Validation Of Treatment Plans Research Articles

Quantum-imaging detection of secondary neutrons in proton radiotherapy fields

Secondary radiation fields encountered in proton radiotherapy environments contain different particle species produced in a broad range of energies and directions. Experimental knowledge of the composition and spectral characteristics of such complex fields is valuable for operation and protection of instruments and personnel, design and optimization of irradiations as well as planning and validation of treatment plans. The neutron component, which are produced with non-negligible yield, is in particular challenging to measure and discriminate from other radiations by conventional detectors. In order to measure in such complex fields the neutron component, both fast and thermal, we make use of the semiconductor pixel detector Timepix3 equipped with a silicon sensor and a neutron converter mask. The detector was before calibrated with well-defined neutron fields. In this work, we characterize the secondary radiation field and examine in particular the neutron component behind a large water-equivalent phantom irradiated by a 190 MeV clinical proton beam. The detected neutrons have a predominant fast neutron component. No thermal neutrons are observed in the measured data. The neutron-induced interactions in the detector are resolved in a high background with enhanced discrimination by quantum-imaging visualization, micrometer scale pattern recognition and high-resolution spectral-sensitive tracking of single particles. Detailed results are provided in wide range in terms of composition of the mixed-radiation field, total and partial fluxes and dose rates as well as particle deposited dose and linear-energy-transfer (LET) spectra.

Abstract ID: 222 Clinical implementation of a Monte Carlo based QA platform for validation of Tomotherapy and Cyberknife treatment plans

This work describes the clinical implementation of a Monte Carlo based platform for treatment plan validation for Tomotherapy and Cyberknife, including a semi-automatic plan evaluation module based on dose constraints for organs-at-risk (OAR). The Monte Carlo-based platform Moderato [1] is based on BEAMnrc/DOSXYZnrc and allows for automated re-calculation of doses planned with Tomotherapy and Cyberknife techniques. The Prescription/Validation module generates a set of dose constraints based on the anatomical region and fractionation scheme considered. Upon achievement of the planning, dose results are displayed with visual warnings in case of constraint violation. The system was tested on 83 patient cases in order to evaluate the influence of difference in calculation algorithms on OAR constraints. The first results with the Tomotherapy plans allowed for detecting and correcting a problem with the CT Hounsfield units when using a large reconstruction diameter (a CT artifact that lead to air voxels with an overestimated density). The Cyberknife results also showed some dose differences associated with different energy thresholds between Moderato and the Monte Carlo algorithm used in the Treatment Planning Station. Regarding OAR constraints, re-calculation generated few violations in thoracic, pelvic and abdominal cases. However, in spinal and head cases, significant differences can appear (-11% to +6%) on optic pathways and spinal cord, leading to doses above the limits. The Moderato platform constitutes a promising tool for the validation of plan quality, offering both dose re-calculation and OAR constraints evaluation. First results show the importance of this verification for some specific regions. Further work is ongoing to optimize the quantity and relevance of the information displayed, before fully introducing the system in clinical routine.

22. Feasibility study for the implementation of aS1000 EPID controls of RapidArc® treatments using FFF photon beams

Introduction The development of VMAT (Volumetric Modulated Arc Therapy) treatments by flattening filter free photon beams (FFF) involves the adaptation of dosimetric verification methods. A number of physical limits (detector saturation, backscattering…) or algorithmic limits, constrain us to use 3D phantoms of which implementation is time-consuming. In order to optimize the patient workflow, the purpose of this study concerns the technical feasibility of using the EPID (Electronic Portal Imaging Device) and the performances of the algorithms PDIP (Varian) and GlAaS (EPIQA) for the verification of plans of treatment Varian RapidArc® FFF. Methods The chain of treatment includes a Varian Clinac 2100C accelerator with an EPID aS1000 imager and the PDIP algorithm V13.6 and EPIQA V2.2.1. A calibration of the portal imager is performed for the energy X6 FFF. To test the criticality of the two algorithms for the local and global gamma index validation methods at 3% 3 mm, the results obtained with the portal imager are compared with those obtained with the ArcCheck. Results The first results show the feasibility of implementing FFF treatment plan validation checks with EPID for CLINAC 2100C accelerators equipped with the aS1000 portal imaging system. Conclusions This study makes it possible to validate the control of the RapidArc® treatment plans by FFF beams using the portal imager. This method is an alternative to 3D phantom validations such as ArcCheck, Delta 4.

