| 11. |
- Ghazal, Mohammed, et al.
(författare)
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6-MV small field output factors: intra-/intermachine comparison and implementation of TRS-483 using various detectors and several linear accelerators
- 2019
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Ingår i: Medical physics (Lancaster). - : WILEY. - 0094-2405 .- 2473-4209. ; 46:11, s. 5350-5359
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Tidskriftsartikel (refereegranskat)abstract
- Purpose To investigate the applicability of output correction factors reported in TRS-483 on 6-MV small-field detector-reading ratios using four solid-state detectors. Also, to investigate variations in 6-MV small-field output factors (OF) among nominally matched linear accelerators (linacs). Methods The TRS-483 Code of Practice (CoP) introduced and provided output correction factors to be applied to measured detector-reading ratios to obtain OFs for several small-field detectors. Detector readings for 0.5 cm x 0.5 cm to 8 cm x 8 cm fields were measured and normalized to that of 10 cm x 10 cm field giving the detector-reading ratios. Three silicon diodes, IBA PFD, IBA EFD (IBA, Schwarzenbruck, Germany), PTW T60017, and one microdiamond, PTW T60019 (PTW, Freiburg, Germany), were used. Output correction factors from the CoP were applied to measured detector-reading ratios. Measurements were performed on six Clinac and six TrueBeam linacs (Varian Medical Systems, Palo Alto, USA). An investigation of the relationship between the size of small fields and corresponding detector-reading ratio among the linacs was performed by measuring lateral dose profiles for 0.5 cm x 0.5 cm fields to determine the full width half maximum (FWHM). The relationship between the linacs focal spot size and the small-field detector-reading ratio was investigated by measuring 10 cm x 10 cm lateral dose profiles and determining the penumbra width reflecting the focal spot size. Measurement geometry was as follows: gantry angle = 0 degrees, collimator angle = 0 degrees, source-to surface distance (SSD) = 90 cm, and depth in water = 10 cm. Results For a given linac and 0.5 cm x 0.5 cm field, the deviations in detector-reading ratios among the detectors were 9%-15% for the Clinacs and 4%-5% for the TrueBeams. Use of output correction factors reduced these deviations to 6%-12% and 3%-4%, respectively. For field sizes equal to or larger than 0.8 cm x 0.8 cm, the deviations were corrected to 1% using output correction factors for both Clinacs and TrueBeams. For a given detector and 0.5 cm x 0.5 cm field, the deviations in detector-reading ratios among the linacs were 11%-17% for the Clinacs and 5-6% for the TrueBeams. For 1 cm x 1 cm the deviations were 1%-2% for Clinacs and 1% for TrueBeams. For field sizes larger than 1 cm x 1 cm the deviations were within 1% for both Clinacs and TrueBeams. No relationship between FWHMs and detector-reading ratios for 0.5 cm x 0.5 cm was observed. For Clinacs, larger 10 cm x 10 cm penumbra width yielded lower 0.5 cm x 0.5 cm detector-reading ratio indicating an effect of the focal spot size. For TrueBeams, the spread of penumbra widths was lower compared to Clinacs and no similar relationship was observed. Conclusions Output correction factors from the TRS-483 CoP are not sufficient for accurate determination of OF for 0.5 cm x 0.5 cm fields but are applicable for 0.8 cm x 0.8 cm to 8 cm x 8 cm fields. Nominally matched Clinacs and TrueBeams show large differences in detector-reading ratios for fields smaller than 1 cm x 1 cm.
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| 12. |
- Gotz, Malte, et al.
