Calculated Dose Distributions Research Articles

A Specific Patient Quality Assurance (PSQA) procedure for a Co-60 source based High Dose Rate Brachytherapy

Introduction: In radiation therapy, accurate dose determination and precise dose delivery to the tumor are directly associated with better treatment outcomes in terms of higher tumor control and lower post radiation therapy complications. The current study aims the development and clinical application of the Patient Specific Quality Assurance (PSQA) procedures for nasopharyngeal and Tongue cancer treated using a cobalt-60 based high-dose rate brachytherapy. Materials and Methods: In this study, patient-specific quality assurance tools for brachytherapy in nasopharyngeal and Tongue cancers were utilized using a radiometric film dosimeter system through a gamma index for comparison. Also, with the association of dose distribution measured at surface area, and applying a reverse method, the calculated dose distribution at the region of interest were compared with those resulted from a treatment planning system. All results were also compared with those achieved by a Monte Carlo simulation classified as gold standard for the project. We also designed and evaluated two phantoms in this study. The first phantom designed based on the BALTAS phantom and the second one was created as a special phantom for brachytherapy applicators used for Nasopharyngeal and Tongue cancers. The same passing criteria used for external beam QA (3%/3mm), were applied. Finally, the PSQA system’s sensitivity to source positional errors was investigated by introducing intentional and controlled mistakes on selected patient’s plans. Results: Results presented in this study demonstrated that radiochromic film dosimetry based PSQA for brachytherapy can be used not only for patient specific quality assurance, but also as a part of the commissioning process and periodic QA . Conclusion: Patient Specific Quality Assurance tasks should be extended for all Radiotherapy tasks including Brachytherapy.

33 Commissioning and dosimetric characteristics of new halcyon system

Introduction A new Halcyon linear accelerator had been installed at clinique Charcot in Sainte-Foy-Les-Lyon. It’s the first one in France. The accelerator head is mounted on a ring gantry. There is no jaws and, for each bank, two MLC (proximal and distal) of 29 and 28 leaves of 1 cm width at isocenter with an offset of 5 mm to minimize the transmission. The maximum speed of the leaves is 5 cm/s. The maximum field size is 28 × 28 cm2. Halcyon system has one unique energy of 6XFFF and a maximum dose rate of 800 UM/min. The MV imaging panel size is 43 × 43 cm2, fxed at a distance source to imager of 154 cm and its resolution is 0.22 mm at isocenter. Currently, Halcyon system has only MV imaging (MV/MV or MV CBCT). Imaging dose is calculated by TPS Eclipse (Varian) and taken into account during optimization. Beam model of Halcyon is already preinstalled and fully modelled in Eclipse. Methods All the preinstalled data in Eclipse were measured in a water tank Blue Phantom2 (IBA) using CC13 (IBA), EDGE (SunNuclear) et Farmer (PTW) detectors in accordance with Varian measurements. PDD, profiles, FOC, transmission factor and DLG measurements were compared with preinstalled beam model. Calculated dose distribution with AAA algorithm for PDD and profiles were compared with our measurements using tolerances from SFPM report n°27. End-to-end and RapidArc commissioning were performed. Imaging dose from clinical radiotherapy plans were calculated from several anatomical regions and measured in an anthropomorphic phantom CIRS with a semiflex (PTW) ionisation chamber. Results A very good superposition of preinstalled data with our measurements was observed for every data. Largest variations were observed for small field profiles 2 × 2 cm2. Mean and maximum variations for FOC measurements were −0.03% and 0.71%. Leaf transmission measured through both banks is 0.005%. Beam quality index (PDD(10) with SSD = 100 cm) is 63.5% (Varian specification 63% ± 1). Maximum depth dose for a 10x10cm2 field size at SSD = 100 cm is 1.28 cm (Varian specification 1.3 cm ± 0.2). For every field sizes 100% of the points for PDD and profiles pass the tolerances from SFPM report n°27. Gantry isocentricity measured with Drum phantom (Varian) is 0.65 mm. Absolute dose difference at isocenter for a VMAT plan measured during an End-to-End test performed in the IMRT Thorax Phantom (CIRS) is 1%. Conclusions An excellent agreement between Varian preinstalled beam model and our measures were observed and therefore no changes to the preinstalled beam model have been made.

