Absolute Dose Measurements Research Articles

Long-term stability of a PTW 34070 large-area parallel ionization chamber in clinical proton scanning beams.

In the modeling of beam data for proton therapy planning systems, absolute dose measurements are performed utilizing a Bragg peak chamber (BPC), which is a parallel-plate ionization chamber. The long-term stability of the BPC is crucial for ensuring accurate absolute dose measurement. The study aims to assess the long-term stability of the BPC in clinical proton pencil beam scanning delivery. The long-term stability evaluation focused on the BPC-Type 34070 (PTW Freiburg, Germany), utilizing clinical proton scanning beams from December 2022 to November 2023. Monthly investigations were conducted to evaluate the response and cross-calibration factor of the BPC and a reference chamber, employing the spread-out Bragg peak (SOBP) field. Additionally, assessments were made regarding the BPC's response to monoenergetic beams, along with an examination of the impact of polarity and ion recombination on the BPC. The response and cross-calibration factor of the BPC varied up to 1.9% and 1.8%, respectively, while the response of the reference chamber remained within a 0.5% range. The BPC's response to the mono-energetic beams varied up to 2.0% across all energies, demonstrating similar variation trends in both the SOBP field and mono-energetic beams. Furthermore, the variations in polarity and ion recombination effect remained stable within a 0.4% range throughout the year. Notably, the reproducibility of the BPC remained high for each measurement conducted, whether for the SOBP field or mono-energetic beams, with a maximum deviation observed at 0.1%. The response and cross-calibration factor of the BPC demonstrated significant variations, with maximum changes of 1.9% and 1.8%, respectively. However, the reproducibility of the BPC remained consistently high for each measurement. It is recommended that when conducting absolute dose measurements using a BPC, its response should be compared and corrected against the reference chamber for each measurement.

1520: Characterization of EBT, EBT-XD and OC-1 Films for Absolute Dose measurement in Electron Beams

1520: Characterization of EBT, EBT-XD and OC-1 Films for Absolute Dose measurement in Electron Beams

Accurate machine-specific reference and small-field dosimetry for a self-shielded neuro-radiosurgical system.

The newly available ZAP-X stereotactic radiosurgical system is designed for the treatment of intracranial lesions, with several unique features that include a self-shielding, gyroscopic gantry, wheel collimation, non-orthogonal kV imaging, short source-axis distance, and low-energy megavoltage beam. Systematic characterization of its radiation as well as other properties is imperative to ensure its safe and effective clinical application. To accurately determine the radiation output of the ZAP-X with a special focus on the smaller diameter cones and an aim to provide useful recommendations on quantification of small field dosimetry. Six different types of detectors were used to measure relative output factors at field sizes ranging from 4to 25mm, including the PTW microSilicon and microdiamond diodes, Exradin W2 plastic scintillator, Exradin A16 and A1SL ionization chambers, and the alanine dosimeter. The 25mm cone served as the reference field size. Absolute dose was determined with both TG-51-based dosimetry using a calibrated PTW Semiflex ion chamber and measurements using alanine dosimeters. The average radiation output factors (maximum deviation from the average) measured with the microDiamond, microSilicon, and W2 detectors were: for the 4mm cone, 0.741 (1.0%); for the 5mm cone: 0.817 (1.0%); for the 7.5mm cone: 0.908 (1.0%); for the 10mm cone: 0.946 (0.4%); for the 12.5mm cone: 0.964 (0.2%); for the 15mm cone: 0.976 (0.1%); for the 20mm cone: 0.990 (0.1%). For field sizes larger than 10mm, the A1SL and A16 micro-chambers also yielded consistent output factors within 1.5% of those obtained using the microSilicon, microdiamond, and W2 detectors. The absolute dose measurement obtained with alanine was within 1.2%, consistent with combined uncertainties, compared to the PTW Semiflex chamber for the 25mm reference cone. For field sizes less than 10mm, the microSilicon diode, microDiamond detector, and W2 scintillator are suitable devices for accurate small field dosimetry of the ZAP-X system. For larger fields, the A1SL and A16 micro-chambers can also be used. Furthermore, alanine dosimetry can be an accurate verification of reference and absolute dose typically measured with ion chambers. Use of multiple suitable detectors and uncertainty analyses were recommended for reliable determination of small field radiation outputs.

A preliminary study of multispectral Cherenkov imaging and a Fricke-xylenol orange gel film (MCIFF) for online, absolute dose measurement.

