Ionization Chamber In Water Research Articles

Technical note: Determination of proton linear energy transfer from the integral depth dose.

Proton linear energy transfer (LET) is associated with the relative biological effectiveness of radiation on tissues. Monte Carlo (MC) simulations have been known to be the preferred method to calculate LET. Detectors have also been built to measure LET, but they need to be calibrated with MC simulations. To propose and test a MC-free method for determining LET from the measured integral depth dose (LFI) of the protons of interest. LFI consists of three steps: (1) IDD measurements, (2) extraction of energy spectrum (ES) from the IDD, and (3) LET determination from the extracted ES and the stopping power of each energy. To validate the accuracy of the extraction of ES, we use Gaussian ES to synthesize IDD, extract ES from the synthesized IDD, and then compare the original (ground truth) and extracted ES. LETs calculated from the original and extracted ES are also compared. To obtain the LET of protons of interest, we measure IDDs by a large-area plane-parallel ionization chamber in water. Finally, TOPAS MC is employed to simulate IDDs, ES, and LETs. From the simulated IDD, the extracted ES and LET are compared with the simulations from TOPAS MC. From the synthesized IDDs, the LETs agreed excellently when the peak energies ≥10 and 1.25 MeV with depth resolutions 0.1and 0.01mm, respectively. For energy<1.25 MeV, even higher depth resolution than 0.01mm is required. From the MC simulated IDDs, our track-averaged LET excellently agreed with MC simulation, but not the LETd. Our LETd was smaller than MC simulated LETd in the shallow region but larger in the distal Bragg peak region. LET can be accurately determined from the IDD. This method can be used in the clinic to commission or validate LETs from other measurement methods or a treatment planning system.

Evaluation of the water-equivalent characteristics of the SP34 plastic phantom for film dosimetry in a clinical linear accelerator.

In this study, some confusing points about electron film dosimetry using white polystyrene suggested by international protocols were verified using a clinical linear accelerator (LINAC). According to international protocol recommendations, ionometric measurements and film dosimetry were performed on an SP34 slab phantom at various electron energies. Scaling factor analysis using ionometric measurements yielded a depth scaling factor of 0.923 and a fluence scaling factor of 1.019 at an electron beam energy of <10 MeV (i.e., R50 < 4.0 g/cm2). It was confirmed that the water-equivalent characteristics were similar because they have values similar to white polystyrene (i.e., depth scaling factor of 0.922 and fluence scaling factor of 1.019) presented in international protocols. Furthermore, percentage depth dose (PDD) curve analysis using film dosimetry showed that when the density thickness of the SP34 slab phantom was assumed to be water-equivalent, it was found to be most similar to the PDD curve measured using an ionization chamber in water as a reference medium. Therefore, we proved that the international protocol recommendation that no correction for measured depth dose is required means that no scaling factor correction for the plastic phantom is necessary. This study confirmed two confusing points that could occur while determining beam characteristics using electron film dosimetry, and it is expected to be used as basic data for future research on clinical LINACs.

Photon and photon–neutron experimental dosimetry in Grid therapy with 18 MV photon beams

AbstractPropose:Spatially fractionated Grid radiation therapy (SFGRT) in an effective technique for bulky and radio-sensitive tumours. SFGRT using a constructed block has been used to evaluate the photon and photo-neutron (PN) dose measurement in 18-MV photon beam energy.Methods and materials:A mounted Grid block on to a Varian Clinac 2100c linear accelerator was used to perform photon dosimetry. The percentage depth dose, in-plane and cross-plane beam profile and output factor was measured by ionization chamber in water. The PN contamination was measured after photon dosimetry using the combination of thermoluminescence dosimetry types 600 and 700, and Polycarbonate Film dosimeters on the surface and in the maximum depth dose (dmax) of solid water™ slabs.Results:The valley-to-peak ration for 6 and 18 MV photon beams obtained from the beam profiles was ~35 and 72%, respectively. Fast and thermal PN equivalent dose decreased in the Grid field compared to an open field (without Grid).Conclusion:The Grid therapy dosimetry compared to the conventional radiotherapy (without the grid) the production of fast and thermal neutrons were reduced. Using of a Grid block in high-energy photon beams for a long period of the treatment continuously might be a new source of contamination due to the interaction of photon beam resulting the activation of the Grid block