36 Monte Carlo simulation of absorbed dose distribution for electron beam using GATE/GEANT4

Introduction Limits of Treatment Planning Systems (TPS) dose calculation for photon beams have been widely studied. There are fewer studies on electron beam dose calculation. Fast Electron Monte Carlo (eMC, Varian, Palo Alto, CA) is a clinical dose distribution calculation algorithm for electron beam, based on an advanced variance reduction technique, Macro Monte Carlo (MMC) [1] . Results for complex cases (heterogeneity, irregular surface contour) are limited. A GATE/GEANT4 model of a radiotherapy linac (Varian, TrueBeam) is implemented. The purpose of this study is to evaluate these algorithms by comparing them to film measurements in complex cases. Methods Validation of GATE model is performed on homogenous water phantom. Percentage depth dose curves (PDD) and lateral dose profiles (LDP) are compared with experimental measurement (6 and 9 MeV). Results of dose distribution calculation for eMC and GATE are comparing with Gafchromic EBT3 (Ashland ISP, Wayne, NJ) films, for complex cases including heterogeneities (lung, bone) and irregular surface (step). Normalization are achieved in a homogenous water region for dose profiles at two different depths (at contact and at a distance of the heterogeneity). Global Gamma Index Pass Rate (GIPR) 1D is used to compare the data sets. Results When comparing GATE and measurements, GIPR 2 % / 2 mm for PDD and LDP in homogenous water phantom is 100% for 6 MeV and 9 MeV. GIPR 3 % / 3 mm (threshold >10% D max ) for complexes cases (lung, bone and step) is >98.5% for GATE (6, 9 MeV) except for only one case at 6 MeV (at contact with lung heterogeneity: 82.9%). But, for eMC in lung case, GIPR are 100% for 9 MeV electrons but only 65.6% and 84.4% at 6 MeV at contact and at 10 mm of the heterogeneity, respectively. In presence of bones, GIPR are 100% (6, 9 MeV) except at 9 MeV, at contact of the heterogeneity (69.7%). For irregular surface cases GIPR of eMC vs. films was superior to 95.2% and 99.2% for 6 and 9 MeV respectively. Conclusions GATE model for electron beams is validated in homogeneous water phantom. In presence of surface irregularities (step) satisfactory results are obtained for both GATE and eMC. Nevertheless, when comparing to measurements for heterogeneous media, and in particular after lung (6 MeV) and bones (9 MeV) eMC was limited while GATE was satisfactory in all configurations. The study is currently carried on for more energy (12 and 18 MeV) and clinical cases (face). GATE could be proposed as a 3D dose check solution for complex electron beams.

18 Dosimetric study of the couch top influence in VMAT

Introduction During these last years, treatment planning system (TPS) algorithms that compute the absorbed dose have improved. A high precision between delivered absorbed dose and prescribed dose is required, limiting the uncertainties up to 5% between delivered dose and prescribed dose according to the ICRU recommendations [1] . One of the parameters contributing to these uncertainties is the couch top, which attenuates the beam from 2% to 15% when it passes through [2] . Last generation’s carbon fiber couch tops act as a bolus material, increasing skin dose while decreasing depth-dose. Therefore, the consideration of the couch top by the TPS is important for avoiding bad tumor control, overdose of organs at risk and skin radiotoxicity effects [2] . Methods The reference technique for the dose distribution calculations is based on Monte Carlo method (MC) [3] . Among the most well-known codes, BEAMnrc/EGSnrc allows to model the accelerator treatment head and to use CT images (DICOM format) in order to simulate the dose distribution to voxelized phantoms. Furthermore, the CT data have to be converted to densities and to the corresponding materials, taking into account the couch top [4] . A software developed by the team using a conversion ramp between Hounsfield Units (HU) and densities/materials, specific to the couch top, will be used to create the voxelized phantoms. Results MC method takes into consideration not only voxel densities, but also materials, if a wrong material assignment is made, it could lead to dosimetric errors in the dose distribution calculations. Different treatment sites will be considered in Volumetric Modulated Arc Therapy (VMAT). Phantoms with the couch top will be created and dose distributions will be compared to TPS dose distributions. Conclusions The results of this study contributed to evaluate different methods of taking into account the couch top, using MC.