Cherenkov luminescence imaging has shown potential for relative dose distribution and field verification in radiation therapy. However, to date, limited research utilizing Cherenkov luminescence for absolute dose calibration has been conducted owing to uncertainties arising from camera positioning and tissue surface optical properties. This paper introduces a novel approach to multispectral Cherenkov luminescence imaging combined with Fricke-xylenol orange gel (FXG) film, termed MCIFF, which can enable online full-field absolute dose measurement. By integrating these two approaches, MCIFF allows for calibration of the ratio between two spectral intensities with absorbed dose, thereby enabling absolute dose measurement. All experiments are conducted on a Varian Clinac 23EX, utilizing an electron multiplying charge-coupled device (EMCCD) camera and a two-way image splitter for simultaneous capture of two-spectral Cherenkov imaging. In the first part of this study, the absorbance curves of the prepared FXG film, which receives different doses, are measured using a fluorescence spectrophotometer to verify the correlation between absorbance and dose. In the second part, the FXG film is positioned directly under the radiation beam to corroborate the dose measurement capacity of MCIFF across various beams. In the third part, the feasibility of MCIFF is tested in actual radiotherapy settings via a humanoid model, demonstrating its versatility with various radiotherapy materials. The results of this study indicate that the logarithmic ratios of spectral intensities at wavelengths of 550±50 and 700±100nm accurately reflect variations in radiation dose (R2>0.96) across different radiation beams, particle energies, and dose rates. The slopes of the fitting lines remain consistent under varying beam conditions, with discrepancies of less than 8%. The optical profiles obtained using the MCIFF exhibit a satisfactory level of agreement with the measured results derived from the treatment planning system (TPS) and EBT3 films. Specifically, for photon beams, the lateral distances between the 80% and 20% isodose lines, referred to as the penumbra (P80-20) values, obtained through TPS, EBT3 films, and MCIFF, are determined as 0.537, 0.664, and 0.848cm, respectively. Similarly, for electron beams, the P80-20 values obtained through TPS, EBT3 films, and MCIFF are found to be 0.432, 0.561, and 0.634cm, respectively. Furthermore, imaging of the anthropomorphic phantom demonstrates the practical application of MCIFF in real radiotherapy environments. By combining an FXG film with Cherenkov luminescence imaging, MCIFF can calibrate Cherenkov luminescence to absorbed dose, filling the gap in online 2D absolute dose measurement methods in clinical practice, and providing a new direction for the clinical application of optical imaging to radiation therapy.

Estimation of Uncertainty of the VMAT Absolute Dose Measurement Due to the Phantom Setup Error Using a Treatment Planning System

When performing single-point dose verification in VMAT, it is necessary to avoid the regions with steep dose gradient. We propose a method to obtain the estimated value ( Uplan) of uncertainty of the absolute dose measurement due to the phantom setup error by using dose gradient calculated from treatment planning system (TPS), for evaluating the appropriate measurement points. The dose gradient was calculated from the planned dose values in the vicinity of the isocenter point using TPS. The phantom setup error was estimated. The Uplan was calculated using the proposed formula after estimating the phantom setup error. Then, the dose gradient was calculated from the measured dose values in the vicinity of the isocenter point specified by TPS using the Tough water phantom with ionization chamber (IC), and Umeas was calculated as in Uplan. The correlation coefficient between Uplan and Umeas was 0.984, which indicates a high correlation. The average of the difference between Umeas and Uplan was -0.24%. We considered that this result was caused by the influence of volume averaging effect of IC. The Uplan obtained from this proposed method reflects the uncertainty of the absolute dose measurement due to the phantom setup error and is useful for evaluating the appropriate measurement points for absolute dose measurement.

Viability of the virtual cone technique using a fixed small multi-leaf collimator field for stereotactic radiosurgery of trigeminal neuralgia.

