Numerical modeling of air-vented parallel plate ionization chambers for ultra-high dose rate applications.

PURPOSE: Air-vented ionization chambers have been the secondary standard for radiation dosimetry since the origins of radiation metrology. However, the feasibility of their use in ultra-high dose rate pulsed beams has been a matter of discussion, as large losses are caused by ion recombinations and no suitable theoretical model is available for their correction. The theories developed by Boag and his contemporaries since the 1950s, which have provided the standard ion recombination correction factor in clinical dosimetry, do not provide an accurate description when used under the limit conditions of ultra-high dose rates (UHDRs). Moreover, the high-ion recombination effects of ionization chambers under extreme dose-rate applications are an obstacle to the development of adequate dosimetry standards. METHODS: In this article, the charge carrier transport equations within a parallel plate ionization chamber (PPIC) have been solved numerically with a double aim. First, this numerical model provides a more accurate tool that can be used to evaluate ion recombination correction for established PPICs in pulsed ultra-high dose rate regimes. Second, studying the chamber behavior in detail allow as to explore the limits of new chamber designs in order to improve their performance under UHDRs. The model presented here has been tested by measuring the instantaneous current of one unit of a Roos chamber (i.e., the time-resolved current during and after the irradiation pulse under UHDR conditions) and comparing these results with the absolute value of the simulated current. RESULTS: The experimental data show consistent agreement with the results obtained using the numerical model. The experimental instantaneous current reveals effects such as the variation of the free electron fraction with the dose per pulse that are supported by the numerical model but cannot be explained in the framework of Boag's theory. CONCLUSIONS: Numerical solutions of the charge carrier released and transport in ionization chambers are able to estimate the effects observed when PPICs are irradiated with ultra-high dose rate beams and to provide new insight into processes related to recombination losses at UHDRs. These models can be reliably extended to include regions where current analytical solutions are not valid. An agreement of better than 5 % between the experimental and simulated effective free electron fraction is found. We were able to reproduce the instantaneous current from a Roos chamber. The discrepancies observed between the experimental data and the numerical simulations can be attributed to the uncertainty about the transport parameters involved in the calculation.

Development of an ultra-thin parallel plate ionization chamber for dosimetry in FLASH radiotherapy.

BACKGROUND: Conventional air ionization chambers (ICs) exhibit ion recombination correction factors that deviate substantially from unity when irradiated with dose per pulse magnitudes higher than those used in conventional radiotherapy. This fact makes these devices unsuitable for the dosimetric characterization of beams in ultra-high dose per pulse as used for FLASH radiotherapy. PURPOSE: We present the design, development, and characterization of an ultra-thin parallel plate IC that can be used in ultra-high dose rate (UHDR) deliveries with minimal recombination. METHODS: The charge collection efficiency (CCE) of parallel plate ICs was modeled through a numerical solution of the coupled differential equations governing the transport of charged carriers produced by ionizing radiation. It was used to find out the optimal parameters for the purpose of designing an IC capable of exhibiting a linear response with dose (deviation less than 1%) up to 10 Gy per pulse at 4 µ $\umu$ s pulse duration. As a proof of concept, two vented parallel plate IC prototypes have been built and tested in different ultra-high pulse dose rate electron beams. RESULTS: It has been found that by reducing the distance between electrodes to a value of 0.25 mm it is possible to extend the dose rate operating range of parallel plate ICs to ultra-high dose per pulse range, at standard voltage of clinical grade electrometers, well into several Gy per pulse. The two IC prototypes exhibit behavior as predicted by the numerical simulation. One of the so-called ultra-thin parallel plate ionization chamber (UTIC) prototypes was able to measure up to 10 Gy per pulse, 4 µ $\umu$ s pulse duration, operated at 300 V with no significant deviation from linearity within the uncertainties (ElectronFlash Linac, SIT). The other prototype was tested up to 5.4 Gy per pulse, 2.5 µ $\umu$ s pulse duration, operated at 250 V with CCE higher than 98.6% (Metrological Electron Accelerator Facility, MELAF at Physikalisch-Technische Bundesanstalt, PTB). CONCLUSIONS: This work demonstrates the ability to extend the dose rate operating range of ICs to ultra-high dose per pulse range by reducing the spacing between electrodes. The results show that UTICs are suitable for measurement in UHDR electron beams.

Small static radiosurgery field dosimetry with small volume ionization chambers.