Independent dose verification system with Monte Carlo simulations using TOPAS for passive scattering proton therapy at the National Cancer Center in Korea

For the independent validation of treatment plans, we developed a fully automated Monte Carlo (MC)-based patient dose calculation system with the tool for particle simulation (TOPAS) and proton therapy machine installed at the National Cancer Center in Korea to enable routine and automatic dose recalculation for each patient. The proton beam nozzle was modeled with TOPAS to simulate the therapeutic beam, and MC commissioning was performed by comparing percent depth dose with the measurement. The beam set-up based on the prescribed beam range and modulation width was automated by modifying the vendor-specific method. The CT phantom was modeled based on the DICOM CT files with TOPAS-built-in function, and an in-house-developed C++ code directly imports the CT files for positioning the CT phantom, RT-plan file for simulating the treatment plan, and RT-structure file for applying the Hounsfield unit (HU) assignment, respectively. The developed system was validated by comparing the dose distributions with those calculated by the treatment planning system (TPS) for a lung phantom and two patient cases of abdomen and internal mammary node. The results of the beam commissioning were in good agreement of up to 0.8 mm2 for B8 option in both of the beam range and the modulation width of the spread-out Bragg peaks. The beam set-up technique can predict the range and modulation width with an accuracy of 0.06% and 0.51%, respectively, with respect to the prescribed range and modulation in arbitrary points of B5 option (128.3, 132.0, and 141.2 mm2 of range). The dose distributions showed higher than 99% passing rate for the 3D gamma index (3 mm distance to agreement and 3% dose difference) between the MC simulations and the clinical TPS in the target volume. However, in the normal tissues, less favorable agreements were obtained for the radiation treatment planning with the lung phantom and internal mammary node cases. The discrepancies might come from the limitations of the clinical TPS, which is the inaccurate dose calculation algorithm for the scattering effect, in the range compensator and inhomogeneous material. Moreover, the steep slope of the compensator, conversion of the HU values to the human phantom, and the dose calculation algorithm for the HU assignment also could be reasons of the discrepancies. The current study could be used for the independent dose validation of treatment plans including high inhomogeneities, the steep compensator, and riskiness such as lung, head & neck cases. According to the treatment policy, the dose discrepancies predicted with MC could be used for the acceptance decision of the original treatment plan.

Clinical implementation of a Monte Carlo based treatment plan QA platform for validation of Cyberknife and Tomotherapy treatments

PurposeThe main focus of the current paper is the clinical implementation of a Monte Carlo based platform for treatment plan validation for Tomotherapy and Cyberknife, without adding additional tasks to the dosimetry department. MethodsThe Monte Carlo platform consists of C++ classes for the actual functionality and a web based GUI that allows accessing the system using a web browser. Calculations are based on BEAMnrc/DOSXYZnrc and/or GATE and are performed automatically after exporting the dicom data from the treatment planning system. For Cyberknife treatments of moving targets, the log files saved during the treatment (position of robot, internal fiducials and external markers) can be used in combination with the 4D planning CT to reconstruct the actually delivered dose. The Monte Carlo platform is also used for calculation on MRI images, using pseudo-CT conversion. ResultsFor Tomotherapy treatments we obtain an excellent agreement (within 2%) for almost all cases. However, we have been able to detect a problem regarding the CT Hounsfield units definition of the Toshiba Large Bore CT when using a large reconstruction diameter. For Cyberknife treatments we obtain an excellent agreement with the Monte Carlo algorithm of the treatment planning system. For some extreme cases, when treating small lung lesions in low density lung tissue, small differences are obtained due to the different cut-off energy of the secondary electrons. ConclusionsA Monte Carlo based treatment plan validation tool has successfully been implemented in clinical routine and is used to systematically validate all Cyberknife and Tomotherapy plans.

Treatment planning validation for symmetric and asymmetric motorized wedged fields

Purpose: Wedged beam are often used in clinical radiotherapy to compensate missing tissues and dose gradients. The Elekta Precise linear accelerator supports an internal motorized wedge, which is a single large, physical wedge on a motorized carriage. In this study, the dosimetric performance of Elekta precise three dimensional treatment planning system (3DTPS) is evaluated by comparing the calculated and measured doses. Methods: The calculations were performed by the 3DTPS for symmetric as well as asymmetric fields in a source to skin distance (SSD) setup at the depth of maximum dose (d max ) as well as at 5, 10, and 20 cm depths in water phantom using 60° motorized wedges for field sizes of 4 × 4, 10 × 10, and 20 × 20 cm 2 for 6 and 15 MV photon beams. Measurements were produced by Elekta Precise linear accelerator using 0.125 cc volume ionization chamber. Results: Good agreement between the measured and calculated isodose lines were found, with the maximum difference not exceed 5%. The difference between the calculated and measured data increases as the field size decreases, and the deviation in symmetric setting was less than that of asymmetric setting. The increase in wedge angle led to increase in the difference between calculated and measured data. Conclusion: The results from this study showed that the accuracy of Elekta Precise 3DTPS used with the motorized wedges for symmetric and asymmetric fields is adequate for the clinical applications under the studied experimental conditions.