Dosimetric pitfalls in the application of shielding disk in breast intraoperative electron radiation therapy

Introduction: Intraoperative electron radiotherapy (IOERT) is an IORT technique in which electron beams with different nominal energies are used for irradiation of microscopic residual of tumor bed after surgery. Application of a shielding disk, which is positioned under the target volume, for protecting the normal tissues such as pectoral muscle, lung, and heart is one of the advantages of IOERT in the breast cancer treatment. A common characteristic of these disks is double-layer format; a high atomic number (Z) layer with the ability to absorb the electron beam to prevent penetration to deeper tissues and a Low-Z layer on top of the former to minimize the backscattered electrons from the lower layer. In this study, we have evaluated the misalignment effects of shielding disk on dose distribution inside the target volume and surrounding healthy tissues through EGSnrc MC Simulation. Materials and Methods: LIAC 12 (Sordina IORT Technologies, S.p.A) is a dedicated IOERT machine that can produce electron beam energies of 6, 8, 10, 12MeV. This machine and its dedicated shielding disks which is made from 3 mm steel and 3 mm PMMA were used in this study. Machine head and employed radio-protecting disk were modeled using the BEAMnrc MC code. Some scoring planes were defined at the bottom of the applicator. Before using the phase-space files, the developed MC models were validated through comparing the Monte Carlo calculated PDDs and TDPS with those measured by ionometric dosimetry. EGSnrc C++ class library was used for simulation and calculation of the dose distribution around the shielding disk in some incorrect positions. Backscatter factor (BSF), shielding factor (SF), and percentage of leakage dose (PLD) was calculated as interested shielding parameters. In addition, some extracted parameters form differential DVH such as V100 and S-Index (Uniformity Index) were used to evaluate the dose distribution in the target. Results: The BSFs for all correct and incorrect setups at different nominal energies varied from 1.03 to 1.11, while experimental BSF values ranged from 1.02 to 1.1. By increasing the degree of disk rotation and beam energy, the SF decreases, while the PLD would be increased. By increasing the disk rotation, V100 decrements while the S-index is increased inside the target area. Conclusion: The analysis of uncertainties in shielding disk setup showed that the increment of disk rotation relative to applicator central axis can considerably influence the dose uncertainty inside the target volume and received dose to the organ at risks. Therefore, evaluating the treatment setup after positioning the disk and applicator through intraoperative C-arm imaging can be considered as a mandatory issue for IOERT quality assurance.

Influence of Compensator Thickness and Field Size on Buildup factor of a Brass Compensator for Intensity- Modulated Radiation Therapy

Introduction: Radiotherapy is one of the effective methods in the treatment of cancer. In the recent methods of radiotherapy, intensity modulated radiotherapy (IMRT) technique by compensators is used. when photon beam passes the compensator, scattered photons can be generated within the compensator. The presence of scattered photons may complicate the design of compensators as well as the subsequent calculations of dose distributions. The buildup factor, ‘B’, is one of these important properties of a material to be used for compensator construction. It directly affects the dose quantity. Materials and Methods: Radiotherapy is one of the effective methods in the treatment of cancer. In the recent methods of radiotherapy, intensity modulated radiotherapy (IMRT) technique by compensators is used. when photon beam passes the compensator, scattered photons can be generated within the compensator. The presence of scattered photons may complicate the design of compensators as well as the subsequent calculations of dose distributions. The buildup factor, ‘B’, is one of these important properties of a material to be used for compensator construction. It directly affects the dose quantity. Results: Our results indicated that the compensator thickness and field size have the most significant effect on the calculation of buildup factor for the 6-MV photon beam. Conclusion: Influence of compensator thickness and field size on calculation buildup factor can reduce the error due to delivered dose to target volume and organs at risk

The dosimetry assessment of Varian Linear Accelerators of 6, 15 and 20 MV by Monte Carlo Method

Introduction: Monte Carlo method is often applied in radiation therapy as utilized in all the branches of science. An important requirement for successful radiotherapy is carefully examine the dose distribution specifications and decrease the difference between these features with experience to an acceptable level. In this study, the characteristics of 6, 15 and 20 MeV incident x-rays are provided for 10×10 cm2 fields. This information is necessary to achieve an accurate treatment planning. Materials and Methods: The MCNP code is employed to simulate 6 MeV photon beam from a major manufacturer of a Varian linac accelerator. The used phantom in this research was water. The output data charts have illustrated, by two software origin and table curve. Results: In order to validate the performance and accuracy of the simulated head, the parameters associated with 5×5 cm2 field were compared to empirical values. In this study, 6, 15, 20 MeV (6, 15, 20 photon modes) energy spectra, percentage depth dose, beam profile are calculated. The comparisons between calculated and experimental results show good agreement (with ±4%). Conclusion: The MCNP code has a suitable precision in calculation of dose distribution for these modes of Varian. There is a good agreement between empirical and simulation result for 6 MV photon beam for both percentage depth dose and beam profiles. For the next two modes, comparing results with references indicates that the results are acceptable as well as other applications in radiotherapy.