Characterizing a Geant4 Monte Carlo model of a multileaf collimator for a TrueBeam™ linear accelerator

PurposeThe purpose of this work was to develop and validate a multileaf collimator (MLC) model for a TrueBeam™ linac using Geant4 Monte Carlo (MC) simulation kit. MethodsA Geant4 application was developed to accurately represent TrueBeam™ linac. Pre-computed phase-space file in a plane just above the jaws was used for radiation transport. A Varian 120 leaf Millennium™ MLC was modeled using geometry and material specifications provided by the manufacturer using Geant4 constructs. Leaf characteristics e.g. tongue-groove design, variable thickness, interleaf gap were simulated. The linac model was validated by comparing simulated dose profiles and depth-doses with experimental data using an ionization chamber in water. Dosimetric characteristics of the MLC such as inter- and intra-leaf leakage, penumbra effect, MLC leaf positioning, and dynamic characteristics were also investigated. ResultsFor the depth dose curves, 99% of the calculated data points agree within 1% of the experimental values for the 4 × 4 cm2 and 10 × 10 cm2 and within 2% of the experimental values for 20 × 20, 30 × 30 and 40 × 40 cm2 jaw defined fields. The cross-plane dose profiles show agreement <2% for depths up to 10 cm and to within 4% beyond 10 cm. MLC dosimetric characterization with MC agree well with film measurements. The rounded leaf penumbra remained constant throughout the range of leaf motion. ConclusionsThe TrueBeam™ linac equipped with 120-leaf MLC was successfully modeled using Geant4. The accuracy of the model was verified by comparing the simulations with experiments. The model may be utilized for independent dose verification and QA of IMRT.

Monte Carlo and experimental determination of correction factors for gamma knife perfexion small field dosimetry measurements

Detector-, field size- and machine-specific correction factors are required for precise dosimetry measurements in small and non-standard photon fields. In this work, Monte Carlo (MC) simulation techniques were used to calculate the and correction factors for a series of ionization chambers, a synthetic microDiamond and diode dosimeters, used for reference and/or output factor (OF) measurements in the Gamma Knife Perfexion photon fields. Calculations were performed for the solid water (SW) and ABS plastic phantoms, as well as for a water phantom of the same geometry. MC calculations for the correction factors in SW were compared against corresponding experimental results for a subset of ionization chambers and diode detectors. Reference experimental OF data were obtained through the weighted average of corresponding measurements using TLDs, EBT-2 films and alanine pellets. values close to unity (within 1%) were calculated for most of ionization chambers in water. Greater corrections of up to 6.0% were observed for chambers with relatively large air-cavity dimensions and steel central electrode. A phantom correction of 1.006 and 1.024 (breaking down to 1.014 from the ABS sphere and 1.010 from the accompanying ABS phantom adapter) were calculated for the SW and ABS phantoms, respectively, adding up to corrections in water. Both measurements and MC calculations for the diode and microDiamond detectors resulted in lower than unit correction factors, due to their denser sensitive volume and encapsulation materials. In comparison, higher than unit results for the ionization chambers suggested field size depended dose underestimations (being significant for the 4 mm field), with magnitude depending on the combination of contradicting phenomena associated with volume averaging and electron fluence perturbations. Finally, the presence of 0.5 mm air-gap between the diodes’ frontal surface and their phantom-inserts may considerably influence OF measurements, reaching 4.6% for the Razor diode.

Basic Study on Visibility and Water Equivalency of a New Colorless Transparent Bolus for Electron Radiotherapy.