Dosimetric uncertainties in very small (≤1.5×1.5 cm2 ) photon fields are remarkably higher, which undermines the validity of the virtual cone (VC) technique with a diminutive and variable MLC fields. We evaluate the accuracy and reproducibility of the VC method with a very small, fixed MLC field setting, called a fixed virtual cone (fVC), for small target radiosurgery such as trigeminal neuralgia (TGN). The fVC is characterized by 0.5 cm x 0.5cm high-definition (HD) MLC field of 10MV FFF beam defined at 100cm SAD, while backup jaws are positioned at 1.5 cm x 1.5cm. A spherical dose distribution equivalent to 5mm (diameter) physical cone was generated using 10-14 non-coplanar, partial arcs. Dosimetric accuracy was validated using SRS diode (PTW 60018), SRS MapCHECK (SNC) measurements. As a quality assurance measure, 10 treatment plans (SRS) for TGN, consisting of various arc ranges at different collimator angles were analyzed using 6 MV FFF and 10 MV FFF beams, including a field-by-field study (n=130 fields). Dose outputs were compared between the Eclipse TPS and measurements (SRS MapCHECK). Moreover, dosimetric changes in the field defining fVC, prompted by a minute (±0.5-1.0mm) leaf shift, was examined among TPS, diode measurements, and Monte Carlo (MC) simulations. The beam model for fVC was validated (≤3% difference) using SRS MapCHECK based absolute dose measurements. The equivalent diameters of the 50% isodose distribution were found comparable to that of a 5mm cone. Additionally, the comparison of field output factors, dose per MU between the TPS and SRS diode measurements using the fVC field, including±1mm leaf shift, yielded average discrepancies within 5.5% and 3.5% for 6 MV FFF and 10 MV FFF beams, respectively. Overall, the fVC method is a credible alternative to the physical cone (5mm) that can be applied in routine radiosurgical treatment of TGN.

Technical note: Point-by-point ion-recombination correction for accurate dose profile measurement in high dose-per-pulse irradiation field.

Although flattening filter free (FFF) beams are commonly used in clinical treatment, the accuracy of dose measurements in FFF beams has been questioned. Higher dose per pulse (DPP) such as FFF beams from a linear accelerator may cause problems in dose profile measurements using an ionization chamber due to the change of the charge collection efficiency. Ionization chambers are commonly used for percent depth dose (PDD) measurements. Changes of DPP due to chamber movement during PDD measurement can vary the ion collection efficiency of ionization chambers. In the case of FF beams, the DPP fluctuation is negligible, but in the case of the FFF beams, the DPP is 2.5 ∼ 4 times larger than that of the FF beam, and the change in ion collection efficiency is larger than that of the FF beam. PDD profile normalized by maximum dose depth, 10 cm depth for example, may therefore be affected by the ion collection efficiency. In this study, we investigate the characteristics of the ion collection efficiency change depending on the DPP of each ionization chamber in the FFF beam. We furthermore propose a method to obtain the chamber- independent PDD by applying a DPP-dependent ion recombination correction. Prior to investigating the relationship between DPP and charge collection efficiency, Jaffe-plots were generated with different DPP settings to investigate the linearity between the applied voltage and collected charge. The absolute dose measurement using eight ionization chambers under the irradiation settings of 0.148, 0.087, and 0.008 cGy/pulse were performed. Applied voltages for the Jaffe-plots were 100, 125, 150, 200, 250, and 300V. The ion recombination correction factor Pion was calculated by the two-voltage analysis (TVA) method at the applied voltages of 300 and 100V. The DPP dependency of the charge collection efficiency for each ionization chamber were evaluated from the DPP- Pion plot. PDD profiles for the 10 MV FFF beam were measured using Farmer type chambers (TN30013, FC65-P, and FC65-G) and mini-type chambers (TN31010, TN31021, CC13, CC04, and FC23-C). The PDD profiles were corrected with ion recombination correction at negative and positive polar applied voltages of 100 and 300V. From the DPP-Pion relation for each ionization chamber with DPP ranging from 0.008 cGy/pulse to 0.148 cGy/pulse, all Farmer and mini-type chambers satisfied the requirements described in AAPM TG-51 addendum. However, Pion for the CC13 was most affected by DPP among tested chambers. The maximum deviation among PDDs using eight ionization chambers for 10 MV FFF was about 1%, but the deviation was suppressed to about 0.5% by applying ion recombination correction at each depth. In this study, the deviation of PDD profile among the ionization chambers was reduced by the ion recombination coefficient including the DPP dependency, especially for high DPP beams such as FFF beams. The present method is particularly effective for CC13, where the ion collection efficiency is highly DPP dependent.

FLASH Radiotherapy and the Use of Radiation Dosimeters.