PURPOSE: To evaluate the response of the four smallest active volume thimble type ionization chambers commercially available (IBA-dosimetry RAZOR Nano Chamber, Standard Imaging Exradin A16, IBA-dosimetry CC01 and PTW T31022) when measuring SRS cone collimated Flattening Filter Free (FFF) fields. METHODS: We employed Monte Carlo simulation for calculating correction factors as defined in IAEA TRS-483. Monte Carlo simulation beam model and ion chamber geometry definitions were supported by an extensive set of measurements. Type A and B uncertainty components were evaluated. RESULTS: Commissioning of Monte Carlo 6 MV and 10 MV FFF beam models yielded relative differences between measured and simulated dose distributions lower than 1.5%. Monte Carlo simulated output factors for 5 mm SRS field agree with experimental values within 1% local relative difference for all chambers. Smallest active volume ion chamber (IBA-dosimetry RAZOR Nano Chamber) exhibits smallest correction, being compatible with unity. Correction factor combined uncertainties range between 0.7% and 0.9%. Smallest uncertainties were recorded for smallest and largest active volume ion chambers, although the latter exhibited largest correction factor. Highest contribution to combined uncertainty was type B component associated with beam model initial electron spatial Full Width Half Maximum (FWHM) uncertainty. CONCLUSIONS: Among the investigated chambers, the IBA RAZOR Nano Chamber was found to be an excellent choice for narrow beam output factor measurement since it requires minimum correction (in line with IAEA TRS-483 recommendations). This is caused by its tiny size and tissue equivalence materials which produce minimum volume averaging and fluence perturbation.

An EGS Monte Carlo model for Varian TrueBEAM treatment units: Commissioning and experimental validation of source parameters.

In this work we have created and commissioned a Monte Carlo model of 6FFF Varian TrueBeam linear accelerator using BEAMnrc. For this purpose we have experimentally measured the focal spot size and shape of three Varian TrueBeam treatment units in 6FFF modality with a slit collimator and several depth dose and lateral beam profiles in a water phantom. The Monte Carlo model of a 6FFF TrueBeam machine was implemented with a primary electron source commissioned as a 2D Gaussian with Full Width Half Maximum selected by comparison of simulated and measured narrow beam profiles. The energy of the primary electron beam was optimized through a simultaneous fit to the measured beam depth dose profiles. Special attention was paid to evaluation of uncertainties of the selected Monte Carlo source parameters. These uncertainties were calculated by analysing the sensitivity of the commissioning process to changes in both primary beam size and energy. Both experimental and Monte Carlo commissioned focus size values were compared and found to be in excellent agreement. The commissioned Monte Carlo model reproduces within 1% accuracy the dose distributions of radiation field size from 3 cm × 3 cm to 15 cm × 15 cm.

Development and clinical characterization of a novel 2041 liquid-filled ionization chambers array for high-resolution verification of radiotherapy treatments.

PURPOSE: The aim of this study was to present a novel 2041 liquid-filled ionization chamber array for high-resolution verification of radiotherapy treatments. MATERIALS AND METHODS: The prototype has 2041 ionization chambers of 2.5 × 2.5 mm2 area filled with isooctane. The detection elements are arranged in a central square grid of 43 × 43, totally covering an area of 107.5 × 107.5 mm2 . The central inline and cross-line are extended to 227 mm and the diagonals to 321 mm to be able to perform profile measurements of large fields. We have studied stability, pixel response uniformity, dose rate dependence, depth and field size dependence and anisotropy. We present results for output factors, tongue-and-groove, garden fence, small field profiles, irregular fields, and verification of dose planes of patient treatments. RESULTS: Comparison with other detectors used for small field dosimetry (SFD, CC13, microDiamond) has shown good agreement. Output factors measured with the device for square fields ranging from 10 × 10 to 100 × 100 mm2 showed relative differences within 1%. The response of the detector shows a strong dependence on the angle of incident radiation that needs to be corrected for. On the other hand, inter-pixel relative response variations in the 0.95-1.08 range have been found and corrected for. The application of the device for the verification of dose planes of several treatments has shown gamma passing rates above 97% for tolerances of 2% and 2 mm. The verification of other clinical fields, like small fields and irregular fields used in the commissioning of the TPS, also showed large passing rates. The verification of garden fence and tongue-and-groove fields was affected by volume-averaging effects. CONCLUSIONS: The results show that the liquid filled ionization chamber prototype here presented is appropriate for the verification of radiotherapy treatments with high spatial resolution. Recombination effects do not affect very much the verification of relative dose distributions. However, verification of absolute dose distributions may require normalization to a radiation field which is representative of the dose rate of the treatment delivered.

Correction factors for ionization chamber dosimetry in CyberKnife: machine-specific, plan-class, and clinical fields.

