deliverability characteristics. A Varian Exact couch model was added to Raystation 4.7 and commissioned for use in VMAT optimization. CDR-VMAT commissioning checks were performed on the linac, including patient-specific QA measurements for 10 test patients using both the ArcCHECK from Sun Nuclear Corporation and COMPASS from IBA Dosimetry. Multi-criteria optimization (MCO) in Raystation was used for CDR-VMAT planning. Results: Raystation 4.7 generated clinically acceptable and deliverable CDR-VMAT plans for the Varian Clinac. VMAT plans were optimized including a model of the Exact couch with both rails in the out positions. CDR-VMAT plans generated with MCO in Raystation were dosimetrically comparable to Raystation MCO-generated IMRT plans. Patient-specific QA measurements with the ArcCHECK on the couch showed good agreement with the treatment planning system prediction. Patient-specific, structure-specific, multi-statistical parameter 3D QA measurements with gantry-mounted COMPASS also showed good agreement. Conclusion: Constant dose rate VMAT was successfully modeled in Raystation 4.7 for a Varian Clinac iX, and Raystation's multicriteria optimization generated constant dose rate VMAT plans which were deliverable and dosimetrically comparable to IMRT plans.

Cadaveric Verification of VMAT Dose Distributions for Spine SBRT Treatments With Titanium Implants

Cadaveric Verification of VMAT Dose Distributions for Spine SBRT Treatments With Titanium Implants

Uncertainty in dose per monitor unit estimates for passively scattered proton therapy: The role of compensator and patient scatter in prostate cases

Standard methods for determining dose per monitor unit values in a patient do not yet exist for proton therapy. Indeed, some aspects of D/MU estimation remain poorly understood, such as the conversion of absorbed dose in a water phantom to absorbed dose in a patient. This study focused on the water-to-patient absorbed dose conversion factor, F CSPS , which accounts for differences in scatter (from the range compensator and internal patient anatomy) between patient treatments and their corresponding calibration irradiation in a homogeneous water-box-phantom. We estimated F CSPS for 32 prostate fields using a pencil beam dose algorithm in the treatment planning system (TPS). The mean F CSPS value was 1.006; its standard deviation of the mean was ±0.001. The lower bound for uncertainty in F CSPS , μF CSPS , was estimated for a sub-set of fields through comparisons of TPS dose predictions with measurements and Monte Carlo (MC) simulations. Comparison of TPS predictions and measurements yielded μF CSPS of 0.4% - 0.8%. Comparison of TPS predictions and MC simulations yielded μF CSPS < 0.3%. For a prostate treatment, a comparison of F CSPS values from TPS predictions with the historical value of 1.0 yielded μF CSPS < 3% and a mean μF CSPS of 0.6%. Regardless of estimation method, μF CSPS was approximately 1%, suggesting that uncertainty in F CSPS for proton treatments of prostate cancer is clinically acceptable.

Two-dimensional in vivo dose verification using portal imaging and correlation ratios.

The electronic portal imaging device (EPID) has the potential to be used for in vivo dosimetry during radiation therapy as an additional dose delivery check. In this study we have extended a method developed by A. Piermattei and colleagues in 2006 that made use of EPID transit images (acquired during treatment) to calculate dose in the isocenter point. The extension allows calculation of two‐dimensional dose maps of the entire radiation field at the depth of isocenter. We quantified the variability of the ratio of EPID signal to dose in the isocenter plane in Solid Water phantoms of various thicknesses and with various field sizes, and designed a field edge dose calculation correction. To validate the method, we designed three realistic conventional radiation therapy treatment plans on a thorax and head anthropomorphic phantom (whole brain, brain primary, lung tumor). Using CT data, EPID transit images, EPID signal‐to‐dose correlation, and our edge correction, we calculated dose in the isocenter plane and compared it with the treatment planning system's prediction. Gamma evaluation (3%, 3 mm) showed good agreement (Pγ<1 ≥ 96.5%) for all fields of the whole brain and brain primary plans. In the presence of lung, however, our algorithm overestimated dose by 7%–9%. This 2D EPID‐based in vivo dosimetry method can be used for posttreatment dose verification, thereby improving the safety and quality of patient treatments. With future work, it may be extended to measure dose in real time and to prevent harmful delivery errors.PACS numbers: 87.55.km, 87.55.Qr, 87.55.T‐

In vivo and phantom measurements versus Eclipse TPS prediction of near surface dose for SBRT treatments

This study reports on Gafchromic EBT2 film skin dose measurements for lung stereotactic body radiotherapy. These measurements were compared to near surface skin doses predicted by Eclipse treatment planning system (TPS) using the Analytical Anisotropic Algorithm (AAA) for a 6 MV photon beam. The accuracy of the predicted near surface dose for 3×3, 5×5 and 10×10 cm2 fields was assessed using an Attix chamber and EBT2 film in a Virtual Water phantom and compared to Monte Carlo calculation. The maximum near surface dose and its location were identified from the patient's treatment plan. For phantom measurements, the TPS dose (nominal 0 mm depth) was higher than the Attix chamber data by up to 24.0 % but in closer agreement with the EBT2 film measurement, which was up to 10.3 % higher than the Attix data. The MC calculated dose values were higher than the Attix data by up to 3.5% for the depths up to 2 mm. The maximum patient skin dose estimated from in vivo EBT2 film measurements was 7.5 – 19.5 Gy per course and depended on the number of overlapping fields, beam weight and/or contact with immobilisation devices. The TPS predicted dose for patient plans was mostly higher than the in vivo dose, by as much as 69.3%, but in two cases was lower, by as much as -23.1%.