Development of computer tools for validation of treatment plans according to RTOG, QUANTEC and SFRO criteria

Development of computer tools for validation of treatment plans according to RTOG, QUANTEC and SFRO criteria

SU‐E‐T‐56: Brain Metastasis Treatment Plans for Contrast‐Enhanced Synchrotron Radiation Therapy

Purpose:Iodine‐enhanced radiotherapy is an innovative treatment combining the selective accumulation of an iodinated contrast agent in brain tumors with irradiations using monochromatic medium energy x‐rays. The aim of this study is to compare dynamic stereotactic arc‐therapy and iodineenhanced SSRT.Methods:Five patients bearing brain metastasis received a standard helical 3D‐scan without iodine. A second scan was acquired 13 min after an 80 g iodine infusion. Two SSRT treatment plans (with/without iodine) were performed for each patient using a dedicated Monte Carlo (MC) treatment planning system (TPS) based on the ISOgray TPS. Ten coplanar beams (6×6 cm2, shaped with collimator) were simulated. MC statistical error objective was less than 5% in the 50% isodose. The dynamic arc‐therapy plan was achieved on the Iplan Brainlab TPS. The treatment plan validation criteria were fixed such that 100% of the prescribed dose is delivered at the beam isocentre and the 70% isodose contains the whole target volume. The comparison elements were the 70% isodose volume, the average and maximum doses delivered to organs at risk (OAR): brainstem, optical nerves, chiasma, eyes, skull bone and healthy brain parenchyma.Results:The stereotactic dynamic arc‐therapy remains the best technique in terms of dose conformation. Iodine‐enhanced SSRT presents similar performances to dynamic arc‐therapy with increased brainstem and brain parenchyma sparing. One disadvantage of SSRT is the high dose to the skull bone. Iodine accumulation in metastasis may increase the dose by 20–30%, allowing a normal tissue sparing effect at constant prescribed dose. Treatment without any iodine enhancement (medium‐energy stereotactic radiotherapy) is not relevant with degraded HDVs (brain, parenchyma and skull bone) comparing to stereotactic dynamic arc‐therapy.Conclusion:Iodine‐enhanced SSRT exhibits a good potential for brain metastasis treatment regarding the dose distribution and OAR criteria.

Brain metastasis treatment plans for contrast-enhanced synchrotron radiation therapy

Introduction Contrast-enhanced stereotactic synchrotron radiotherapy (SSRT) is an innovative treatment based on localized dose enhancement effects obtained by reinforced photoelectric absorption in brain tumors. Tumors previously loaded with high-Z elements are irradiated using medium-energy monochromatic x-rays (50–100 keV). The aim of this study is to compare SSRT treatment in the presence and absence of iodine enhancement. For a single case, SSRT was compared to conventional treatments: 3D conformational radiotherapy, modulated radiation therapy (IMRT) and dynamic stereotactic arc-therapy. Material and methods Patients bearing brain metastasis received a standard helical 3D-scan without iodine. A second scan was acquired 13 min after an 80 g iodine infusion. Two SSRT treatment plans (with/without iodine) were performed for each patient. A dedicated Monte Carlo (MC) treatment planning system (TPS) based on the ISOgray TPS was developed for this purpose. Ten coplanar beams (6 * 6 cm 2 , shaped with collimator) were simulated. MC statistical error objective was less than 5% in the 50% isodose. The treatment plan validation criteria were fixed such as 100% of the prescribed dose is delivered at the beam isocentre and the 70% isodose contains the whole target volume. The comparison elements were the 70% isodose volume, the average and maximal doses delivered to organs at risk (OAR): brainstem, optical nerves, chiasma, eyes, skull bone and healthy brain parenchyma. For the single case study, the 3D conformational radiotherapy and IMRT treatment plans were established using the Eclipse ® -Varian TPS. The dynamic arc-therapy plan was achieved on the Iplan ® -Brainlab TPS. Results Iodine-enhanced SSRT exhibits a good potential for brain metastasis treatment regarding the dose distribution and OAR criteria. Iodine accumulation in metastasis may increase the dose of 20– 30% (sparing effect). Treatment without any iodine enhancement (medium-energy stereotactic radiotherapy) is not relevant with degraded HDVs (brain, parenchyma and skull bone) comparing to conventional stereotactic surgery. In the single case study, the stereotactic dynamic arc-therapy remains the best technique in terms of dose conformation. However, OAR sparing is increased with iodine-enhanced SSRT: the dose delivered to the brainstem and the brain parenchyma is significantly reduced. Conclusion This study shows the potential dosimetric advantages of contrast-enhanced SSRT for treating brain metastasis.