PURPOSE: The aim of this work is the application of the formalism for ionization chamber reference dosimetry of small and nonstandard fields [R. Alfonso, P. Andreo, R. Capote, M. S. Huq, W. Kilby, P. Kjäll, T. R. Mackie, H. Palmans, K. Rosser, J. Seuntjens, W. Ullrich, and S. Vatnitsky, "A new formalism for reference dosimetry of small and nonstandard fields," Med. Phys. 35, 5179-5186 (2008)] to the CyberKnife robotic radiosurgery system. Correction factors for intermediate calibration fields, a machine-specific reference field (msr) and two plan-class specific reference fields (pcsr), have been studied. Furthermore, the applicability of the new formalism to clinical dosimetry has been analyzed through the investigation of two clinical treatments. METHODS: PTW31014 and Scanditronix-Wellhofer CC13 ionization chamber measurements were performed for the fields under investigation. Absorbed dose to water was determined using alanine reference dosimetry, and experimental correction factors were calculated from alanine to ionization chamber readings ratios. In addition, correction factors were calculated for the intermediate calibration fields and one of the clinical treatment fields using the Monte Carlo method and these were compared with the experimental values. RESULTS: Overall correction factors deviating from unity by approximately 2% were obtained from both measurements and simulations, with values below and above unity for the studied intermediate calibration fields and clinical fields for the ionization chambers under consideration. Monte Carlo simulations yielded correction factors comparable with those obtained from measurements for the machine-specific reference field, although differences from 1% to 3.3% were observed between measured and calculated correction factors for the composite intermediate calibration fields. Dose distribution inhomogeneities are thought to be responsible for such discrepancies. CONCLUSIONS: The differences found between overall correction factors associated with the proposed intermediate calibration fields and the clinical fields under investigation show that depending on the clinical field and the detector used, either a machine-specific reference field or a plan-class specific reference field is more representative for the clinical field. Given the experimental and numerical uncertainties and the small number of clinical fields considered in this study the significance of these observations is limited and it remains unclear for the CyberKnife if there would be a significant gain in using a pcsr field rather than a msr field as reference field for relative dosimetry.

A two-dimensional liquid-filled ionization chamber array prototype for small-field verification: characterization and first clinical tests.

In this work we present the design, characterization and first clinical tests of an in-house developed two-dimensional liquid-filled ionization chamber prototype for the verification of small radiotherapy fields and treatments containing such small fields as in radiosurgery, which consists of 2 mm × 2 mm pixels arranged on a 16×8 rectangular grid. The ionization medium is isooctane. The characterization of the device included the study of depth, field-size and dose-rate dependences, which are sufficiently moderate for a good operation at therapy radiation levels. However, the detector presents an important anisotropic response, up to ≃ 12% for front versus near-lateral incidence, which can impact the verification of full treatments with different incidences. In such a case, an anisotropy correction factor can be applied. Output factors of small square fields measured with the device show a small systematic over-response, less than 1%, when compared to unshielded diode measurements. An IMRT radiosurgery treatment has been acquired with the liquid-filled ionization chamber device and compared with film dosimetry by using the gamma method, showing good agreement: over 99% passing rates for 1.2% and 1.2 mm for an incidence-per-incidence analysis; 100% passing rates for tolerances 1.8% and 1.8 mm when the whole treatment is analysed and the anisotropy correction factor is applied. The point dose verification for each incidence of the treatment performed with the liquid-filled ionization chamber agrees within 1% with a CC01 ionization chamber. This prototype has shown the utility of this kind of technology for the verification of small fields/treatments. Currently, a larger device covering a 5 cm × 5 cm area is under development.

Correction factors for A1SL ionization chamber dosimetry in TomoTherapy: machine-specific, plan-class, and clinical fields.

PURPOSE: Recently, an international working group on nonstandard fields presented a new formalism for ionization chamber reference dosimetry of small and nonstandard fields [Alfonso et al., Med. Phys. 35, 5179-5186 (2008)] which has been adopted by AAPM TG-148. This work presents an experimental determination of the correction factors for reference dosimetry with an Exradin A1SL thimble ionization chamber in a TomoTherapy unit, focusing on: (i) machine-specific reference field, (ii) plan-class-specific reference field, and (iii) two clinical treatments. METHODS: Ionization chamber measurements were performed in the TomoTherapy unit for intermediate (machine-specific and plan-class-specific) calibration fields, based on the reference conditions defined by AAPM TG-148, and two clinical treatments (lung and head-and-neck). Alanine reference dosimetry was employed to determine absorbed dose to water at the point of interest for the fields under investigation. The corresponding chamber correction factors were calculated from alanine to ionization chamber measurements ratios. RESULTS: Two different methods of determining the beam quality correction factor k(Q,Q(0) ) for the A1SL ionization chamber in this TomoTherapy unit, where reference conditions for conventional beam quality determination cannot be met, result in consistent values. The observed values of overall correction factors obtained for intermediate and clinical fields are consistently around 0.98 with a typical expanded relative uncertainty of 2% (k = 2), which when considered make such correction factors compatible with unity. However, all of them are systematically lower than unity, which is shown to be significant when a hypothesis test assuming a t-student distribution is performed (p=1.8×10(-2)). Correction factors k(Q(clin),Q(pcsr) ) (f(clin),f(pcsr) ) and k(Q(clin),Q(msr) ) (f(clin),f(msr) ), which are needed for the computation of field factors for relative dosimetry of clinical beams, have been found to be very close to unity for two clinical treatments. CONCLUSIONS: The results indicate that the helical field deliveries in this study (including two clinical fields) do not introduce changes on the ion chamber correction factors for dosimetry. For those two specific clinical cases, ratios of chamber readings accurately represent field output factors. The values observed here for intermediate calibration fields are in agreement with previously published data based on alanine dosimetry but differ from values recently reported obtained via radiochromic dosimetry.