(författare)
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Correction for volume recombination in liquid ionization chambers at high dose-per-pulse
- 2019
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Ingår i: Medical physics (Lancaster). - : WILEY. - 0094-2405 .- 2473-4209. ; 46:8, s. 3692-3699
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Tidskriftsartikel (refereegranskat)abstract
- Purpose To determine the volume recombination at high dose-per-pulse in liquid ionization chambers (LIC) and to ascertain whether existing calculation methods verified in air-filled chambers may be used to calculate a correction factor. Methods Two LICs, one filled with 2,2,4-trimethylpentane (isooctane) the other with tetramethylsilane (TMS), were irradiated in a pulsed, 20 MeV electron beam. Via reference measurements with a Faraday cup, the saturation correction for volume recombination was determined for dose-per-pulse values ranging from about 5 mGy to 1 Gy for both chambers at a pulse duration of 693 ns. In addition, the isooctane chamber was irradiated with pulses of varying duration, ranging from 5 ps to 10 ms, at a dose-per-pulse of about 76.5 mGy. The dose-per-pulse-dependent measurements were compared to calculations based on Boag's models (with and without a free electron fraction), the two-dose-rate method, and a numerical calculation. The pulse duration dependent measurements were compared only to a numerical calculation that iteratively calculates the charge transport and loss in a 1D model of an ionization chamber. Results In TMS only Boag's model with a free electron fraction and the numerical calculation are in good agreement with the experimental data. However, in isooctane, good agreement is observed between the experimental data, the numerical calculation as well as the two-dose-rate method, and Boag's model including a free electron fraction. Only Boag's model without a free electron fraction shows a good agreement with lesser extend. Furthermore, the pulse duration-dependent data for isooctane are well described by the numerical model. Conclusion With isooctane as an active medium, a LIC could be directly used in a field with high dose-per-pulse utilizing the well-established two-dose-rate method to correct the volume recombination. In addition, pulsed fields with variable pulse duration are easily modeled for this medium using a numerical calculation. Other media, as exemplified by the TMS-filled chamber, might require additional considerations, such as including a fraction of free electrons in the consideration of volume recombination.
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| 13. |
- Götstedt, Julia, et al.
(författare)
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Development and evaluation of aperture-based complexity metrics using film and EPID measurements of static MLC openings.
- 2015
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Ingår i: Medical physics. - : Wiley. - 2473-4209 .- 0094-2405. ; 42:7, s. 3911-21
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Tidskriftsartikel (refereegranskat)abstract
- Complexity metrics have been suggested as a complement to measurement-based quality assurance for intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT). However, these metrics have not yet been sufficiently validated. This study develops and evaluates new aperture-based complexity metrics in the context of static multileaf collimator (MLC) openings and compares them to previously published metrics.This study develops the converted aperture metric and the edge area metric. The converted aperture metric is based on small and irregular parts within the MLC opening that are quantified as measured distances between MLC leaves. The edge area metric is based on the relative size of the region around the edges defined by the MLC. Another metric suggested in this study is the circumference/area ratio. Earlier defined aperture-based complexity metrics-the modulation complexity score, the edge metric, the ratio monitor units (MU)/Gy, the aperture area, and the aperture irregularity-are compared to the newly proposed metrics. A set of small and irregular static MLC openings are created which simulate individual IMRT/VMAT control points of various complexities. These are measured with both an amorphous silicon electronic portal imaging device and EBT3 film. The differences between calculated and measured dose distributions are evaluated using a pixel-by-pixel comparison with two global dose difference criteria of 3% and 5%. The extent of the dose differences, expressed in terms of pass rate, is used as a measure of the complexity of the MLC openings and used for the evaluation of the metrics compared in this study. The different complexity scores are calculated for each created static MLC opening. The correlation between the calculated complexity scores and the extent of the dose differences (pass rate) are analyzed in scatter plots and using Pearson's r-values.The complexity scores calculated by the edge area metric, converted aperture metric, circumference/area ratio, edge metric, and MU/Gy ratio show good linear correlation to the complexity of the MLC openings, expressed as the 5% dose difference pass rate, with Pearson's r-values of -0.94, -0.88, -0.84, -0.89, and -0.82, respectively. The overall trends for the 3% and 5% dose difference evaluations are similar.New complexity metrics are developed. The calculated scores correlate to the complexity of the created static MLC openings. The complexity of the MLC opening is dependent on the penumbra region relative to the area of the opening. The aperture-based complexity metrics that combined either the distances between the MLC leaves or the MLC opening circumference with the aperture area show the best correlation with the complexity of the static MLC openings.
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| 14. |
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| 15. |
- Kaveckyte, Vaiva, et al.