51. A novel algorithm (DirectDensity™) for reconstruction of simulation CT images used in radiotherapy treatment planning: dosimetric evaluation

Purpose CT imaging in radiotherapy represents the standard for treatment planning. The calculation of dose distribution requires appropriate energy-dependent calibration curves to convert Hounsfield Units (HU) into electronic densities relative to water (rEDs). To obtain optimal image quality and reduce dose, it could be useful to modulate X-ray tube potential based on patient size, but in radiotherapy treatment planning this practice requires the use of several calibration curves. A novel algorithm (DirectDensity™, DD™ recently developed by Siemens Healthcare, uses CT projection data to reconstruct images directly proportional to rEDs, independently of kVp used, thus removing the need of different curves. The purpose of the study is to evaluate this algorithm for clinical use. Methods The CIRS Density Phantom with different tissue equivalent inserts was scanned at 70, 80, 100, 120 and 140 kVp with the new Siemens Confidence CT scanner. Images were reconstructed with the DD™ algorithm to assess the correspondence of rEDs obtained from imaging with those certified. Afterwards the images of 7 patients scanned at 120 kVp were reconstructed with standard filtered backprojection (FBP) and DD™ algorithm. Treatment plans were calculated on both FBP and DD™ images by using standard HU-rEDs curve and linear CTvalue-rEDs relationship respectively (Fig. 1). Dose differences were evaluated through γ -analysis (1 mm, 1%). Results The rEDs measured from images (by reversing the linear relationship between CTvalue and rED) in regions corresponding to phantom inserts were compared to certified rEDs: differences were reported in Table 1 . Minimal dosimetric differences were found between treatment plans calculated on both FBP and DD™ images. DVHs showed differences in Dmean, D98, D2 and V95 for PTVs and OARs smaller than 1%. γ -analysis indicated negligible differences between dose distributions, with lowest passing rate at 97%. Conclusions This study proved the equivalence between DD™ and FBP, including the possibility to modulate kVp during CT acquisition, thus reducing patient dose without complicating treatment planning workflow.

168. Evaluation of a machine QA software tool for MLC performance checks

PurposeOne of the main requirement for a correct intensity modulated treatment (IMRT) delivery, is a periodic check of Multileaf Collimator (MLC) parameters. In some Treatment Planning System (TPS), the Dosimetric Leaf Gap (DLG) is one of the most important parameter for the correct calculation of dose distribution when using dynamic MLC. The purpose of this work is to evaluate the feasibility and time saving in using a dedicated machine quality assurance (QA) software tool to monitor over time the DLG parameter and leaves positions. MethodsTwo 6 MV Linac equipped with a 120 leaf MLC were compared to check the measured constancy of the DLG and leaves positions through use of a 3D diode array equipped with a dedicated machine QA software tool. The MLC checks were performed at 50° and 320° gantry angles due to phantom’s plate geometry. In this way it was possible also to verify gravity effects on leaves banks. The analysis was carried out with a version of picket fence test implemented in the software. ResultsThe Veroli mean DLG values was 2,0 ± 0.4 mm for 50° gantry angle and 2.1 ± 0.5 mm for 320° gantry angle. The Rieti mean DLG values was 1.9 ± 0.1 for 50° gantry angle and 2.0 ± 0.1 mm for 320° gantry angle. The check on the leaves movement has shown gravity effect on two different angles. In particular the Rieti DLG value carried out at gantry 50° was 1.6 ± 0.1 mm when there was a problem to leaf motor. ConclusionThe picket fence test, originally used with radiographic film to check the reproducibility of leaf gap, is extended in this machine QA software tool, including checks for gap width and leaf positions. This tool can be used as a routine MLC QA check for mean leaf deviation, DLG, and absolute positions of the leaves.

The comparison between 6 MV Primus LINAC simulation output using EGSnrc and commissioning data

Introduction: Monte Carlo calculation method is considered to be the most accurate method for dose calculation in radiotherapy. The purpose of this research is comparison between 6 MV Primus LINAC simulation output with commissioning data using EGSnrc and build a Monte Carlo geometry of 6 MV Primus LINAC as realistically as possible. The BEAMnrc and DOSXYZnrc (EGSnrc package) Monte Carlo model of the LINAC head was used as a benchmark. Materials and Methods In the first part, the BEAMnrc was used for the designing of the LINAC treatment head. In the second part, dose calculation and for the design of 3D dose file were produced by DOSXYZnrc. The simulated PDD and beam profile obtained were compared with that calculated using commissioning data. Good agreement was found between calculated PDD (1·1%) and beam profile using Monte Carlo simulation and commissioning data. After validation, TPR20,10, TMR and Sp values were calculated in five different field. Results Good agreement was found between calculated values by using Monte Carlo simulation and commissioning data. Average differences for five field sizes in this approach is about 0·83% for Sp. For TPR20,10 differences for field sizes 10 × 10 cm2 is 0·29% and for TMR in five field sizes, the average value is ~ 1·6%. Conclusion: In conclusion, the BEAMnrc and DOSXYZnrc codes package have very good accuracy in calculating dose distribution for 6 MV photon beam and it can be considered as a promising method for patient dose calculations and also the Monte Carlo model of primus linear accelerator built in this study can be used as method to calculate the dose distribution for cancer patients.