Radiotherapy (RT) using ultra-high dose rate (UHDR) radiation, known as FLASH RT, has shown promising results in reducing normal tissue toxicity while maintaining tumor control. However, implementing FLASH RT in clinical settings presents technical challenges, including limited depth penetration and complex treatment planning. Monte Carlo (MC) simulation is a valuable tool for dose calculation in RT and has been investigated for optimizing FLASH RT. Various MC codes, such as EGSnrc, DOSXYZnrc, and Geant4, have been used to simulate dose distributions and optimize treatment plans. Accurate dosimetry is essential for FLASH RT, and radiation detectors play a crucial role in measuring dose delivery. Solid-state detectors, including diamond detectors such as microDiamond, have demonstrated linear responses and good agreement with reference detectors in UHDR and ultra-high dose per pulse (UHDPP) ranges. Ionization chambers are commonly used for dose measurement, and advancements have been made to address their response nonlinearities at UHDPP. Studies have proposed new calculation methods and empirical models for ion recombination in ionization chambers to improve their accuracy in FLASH RT. Additionally, strip-segmented ionization chamber arrays have shown potential for the experimental measurement of dose rate distribution in proton pencil beam scanning. Radiochromic films, such as GafchromicTM EBT3, have been used for absolute dose measurement and to validate MC simulation results in high-energy X-rays, triggering the FLASH effect. These films have been utilized to characterize ionization chambers and measure off-axis and depth dose distributions in FLASH RT. In conclusion, MC simulation provides accurate dose calculation and optimization for FLASH RT, while radiation detectors, including diamond detectors, ionization chambers, and radiochromic films, offer valuable tools for dosimetry in UHDR environments. Further research is needed to refine treatment planning techniques and improve detector performance to facilitate the widespread implementation of FLASH RT, potentially revolutionizing cancer treatment.

Monte Carlo flattening filter design to high energy intraoperative electron beam homogenization

Intraoperative radiotherapy using mobile linear accelerators is used for a wide variety of malignancies. However, when large fields are used in combination with high energies, a deterioration of the flatness dose profile is measured with respect to smaller fields and lower energies. Indeed, for the LIAC HWL of Sordina, this deterioration is observed for the 12 MeV beam combined with 10 cm (or larger) diameter applicator. Aimed to solve this problem, a flattening filter has been designed and validated evaluating the feasibility of its usage at the upper part of the applicator. The design of the filter was based on Monte Carlo simulations because of its accuracy in modeling components of clinical devices, among other purposes. The LIAC 10 cm diameter applicator was modeled and simulated independently by two different research groups using two different MC codes, reproducing the heterogeneity of the 12 MeV energy beam. Then, an iterative process of filter design was carried out. Finally, the MC designed conical filter with the optimal size and height to obtain the desired flattened beam was built in-house using a 3D printer. During the experimental validation of the applicator-filter, percentage depth dose, beam profiles, absolute and peripheral dose measurements were performed to demonstrate the effectiveness of the filter addition in the applicator. These measurements conclude that the beam has been flattened, from 5.9% with the standard configuration to 1.6% for the configuration with the filter, without significant increase of the peripheral dose. Consequently, the new filter-applicator LIAC configuration can be used also in a conventional surgery room. A reduction of 16% of the output dose and a reduction of 1.1 mm in the D50 of the percentage depth dose was measured with respect to the original configuration. This work is a proof-of-concept that demonstrates that it is possible to add a filter able to flatten the beam delivered by the Sordina LIAC HWL. Future studies will focus on more refined technical solutions fully compatible with the integrity of the applicator, including its sterilization, to be safely introduced in the clinical practice.

Auto-commissioning of a Monte Carlo electron beam model with application to photon MLC shaped electron fields

Objective. Presently electron beam treatments are delivered using dedicated applicators. An alternative is the usage of the already installed photon multileaf collimator (pMLC) enabling efficient electron treatments. Currently, the commissioning of beam models is a manual and time-consuming process. In this work an auto-commissioning procedure for the Monte Carlo (MC) beam model part representing the beam above the pMLC is developed for TrueBeam systems with electron energies from 6 to 22 MeV. Approach. The analytical part of the electron beam model includes a main source representing the primary beam and a jaw source representing the head scatter contribution each consisting of an electron and a photon component, while MC radiation transport is performed for the pMLC. The auto-commissioning of this analytical part relies on information pre-determined from MC simulations, in-air dose profiles and absolute dose measurements in water for different field sizes and source to surface distances (SSDs). For validation calculated and measured dose distributions in water were compared for different field sizes, SSDs and beam energies for eight TrueBeam systems. Furthermore, a sternum case in an anthropomorphic phantom was considered and calculated and measured dose distributions were compared at different SSDs. Main results. Instead of the manual commissioning taking up to several days of calculation time and several hours of user time, the auto-commissioning is carried out in a few minutes. Measured and calculated dose distributions agree generally within 3% of maximum dose or 2 mm. The gamma passing rates for the sternum case ranged from 96% to 99% (3% (global)/2 mm criteria, 10% threshold). Significance. The auto-commissioning procedure was successfully implemented and applied to eight TrueBeam systems. The newly developed user-friendly auto-commissioning procedure allows an efficient commissioning of an MC electron beam model and eases the usage of advanced electron radiotherapy utilizing the pMLC for beam shaping.

