IBA Dosimetry Research Articles

Beam monitor chamber calibration of a synchro-cyclotron high dose rate per pulse pulsed scanned proton beam

Objective. Ionization chambers, mostly used for beam calibration and for reference dosimetry, can show high recombination effects in pulsed high dose rate proton beams. The aims of this paper are: first, to characterize the linearity response of newly designed asymmetrical beam monitor chambers (ABMC) in a 100–226 MeV pulsed high dose rate per pulse scanned proton beam; and secondly, to calibrate the ABMC with a PPC05 (IBA Dosimetry) plane parallel ionization chamber and compare to calibration with a home-made Faraday cup (FC). Approach. The ABMC response linearity was evaluated with both the FC and a PTW 60019 microDiamond detector. Regarding ionometry-based ABMC calibration, recombination factors were evaluated theoretically, then numerically, and finally experimentally measured in water for a plane parallel ionization chamber PPC05 (IBA Dosimetry) through k s saturation curves. Finally, ABMC calibration was also achieved with FC and compared to the ionometry method for 7 energies. Main results. Linearity measurements showed that recombination losses in the new ABMC design were well taken into account for the whole range of the machine dose rates. The two-voltage-method was not suitable for recombination correction, but Jaffé’s plots analysis was needed, emphasizing the current IAEA TRS-398 reference protocol limitations. Concerning ABMC calibration, FC based absorbed dose estimation and PPC05-based absorbed dose estimation differ by less than 6.3% for the investigated energies. Significance. So far, no update on reference dosimetry protocols is available to estimate the absorbed dose in ionization chambers for clinical high dose rate per pulse pulsed scanned proton beams. This work proposes a validation of the new ABMC design, a method to take into account the recombination effect for ionometry-based ABMC calibration and a comparison with FC dose estimation in this type of proton beams.

Commissioning and clinical implementation of an independent dose calculation system for scanned proton beams.

Experimental patient-specific QA (PSQA) is a time and resource-intensive process, with a poor sensitivity in detecting errors. Radiation therapy facilities aim to substitute it by means of independent dose calculation (IDC) in combination with a comprehensive beam delivery QA program. This paper reports on the commissioning of the IDC software tool myQAiON (IBA Dosimetry) for proton therapy and its clinical implementation at the MedAustron Ion Therapy Center. The IDC commissioning work included the validation of the beam model, the implementation and validation of clinical CT protocols, and the evaluation of patient treatment data. Dose difference maps, gamma index distributions, and pass rates (GPR) have been reviewed. The performance of the IDC tool has been assessed and clinical workflows, simulation settings, and GPR tolerances have been defined. Beam model validation showed agreement of ranges within±0.2mm, Bragg-Peak widths within±0.1mm, and spot sizes at various air gaps within±5% compared to physical measurements. Simulated dose in 2D reference fields deviated by-0.3%±0.5%, while 3D dose distributions differed by 1.8% on average to measurements. Validation of the CT calibration resulted in systematic differences of 2.0% between IDC and experimental data for tissue like samples. GPRs of 99.4±0.6% were found for head, head and neck, and pediatric CT protocols on a 2%/2mm gamma criterion. GPRs for the adult abdomen protocol were at 98.9% on average with 3%/3mm. Root causes of GPR outliers, for example, implants were identified and evaluated. IDC has been successfully commissioned and integrated into the MedAustron clinical workflow for protons in 2021. IDC has been stepwise and safely substituting experimental PSQA since February 2021. The initial reduction of proton experimental PSQA was about 25% and reached up to 90% after 1 year.