188 Orthovoltage rotational breast radiotherapy: A Monte Carlo and experimental study

Purpose To investigate depth-dose characteristics of a new technique for orthovoltage rotational breast radiotherapy (RT) via Monte Carlo (MC) simulations and experimental phantom study, following the initial work by Boone [1] . Methods Geant4 MC simulations were developed to calculate dose distributions generated by an orthovoltage (320 kVp, 4 mm Cu filtration) or kilovoltage (150 kV, 1 mm Cu) X-ray beams rotating 360° around a cylindrical phantom (polyethylene or PMMA) placed at isocentre to simulate prone patient’s breast. Horizontal beam collimation was 1–14 cm. Radial and longitudinal depth-dose distributions were performed for homogeneous phantoms (with or without lesions simulated for different depths in phantom). Experimental validation was performed on the same geometry with the 150 kV beam. Farmer-type air ionization chamber and radiochromic films are used to measure radial dose profiles. Results MC simulations at 320 kVp show that rotational kilovoltage radiotherapy assures sufficient skin sparing during breast RT. Centre to periphery dose ratio is less than 20:1 (1 cm collimation in the horizontal plane at 320 kVp). The vertical dose spread profile shows limited influence of the scatter dose at distance of 10 mm from the irradiated slice. MC simulations will be compared to laboratory measurements at 150 kV. Conclusions We showed the potential of orthovoltage external beam rotational RT for breast cancer with satisfactory skin sparing, though using a kilovoltage beam.

171. Dosimetric validation for an automatic multiple brain-metastases planning system using a single isocenter and conformal arc

PurposeA commercial planning software using this technique, called Multiple Brain Mets SRS (MBM) Element from Brainlab AG (Feldkirchen, Germany), version 1.5, has been installed in our institution. It uses a preconfigured set of dynamic noncoplanar, conformal arcs to treat up to 15 brain-metastases with a single isocenter. It optimizes the weight of each arc to achieve the best conformity for all targets. Planning time for optimization and dose calculation is 2∼4 min, which improves the planning efficiency dramatically in our clinic. In this study we describe the method used in our clinic to measure the delivered dose and then compare to the calculated dose distribution. MethodsWe included in this study 10 patients whith multiple brain mets. For these 10 pz the number of lesions planned ranges from a minimum of two to a maximum of six. The verification plan is calculated by Brainlab Element for the QA and measured in Easycube phantom equipped with a CC01 ionization chamber and EBT film. Furthermore the comparison of the plans, computed by MBM Element and Varian Eclipse, version 13.0, is realised. ResultsThe percentage difference between calculated by MBM Element and measured is included in 5%.Treatment plans generated using MBM Element and Eclipse resulted in a good agreement, the difference of the maximun dose is lower 2%. The PTV mean dose values difference between MBM Element and Eclipse have resulted less 1,7%. ConclusionsSatisfactory absolute dose and distribution agreement were observed. We conclude that MBM Element was in clinical use after these validations.

Benchmarking of Monte Carlo model of 6 Mv photon beam produced by Siemens Oncor® linear accelerator: determination of initial electron beam parameters in comparison with measurement

Introduction: The aim of this study was to investigate the initial electron beam parameters for Monte Carlo model of 6MV photon beam produced by Siemens Oncor® linear accelerator. Materials and Methods: In this study, the EGSnrc Monte Carlo user codes BEAMnrc and DOSXYZnrc were used. The beamnrc code were used for modelling of a 6 MV photon beam produced by Siemens Oncor® linac’s head and DOSXYZnrc code utilized for calculating dose distributions in a virtual water phantom. The simulations were started for 10×10 cm2 and 40×40 cm2 field sizes. First the electron energy was changed to match percent depth dose curves of simulations with those of measurements. Second the beam width of primary electron source was tweaked to match between dose profile curves of simulation with those of measurement. For data comparison a 1-dimensional Gamma analyses were used with criteria of 3%/ 3mm using an inhouse-matlab script. The gamma analyses were performed while dose distributions of Monte Carlo and measurements were set as an evaluated and reference dose. Results: The results of gamma analyses showed that for percent depth dose curves a passing rate of close to 100% evaluated points. Also for profile curves, the passing rates were above 95% of evaluated points. Therefore, all depth dose and dose profile curves were in good agreement. The agreements were found at 5.75 MeV primary electron and 0.35 cm beam width respectively. Conclusion: The Monte Carlo model of 6 MV photon beam produced by Siemens Oncor® linear accelerator was accurately benchmarked using measured data. Since the profile curves of large field sizes are much more sensitive to the variations of beam width than small field size, it is recommended that for tuning process both field sizes are considered.