Boluses used in electron radiotherapy need to have radiation field visibility and water equivalence. In this report, we have examined field visibility and water equivalence of a new colorless transparent bolus. We examined field visibility, water equivalence, and dose profile. Field visibility was evaluated by comparison to conventional bolus. Water equivalence was investigated by a measured fluence scaling factor. The dose profile was measured by using radiochromic film with the bolus and an ionization chamber in water. We confirmed that the irradiation field could clearly be seen through the transparent colorless bolus. The bolus did not cast a field edge as compared with the conventional bolus. The fluence scaling factor was less than 0.8% as compared to water. We confirmed that the colorless transparent bolus was treated as a water equivalent material. The percentage depth dose (PDD) measured by using radiochromic film with the bolus matched the PDD measured with an ionization chamber in water. R50 was less than 1 mm as compared to PDD measured with an ionization chamber. It was confirmed that the colorless transparent bolus can use to set up patient without losing visibility on flat ground planes. The fluence scaling factor and dose profile measured by using the bolus matched the results measured in water. Therefore, the new colorless transparent bolus has feasibility to improve patient setup efficiency and can improve calculation accuracy by using the fluence scaling factor.

Measurement of Energy Parameters for Electron Gun Heater Currents and Output Dose Rate for Electron Beams from a Prototype Linac

The dosimetric characteristics were experimentally evaluated for electron beams from the prototype linac developed for radiotherapy units. This paper focuses on the electron beam output and energy variations as a function of electron gun heater current. The electron energy was derived from its mean and most probable energies measured by film dosimetry. The electron beam output at the maximum electron energy was measured with the plane parallel ionization chamber in water using TRS-398 dosimetry protocol. The mean energy and the most probable energy of the electron beam were 6.54∼3.31 MeV and 5.94∼2.80 MeV at electron gun current of 2.02∼2.50 A respectively. The output dose rate for an electron beam of mean energy 6.54 MeV was 5.41 Gy/min ±1.5% at the reference depth in water.

WE‐EF‐207‐11: Energy and Depth Response of Thermoluminescent Dosimeters and Ionization Chambers in Water for Kilovoltage X‐Ray Beams

Purpose:To assess the effects of changes in beam quality on detector response in the kilovoltage energy range by modulating the x‐ray tube voltage and the measurement depth in water.Methods:Measurements were performed with TLD‐100 and TLD‐100H thermoluminescent dosimeters and an A12 farmer‐type ionization chamber. To assess the energy response of the detectors, irradiations were performed at a depth of 3 cm in a custom‐built thin‐window water phantom using the moderately filtered x‐ray beams at the UWADCL (20 kVp‐250 kVp) and a Co‐60 beam.The x‐ray beams and detectors were modeled using the EGSnrc Monte Carlo code. The model was validated by simulating dose to the collecting volume of an A12 farmer chamber and comparing it with measured A12 signal as a function of depth. Dose was tallied to each detector and to water for comparison with measurements. Simulations were used to calculate the predicted energy response, which was compared to the measured response of each detector. Dose to each detector and dose to water as a function of depth were also simulated.Results:Detector output per dose to water was found to deviate by up to 15%, 20% and 30% as a function of energy relative to Co‐60 for the A12, TLD‐100H and TLD‐100, respectively. The EGSnrc simulations produced results similar to the measurements for ionization chambers, but discrepancies of up to 30% were observed for TLD‐100H. Simulated detector response as a function of depth was found to vary by up to 3%.Conclusion:These results suggest that changes in beam quality in kilovoltage x‐ray beams can have a significant impact on detector response. In‐water detector response was found to differ from the previously investigated in‐air response. Deviations in detector response as a function of depth were less significant, but could potentially cause dosimetric errors if ignored.

A dual two dimensional electronic portal imaging device transit dosimetry model based on an empirical quadratic formalism.