Comparison of CTDIw and Homogeneity Index on CTDI Phantoms

The study was conducted to compare the Computed Tomography Dose Index Weighted (CTDIw) value values and homogeneity index on head and body phantoms with tube voltage variations. Two CTDI phantoms are Gammex (Sun Nuclear, Florida, United States) and IBA (IBA Dosimetry, Schwarzenbruck, Germany). The pencil ionization chamber was used for the measurement of CTDI. The measurements were carried out with a Toshiba Alexion 16 MSCT in a single rotation of axial mode with detector position in the phantom’s center, top, bottom, right, and left. Tube voltage values are 80 kVp, 100 kVp, and 120 kVp. Then, the homogeneity test of the phantom was carried out. The homogeneity value was obtained by measuring the average CT number in the image by determining the region of interest (ROI) at positions namely a, b, c, d, and e, In addition the ratio of the two phantoms was also carried out. The ratio was obtained from the difference of the CTDI100 value at the edge to the CTDI100 value at the center of the head and body phantom from Gammex and IBA. The results showed that the CTDIwof the Gammex head phantom are 26.83 mGy (80 kV), 53.32 mGy (100 kV) and 83.32 mGy (120 kV). While the CTDIw of the Gammex body phantom are 11.73 mGy (80 kV), 21.58 mGy (100 kV) and 36.45 mGy (120 kV). In comparison, CTDIw of the IBA head phantom are 27.01 mGy (80 kV), 55.33 mGy (100 kV) and 81.69 mGy (120 kV). While the CTDIw of the IBA body phantom are 11.85 mGy (80 kV), 23.32 mGy (100 kV) and 35.00 mGy (120 kV). The differences in CTDIw of the two phantoms were within (head phantom is 0.18 % – 2.01 %) and (body phantom is 0.13 % – 1.75 %). The difference below 5% with the p-value of the head phantom is 0.87 and body phantom is 0.89 (more than 0.05) indicates that the two phantoms are not significantly different because the two phantoms are made of the same material. The average ratio for the Gammex head phantom is 1.12 – 1.28, while the IBA head phantom is 1.07 – 1.28. Then the average ratio for the Gammex body phantom is 2.03 – 2.56, while for the IBA body phantom is 1.91 – 2.59 which indicates that the head phantom produces a more uniform dose distribution compared to a body phantom. The average homogeneity value of the IBA phantom is 90.52 % and the average homogeneity value of the Gammex phantom is 87.15 % (a difference of around 3.37%). This value shows that Gammex and IBA phantom have fairly good homogeneity

Is it still necessary to perform measured based pre-treatment patient-specific QA for SRS HyperArc treatments?

The Mobius3D system was validated as a modern secondary check dosimetry system. In particular, our objective has been to assess the suitability of the M3D as pre-treatment patient-specific Quality Assurance (QA) tool for Stereotactic Radiosurgery (SRS) HyperArc (HA) treatments. We aimed to determine whether Mobius3D could safely replace the measurements-based patient-specific QA for this type of treatment. 30 SRS HA treatment plans for brain were selected. The dose distributions, calculated by Mobius and our routinely used algorithm (AcurosXB v.15.6), were compared using gamma analysis index and DVH parameters based on the patient's CT dataset. All 30 plans were then delivered across the ionization chamber in a homogeneous phantom and the measured dose was compared with both M3D and TPS calculated one. The plans were delivered and verified in terms of PSQA using the electronic portal imaging device (EPID) with Portal Dosimetry (PD) and myQA SRS (IBA Dosimetry) detector. Plans that achieved a global gamma passing rate (GPR) ≥ 97% based on 2%/2mm criteria, with both Mobius3D and the conventional methods were evaluated acceptable. Finally, we assessed the capability of the M3D system to detect errors related to the position of the Multi-Leaf Collimator (MLC) in comparison to the analyzed measurement-based systems. No relevant differences were observed in the comparison between the dose calculated on the CT-dataset by M3D and the TPS. Observed discrepancies are imputable to different used algorithms, but no discrepancies related to goodness of plans have been found. Average differences between calculated (M3D and TPS) vs measured dose with ionization chamber were 2.5% (from 0.41% to 3.2%) and 1.81% (from 0.66% to 2.65%), for M3D and TPS, respectively. All plans passed with a gamma passing rate>97% using conventional PSQA methods with a gamma criterion of 2% dose difference and 2mm distance-to-agreement. The average gamma passing rate for the M3D system was determined to be 99.4% (from 97.3% to 100%). Results from this study also demonstrated Mobius has better error detectability than conventional measurement-based systems. Our study shows Mobius3D could be a suitable alternative to conventional measured based QA methods for SRS HyperArc treatments.

Adaptation of a Clinical Proton Pencil Beam Scanning System for FLASH Experiments