39 Use of the GATE/GEANT4 platform in high dose rate brachytherapy: Dosimetric study of a Cobalt 60 source

Introduction The Monte-Carlo GATE/GEANT4 simulation platform offers a powerful tool for dosimetry applications in brachytherapy [1] . This work aims to use this platform to study the dosimetric parameters of a source of Cobalt-60 used in high dose rate brachytherapy (HDR). Methods In this work a source of cobalt 60 (model Co0-A86 from Eckert & Ziegler BEBIG) was simulated by the platform GATE/GEANT4 version 7.2. In a first step, all dosimetric parameters: dose rate constant, air kerma strength, radial dose function and anisotropy function were studied according to the recommendations of the AAPM (American Association of Physicists in Medicine) Working Group No. 43 [2] and the report of AAPM and ESTRO [3] (European Society for Radiotherapy & Oncology). In a second step, a simulation of the dose profile curves was performed on different axes of the source. In addition, we reproduced the results of the simulation of the profiles on the treatment planning system HDRPlus (Eckert & Ziegler BEBIG). The comparison of the profiles is done by the tool gamma-index [4] . Results For the parameters of the TG-43 formalism, the results obtained show a good agreement between the GATE/ GEANT4 platform and the studies taken as reference [5] , [6] . For the comparison of the dose profiles a gamma-index of 100% was obtained for 2% difference-in-dose criteria and DTA (Distance-To-Agreement) of 2 mm. Conclusions This work shows that the GATE/GEANT4 simulation platform can be a reliable tool for high dose rate (HDR) brachytherapy dose distribution calculations.

Total skin electron therapy (TSET): Monte Carlo Simulation and implementation

Introduction: Total skin electron irradiation technique is used in treatment of the mycosis fungoid. The implementation of this technique requires non-standard measurements and complex dosimetry methods. Operating procedures for total skin electron irradiation and its dosimetry vary in different radiation therapy centers in the world. In this article, validation of TSET technique dosimetry data by Monte Carlo simulation is done. Materials and Methods: The electron beam characteristic of RTSEI for the only electron accelerator, located at the radiation center of the Seyed Alshohada Hospital of Isfahan (NEPTUN 10PC) determined by performing Monte Carlo simulations and using EGSnrc-based codes (BEAMnrc and DOSXYZnrc). For the best uniformity of the vertical profile, the optimal angle of gantry was defined at SSD=300 cm. The effect of the degrader plane that is located at a distance of 20 cm from the patient surface, was evaluated on the amount of energy reduction of the beam, the opening of the electron beam field and the homogeneity of the dose distribution. The transversal dose distribution from the whole treatment with Rotational technique was simulated in a CT- based anthropomorphic phantom. Also, the percentage depth dose in the head, neck, thorax, abdomen and legs was obtained for RTSET technique. TSET dosimetry requires measurements in nonreference conditions. Because of the complexity of the required measurements for the commissioning process of this technique, different dosimetric systems were employed such as radiochromic films (EBT3) and an ionization chamber. In particular, for dual-field beams irradiation, the optimal tilt angle was investigated and the dose distribution in the treatment plane was measured by radiochromic films. Dose distributions and percentage depth dose measurements for a total skin electron therapy were measured in an anthropomorphic phantom. Results: The optimal angle of 20o would give the most uniform total profile and the use of a 0.8 cm PMMA degrader in front of the patient leads to a homogeneous distribution of the dose in all directions (the mean relative dose value was 97%±5%, normalizing to 100% at the calibration point level). The percentage depth dose curves in different organs of anthropomorphic phantom for RTSEI indicates that the depth of maximum dose is on the surface of the phantom, Isodose curve of 80% is formed at a depth less than 4 mm and at the depth less than 1.5 mm, the dose decreases to 20% of the maximum dose. Conclusion: The main purpose of this study was to commission and optimize a TSET technique for the treatment of mycosis fungiodes with the NEPTUN 10PC linear accelerator. This was done through an extensive set of measurements and a large number of MC simulations. The results of Monte Carlo calculations were found to be in general agreement with the measurements, providing a promising tool for further studies of dose distribution calculations in TSEI.