This study describes a two dimensional electronic portal imaging device (EPID) transit dosimetry model that can predict either: (1) in-phantom exit dose, or (2) EPID transit dose, for treatment verification. The model was based on a quadratic equation that relates the reduction in intensity to the equivalent path length (EPL) of the attenuator. In this study, two sets of quadratic equation coefficients were derived from calibration dose planes measured with EPID and ionization chamber in water under reference conditions. With two sets of coefficients, EPL can be calculated from either EPID or treatment planning system (TPS) dose planes. Consequently, either the in-phantom exit dose or the EPID transit dose can be predicted from the EPL. The model was tested with two open, five wedge and seven sliding window prostate and head and neck intensity-modulated radiation therapy (IMRT) fields on phantoms. Results were analysed using absolute gamma analysis (3%/3 mm). The open fields gamma pass rates were >96.8% for all comparisons. For wedge and IMRT fields, comparisons between predicted and TPS-computed in-phantom exit dose resulted in mean gamma pass rate of 97.4% (range, 92.3-100%). As for the comparisons between predicted and measured EPID transit dose, the mean gamma pass rate was 97.5% (range, 92.6-100%). An EPID transit dosimetry model that can predict in-phantom exit dose and EPID transit dose was described and proven to be valid. The described model is practical, generic and flexible to encourage widespread implementation of EPID dosimetry for the improvement of patients' safety in radiotherapy.

Upgrade and benchmarking of a 4D treatment planning system for scanned ion beam therapy

Upgrade and benchmarking of a research 4D treatment planning system (4DTPS) suitable for realistic patient treatment planning and treatment simulations taking into account specific requirements for scanned ion beam therapy, i.e., modeling of dose heterogeneities due to interplay effects and range changes caused by patient motion and dynamic beam delivery. The 4DTPS integrates data interfaces to 4D computed tomography (4DCT), deformable image registration and clinically used motion monitoring devices. The authors implemented a novel data model for 4D image segmentation using Boolean mask volume datasets and developed an algorithm propagating a manually contoured reference contour dataset to all 4DCT phases. They further included detailed treatment simulation and dose reconstruction functionality, based on the irregular patient motion and the temporal structure of the beam delivery. The treatment simulation functionality was validated against experimental data from irradiation of moving radiographic films in air, 3D moving ionization chambers in a water phantom, and moving cells in a biological phantom with a scanned carbon ion beam. The performance of the program was compared to results obtained with predecessor programs. The measured optical density distributions of the radiographic films were reproduced by the simulations to (-2 ± 12)%. Compared to earlier versions of the 4DTPS, the mean agreement improved by 2%, standard deviations were reduced by 7%. The simulated dose to the moving ionization chambers in water showed an agreement with the measured dose of (-1 ± 4)% for the typical beam configuration. The mean deviation of the simulated from the measured biologically effective dose determined via cell survival was (617 ± 538) mGy relative biological effectiveness corresponding to (10 ± 9)%. The authors developed a research 4DTPS suitable for realistic treatment planning on patient data and capable of simulating dose delivery to a moving patient geometry for scanned ion beams. The accuracy and reliability of treatment simulations improved considerably with respect to earlier versions of the 4DTPS.

The effective depth of cylindrical ionization chambers in water for clinical proton beams

In this study, we have developed an explicit analytical model to compute the effective depth of cylindrical ionization chambers in water for clinical proton beams. We have compared our explicit analytical model with an existing series expansion model. We have calculated the shift of water equivalent depth for different cylindrical ionization chambers and have compared our results with the IAEA recommendations and series expansion model. We have developed a method to compute the elliptic integral in an explicit analytical form. Using this integral form, the shift of the water equivalent depth has been computed by accounting for individual contributions of the ionization chamber cavity, wall, central electrode and sleeve for proton energies ranging from 1.0 keV to 1.0 GeV for 34 commercially available ionization chambers. The comparison of an explicit analytical expression with a series expansion reveals that integrations calculated by the series expansion fail to converge to a precise value when the ratio of radii of cylindrical ionization chambers is greater than 0.5. For all the ion chambers selected in this study, our results vary at a maximum of 0.5 mm from the IAEA recommendations, whereas the maximum variation for the series expansion model is 1.5 mm. The findings of this study suggest that the developed analytical model is reliable for the calculation of the effective depth in water. Furthermore, the verification of these results with Monte Carlo calculation may suggest the need for a review of the standards for all commercial ionization chambers.