To characterize a proton pencil beam scanning system for ultra-high dose rate (UHDR) irradiations and validate it with FLASH preclinical experiments. After modifications to the beamline to maximize the beam current at isocenter in our gantry room, we characterized the UHDR beam in terms of: 1) Size and shape of the beam spot in three configurations; pristine beam, 75 mm water-equivalent-thickness (WET) range shifter (RS), and custom-built 135 mm WET RS mounted 310 mm upstream of the aperture in the snout housing. These configurations were analyzed to determine which one achieved the highest dose rate; 2) Beam transport efficiency and beam output. We compared the signal in the monitor chambers of the proton system with a Faraday cup and plane parallel ionization chamber (PPC05, IBA dosimetry) for beam current at the cyclotron from 7.5 nA to 800 nA; 3) Dose homogeneity, beam penumbra, and dose rate for the fields to be used in preclinical irradiations. All measurements were performed at isocenter, in air or at 1 cm depth in solid water, using the highest energy (about 230 MeV), which corresponded to a nominal range of 32.9 cm in water. We modeled the UHDR beam in our treatment planning system (TPS) to optimize the dose homogeneity and lateral penumbra of the irradiation fields. We performed the preclinical experiments in single fractions of 19 Gy (RBE), 21 Gy (RBE) and 23 Gy (RBE) (RBE = 1.1), targeting the pelvis of C57BL/6 mice and using survival as the endpoint. Each arm included 6-10 mice. The proton beam was used in transmission mode, positioning the center of the mouse pelvis at isocenter, and irradiating the pelvis with a 2x6 cm^2 field. Apertures were placed at 9cm from the isocenter to sharpen the lateral penumbra. The range measurements with a multi-layer ionization chamber were consistent within 1 mm with the nominal range. In UHDR mode, the spot size at the isocenter varied from 4.5 mm for the pristine beam to 9.2 mm for the 135 mm RS. The spot size at isocenter remained constant when the beam intensity varied from 7.5 nA to 800 nA at the cyclotron exit. By employing the configuration with the 135 mm RS and optimizing the fields in the TPS, we achieved a dose rate of 1 Gy (RBE)/s for the conventional regime and 75(RBE) Gy/s for the UHDR regime. The monitor chambers of the proton system were affected by recombination at high dose rates: we observed about 35% higher output for the same number of monitor units delivered at 800 nA vs 7.5 nA. The delivered dose was determined with the PPC05 for each field, as this detector did not show recombination effects. When preclinical irradiations were independently monitored, the delivered dose was typically within 1% of the intended value. In three independent experiments, a dose of 21 Gy (RBE) or higher was associated with an increased survival in the UHDR arm compared to the conventional arm. We adapted a clinical proton system for preclinical irradiations at UHDR. Our results confirm the presence of the FLASH effect.

FLASH dose rate calculation based on log files in proton pencil beam scanning therapy.

In radiation therapy, irradiating healthy normal tissues in the beam trajectories is inevitable. This unnecessary dose means that patients undergoing treatment risk developing side effects. Recently, FLASH radiotherapy delivering ultra-high-dose-rate beams has been re-examined because of its normal-tissue-sparing effect. To confirm the mean and instantaneous dose rates of the FLASH beam, stable and accurate dosimetry is required. Detailed verification of the FLASH effect requires dosimeters and a method to measure the average and instantaneous dose rate stably for 2- or 3-dimensional dose distributions. To verify the delivered FLASH beam, we utilized machine log files from the built-in monitor chamber to develop a dosimetry method to calculate the dose and average/instantaneous dose rate distributions in two or three dimensions in a phantom. To create a spread-out Bragg peak (SOBP) and deliver a uniform dose in a target, a mini-ridge filter was created with a 3D printer. Proton pencil beam line scanning plans of 2×2 cm2 , 3×3 cm2 , 4×4 cm2 , and round shapes with 2.3cm diameter patterns delivering 230 MeV energy protons were created. The absorbed dose in the solid water phantom of each plan was measured using a PPC05 ionization chamber (IBA Dosimetry, Virginia, USA) in the SOBP region, and the log files for each plan were exported from the treatment control system console. Using these log files, the delivered dose and average dose rate were calculated using two methods: a direct method and a Monte Carlo (MC) simulation method that uses log file information. The computed and average dose rates were compared with the ionization chamber measurements. Additionally, instantaneous dose rates in user-defined volumes were calculated using the MC simulation method with a temporal resolution of 5ms. Compared to ionization chamber dosimetry, 10 of 12 cases using the direct calculation method and 9 of 11 cases using the MC method had a dose difference below ±3%.Nine of 12 cases using the direct calculation method and 8 of 11 cases using the MC method had dose rate differences below ±3%.The average and maximum dose differences for the direct calculation and MC method were-0.17, +0.72%, and -3.15, +3.32%, respectively. For the dose rate difference, the average and maximum for the direct calculation and MC method were +1.26, +1.12%, and +3.75, +3.15%, respectively. In the instantaneous dose rate calculation with the MC simulation, a large fluctuation with a maximum of 163Gy/s and a minimum of 4.29Gy/s instantaneous dose rate was observed in a specific position, whereas the mean dose rate was 62Gy/s. We successfully developed methods in which machine log files are used to calculate the dose and the average and instantaneous dose rates for FLASH radiotherapy and demonstrated the feasibility of verifying the delivered FLASH beams.