Development of a compact linear accelerator to generate ultrahigh dose rate high-energy X-rays for FLASH radiotherapy applications.

In recent years, the FLASH effect, in which ultrahigh dose rate (UHDR) radiotherapy (RT) can significantly reduce toxicity to normal tissue while maintaining antitumor efficacy, has been verified in many studies and even applied in human clinical cases. This work evaluates whether a room-temperature radio-frequency (RF) linear accelerator (linac) system can produce UHDR high-energy X-rays exceeding a dose rate of 40Gy/s at a clinical source-surface distance (SSD), exploring the possibility of a compact and economical clinical FLASH RT machine suitable for most hospital treatmentrooms. A 1.65m long S-band backward-traveling-wave (BTW) electron linac was developed to generate high-current electron beams, supplied by a commercial klystron-based power source. A tungsten-copper electron-to-photon conversion target for UHDR X-rays was designed and optimized with Monte Carlo (MC) simulations using Geant4 and thermal finite element analysis (FEA) simulations using ANSYS. EBT3 and EBT-XD radiochromic films, which were calibrated with a clinical machine Varian VitalBeam, were used for absolute dose measurements. A PTW ionization chamber detector was used to measure the relative total dose and a plane-parallel ionization chamber detector was used to measure the relative normalized dose of each pulse. The BTW linac generated 300-mA-pulse-current 11 MeV electron beams with 29kW mean beam power, and the conversion target could sustain this high beam power within a maximum irradiation duration of 0.75 s. The mean energy of the produced X-rays was 1.66 MeV in the MC simulation. The measured flat-filter-free (FFF) maximum mean dose rate of the room-temperature linac exceeded 80Gy/s at an SSD of 50cm and 45Gy/s at an SSD of 67.9cm, both at a 2.1cm depth of the water phantom. The FFF radiation fields at 50cm and 67.9cm SSD at a 2.1cm depth of the water phantom showed Gaussian-like distributions with 14.3 and 20cm full-width at half-maximum (FWHM) values, respectively. This work demonstrated the feasibility of UHDR X-rays produced by a room-temperature RF linac, and explored the further optimization of system stability. It shows that a simple and compact UHDR X-ray solution can be facilitated for both FLASH-RT scientific research and clinical applications.

Calorimeter measurements of absolute dose in aluminum, a surrogate of bone, to validate dose-to-medium in Acuros XB

Objective. While the accuracy of dose calculations in water with Acuros XB is well established, experimental validation of dose in bone is limited. Acuros XB reports both dose-to-medium and dose-to-water, and these values differ in bone, but there are no reports of measurements of validation in bone. This work compares Acuros XB calculations to measurements of absolute dose in aluminum (medium similar to bone). The validity of using selected relative dosimeters in aluminum is also investigated. Approach. A calorimeter with an aluminum core embedded in an aluminum phantom was selected as bone surrogate for the measurement of absolute dose. Matching the medium of the core to the medium of the phantom allowed eliminating the calculation of the conversion between media. The dose was measured at the fixed depth of 3.3 cm in aluminum (∼9 g·cm−2) with 6X, 10X, 6FFF and 10FFF photon beams from a TrueBeam Varian linac. In addition, experimental cross-calibration between water and aluminum was performed for an IBA CC13 ionization chamber, a PTW microDiamond and EBT3 Gafchromic film. Main results. Calculations with Acuros XB dose-to-medium in aluminum differed from the calorimetry data by −2.8% to −3.5%, depending on the beam. Use of dose-to-water would have resulted in about 39% discrepancy. The cross calibration coefficient between water and aluminum yielded values of about 0.87 for the CC13 chamber, 0.91 for the microDiamond, and 0.88 for the film, and independent of the beam within about ±1%. Significance. It was demonstrated the value of the dose-to-medium in aluminum (surrogate of bone) computed with Acuros XB is close to the value of the absolute dose measured with a calorimeter, and there is a significant discrepancy when dose-to-water is used instead. The use of an ionization chamber, a microDiamond and Gafchromic film in aluminum required a considerable correction from calibration in water.