TU‐G‐BRB‐09: The Effective Depth of Cylindrical Ionization Chambers in Water for Clinical Proton Beams

Purpose: In this study, we have developed an explicit analytical model to compute the effective depth of cylindrical ionization chambers in water for clinical proton beams. We have compared our explicit analytical model with an existing series expansion model. We have calculated the shift of water equivalent depth for different cylindrical ionization chambers and have compared our results with the IAEA recommendations and series expansion model. Methods: We have developed a method to compute the elliptic integral in an explicit analytical form. Using this integral form, the shift of water equivalent depth has been computed by accounting for individual contributions of ionization chamber cavity, wall, central electrode and sleeve for proton energies ranging from 1.0 Kev to 1.0 Gev for thirty four commercially available ionization chambers. Results: The comparison of explicit analytical expression with series expansion reveals that integrations calculated by series expansion fail to converge when the ratio of radii of cylindrical ionization chambers is greater than 0.5. For all the ion chambers selected in this study, our results vary at maximum of 0.5 mm from the IAEA recommendations, whereas the maximum variation for the series expansion model is 1.5 mm. Conclusions: The findings of this study suggest that the developed analytical model is reliable for calculation of the effective depth in water. Further, verification of these results with Monte Carlo calculation may suggest the need for review of the standards for all commercial ionization chambers.

Establishing radiation therapy treatment planning effects involving implantable pacemakers and implantable cardioverter‐defibrillators

Recent improvements to the functionality and stability of implantable pacemakers and cardioverter‐defibrillators involve changes that include efficient battery power consumption and radiation hardened electrical circuits. Manufacturers have also pursued MRI‐compatibility for these devices. While such newer models of pacemakers and cardioverter‐defibrillators are similar in construction to previously marketed devices – even for the recent MRI‐compatible designs currently in clinical trials – there is increased interest now with regard to radiation therapy dose effects when a device is near or directly in the field of radiation. Specifically, the limitation on dose to the device from therapeutic radiation beams is being investigated for a possible elevation in limiting dose above 200 cGy. We present here the first‐ever study that evaluates dosimetric effects from implantable pacemakers and implantable cardioverter‐defibrillators in high energy X‐ray beams from a medical accelerator. Treatment plan simulations were analyzed for four different pacemakers and five different implantable cardioverter‐defibrillators and intercompared with direct measurements from a miniature ionization chamber in water. All defibrillators exhibited the same results and all pacemakers were seen to display the same consequences, within only a a±1.8% deviation for all X‐ray energies studied. Attenuation, backscatter, and lateral scatter were determined to be −13.4%, 2.1% and 1.5% at 6 MV, and −6.1%, 3.1% and 5.1% at 18 MV for the defibrillator group. For the pacemaker group, this research showed results of −15.9%, 2.8% and 2.5% at 6 MV, and −9.4%, 3.4% and 5.7% at 18 MV, respectively. Limited results were discovered from scattering processes through computer modeling. Strong verification from measurements was concluded with respect to simulating attenuation characteristics. For IP and ICD leads, measured dose changes were less than 4%, existing as attenuation processes only, and invariant with regard to X‐ray energy.PACS number: 87.53.Bn, 87.53.Dq, 87.53.Tf, 87.66.Jj