Dosimetric effects and impacts caused by a carbon fiber table and its accessories in a linear accelerator

Objective : Most contemporary treatment planning systems (TPS) exclude the couch during treatment planning. The present investigation aimed to evaluate the impact of couches and accessories on radiation therapy planning by quantifying the degree of attenuation for two-photon beam energies, 6 MV and 10 MV, at two different field sizes, 5 cm x 5 cm and 10 cm x 10 cm. 
 Methods: An x-ray radiation beam at two energies (6 MV and 10 MV) generated by a linear accelerator (Elekta Synergy). The output dose was measured using a digital parallel plate ionization chamber of the IBA cc13 type (IBA Dosimetry, Germany), and the readings were recorded with a 2944 Farmer Type chamber. Different commercially available couch tops and accessories evaluated in this study.
 Results: The results show that the highest attenuation values were observed at 1300 gantry angles for both energies and field sizes. The lowest attenuation values were recorded at 1000 gantry angles with 6 MV energy for 5cm x 5cm and 10cm x 10cm field sizes. The breast board with D level demonstrated the highest attenuation of 6.27% and 5.51% for 6 MV and 10 MV energies, respectively.
 Conclusion: the findings indicate that the degree of attenuation is not uniform across all angles and may exceed tolerable limits, indicating the need for careful consideration during treatment planning to ensure optimal treatment outcomes.

Validation of Monte Carlo multithreading TOpas 3.6 software in order to simulate the 6 MV Elekta Synergy MLCi2 platform linac.

Monte Carlo simulation is generally appreciated as an extraordinary technique to investigate particle physics processes and interactions in nuclear medicine and Radiation Therapy. The present task validates a new methodology of Monte Carlo simulation based on the Multithreading technique to reduce CPU time to simulate a 6 MV photon beam provided by the Elekta Synergy MLCi2 platform medical linear accelerator treatment head utilizing TOpas version 3.6 Monte Carlo software and the Slurm Marwan cluster. The simulation includes the linear accelerator (LINAC) major components. Calculations are performed for the photon beam with several treatment field sizes varying from 3 × 3 to 10 × 10 cm2 at a 100 cm of distance from the source to the surface of the IBA dosimetry water box. The simulation was wholly approved by comparison with experimental distributions. To evaluate simulation accuracy, gamma index formalism for (2%/2mm) and (3%/2mm) criteria, Distance To Agreement DTA, and the estimator standard error ɛ and ɛ max are used. Good agreement between simulations and measurements was observed for depth doses and lateral dose profiles, respectively. The gamma index comparisons also highlighted this agreement; more than 97% of the points for all simulations satisfy the quality assurance criteria of (2%/2mm). Regarding calculation performance, the event processing speed is faster using Gate-[mp] compared to TOpas-[mt] mode when running the identical simulation code for both. Consequently, according to the achieved results, the proposed methodology shows the first validation of TOpas in radiotherapy linacs simulations and a reduction in calculation time, capping simulation accuracy as much as possible. For this reason, this software is recommended to be serviceable for Treatment Planning Systems (TPS) purposes.

Determination of Integral Depth Dose in Proton Pencil Beam Using Plane-parallel Ionization Chambers.

PurposeThis study aimed to determine the integral depth-dose curves and assess the geometric collection efficiency of different detector diameters in proton pencil beam scanning.Materials and MethodsThe Varian ProBeam Compact spot scanning system was used for this study. The integral depth-dose curves with a proton energy range of 130 to 220 MeV were acquired with 2 types of Bragg peak chambers: 34070 with 8-cm diameter and 34089 with 15-cm diameter (PTW), multi-layer ionization chamber with 12-cm diameter (Giraffe, IBA Dosimetry), and PeakFinder with 8-cm diameter (PTW). To assess geometric collection efficiency, the integral depth-dose curves of 8- and 12-cm chamber diameters were compared to a 15-cm chamber diameter as the largest detector.ResultsAt intermediate depths of 130, 150, 190, and 220 MeV, PTW Bragg peak chamber type 34089 provided the highest integral depth-dose curves followed by IBA Giraffe, PTW Bragg peak chamber type 34070, and PTW PeakFinder. Moreover, PTW Bragg peak chamber type 34089 had increased geometric collection efficiency up to 3.8%, 6.1%, and 3.1% when compared to PTW Bragg peak chamber type 34070, PTW PeakFinder, and IBA Giraffe, respectively.ConclusionA larger plane-parallel ionization chamber could increase the geometric collection efficiency of the detector, especially at intermediate depths and high-energy proton beams.

Determination of correction factors in small MLC-defined fields for the Razor and microSilicon diode detectors and evaluation of the suitability of the IAEA TRS-483 protocol for multiple detectors.