ANALYSIS BY SYNTHESIS FOR ESTIMATION OF DOSE CALCULATION WITH gMOCREN AND GEANT4 IN MEDICAL IMAGE

The use of GEANT4 simulation toolkit has increased in the radiation medical field for the design of treatment system and the calibration or validation of treatment plans. Moreover, it is used especially on calculating dose simulation using medical data for radiation therapy. However, using internal visualization tool of GEANT4 detector constructions on expressing dose result has deficiencies because it cannot display isodose line. No one has attempted to use this code to a real patient’s data. Therefore, to complement this problem, using the result of gMocren that is a three-dimensional volume-visualizing tool, we tried to display a simulated dose distribution and isodose line on medical image. In addition, we have compared cross-validation on the result of gMocren and GEANT4 simulation with commercial radiation treatment planning system. We have extracted the analyzed data of dose distribution, using real patient’s medical image data with a program based on Monte Carlo simulation and visualization tool for radiation isodose mapping.

TU-C-BRB-12: Treatment Plan Validation through Graphical Fingerprint

Purpose: The treatment plan validation by the human is slow and error‐ prone due to the human limitation. We present a possible solution to improve the validation efficiency and quality by adopting graphical fingerprint. We aim to produce a digital fingerprint from various treatment plan parameters and present the fingerprint in a graphical format for easy recognition by human. Method and Materials: Since large amount information exists in the treatment plan, it is not realistic to ask the therapists to verify manually. We resort to use the hash function to generate a fingerprint for each copy of the plan. In this study, Message‐Digest algorithm 5 (MD5) is used mainly due to its relative short output (128bit). To make sure the difference can be readily recognized by human, we adopt the approach used in random art to produce an image (refereed as to graphical fingerprint) for each MD5 output. To ensure there is no result collision, we employed a result checking mechanism to ensure all images produced are valid. Results: 1000 MD5 stings are generated to produce the images we need. The 1000 images are randomly selected to form 20 groups, where each group contains 100 images. Each group is printed out on paper and visually inspected by human. It turns out all groups reach the same conclusion that they contains completely different images and none of the inspections exceeds 2 minutes. Conclusion: Wpresented a possible treatment plan validation approach through graphical fingerprint. It aims to address the human weakness during the manual validation. The approach allows fast recognition of errors by providing an graphical representation for the output of hash functions.

TH-C-301-05: Practical Medical Physics Session

Among dosimetry tools radiographic films have the unique advantage of providing the capability for dose measurements at very high spatial resolution; and between dosimetry films, radiochromic films are particularly advantageous for their versatility. Thus radiochromic films can be handled in light, easily cut to any size and shape, bent to conform to curved surfaces and immersed in water. All these attributes are embodied in the GAFCHROMIC® family of dosimetry films as used in a broad spectrum of medical applications including external beam therapy, brachytherapy, machine quality assurance and radiology. Current trends in external beam therapy are towards ever more precise treatment protocols with less dose fractionation. Both these factors increase the importance of pre‐treatment validation of treatment plans and place a high and increasing value on those measurement techniques that are able to deliver the ability for excellent resolution in the spatial frequency and dose domains. The talk will show, by example, the range of medical applications covered by radiochromic dosimetry films. It will demonstrate how the attributes of these films are being used to advantage in a full spectrum of radiotherapy applications. The presentation will finish by introducing another advantage of radiochromic film and showing the benefit of multi‐channel film dosimetry as a means for improving the integrity of dose measurements by separating dose‐independent artifacts and removing them from the response measurement.