Comparison and calibration of dose delivered by 137Cs and x-ray irradiators in mice

Objective. The Office of Radiological Security, U.S. Department of Energy's National Nuclear Security Administration, is implementing a radiological risk reduction program which seeks to minimize or eliminate the use of high activity radiological sources, including 137Cs, by replacing them with non-radioisotopic technologies, such as x-ray irradiators. The main goal of this paper is to evaluate the equivalence of the dose delivered by gamma- and x-ray irradiators in mice using experimental measurements and Monte Carlo simulations. We also propose a novel biophantom as an in situ dose calibration method. Approach. We irradiated mouse carcasses and 3D-printed mouse biophantoms in a 137Cs irradiator (Mark I-68) and an x-ray irradiator (X-Rad320) at three voltages (160 kVp, 225 kVp and 320 kVp) and measured the delivered radiation dose. A Geant4-based Monte Carlo model was developed and validated to provide a comprehensive picture of gamma- and x-ray irradiation in mice. Main Results. Our Monte Carlo model predicts a uniform dose delivered in soft-tissue for all the explored irradiation programs and in agreement with the absolute dose measurements. Our Monte Carlo model shows an energy-dependent difference between dose in bone and in soft tissue that decreases as photon energy increases. Dose rate depends on irradiator and photon energy. We observed a deviation of the measured dose from the target value of up to −9% for the Mark I-68, and up to 35% for the X-Rad320. The dose measured in the 3D-printed phantoms are equivalent to that in the carcasses within 6% uncertainty. Significance. Our results suggest that 320 kVp irradiation is a good candidate to substitute 137Cs irradiation barring a few caveats. There is a significant difference between measured and targeted doses for x-ray irradiation that suggests a strong need for in situ calibration, which can be achieved with 3D-printed mouse biophantoms. A dose correction is necessary for bone doses, which can be provided by a Monte Carlo calculation. Finally, the biological implications of the differences in dose rates and dose per photon for the different irradiation methods should be carefully assessed for each small-animal irradiation experiment.

Measurement of Absorbed Dose in Water Using Co-60 gamma source Teletherapy Units According to IAEA Dosimetry Protocols

Modern radiotherapy facilities like 3-Dimensional conformal radio therapy (3DCRT), Intensity Modulated Radiotherapy (IMRT), were recently suggested in Co-60 machine with Multi-leaf Collimator (MLC). In this study, two reference chambers NE-2571#1205 and NE2581#537 were used for absolute dose measurement in Equinox accelerator. A comparison of dose measurement by two different IAEA protocols TRS-277 and TRS-398 has been studied. Analyzing TRS-398, a common shaped empirical formula was developed for the four Co-60 units of four Medical Colleges in Bangladesh with fitting parameters. It was found that an average discrepancy in the determination of absorbed dose in water among the two different protocols TRS-277 and TRS-398 were 1.33 % for the chamber NE-2571#1205 and 0.65 % for the chamber NE2581#537 with combined uncertainty ±1.59 % (k=1). A good convergence has been obtained in the concepts and methods in this study.

Technical note: Absolute dose measurements of a vault-free radiosurgery system.

Methods for accurate absolute dose (AD) calibration are essential for the proper functioning of radiotherapy treatment machines. Many systems do not conform to TG-51 calibration standards, and modifications are required. TG-21 calibration is also a viable methodology for these situations with the appropriate setup, equipment, and factors. It has been shown that both these methods result in minimal errors. A similar approach has been taken in calibrating the dose for a recent vault-free radiosurgery system. To evaluate modified TG-21 and TG-51 protocols for AD calibrations of the ZAP-X radiosurgery system using ion chambers, film, and thermoluminescent dosimeters (TLDs). The current treatment planning system for ZAP-X requires AD calibration at dmax (7mm) and 450mm source-to-axis distance. Both and Nx [R/C] calibration coefficients were provided by an accredited dosimetry calibration laboratory for a physikalisch technische werkstatten (PTW) 31010 chamber (0.125 cc). The vendor provides an f-bracket that can be mounted on the collimator. Various phantoms can then be attached to the f-bracket. A custom acrylic phantom was designed based on recommendations from TG-21 and technical report series-398 that places the chamber at 500mm from the source with a depth of 44-mm acrylic and 456-mm SSD. Nx along with other TG-21 parameters was used to calculate the AD. Measurements using a PTW MP3-XS water tank and the same chamber were used to calculate AD using and TG-51 factors. Dose verification was performed using Gafchromic film and 3rd party TLDs. Measurements from TG-51, TG-21 (utilizing the custom acrylic phantom), film, and TLDs agreed to within±2%. A modified TG-51 AD calculation in water is preferred but may not be practical due to the difficulty in tank setup. The TG-21 modified protocol using a custom acrylic phantom is an accurate alternative option for dose calibration. Both of these methods are within acceptable agreement and provide confidence in the system's AD calibration.