SU-E-T-331: Dose Comparison of IVD2 Measurement and Eclipse Treatment Planning Calculation for Whole Brain Irradiation

Purpose: To compare the dmax doses measured by Sun Nuclear IVD2 system with the doses calculated by Varian Eclipse treatment planning system (TPS) in the whole brain irradiation (WBI) of a humanoid unsliced phantom using standard lateral fields. Methods: The IVD2 system was carefully calibrated for measuring the dmax dose (at 1.5cm) of Varian Trilogy 6 MV photon beams. This system was also experimentally validated for the dependencies of field size, SSD, dose rate, and beam incident angle. The head and neck part of the phantom was scanned by a Phillips CT at the slice thickness of 2mm. A WBI plan was created using 6MV photon beams and parallel opposed lateral fields with the multiple leave collimator to spare the orbital structures. The plan was normalized to the weight point and with >=98% of brain volume covered by >=98% of prescription dose (300cGy/fraction). The phantom setup and target location were verified by the kV imaging and CBCT; the diodes were carefully placed at various locations. The comparison was made between the measured dose and the planned dose for all interested anatomical sites. Results: Our results indicated that the measured doses agree with the planned dose within 7.9% for the sites in the treatment fields such as mid right/left parietal bones. For the sites outside or partially outside the fields, such as the low chin and right/left orbits, the difference between the measurement and the TPS prediction was varied between 8.3% and 33.8%. Conclusion: Though the human head is relatively small in size, the dose at dmax of WBI can be accurately predicted by TPS and measured by IVD2 system if the points of the interest are inside of the fields. A difference up to 33.8% could be expected if the points are located on the boarder of the fields.

VMAT In Vivo Dose Delivery Verification

Since the VMAT beam is non-uniform we use three MOSFET detectors placed immediately adjacent to each other at the point of interest on the patient’s skin to improve our measurement statistics. If the skin is not easily accessible due to an immobilization mask, the MOSFETS are placed on an appropriate location on the mask. A 3.0 cm x 3.0 cm minibolus of thickness Dmax for the given x-ray energy is placed over the detectors and the treatment is delivered to the patient. The three readings are averaged and compared to the treatment planning system prediction where the effect of the mini-bolus has been taken into account. CONCLUSIONS In addition to the pre-treatment quality assurance performed by measuring the absolute dose and planar dose distribution in a phantom and comparing the results to the values as predicted by the treatment planning system, it was desired to also have a dose measurement during the first or second treatment fraction. This was accomplished by the use of MOSFET detectors. Other methods have also been reported. (1,2)

SU-E-T-156: Robust Algorithms for Correction of Predicted Electronic Portal Imager Response.

Predicted electronic portal imaging device (EPID) response, as calculated by a commercial treatment planning system (TPS), is up to 15% lower than measured EPID response for off-axis IMRT fields. Two original algorithms are presented to correct for EPID prediction errors. The EPID prediction algorithm and a recent image-to-dose conversion algorithm are each tested for ability to identify TPS dose calculation errors. By comparing test images to respective predictions, correction factors were calculated to modify the EPID diagonal calibration profile (applied via radial symmetry). Secondly, image/prediction comparisons were used to compute a 2D correction matrix for EPID predictions, to account for radially-asymmetric errors. Over 50 IMRT fields of varying complexity were tested with each correction technique, and with a diode array. Absolute dose and beam-profile errors were separately induced into the TPS and a number of IMRT plans were recalculated and measured with three systems - an EPID prediction system, an EPID image-to-dose conversion system, and a diode array - for comparison to verification plans. With the profile correction, TPS predictions agree much better with EPID measurements, yielding improvement in gamma pass rates (3%,3mm) of over 30% on average for off-axis IMRT fields. Since off-axis prediction errors are not radially-symmetric, the matrix correction further improves pass rates by 5% on average (up to 30%) for fields where the profile correction is limited. The EPID prediction system was unable to catch either induced TPS error, while both the image-to-dose conversion system and the diode array indicated both errors. Profile correction is effective and efficient though approximate, due to radial symmetry. The matrix correction is comprehensive but requires computational manipulation of DICOM images. Users must be aware that EPID prediction systems may be unable to catch delivered IMRT inaccuracies due to calculation errors downstream from the actual fluence calculation.