Small field output factors for Multileaf collimator (MLC)‐defined field sizes between 0.5 × 0.5 cm2 and 3 × 3 cm2 were measured with six different detectors for a Varian TrueBeam in 6‐MV, 6‐FFF, 10‐MV, and 10‐FFF photon beams. Correction factors kQclin,Qreffclin,fref from the IAEA publication TRS‐483 were used to correct the measured output factors. The corrected output factors from the six detectors were used to calculate correction factors for the PTW microSilicon T60023 (PTW, Freiburg, Germany) and IBA Razor (IBA Dosimetry, Schwarzenbruck, Germany) detectors. The uncertainty of the output and correction factors in this study was calculated and the calculations presented in detail. The application of the TRS‐483 correction factors significantly reduced the variation in output factors between the various detectors, with the exception of the PTW 60016 diode in 6‐MV and 6‐FFF beams, and the IBA PFD in 10‐MV and 10‐FFF beams. Correction factors calculated for the Razor agreed within 2.9% of existing literature for all energies, while the microSilicon correction factors agreed within 1.6% to the literature for 6‐MV beams. The uncertainty in the microSilicon and Razor correction factors was calculated to be less than 0.9% (k = 1). This study shows that TRS‐483 correction factors reduce the variation in output factors between the detectors used in this study and presents a suitable method for determining correction factors for detectors with unpublished values.

Evaluation of Two-Voltage and Three-Voltage Linear Methods for Deriving Ion Recombination Correction Factors in Proton FLASH Irradiation

Ultrahigh dose-rate (FLASH) proton therapy is of great interest due to potential reduced normal tissue toxicities without compromising tumor-killing effect compared to current clinical proton practices. However, the ionization chamber response to proton beams under ultrahigh dose rates (&#x003E;40 Gy/s) has not been thoroughly investigated. In this study, four different ion chambers (PTW 34045 Advanced Markus, PPC-40, CC-04, and CC-13 from IBA Dosimetry) were irradiated with 230 MeV proton beams at 1.5, 63.7, and 127.6 Gy/s dose rates. Theoretical values of ion recombination correction factor (<inline-formula> <tex-math notation="LaTeX">${k}_{s}$ </tex-math></inline-formula>) were calculated from saturation curves using Niatel&#x2019;s model. The theoretical <inline-formula> <tex-math notation="LaTeX">${k}_{s}$ </tex-math></inline-formula> values were compared to the values using the two-voltage (2V) method from standard dosimetry protocols and the three-voltage linear (3VL) method proposed by Rossomme <i>et al.</i> Both parallel plate chambers and CC-04 demonstrated adequate ion collection efficiency at the highest dose rate. For these three chambers, there is no statistically significant difference between theoretical <inline-formula> <tex-math notation="LaTeX">${k}_{s}$ </tex-math></inline-formula> values and those calculated with 2V and 3VL methods. However, significant ion recombination correction was found in CC-13 (<inline-formula> <tex-math notation="LaTeX">${k}_{s}&gt;1.50$ </tex-math></inline-formula>) when dose rate reached 63.7 Gy/s. The assumption of insignificant initial recombination in standard dosimetry protocols also underestimated the ion recombination effect in this scenario.

Validation of Dosimetric Comissioning Accuracy of IMRT for Versa HD Linear Accelerator with AAPM TG-119 protocol

Purpose: Verification of the IMRT prior to the introduction the new Versa HD (Elekta) Linac into operation using test examples given in the AAPM TG-119 report.&#x0D; Materials and methods: The verification of plans created both on the basis of the dose prescriptions given in the AAPMTG-119 report and on the basis of the constraints used in the NMRRC was made. The&#x0D; plans were verified using a matrix of ionization chambers (MatriXX Evolution) placed in a mini Phantom&#x0D; (IBA Dosimetry), using gamma criteria of 3 %/3 mm and 3 %/2 mm at a dose threshold of 10 %. In addition to checking the coincidence of the calculated and measured dose distribution in the plane, a&#x0D; comparison of the dose at the point calculated in the Monaco planning system (Elekta) and measured&#x0D; by the CC13 ionization chamber (0.13 cm3&#x0D; , Iba Dosimetry) inside the PTV and in the organ at risk was&#x0D; carried out. &#x0D; Results: The difference in the dose measured by the ionization chamber and calculated in the planning&#x0D; system was less than 2 %, and correlates well with the results of accredited clinics. The measured dose&#x0D; distribution in the plane well (97-100 % of the points) coincided with the calculated one for all localizations. An exception was the test case with several targets, in which the assessment was carried out&#x0D; according to the 3 %/2 mm gamma criterion. &#x0D; Conclusions: The results obtained when testing IMRT according to the protocol described in the AAPM&#x0D; TG-119 report, can be used to compare their results with the results of other accredited clinics.