eIMRT: a web platform for the verification and optimization of radiation treatment plans.

The eIMRT platform is a remote distributed computing tool that provides users with Internet access to three different services: Monte Carlo optimization of treatment plans, CRT & IMRT treatment optimization, and a database of relevant radiation treatments/clinical cases. These services are accessible through a user-friendly and platform independent web page. Its flexible and scalable design focuses on providing the final users with services rather than a collection of software pieces. All input and output data (CT, contours, treatment plans and dose distributions) are handled using the DICOM format. The design, implementation, and support of the verification and optimization algorithms are hidden to the user. This allows a unified, robust handling of the software and hardware that enables these computation-intensive services. The eIMRT platform is currently hosted by the Galician Supercomputing Center (CESGA) and may be accessible upon request (there is a demo version at http://eimrt.cesga.es:8080/eIMRT2/demo; request access in http://eimrt.cesga.es/signup.html). This paper describes all aspects of the eIMRT algorithms in depth, its user interface, and its services. Due to the flexible design of the platform, it has numerous applications including the intercenter comparison of treatment planning, the quality assurance of radiation treatments, the design and implementation of new approaches to certain types of treatments, and the sharing of information on radiation treatment techniques. In addition, the web platform and software tools developed for treatment verification and optimization have a modular design that allows the user to extend them with new algorithms. This software is not a commercial product. It is the result of the collaborative effort of different public research institutions and is planned to be distributed as an open source project. In this way, it will be available to any user; new releases will be generated with the new implemented codes or upgrades.

Monte Carlo verification of IMRT treatment plans on grid.

The eIMRT project is producing new remote computational tools for helping radiotherapists to plan and deliver treatments. The first available tool will be the IMRT treatment verification using Monte Carlo, which is a computational expensive problem that can be executed remotely on a GRID. In this paper, the current implementation of this process using GRID and SOA technologies is presented, describing the remote execution environment and the client.

Automatic determination of primary electron beam parameters in Monte Carlo simulation.

In order to obtain realistic and reliable Monte Carlo simulations of medical linac photon beams, an accurate determination of the parameters that define the primary electron beam that hits the target is a fundamental step. In this work we propose a new methodology to commission photon beams in Monte Carlo simulations that ensures the reproducibility of a wide range of clinically useful fields. For such purpose accelerated Monte Carlo simulations of 2 x 2, 10 x 10, and 20 x 20 cm2 fields at SSD = 100 cm are carried out for several combinations of the primary electron beam mean energy and radial FWHM. Then, by performing a simultaneous comparison with the correspondent measurements for these same fields, the best combination is selected. This methodology has been employed to determine the characteristics of the primary electron beams that best reproduce a Siemens PRIMUS and a Varian 2100 CD machine in the Monte Carlo simulations. Excellent agreements were obtained between simulations and measurements for a wide range of field sizes. Because precalculated profiles are stored in databases, the whole commissioning process can be fully automated, avoiding manual fine-tunings. These databases can also be used to characterize any accelerators of the same model from different sites.

Remote radiotherapy planning: the eIMRT project.

In this paper, we present the eIMRT project which is currently carried out by diverse institutions in Galicia (Spain) and the USA. The eIMRT project will offer radiotherapists a set of algorithms to optimize and validate radiotherapy treatments, both CRT- and IMRT-based, hiding the complexity of the computer infrastructure needed to solve the problem using GRID technologies. The new platform is designed to be independent from the medical accelerator models, scalable and open. Having a web portal as client, it is designed in three layers using web services, which will allow users to access the platform directly from any front-end and client. It has three main components, namely remote characterization of linear accelerators for Monte Carlo and convolution/superposition (C/S) dose-calculation techniques, remote Grid-enabled radiotherapy treatment planning optimization and verification and data depository.