SU‐E‐T‐697: Online Monte‐Carlo‐Accurate Real‐Time Proton Dose Calculator

Purpose: Demonstrate the feasibility of a online tool to provide Monte‐Carlo accurate therapeutic proton dose distributions. Methods: The Fast Dose Calculator (FDC), a fast track‐repeating algorithm for proton therapy, has been deployed on the High Performance Research Blue BioU IBM computer cluster at Rice University. FDC utilizes a database of discretized proton trajectories in water generated with GEANT4. Information about the energy deposited, length and direction is stored for each trajectory step. Proton trajectories and dose distributions in any other material are extrapolated from the database by scaling step lengths and angles. A web interface to upload anonymized patient and field data to be processed by the Blue BioU implementation of FDC has been developed. Once the data is uploaded, the website user can request a calculation of the dose distribution for a particular field configuration. The results can be displayed on the website and compared with dose distributions calculated with alternative methods. Results: Doses calculated with FDC reproduced the results obtained with traditional Monte Carlos, i.e. GEANT4, within 2%. The web interface with the Blue BioU back‐end can deliver Monte‐Carlo accurate dose distributions for proton treatment plans within a few seconds. Conclusions: An online real‐time tool for proton dose calculations has been developed. The tool could be utilized for treatment plan validation and radiation treatment research.

Quality assurance for radiotherapy in prostate cancer: Point dose measurements in intensity modulated fields with large dose gradients

Purpose: We aimed to evaluate an optimization algorithm designed to find the most favorable points to position an ionization chamber (IC) for quality assurance dose measurements of patients treated for prostate cancer with intensity-modulated radiotherapy (IMRT) and fields up to 10 cm × 10 cm. Methods and Materials: Three cylindrical ICs (PTW, Freiburg, Germany) were used with volumes of 0.6 cc, 0.125 cc, and 0.015 cc. Dose measurements were made in a plastic phantom (PMMA) at 287 optimized points. An algorithm was designed to search for points with the lowest dose gradient. Measurements were made also at 39 nonoptimized points. Results were normalized to a reference homogeneous field introducing a dose ratio factor, which allowed us to compare measured vs. calculated values as percentile dose ratio factor deviations ΔF (%). A tolerance range of ΔF (%) of ±3% was considered. Results: Half of the ΔF (%) values obtained at nonoptimized points were outside the acceptable range. Values at optimized points were widely spread for the largest IC (i.e., 60% of the results outside the tolerance range), whereas for the two small-volume ICs, only 14.6% of the results were outside the tolerance interval. No differences were observed when comparing the two small ICs. Conclusions: The presented optimization algorithm is a useful tool to determine the best IC in-field position for optimal dose measurement conditions. A good agreement between calculated and measured doses can be obtained by positioning small volume chambers at carefully selected points in the field. Large chambers may be unreliable even in optimized points for IMRT fields ≤10 cm × 10 cm.

Automated Intracranial Contouring for Brain Cancer Patients: Treatment Plan Validation

Automated Intracranial Contouring for Brain Cancer Patients: Treatment Plan Validation

TU‐D‐T‐617‐07: Treatment Plan Validation of the Xoft AXXENT™ X‐Ray Source

Purpose: Accuracy of delivered dose compared to BrachyVision™ generated treatment plans was tested using both a calibrated miniature air ionization chamber to measure dose at the prescription point, and radiochromic film to map isodose contour lines. Quantitative and qualitative comparisons of the total dose and contour shapes were made. Method and Materials: Xoft applicator balloons were held in a water phantom with a Solid Water™ fixture. A PTW model 34013 ionization chamber connected to a PTW UniDos electrometer, and read into a computer, were used to measure the dose at the prescription point. Films were held in the fixture in the plane of the source, offset by 5 mm. The Xoft Axxent controller performed sample treatments including stepping of the source for four combinations of balloon size and operating voltage. Film samples were digitized and converted to dose values through a specially developed calibration curve. Comparison of isodose contours with the film‐measured dose was accomplished with specially written image analysis algorithms, which compared the measured values with predicted along the isodose contours. Results: Dosimeter based dose measurements agreed with prescription to within 11% worst case, with an RMS error of 8%. Film analyses quantified deviation from isodose contours as a function of polar angle, with relative RMS deviations well under 10%. However limitations in the accuracy of film calibration limited absolute accuracy to just over 20%, with systematic behavior attributable to limitations of the fitting function. Conclusion: Validation of treatment plans were performed using direct dosimetry and film based measurements. Novel image analysis tools were developed to allow numerical evaluation of film dose to isodose contours. Conflict of Interest: Research was supported by Xoft, Inc.