A review on reference dosimetry in radiation therapy with proton and light ion beams: status and impact of new developments

This review focuses on reference dosimetry of proton and light ion beams used for cancer treatments. The background is the recommendation from the International Atomic Energy Agency in the Technical Report Series 398 and the recent publication of the International Commission on Radiation Units and Measurements Report 93 (Prescribing, Recording, and Reporting Light Ion Beam Therapy). These recommendations are briefly summarised, focusing on protons and carbon ions, as the most used ion types worldwide. Following these guidelines, the different aspects involving absolute dose measurements with ionisation chambers are discussed. A description of the current challenges and adaptions for the future is given for new developments. One of them, with a direct impact on ionisation chambers, is the influence of magnetic fields since efforts are made to integrate magnetic resonance imaging techniques into ion beam treatments, allowing online imaging with high soft-tissue contrast. Additionally, upcoming treatments with helium beams call for more experimental studies and simulations characterising the response of ionisation chambers, and at the same time following the international recommendations. Lastly, since the use of helium ion beams may present the advantage of reduced number of secondary particles as compared to carbon ion beams, the role of secondary neutrons and their impact on secondary cancer incidence needs to be assessed.

The fast calibration model for dosimetry with an electronic portal imaging device.

The aim of this study was to develop an algorithm that corrects the image of an electronic portal imaging device (EPID) of a linear accelerator so that it can be used for dosimetric purposes, such as in vivo dosimetry or quality assurance for photon radiotherapy. For that purpose, the impact of the field size, phantom thickness, and the varying spectral photon distribution within the irradiation field on the EPID image was investigated. The EPID measurements were verified using reference measurements with ionization chambers. Therefore, absolute dose measurements with an ionization chamber and relative dose measurements with a detector array were performed. An EPID calibration and correction algorithm was developed to convert the EPID image to a dose distribution. The algorithm was validated by irradiating inhomogeneous phantoms using square fields as well as irregular IMRT fields. It was possible to correct the influence of the field size, phantom thickness on the EPID signal as well as the homogenization of the image profile by several correction factors within 0.6%. A gamma index analysis (3%, 3mm) of IMRT fields showed a pass rate of above 99%, when comparing to the planning system. The developed algorithm enables an online dose measurement with the EPID during the radiation treatment. The algorithm is characterized by a robust, non-iterative, and thus real-time capable procedure with little measuring effort and does not depend on system-specific parameters. The EPID image is corrected by multiplying three independent correction factors. Therefore, it can easily be extent by further correction factors for other influencing variables, so it can be transferred to other linear accelerators and EPID configurations.

Commissioning and validation of RayStation treatment planning system for CyberKnife M6.

BackgroundRaySearch (AB, Stockholm) has released a module for CyberKnife (CK) planning within its RayStation (RS) treatment planning system (TPS).PurposeTo create and validate beam models of fixed, Iris, and multileaf collimators (MLC) of the CK M6 for Monte Carlo (MC) and collapsed cone (CC) algorithms in the RS TPS.MethodsMeasurements needed for the creation of the beam models were performed in a water tank with a stereotactic PTW 60018 diode. Both CC and MC models were optimized in RS by minimizing the differences between the measured and computed profiles and percentage depth doses. The models were then validated by comparing dose from the plans created in RS with both single and multiple beams in different phantom conditions with the corresponding measured dose. Irregular field shapes and off‐axis beams were also tested for the MLC. Validation measurements were performed using an A1SL ionization chamber, EBT3 Gafchromic films, and a PTW 1000 SRS detector. Finally, patient‐specific QAs with gamma criteria of 3%/1 mm were performed for each model.ResultsThe models were created in a straightforward manner with efficient tools available in RS. The differences between computed and measured doses were within ±1% for most of the configurations tested and reached a maximum of 3.2% for measurements at a depth of 19.5‐cm. With respect to all collimators and algorithms, the maximum averaged dose difference was 0.8% when considering absolute dose measurements on the central axis. The patient‐specific QAs led to a mean result of 98% of points fulfilling gamma criteria.ConclusionsWe created both CC and MC models for fixed, Iris, and MLC collimators in RS. The dose differences for all collimators and algorithms were within ±1%, except for depths larger than 9 cm. This allowed us to validate both models for clinical use.

Ultrahigh dose rate pencil beam scanning proton dosimetry using ion chambers and a calorimeter in support of first in-human FLASH clinical trial.