Clinical experience using Delta 4 phantom for pretreatment patient-specific quality assurance in modern radiotherapy

AbstractPurposeIn advanced radiotherapy techniques such as intensity-modulated radiation therapy (IMRT), the quality assurance (QA) process is essential. The aim of the study was to assure the treatment planning dose delivered during delivery of complex treatment plans. The QA standard is to perform patient-specific comparisons between planned doses and doses measured in a phantom.Materials and methodThe Delta 4 phantom (Scandidos, Uppsala, Sweden) has been used in this study. This device consists of diode matrices in two orthogonal planes inserted in a cylindrical acrylic phantom. Each diode is sampled per beam pulse so that the dose distribution can be evaluated on segment-by-segment, beam-by-beam, or as a composite plan from a single set of measurements. Ninety-five simple and complex radiotherapy treatment plans for different pathologies, planned using a treatment planning system (TPS) were delivered to the QA device. The planned and measured dose distributions were then compared and analysed. The gamma index was determined for different pathologies.ResultsThe evaluation was performed in terms of dose deviation, distance to agreement and gamma index passing rate. The measurements were in excellent agreement between with the calculated dose of the TPS and the QA device. Overall, good agreement was observed between measured and calculated doses in most cases with gamma values above 1 in >95% of measured points. Plan results for each test met the recommended dose goals.ConclusionThe delivery of IMRT and volumetric-modulated arc therapy (VMAT) plans was verified to correspond well with calculated dose distributions for different pathologies. We found the Delta 4 device is accurate and reproducible. Although Delta4 appears to be a straightforward device for measuring dose and allows measure in real-time dosimetry QA, it is a complex device and careful quality control is required before its use.

Novel Monte Carlo dose calculation algorithm for robotic radiosurgery with multi leaf collimator: Dosimetric evaluation

PurposeAt introduction in 2014, dose calculation for the first MLC on a robotic SRS/SBRT platform was limited to a correction-based Finite-Size Pencil Beam (FSPB) algorithm. We report on the dosimetric accuracy of a novel Monte Carlo (MC) dose calculation algorithm for this MLC, included in the Precision™ treatment planning system. MethodsA phantom was built of one slab (5.0 cm) of lung-equivalent material (0.09…0.29 g/cc) enclosed by 3.5 cm (above) and 5 cm (below) slabs of solid water (1.045 g/cc). This was irradiated using rectangular (15.4 × 15.4 mm2 to 53.8 × 53.7 mm2) and two irregular MLC-fields. Radiochromic film (EBT3) was positioned perpendicular to the slabs and parallel to the beam. Calculated dose distributions were compared to film measurements using line scans and 2D gamma analysis. ResultsMeasured and MC calculated percent depth dose curves showed a characteristic dose drop within the low-density region, which was not correctly reproduced by FSPB. Superior average gamma pass rates (2%/1 mm) were found for MC (91.2 ± 1.5%) compared to FSPB (55.4 ± 2.7%). However, MC calculations exhibited localized anomalies at mass density transitions around 0.15 g/cc, which were traced to a simplification in electron transport. Absence of these anomalies was confirmed in a modified build of the MC engine, which increased gamma pass rates to 96.6 ± 1.2%. ConclusionsThe novel MC algorithm greatly improves dosimetric accuracy in heterogeneous tissue, potentially expanding the clinical use of robotic radiosurgery with MLC. In-depth, independent validation is paramount to identify and reduce the residual uncertainties in any software solution.

Performance of a Semi-Analytical Dose Engine Designed for Spot Scanning Proton Therapy with Small Spot Sizes