Monte Carlo simulation of an x‐ray volume imaging cone beam CT unit

In this work the authors characterized the radiation field produced by a kilovolt cone beam computed tomography (CBCT) unit integrated in the Elekta Synergy linear accelerator. The x-ray volume imaging (XVI) radiation unit was modeled in detail using the BEAMNRC Monte Carlo (MC) code system. The simulations of eight collimator cassettes and the neutral filter F0 were successfully carried out. MC calculations from the EGSNRC code DOSXYZNRC were benchmarked against measurements in water. A large set of depth dose and lateral profiles was acquired with the ionization chamber in water, with the x-ray tube in a stationary position, and with the beam energy set to 120 kV. Measurements for all the available collimator cassettes were compared with calculations, showing very good agreement (< 2% in most cases). Furthermore, half value layer measurements were carried out and used to validate the MC model of the XVI unit. In this case dose calculations were performed with the EGSNRC code cavity and these showed excellent agreement. In this manuscript the authors also report on the optimization work of the relevant parameters that influenced the development of the MC model. The dosimetric part of this work was very useful in characterizing the XVI radiation output for the energy of interest. The detailed simulation part of the work is the first step toward an accurate MC based assessment of the dose delivered to patients during routine CBCT scans for image and dose guided radiotherapy.

Verification of the plan dosimetry for high dose rate brachytherapy using metal–oxide–semiconductor field effect transistor detectors

The feasibility of a recently designed metal-oxide-semiconductor field effect transistor (MOSFET) dosimetry system for dose verification of high dose rate (HDR) brachytherapy treatment planning was investigated. MOSFET detectors were calibrated with a 0.6 cm3 NE-2571 Farmer-type ionization chamber in water. Key characteristics of the MOSFET detectors, such as the energy dependence, that will affect phantom measurements with HDR 192Ir sources were measured. The MOS-FET detector was then applied to verify the dosimetric accuracy of HDR brachytherapy treatments in a custom-made water phantom. Three MOSFET detectors were calibrated independently, with the calibration factors ranging from 0.187 to 0.215 cGy/mV. A distance dependent energy response was observed, significant within 2 cm from the source. The new MOSFET detector has a good reproducibility (<3%), small angular effect (<2%), and good dose linearity (R2=1). It was observed that the MOSFET detectors had a linear response to dose until the threshold voltage reached approximately 24 V for 192Ir source measurements. Further comparison of phantom measurements using MOSFET detectors with dose calculations by a commercial treatment planning system for computed tomography-based brachytherapy treatment plans showed that the mean relative deviation was 2.2 +/- 0.2% for dose points 1 cm away from the source and 2.0 +/- 0.1% for dose points located 2 cm away. The percentage deviations between the measured doses and the planned doses were below 5% for all the measurements. The MOSFET detector, with its advantages of small physical size and ease of use, is a reliable tool for quality assurance of HDR brachytherapy. The phantom verification method described here is universal and can be applied to other HDR brachytherapy treatments.

SU‐FF‐T‐357: Physical and Biological Characteristics of Megavoltage Grid Radiation Therapy

Purpose: Dosimetric characteristics of a newly designed GRID block for megavoltage radiotherapy were investigated using TLD, film, and ionization chamber. Therapeutic advantage of this treatment modality was determined using a linear quadratic model in terms of treatment dose, GRID hole size, center‐to‐center spacing between the holes, and sensitivity of tissues to radiation. Method and Materials: Recently a newly redesigned GRID block has been introduced by Radiation Product Design, Inc. for different linear accelerators. Dosimetric characteristics (i.e. GRID output, dose profiles, and %DD) of three GRID blocks with different hole size and spacing were experimentally investigated. These GRID blocks were designed for a Varian 21EX linear accelerator. The measurements were performed using radiographic films and thermoluminescent dosimeters (TLDs) in Solid Water™ phantom materials and an ionization chamber in water. The measured dose profiles were then utilized to evaluate the therapeutic advantage of the GRID therapy as a function of dose, grid hole size and spacing, and also radiosensitivity of tissues with a linear‐quadratic (LQ) model. Therapeutic advantage of these GRID blocks was determined for a single fraction dose of up to 30 Gy. Results: Dosimetric characteristics of the GRID field were determined as a function of field size, beam energy, and Grid geometry. The results had shown that the therapeutic advantage of GRID increases by increasing the hole size, decreasing the spacing between the holes, and increasing the treatment dose. Moreover, the advantage of the GRID therapy was more pronounced for the radioresistant cells than radiosensitive cells. Conclusion: GRID radiotherapy exhibited a significant therapeutic advantage over the open field radiotherapy when the tumor cells were more radioresistant than normal cells. In addition, the therapeutic advantage of the GRID therapy is dependent on the treatment dose, GRID hole size and spacing between the holes.