PurposeTo provide ultrahigh dose rate (UHDR) pencil beam scanning (PBS) proton dosimetry comparison of clinically used plane‐parallel ion chambers, PTW (Physikalisch‐Technische Werkstaetten) Advanced Markus and IBA (Ion Beam Application) PPC05, with a proton graphite calorimeter in a support of first in‐human proton FLASH clinical trial.MethodsAbsolute dose measurement intercomparison of the plane‐parallel plate ion chambers and the proton graphite calorimeter was performed at 5‐cm water‐equivalent depth using rectangular 250‐MeV single‐layer treatment plans designed for the first in‐human FLASH clinical trial. The dose rate for each field was designed to remain above 60 Gy/s. The ion recombination effects of the plane‐parallel plate ion chambers at various bias voltages were also investigated in the range of dose rates between 5 and 60 Gy/s. Two independent model‐based extrapolation methods were used to calculate the ion recombination correction factors ks to compare with the two‐voltage technique from most widely used clinical protocols.ResultsThe mean measured dose to water with the proton graphite calorimeter across all the predefined fields is 7.702 ± 0.037 Gy. The average ratio over the predefined fields of the PTW Advanced Markus chamber dose to the calorimeter reference dose is 1.002 ± 0.007, whereas the IBA PPC05 chamber shows ∼3% higher reading of 1.033 ± 0.007. The relative differences in the ks values determined from between the linear and quadratic extrapolation methods and the two‐voltage technique for the PTW Advanced Markus chamber are not statistically significant, and the trends of dose rate dependence are similar. The IBA PPC05 shows a flat response in terms of ion recombination effects based on the ks values calculated using the two‐voltage technique. Differences in ks values for the PPC05 between the two‐voltage technique and other model‐based extrapolation methods are not statistically significant at FLASH dose rates. Some of the ks values for the PPC05 that were extrapolated from the three‐voltage linear method and the semiempirical model were reported less than unity possibly due to the charge multiplication effect, which was negligible compared to the volume recombination effect in FLASH dose rates.ConclusionsThe absolute dose measurements of both PTW Advanced Markus and IBA PPC05 chambers are in a good agreement with the National Physical Laboratory graphite calorimeter reference dose considering overall uncertainties. Both ion chambers also demonstrate good reproducibility as well as stability as reference dosimeters in UHDR PBS proton radiotherapy. The dose rate dependency of the ion recombination effects of both ion chambers in cyclotron generated PBS proton beams is acceptable and therefore, both chambers are suitable to use in clinical practice for the range of dose rates between 5 and 60 Gy/s.

Technical note: Evaluation and second check of a commercial Monte Carlo dose engine for small-field apertures in pencil beam scanning proton therapy.

To evaluate the accuracy of the RayStation Monte Carlo dose engine (RayStation MC) in modeling small-field block apertures in proton pencil beam scanning. Furthermore, we evaluate the suitability of MCsquare as a second check for RayStation MC. We have enhanced MCsquare to model block apertures. To test the accuracy of both RayStation MC and the newly enhanced MCsquare, we compare the dose predictions of each to in-water dose measurements obtained using diode detectors and radiochromic film. Nine brass apertures with openings of 1, 2, 3, 4, and 5cm and either 2cm or 4cm thickness were used in the irradiation of a water phantom. Two measurement setups were used, one with a range shifter and 119.7 MeV proton beam energy and the other with no range shifter and 147 MeV proton beam energy. To further test the validity of RayStation MC and MCsquare in modeling block apertures and to evaluate MCsquare as a second check tool, 10 small-field (average target volume 8.3 cm3 ) patient treatment plans were calculated by each dose engine followed by a statistical comparison. Comparing to the absolute dose measurements in water, RayStation MC differed by 1.2% ± 1.0% while MCsquare differed by -1.8% ± 3.7% in the plateau region of a pristine Bragg peak. Compared to the in-water film measurements, RayStation MC and MCsquare both performed well with an average 2D-3D gamma passing rate of 99.4% and 99.7% (3%/3mm), respectively. A t-test comparing the agreement with the film measurements between RayStation MC and MCsquare suggested that the relative spatial dose distributions calculated by MCsquare and RayStation MC were statistically indistinguishable. Directly comparing the dose calculations between MCsquare and RayStation MC over 10 patients resulted in an average 3D-3D gamma passing rates of 98.5% (3%/3mm) and 94.1% (2%/2mm), respectively. The validity of RayStation MC algorithm for use with patient-specific apertures has been expanded to include small apertures. MCsquare has been enhanced to model apertures and was found to be an adequate second check of RayStation MC in this scenario.