SU-GG-T-100: Comparison of Measured D-Max for Irregular Electron Cutout Fields to Treatment Planning System Prediction and Nominal Open Field D-Max

Purpose: For irregular electron cutouts, not only output factors but also depths of maximum dose (D‐max) are often measured experimentally, increasing workload of clinical physicists and slowing clinical workflow. The purpose of this study is to investigate whether the D‐max of the corresponding open field or the D‐max of the cutout derived from the treatment planning system (TPS) can be used. Method and Materials: Seven cutouts for electron beams with energies ranging from 9 to 21 MeV were planned for patients using Pinnacle 8.0m without heterogeneity correction. To determine D‐max, the depth doses (PDDs) of these cutouts were generated in the TPS with flat‐surfaced water phantom geometry. PDDs were then compared with PDDs measured in a PTW scanning water phantom and corresponding open field PDDs. Because of the broad characteristic of PDDs near D‐max of these energy electron beams, a range of depths with PDD values > 99.5% on either side of D‐max were compared. Results: Within 0.5% dose variation, the D‐max ranges obtained from the TPS overlapped with those from measurements and nominal D‐max of open fields. For five of the seven cutouts, output factors measured at depths obtained from TPS, measurement, or corresponding open field resulted in less than 1% difference. For the other two cutouts, which showed the least overlap, less than 2% difference in output factors measured at the nominal depth of open‐field or measured Dmax was observed. Conclusion: For irregular electron cutouts, it is not necessary to measure the unique D‐max for the cutout field. Cutout factors within 2% measured at either nominal D‐max of the corresponding open field or the calculated D‐max from the treatment planning system can be achieved.

WE‐E‐BRD‐06: Validation of An MCNPX Monte Carlo Model of a Discrete‐Spot Scanning Beam Nozzle and Evaluation of the Beamlet Lateral Envelope of Low Doses

Purpose: To build and experimentally validate an accurate Monte Carlo (MC) model of a discrete spot scanning beam nozzle and to use this model to assess the dosimetric consequences of the “halo” dose (i.e., that due to scattering from beam line components and nuclear interactions). Method and Materials: An accurate model of the scanning nozzle at our institution was implemented using the MC system MCNPX version 2.5.0 according to the blueprints provided by the manufacturer of the proton therapy system (Hitachi Ltd., Tokyo, Japan). The MC model was validated against measurements of percentage depth doses (PDDs), lateral profiles in‐air (LPA) and in‐water (LPW), field size factors (FSFs), and spread out Bragg peaks. The validated MC model presented in this study was used to generate data for clinical configuration of our treatment planning system (TPS). Results: Distance to agreement between measured and simulated ranges, LPAs and LPWs were within 0.16 cm, 0.05 cm and 0.1 cm, respectively. Comparisons between measured FSFs and TPS predictions, which may not model the halo dose properly, reveled differences of up to 9.5%, whereas comparison with MC simulations showed maximum differences of 3.6%. We found that ion chambers with radius as large as 4.2 cm are insufficient to measure the integral energy deposition. Conclusion: Because of the halo dose, which broadens lateral sizes of the proton beamlets, care should be taken when measuring integral energy deposition for configuration of the TPSs. Furthermore, the halo dose must be properly modeled in the dose algorithm used by TPS for accurate prediction of scanned proton beam doses.

SU‐FF‐T‐357: Clinical Implementation of the OneDosePlusTM MOSFET Detector for in Vivo Dose Measurement

Purpose: To demonstrate the clinical implementation of the OneDosePlus™ MOSFET detector for in vivo dose measurement. Method and Materials: The ratio of the mean dose reported by OneDosePlus™ MOSFETs placed on the surface of a 90 cm × 30 cm × 9 cm water‐equivalent plastic phantom to the mean dose measured by an ionization chamber placed at dmax within the phantom, was determined as a function of (i) gantry angle and detector orientation, (ii) square‐field size, (iii) SSD, (iv) wedge angle, (v) monitor units, and (vi) dose rate, over clinically relevant ranges, and separately for the 6 MV and 15 MV beams of our clinical linear accelerator. A least‐squares straight‐line fit was performed to the response ratio as a function of each of these parameters in turn, and normalized to the ratio evaluated under reference conditions ((i) 0°, (ii) 10 cm, (iii) 100 cm, (iv) 0°, (v) 100, (vi) 400 MU min−1). From these, a formalism was derived to predict the dose reading by OneDosePlus™ and its uncertainty for arbitrary treatment fields for comparison with the treatment planning system prediction. Results: At 6 MV (15 MV), in order to achieve agreement with the ionization chamber, corrections of &lt;3.5% (&lt;11.5%) to the reported OneDosePlus™ dose are required at 0° gantry angle across the full clinical range of all other variables, increasing rapidly as the gantry rotates beyond 50° to 27.5% (22.1%) at 80°. For non‐zero gantry angles, OneDosePlus™ was found to exhibit a stronger angular dependence when its contacts pointed towards the linac head. Conclusion: The OneDosePlus™ MOSFET is well‐suited to in vivo dose measurement, providing a highly convenient alternative to TLDs and diodes. Provided that appropriate corrections are made, verification of the expected dose should be possible to within 3.3% (6 MV) and 3.7% (15 MV) for arbitrary treatment fields.