SU‐FF‐T‐458: MC Simulations in Support of Developing and Testing An Analytical Dose Algorithm in Ocular Proton Therapy

Purpose: To create a Monte Carlo (MC) model of an ocular proton therapy device, or ocular nozzle, that can predict relative doses to within 3% or 0.5 mm, D/MU values, and be used to develop and commission an analytical algorithm to predict doses and D/MU values to within similar criteria. Methods and Materials: Depth dose profiles and D/MU values were measured in eight ocular proton therapy fields including four unmodulated and four modulated beams using a parallel‐plane ionization chamber in water. Crossfield profiles were measured at three or more depths for each of the modulated beams using film. A MC model of the ocular nozzle was constructed using the MCNPX radiation transport code. After benchmark tests against the measurements, the MC model was used to provide dosimetric data needed to develop a broad‐beam algorithm capable of predicting relative and absolute doses and to test the algorithm's predictions more comprehensively than could be afforded with measured data. Specifically, MC and analytical predictions were calculated and compared in two‐dimensions in a water phantom and in an anthropomorphic model of the eye. Results: Relative depth dose profiles between MC and measurements agreed to within 3% or 0.5 mm. MC predicted D/MU values agreed to within 1% of the measured values. Similarly, the analytical algorithm accurately predicted dose distributions where the predictions were generally within 3% or 0.5 mm of measured and simulated values at water depth &gt; 6 mm. At shallower depths, the analytical model underestimated the dose by approximately 3% to 6%, which MC simulations revealed was due to edge‐scattered protons from the nozzle's collimators. Conclusion: The MC model accurately predicted dose distributions and D/MU values in ocular proton therapy beams. Furthermore, the present work demonstrates the value of the MC method for developing and testing contemporary analytical dose algorithms.

Perturbation correction factors for the NACP-02 plane-parallel ionization chamber in water in high-energy electron beams

Recent dosimetry protocols for clinical high-energy electron beams recommend measurements of absorbed dose-to-water with a plane-parallel or cylindrical ionization chamber. For well-guarded plane-parallel ionization chambers, the ionization chamber perturbation factor in water, pQ, has a recommended value of unity in all protocols. This assumption was investigated in detail in this study for one of the recommended ionization chambers in the protocols: the Scanditronix NACP-02 plane-parallel ionization chamber. Monte Carlo (MC) simulations of the NACP-02 ionization chamber with the EGSnrc code were validated against backscatter experiments. MC simulations were then used to calculate pwall, pcav and pQ perturbation factors and water-to-air Spencer–Attix stopping powers in 4–19 MeV electron beams of a calibration laboratory (NPL), and in 6–22 MeV clinical electron beams from a Varian CL2300 accelerator. Differences between calculated and the currently recommended (Burns et al 1996 Med. Phys. 23 383–8) stopping powers, water-to-air, were found to be limited to 0.9% at depths between the reference depth zref and the depth where the dose has decreased to 50% of the maximum dose, R50. pwall was found to exceed unity by 2.3% in the 4 MeV NPL calibration beam at zref. For higher energy electron beams pwall decreased to a value of about 1%. Combined with a pcav about 1% below unity for all energies at zref, this was found to cause pQ to exceed unity significantly for all energies. In clinical electron beams all three perturbation factors were found to increase with depth. Our findings indicate that the perturbation factors have to be taken into account in calibration procedures and for clinical depth dose measurements with the NACP-02 ionization chamber.