(författare)
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Suitability of microDiamond detectors for the determination of absorbed dose to water around high-dose-rate Ir-192 brachytherapy sources
- 2018
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Ingår i: Medical physics (Lancaster). - : WILEY. - 0094-2405 .- 2473-4209. ; 45:1, s. 429-437
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: Experimental dosimetry of high-dose-rate (HDR) Ir-192 brachytherapy (BT) sources is complicated due to high dose and dose-rate gradients, and softening of photon energy spectrum with depth. A single crystal synthetic diamond detector microDiamond (PTW 60019, Freiburg, Germany) has a small active volume, high sensitivity, direct readout, and nearly water-equivalent active volume. The purpose of this study was to evaluate the suitability of microDiamond detectors for the determination of absorbed dose to water around HDR Ir-192 BT sources. Three microDiamond detectors were used, allowing for the comparison of their properties. Methods: In-phantom measurements were performed using microSelectron and VariSource iX HDR Ir-192 BT treatment units. Their treatment planning systems (TPSs), Oncentra (v. 4.3) and BrachyVision (v. 13.6), respectively, were used to create irradiation plans for a cubic PMMA phantom with the microDiamond positioned at one of three source-to-detector distances (SDDs) (1.5, 2.5, and 5.5 cm) at a time. The source was stepped in increments of 0.5 cm over a total length of 6 cm to yield absorbed dose of 2 Gy at the nominal reference-point of the detector. Detectors were calibrated in Co-60 beam in terms of absorbed dose to water, and Monte Carlo (MC) calculated beam quality correction factors were applied to account for absorbed-dose energy dependence. Phantom correction factors were applied to account for differences in dimensions between the measurement phantom and a water phantom used for absorbed dose calculations made with a TPS. The same measurements were made with all three of the detectors. Additionally, dose-rate dependence and stability of the detectors were evaluated in Co-60 beam. Results: The percentage differences between experimentally determined and TPS-calculated absorbed doses to water were from -1.3% to +2.9%. The values agreed to within experimental uncertainties, which were from 1.9% to 4.3% (k = 2) depending on the detector, SDD and treatment delivery unit. No dose-rate or intrinsic energy dependence corrections were applied. All microDiamonds were comparable in terms of preirradiation dose, stability of the readings and energy response, and showed a good agreement. Conclusions: The results indicate that the microDiamond is potentially suitable for the determination of absorbed dose to water around HDR Ir-192 BT sources and may be used for independent verification of TPSs calculations, as well as for QA measurements of HDR Ir-192 BT treatment delivery units at clinical sites. (C) 2017 American Association of Physicists in Medicine
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| 16. |
- Kjellsson Lindblom, Emely, et al.
(författare)
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Impact of SBRT fractionation in hypoxia dose painting - accounting for heterogeneous and dynamic tumour oxygenation
- 2019
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405 .- 2473-4209. ; 46:5, s. 2512-2521
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Tidskriftsartikel (refereegranskat)abstract
- PurposeTumor hypoxia, often found in nonsmall cell lung cancer (NSCLC), implies an increased resistance to radiotherapy. Pretreatment assessment of tumor oxygenation is, therefore, warranted in these patients, as functional imaging of hypoxia could be used as a basis for dose painting. This study aimed at investigating the feasibility of using a method for calculating the dose required in hypoxic subvolumes segmented on 18F‐HX4 positron emission tomography (PET) imaging of NSCLC.MethodsPositron emission tomography imaging data based on the hypoxia tracer 18F‐HX4 of 19 NSCLC patients were included in the study. Normalized tracer uptake was converted to oxygen partial pressure (pO2) and hypoxic target volumes (HTVs) were segmented using a threshold of 10 mmHg. Uniform doses required to overcome the hypoxic resistance in the target volumes were calculated based on a previously proposed method taking into account the effect of interfraction reoxygenation, for fractionation schedules ranging from extremely hypofractionated stereotactic body radiotherapy (SBRT) to conventionally fractionated radiotherapy.ResultsGross target volumes ranged between 6.2 and 859.6 cm3, and the hypoxic fraction < 10 mmHg between 1.2% and 72.4%. The calculated doses for overcoming the resistance of cells in the HTVs were comparable to those currently prescribed in clinical practice as well as those previously tested in feasibility studies on dose escalation in NSCLC. Depending on the size of the HTV and the distribution of pO2, HTV doses were calculated as 43.6–48.4 Gy for a three‐fraction schedule, 51.7–57.6 Gy for five fractions, and 59.5–66.4 Gy for eight fractions. For patients in whom the HTV pO2 distribution was more favorable, a lower dose was required despite a bigger volume. Tumor control probability was lower for single‐fraction schedules, while higher levels of tumor control probability were found for schedules employing several fractions.ConclusionsThe method to account for heterogeneous and dynamic hypoxia in target volume segmentation and dose prescription based on 18F‐HX4‐PET imaging appears feasible in NSCLC patients. The distribution of oxygen partial pressure within HTV could impact the required prescribed dose more than the size of the volume.