Development and validation of an automatic commissioning tool for the Monte Carlo dose engine in myQA iON

Independent dose verification with Monte Carlo (MC) simulations is an important feature of proton therapy quality assurance (QA). However, clinical integration of such tools often generates an additional and complex workload for medical physicists. The preparation of the necessary clinical inputs, such as the machine beam model, should therefore be automated. In this work, a methodology for automatic MC commissioning has been devised, validated, and developed into a MATLAB tool for the users of myQA iON, the recent QA platform of IBA Dosimetry. With this workflow, all necessary parameters can easily be tuned using dedicated optimization methods. For the geometrical beam parameters (phase space), the assumption of a single or double Gaussian is made. To model the energy spectrum, a Gaussian function is assumed and parameters are optimized using either MC simulations or a library of pre-computed Bragg peaks. For the absolute dose calibration, commissioning fields can be reproduced with the dose engine to retrieve the necessary parameters. We discuss in a first time the tool efficiency and show that one can optimize all parameters in less than 4 min per energy with excellent accuracy. We then validate a beam model obtained with the tool by simulating homogeneous spread-out Bragg peaks (SOBPs) and patient QA plans previously measured in water. An average range agreement of 0.29 ± 0.34 mm is achieved for the SOBPs while 3%/3 mm local gamma passing rates reach 99.3% on average over all 62 measured patient QA planes, which is well within clinical tolerances.

Quantification of the temperature equilibrium time of the cavity in parallel-plate-type ionization chambers by thermal analysis.

Temperature corrections are necessary to account for the varying mass of air in the cavity volume of a vented ionization chamber. The temporal response resulting from temperature changes in a cylindrical and/or Farmer-type ionization chamber, which is the standard dosimeter, has been thoroughly discussed by some researchers. The purpose of this study was to characterise and analyse the dependence of the cavity air temperature of the parallel-plate-type ionization chamber on changes in the ambient temperature. Ionization chambers NACP-02 (IBA Dosimetry, GmbH) and Advanced Markus TN34045 (PTW, Freiburg) were modelled using thermal analysis software to present the temperature equilibrium time and the entire ionization chamber temperature distribution. The temporal response of each ionization chamber was measured for comparing the calculation results of the thermal analysis. The ionization chamber cavities of NACP-02 and TN34045 reached complete equilibrium in 670 and 750 s, respectively. Heat transfer occurred faster at the centre of the front wall of TN34045 than at the outside of the centre except for the edges. Further, the non-uniformity of temperature in the cavity was in the range of 24.2–24.8°C for NACP-02 and 23.7–24.4°C for TN34045 at 200 s after the ionization chamber was installed in the water phantom. The previous proposal to wait for about 15 mins after submerging the chamber in a water phantom before the measurement is demonstrated to be appropriate for parallel-plate-type ionization chambers.

Effect of Multileaf Collimator Leaf Position Error Determined by Picket Fence Test on Gamma Index Value in Patient-Specific Quality Assurance of Volumetric-Modulated Arc Therapy Plans.

AimThe correlation between the MLC QA (IBA Dosimetry, Germany) results of the picket fence test created with intentional errors and the patient's quality assurance (QA) evaluation was investigated to assess the impact of multileaf collimator (MLC) positioning error on patient QA.Materials and methodsThe picket fence, including error-free and intentional MLC errors, defined in Bank In, Bank Out, and Bank Both were analyzed using MLC QA. The QA of 15 plans consisting of stereotactic radiosurgery (SRS), stereotactic body radiotherapy (SBRT), and conventionally fractionated volumetric-modulated arc therapy (VMAT) acquired with electronic portal imaging devices (EPID) was evaluated in the presence of error-free and MLC errors. The QA of plans were analyzed with 2%/2 mm and 3%/3 mm criteria.ResultsThe passing rates of the picket fence test were 97%, 92%, 91%, and 87% for error-free and intentional errors. The criterion of 3%/3 mm wasn’t able to detect an MLC error for either SRS/SBRT or conventionally fractionated VMAT. The criterion of 2%/2mm was more sensitive to detect MLC error for the conventionally fractionated VMAT than SRS/SBRT. While only two of SBRT plans had <90%, four of conventionally fractionated VMAT plans had a <90% passing rate.ConclusionWe found that the systematic MLC positioning errors defined with picket fence have a smaller but measurable impact on SRS/SBRT than the VMAT plan for a conventionally fractionated and relatively complex plan such as head and neck and endometrium cases.