Comparison of dose calculation algorithms in phantoms with lung equivalent heterogeneities under conditions of lateral electronic disequilibrium

An extensive set of benchmark measurement of PDDs and beam profiles was performed in a heterogeneous layer phantom, including a lung equivalent heterogeneity, by means of several detectors and compared against the predicted dose values by different calculation algorithms in two treatment planning systems. PDDs were measured with TLDs, plane parallel and cylindrical ionization chambers and beam profiles with films. Additionally, Monte Carlo simulations by means of the PENELOPE code were performed. Four different field sizes (10 x 10, 5 x 5, 2 x 2, and 1 x 1 cm2) and two lung equivalent materials (CIRS, p(w)e=0.195 and St. Bartholomew Hospital, London, p(w)e=0.244-0.322) were studied. The performance of four correction-based algorithms and one based on convolution-superposition was analyzed. The correction-based algorithms were the Batho, the Modified Batho, and the Equivalent TAR implemented in the Cadplan (Varian) treatment planning system and the TMS Pencil Beam from the Helax-TMS (Nucletron) treatment planning system. The convolution-superposition algorithm was the Collapsed Cone implemented in the Helax-TMS. The only studied calculation methods that correlated successfully with the measured values with a 2% average inside all media were the Collapsed Cone and the Monte Carlo simulation. The biggest difference between the predicted and the delivered dose in the beam axis was found for the EqTAR algorithm inside the CIRS lung equivalent material in a 2 x 2 cm2 18 MV x-ray beam. In these conditions, average and maximum difference against the TLD measurements were 32% and 39%, respectively. In the water equivalent part of the phantom every algorithm correctly predicted the dose (within 2%) everywhere except very close to the interfaces where differences up to 24% were found for 2 x 2 cm2 18 MV photon beams. Consistent values were found between the reference detector (ionization chamber in water and TLD in lung) and Monte Carlo simulations, yielding minimal differences (0.4%+/-1.2%). The penumbra broadening effect in low density media was not predicted by any of the correction-based algorithms, and the only one that matched the experimental values and the Monte Carlo simulations within the estimated uncertainties was the Collapsed Cone Algorithm.

Depth ionization curves for an unmodulated proton beam measured with different ionization chambers.

Differences in depth dose curves for a 78 MeV unmodulated proton beam were measured with four commercially available ionization chambers. Measurements were performed both in water and in a commercially available solid water phantom. A depth scaling factor (Cpl) was determined from the ratio of depths distal to the Bragg peak where the dose is reduced to 80% of the maximum dose in water and in the solid water phantom. This scaling factor provides good agreement between the ionization curves at all depths in water and in this solid water phantom. There is no significant difference in the value of the depth scaling factor between the ratios (R80wat/R80med) and (R50wat/R50med), or (R100wat/R100med) for 78 MeV unmodulated proton beams. The depth scaling factor for this commercially available solid water phantom is 1.023. An effective point of measurement for a cylindrical ionization chamber was found to be slightly greater than the 50% of the cavity radius proposed by the AAPM-TG25 dosimetry protocol for electron beams and amounts to 62.5% of the cavity radius of cylindrical ionization chambers. The ion collection efficiency, Pion, and the polarity correction factor, Ppol, for all the ionization chambers studied are within 1% and 0.4% of unity, respectively. Absolute doses measured with a parallel plate ionization chamber in water and in the solid water phantom show that the doses measured in the solid water phantom are 1.4% +/- 0.5% lower than in water. The dose rate dependent response of the beam line monitor chamber was also investigated. Agreement between all the chambers was within 1.5% at the dose rates studied but the results showed that all four ionization chambers are less dose rate dependent than the monitor chamber.