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| 17. |
- Källman, Hans-Erik, PhD, 1960-, et al.
(författare)
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Toward automated and personalized organ dose determination in CT examinations : A comparison of two tissue characterization models for Monte Carlo organ dose calculation with a Therapy Planning System
- 2019
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Ingår i: Medical physics (Lancaster). - : WILEY. - 0094-2405 .- 2473-4209. ; 46:2, s. 1012-1023
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: Computed tomography (CT) is a versatile tool in diagnostic radiology with rapidly increasing number of examinations per year globally. Routine adaption of the exposure level for patient anatomy and examination protocol cause the patients' exposures to become diversified and harder to predict by simple methods. To facilitate individualized organ dose estimates, we explore the possibility to automate organ dose calculations using a radiotherapy treatment planning system (TPS). In particular, the mapping of CT number to elemental composition for Monte Carlo (MC) dose calculations is investigated.Methods: Organ dose calculations were done for a female thorax examination test case with a TPS (Raystation, Raysearch Laboratories AB, Stockholm, Sweden) utilizing a MC dose engine with a CT source model presented in a previous study. The TPS's inherent tissue characterization model for mapping of CT number to elemental composition of the tissues was calibrated using a phantom with known elemental compositions and validated through comparison of MC calculated dose with dose measured with Thermo Luminescence Dosimeters (TLD) in an anthropomorphic phantom. Given the segmentation tools of the TPS, organ segmentation strategies suitable for automation were analyzed for high contrast organs, utilizing CT number thresholding and model-based segmentation, and for low contrast organs utilizing water replacements in larger tissue volumes. Organ doses calculated with a selection of organ segmentation methods in combination with mapping of CT numbers to elemental composition (RT model), normally used in radiotherapy, were compared to a tissue characterization model with organ segmentation and elemental compositions defined by replacement materials [International Commission on Radiological Protection (ICRP) model], frequently favored in imaging dosimetry.Results: The results of the validation with the anthropomorphic phantom yielded mean deviations from the dose to water calculated with the RT and ICRP model as measured with TLD of 1.1% and 1.5% with maximum deviations of 6.1% and 8.7% respectively over all locations in the phantom. A strategy for automated organ segmentation was evaluated for two different risk organ groups, that is, low contrast soft organs and high contrast organs. The relative deviation between organ doses calculated with the RT model and with the ICRP model varied between 0% and 20% for the thorax/upper abdomen risk organs.Conclusions: After calibration, the RT model in the TPS provides accurate MC dose results as compared to measurements with TLD and the ICRP model. Dosimetric feasible segmentation of the risk organs for a female thorax demonstrates a possibility for automation using the segmentation tool available in a TPS for high contrast organs. Low contrast soft organs can be represented by water volumes, but organ dose to the esophagus and thyroid must be determined using standardized organ shapes. The uncertainties of the organ doses are small compared to the overall uncertainty, at least an order of magnitude larger, in the estimates of lifetime attributable risk (LAR) based on organ doses. Large-scale and automated individual organ dose calculations could provide an improvement in cancer incidence estimates from epidemiological studies.
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| 18. |
- Ma, Yunzhi, et al.