Validation of a 3D Pretreatment Quality Assurance Tool for Volumetric Modulated Arc Therapy (VMAT)

Implementation of a pretreatment quality assurance (QA) system needs a strong validation process and a good comprehension of the tool. The aim of this study is to validate COMPASS (IBA Dosimetry, Germany) as our 3D pre- treatment QA tool for system of Pinnacle as treatment planning system and an ELEKTA synergy linear accelerator. Validation of the systems was performed with static and dynamic plans on AAPM TG-119 phantom and 10 real VMAT plans for prostate and head and neck. Comparison between point dose from TPS and COMPASS was performed to evaluate the confidence limit in high dose region (98.5% and 98.6%) and low dose region (94.1% and 95.2%) for COMPASS computed and reconstructed dose respectively. For planar dose the confidence limit was respectively 95.37% and 96.35% for COMPASS computed and reconstructed dose. Clinical validation was evaluated by comparing dose-volume parameters for real VMAT plans with TPS values (mean differences were below of 1% for the target). The comparison between 2D dose distribution from TPS and dose extracted from COMPASS computed and reconstructed for real VMAT plans were also performed (mean global gamma passing rate better than 94% and 98% for the 2%/2mm and 3%/3mm criteria). The 3D dose distribution comparison between TPS and COMPASS was also performed with good gamma score for global and local analysis. COMPASS was successfully evaluated as our 3D pretreatment system.

The Influence of the Minimum Dynamic Leaf Gap on VMAT Plans Quality

The aim of the present work is to investigate the influence of dynamic minimum leaf gap into Pinnacle3 on VMAT plans quality. Three treatment machines were modeled in our TPS with a different value of dynamic minimum leaf gap of 5, 10 and 20 mm. VMAT plans of AAPM TG-119 phantom and twenty clinical real cases were planned on each machine. Based on AAPM TG-119 guidelines, we compared the machine on their ability to fulfill the dose goals; the pretreatment quality assurance was done with COMPASS QA system (IBA dosimetry, Germany). In order to evaluate how the measured and the planned data from each machine are closed. The monitor units’ numbers for each site and machine were also compared. For simple plans, all the three machines easily meet the goals; however, for complex shape case, only the 5 and 10 mm minimum leaf gap machines allow the user to reach the goals. Otherwise, the 20 mm minimum leaf gap machine presents lower difference between planned and measured dose and the best gamma scores than the two others machines. It also has the lower MUs per clinical site. Based on this investigation, 10 mm is the best compromise value for the dynamic minimum leaf gap into Pinnacle. It allows the planner to reach high complex goals during planning process and in another hand gives good agreement between planned and measured doses.

Single pencil beam benchmark of a module for Monte Carlo simulation of proton transport in the PENELOPE code.

PENH is a recently coded module for simulation of proton transport in conjunction with the Monte Carlo code PENELOPE. PENELOPE applies class II simulation to all type of interactions, in particular, to elastic collisions. PENH uses calculated differential cross sections for proton elastic collisions that include electron screening effects as well as nuclear structure effects. Proton-induced nuclear reactions are simulated from information in the ENDF-6 database or from alternative nuclear databases in ENDF format. The purpose of this work is to benchmark this module by simulating absorbed dose distributions from a single finite spot size proton pencil beam in water. Monte Carlo simulations with PENH are compared with simulation results from TOPAS Monte Carlo (v3.1p2) and RayStation Monte Carlo (v6). Different beam models are examined in terms of mean energy and energy spread to match the measured profiles. The phase-space file is derived from experimental measurements. Simulated absorbed dose distributions are compared to experimental data obtained with the ionization chamber array MatriXX 2D detector (IBA Dosimetry) in a water tank. The experiments were conducted with a clinical IBA pencil beam scanning dedicated nozzle. In all simulations a Fermi-Eyges phase-space representation of a single finite spot size proton pencil beam is used. In general, there is a good agreement between simulated results and experimental data up to a distance of 3cm from the central axis. In the core region (region where the dose is more than 10% of the maximum dose) PENH shows, overall, the smallest deviations from experimental data, with the largest radial rms (root mean square) smaller than 0.2. The results achieved by TOPAS and RayStation in that region are very close to those of PENH. For the halo region, that is the area of the dose distribution outside the core region reaching down to 0.01% of the maximum intensity, the largest rms achieved by TOPAS is always smaller than 0.5, yielding better results than the rest of the codes. The physics modeling of the PENELOPE/PENH code yields results consistent with measurements in the dose range relevant for proton therapy. The discrepancies between PENH appearing at distances larger than 3cm from the central-beam axis are accountable to the lack of neutron simulation in this code. In contradistinction, TOPAS has a better agreement with experimental data at large distances from the central-beam axis because of the simulation of neutrons.