To study the clinical performance of a semi-analytical dose engine developed for spot scanning proton delivery systems that feature small spot sizes. The dose engine used a ray-tracing pencil beam algorithm with three lateral Gaussian components. It was commissioned using Monte Carlo (MC) data generated by an experimentally well-benchmarked Geant4 code and then fine-tuned using point dose measurements with various combinations of energy layers, field sizes, depths, and SOBPs. The dose engine used water-equivalent distance in both the beam direction and the lateral direction to account for inhomogeneity. Ten patients representing different disease sites were randomly selected for validation. Comparisons were done in water with data calculated from a fast MC code and with ionization chamber array measurements for patient-specific QA (including point doses, dose profiles, 2D-3D, and 3D-3D gamma analysis) and in patient geometries with MC data (including dose volume histogram indices and 3D-3D gamma analysis). In-water gamma passing rates using 3%/3mm criteria for these ten patients were greater than 97% when compared to MC and greater than 95% when compared to dose plane measurements. Excellent agreement was observed for dose profiles and point dose measurements in water. Good agreement between the semi-analytical engine and the MC code was also observed in patient geometries. This dose engine has been successfully used at our institution as a fast, independent second check for over two years. A ray-tracing dose engine with three lateral Gaussian components can accurately calculate dose distributions for our proton delivery system.Abstract 1167; TableExample treatment plan DVH indicesMonte CarloDose engineCTV statisticsMean dose [cGy(RBE)]5228.65229.1D95% [cGy(RBE)]4943.45006.6Head and NeckBrain D1% [cGy(RBE)]4336.34330.6OARsBrain stem D1% [cGy(RBE)]3906.94062.2Spinal cord D1% [cGy(RBE)]2684.32728.5Eye mean dose (L/R) [cGy(RBE)]1362.7 / 1947.21389.7 / 1890.1 Open table in a new tab

Dosimetric and volumetric effects in clinical target volume and organs at risk during postprostatectomy radiotherapy.

To assess the reproducibility of the dose-volume distribution of the initial simulation CT, generated using volumetric modulated arc therapy (VMAT) planning, during the radiotherapy of the prostatic bed based on weekly cone beam CTs (CBCT). Twenty-three patients, after radical prostatectomy were treated with adjuvant or salvage radiotherapy between July and December 2016 and considered for this evaluation. Weekly CBCT scans (n = 138) were imported into the treatment planning system, and the clinical tumor volume (CTV), the rectum and the bladder were contoured. The initially calculated dose distribution and the dose-volume histograms generated from weekly CBCTs were compared. The prostatic fossa dose coverage was assessed by the proportion of the CTV fully encompassed by the 95% and 98% isodose lines. Rectal and bladder volumes receiving 50, 60 and 65 Gy during the treatment were compared to the initial plan, with statistical significance determined using the one-sample t‑test. Marked variations in the total organ volume of the rectum and the bladder were observed. The correlation between rectum volume and V50 was not significant (p = 0.487), while the bladder volume and V50 demonstrated asignificant correlation. There was no correlation between urinary bladder volume and CTV. The change in rectal volume correlated significantly with CTV. The dose coverage (D98% and D95%) to the prostatic bed could be achieved for all patients due to the ventral shift in the volume differences of the rectum. Weekly CBCTs can be considered as adequate verification tools to assess the interfractional variability of the CTV and organs at risk. The proven volume changes in the urinary bladder and the rectum do not compromise the final delivered dose in the CTV.

Implementation of planar proton minibeam radiation therapy using a pencil beam scanning system: A proof of concept study.

Proton minibeam radiation therapy (pMBRT) is an innovative approach that combines the advantages of minibeam radiation therapy with the more precise ballistics of protons to further reduce the side effects of radiation. One of the main challenges of this approach is the generation of very narrow proton pencil beams with an adequate dose-rate to treat patients within a reasonable treatment time (several minutes) in existing clinical facilities. The aim of this study was to demonstrate the feasibility of implementing pMBRT by combining the pencil beam scanning (PBS) technique with the use of multislit collimators. This proof of concept study of pMBRT with a clinical system is intended to guide upcoming biological experiments. Monte Carlo simulations (TOPAS v3.1.p2) were used to design a suitable multislit collimator to implement planar pMBRT for conventional pencil beam scanning settings. Dose distributions (depth-dose curves, lateral profiles, Peak-to-Valley Dose Ratio (PVDR) and dose-rates) for different proton beam energies were assessed by means of Monte Carlo simulations and experimental measurements in a water tank using commercial ionization chambers and a new p-type silicon diode, the IBA RAZOR. An analytical intensity-modulated dose calculation algorithm designed to optimize the weight of individual Bragg peaks composing the field was also developed and validated. Proton minibeams were then obtained using a brass multislit collimator with five slits measuring 2cm×400μm in width with a center-to-center distance of 4mm. The measured and calculated dose distributions (depth-dose curves and lateral profiles) showed a good agreement. Spread-out Bragg peaks (SOBP) and homogeneous dose distributions around the target were obtained by means of intensity modulation of Bragg peaks, while maintaining spatial fractionation at shallow depths. Mean dose-rates of 0.12 and 0.09Gy/s were obtained for one iso-energy layer and a SOBP conditions in the presence of multislit collimator. This study demonstrates the feasibility of implementing pMBRT on a PBS system. It also confirms the reliability of RAZOR detector for pMBRT dosimetry. This newly developed experimental methodology will support the design of future preclinical research with pMBRT.