Dosimetric comparison of intensity-modulated radiation therapy (IMRT) and field-in-field (FIF) technique for head-and-neck cancer

AbstractPurpose:This study compared the plan dosimetry between the intensity-modulated radiation therapy (IMRT) and field-in-field (FIF) technique for head-and-neck cancer using the Elekta Monaco treatment planning system (TPS).Materials and methods:A total of 20 head-and-neck cancer patients were selected in this study. IMRT and FIF plans for the patients were created on the Monaco TPS (ver. 5.11.02) using the 6-MV photon beam generated by the Elekta Synergy linear accelerator. The dose–volume histograms, maximum doses, minimum doses, mean doses of the target volumes and organs-at-risk (OARs), conformity index (CI), homogeneity index (HI) and monitor units (MUs) were determined for each IMRT and FIF plan. All IMRT plans passed the patient-specific quality assurance tests from the 2D diode array measurements (MatriXX Evolution System, IBA Dosimetry, Germany).Results:The results showed that the dose distribution to the target volumes of IMRT plans was better than FIF plans, while the dose (mean or max dose) to the OAR was significantly lower than FIF plan, respectively. IMRT and FIF resulted in planning target volume coverage with mean dose of 71·32 ± 0·76 and 73·12 ± 0·62 Gy, respectively, and HI values of 0·08 ± 0·01 (IMRT) and 0·19 ± 0·06 (FIF). The CI for IMRT was 0·98 ± 0·01 and FIF was 0·97 ± 0·01. For the spinal cord tolerance (maximum dose &lt; 45 Gy), IMRT resulted in 39·85 ± 2·04 Gy compared to 41·37 ± 2·42 Gy for FIF. In addition, the mean doses to the parotid grand were 27·27 ± 7·48 and 48·68 ± 1·62 Gy for the IMRT and FIF plans, respectively. Significantly more MUs were required in IMRT plans than FIF plans (on average, 846 ± 100 MU in IMRT and 467 ± 41 MU in FIF).Conclusions:It is concluded that the IMRT technique could provide a better plan dosimetry than the FIF technique for head-and-neck patients.

Analysis of the applicability of two-dimensional detector arrays in terms of sampling rate and detector size to verify scanned intensity-modulated proton therapy plans.

The introduction of advanced treatment techniques in proton therapy, such as intensity-modulated proton therapy, leads to an increased need for patient-specific quality assurance, especially an accurate treatment plan verification becomes inevitable. In this study, signal theoretical analysis of dose distributions in scanned proton therapy is performed to investigate the feasibility and limits of two-dimensional (2D) detector arrays for treatment plan verification. 2D detector arrays are characterized by two main aspects: the distance between the single detectors on the array or the sampling frequency; and the lateral response functions of a single detector. The analysis is based on single spots, reference fields and on measured and calculated dose distributions of typical intensity-modulated proton therapy treatment plans with and without range shifter. Measurements were performed with Gafchromic EBT3 films (Ashland Speciality Ingredients G.P., Bridgewater, NJ, USA), the MatriXX PT detector array (IBA Dosimetry, Schwarzenbruck, Germany) and the OCTAVIUS detector array 1500XDR (PTW-Freiburg, Germany) at an IBA Proteus PLUS proton therapy system (Ion Beam Applications, Louvain-la-Neuve, Belgium). Dose calculations were performed with the treatment planning system RayStation 6 or 8 (RaySearch Laboratories, Sweden). The Fourier analysis of the data of the treatment planning system and film measurements show maximum frequencies of 0.06/mm for the plan with range shifter and 0.083/mm for the plan without range shifter. According to the Nyquist theorem, this corresponds to minimum required sampling distances of 8.3 and 6mm, respectively. By comparison, the sampling distances of the arrays of 7.6mm (MatriXX PT) and 7.1mm (OD1500XDR) are sufficient to reconstruct the dose distributions adequately from measurements if range shifters are used, whereas some fields of the plans without range shifter violated the Nyquist requirement. The lateral dose response functions of the single detectors within the arrays have clearly higher frequencies than the treatment plans and thus the volume effect only slightly influences the measurements. Consequently, the array measurements show high gamma passing rates with at least 96 % and a good agreement between the investigated line profiles. The results indicate that the detector dimensions and sampling distances of the arrays are in most studied cases adequate not to substantially influence the measurement process when they are used for analyzing typical intensity-modulated proton therapy treatment plans. Nevertheless, clinical conditions have been identified, for instance treatment plans without range shifter, under which the Nyquist theorem is violated such that a full representation of the dose distributions with the measurements is not feasible. In these cases, analysis of measurements is limited to pointwise comparisons.