(författare)
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A generic TG-186 shielded applicator for commissioning model-based dose calculation algorithms for high-dose-rate Ir-192 brachytherapy
- 2017
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Ingår i: Medical physics (Lancaster). - : WILEY. - 0094-2405 .- 2473-4209. ; 44:11, s. 5961-5976
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Tidskriftsartikel (refereegranskat)abstract
- PurposeA joint working group was created by the American Association of Physicists in Medicine (AAPM), the European Society for Radiotherapy and Oncology (ESTRO), and the Australasian Brachytherapy Group (ABG) with the charge, among others, to develop a set of well-defined test case plans and perform calculations and comparisons with model-based dose calculation algorithms (MBDCAs). Its main goal is to facilitate a smooth transition from the AAPM Task Group No. 43 (TG-43) dose calculation formalism, widely being used in clinical practice for brachytherapy, to the one proposed by Task Group No. 186 (TG-186) for MBDCAs. To do so, in this work a hypothetical, generic high-dose rate (HDR) Ir-192 shielded applicator has been designed and benchmarked. MethodsA generic HDR Ir-192 shielded applicator was designed based on three commercially available gynecological applicators as well as a virtual cubic water phantom that can be imported into any DICOM-RT compatible treatment planning system (TPS). The absorbed dose distribution around the applicator with the TG-186 Ir-192 source located at one dwell position at its center was computed using two commercial TPSs incorporating MBDCAs (Oncentra((R)) Brachy with Advanced Collapsed-cone Engine, ACE, and BrachyVision ACUROS) and state-of-the-art Monte Carlo (MC) codes, including ALGEBRA, BrachyDose, egs_brachy, Geant4, MCNP6, and Penelope2008. TPS-based volumetric dose distributions for the previously reported source centered in water and source displaced test cases, and the new source centered in applicator test case, were analyzed here using the MCNP6 dose distribution as a reference. Volumetric dose comparisons of TPS results against results for the other MC codes were also performed. Distributions of local and global dose difference ratios are reported. ResultsThe local dose differences among MC codes are comparable to the statistical uncertainties of the reference datasets for the source centered in water and source displaced test cases and for the clinically relevant part of the unshielded volume in the source centered in applicator case. Larger local differences appear in the shielded volume or at large distances. Considering clinically relevant regions, global dose differences are smaller than the local ones. The most disadvantageous case for the MBDCAs is the one including the shielded applicator. In this case, ACUROS agrees with MC within [-4.2%, +4.2%] for the majority of voxels (95%) while presenting dose differences within [-0.12%, +0.12%] of the dose at a clinically relevant reference point. For ACE, 95% of the total volume presents differences with respect to MC in the range [-1.7%, +0.4%] of the dose at the reference point. ConclusionsThe combination of the generic source and generic shielded applicator, together with the previously developed test cases and reference datasets (available in the Brachytherapy Source Registry), lay a solid foundation in supporting uniform commissioning procedures and direct comparisons among treatment planning systems for HDR Ir-192 brachytherapy.
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| 19. |
- Meaney, Paul M, 1960, et al.
(författare)
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y Two-step inversion with a logarithmic transformation for microwave breast imaging
- 2017
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Ingår i: Medical Physics. - : Wiley. - 2473-4209 .- 0094-2405. ; 44:8, s. 4239-4251
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: The authors have developed a new two-step microwave tomographic image reconstruction process specifically designed to incorporate logarithmic transformed microwave imaging algorithms as a means of significantly improving spatial resolution and target property recovery. Log transform eliminates the need for a priori information, but spatial filtering often integrated as part of the regularization required to stabilize image recovery, generally smooths image features and reduces object definition. The new implementation begins with this smoothed image as the first step, but then utilizes it as the starting estimate for a second step which continues the iterative process with a standard weighted Euclidean distance regularization. The penalty term of the latter restricts the new image to a multi-dimensional location close to the original but allows the algorithm to optimize the image without excessive smoothing. Methods: The overall approach is based on a Gauss-Newton iterative scheme which incorporates a log transformation as a way of making the reconstruction more linear. It has been shown to be robust and not require a priori information as a condition for convergence, but does produce somewhat smoothed images as a result of associated regularization. The new two-step process utilizes the previous technique to generate a smoothed initial estimate and then uses the same reconstruction process with a weighted Euclidean distance penalty term. A simple and repeatable method has been implemented to determine the weighting factor without significant computational burden. The reconstructions are assessed according to conventional parameter estimation metrics. Results: We apply the approach to phantom experiments using large, high contrast canonical shapes followed by a set of images recovered from an actual patient exam. The image improvements are substantial in regards to improved property recovery and feature delineation without inducing unwanted artifacts. Analysis of the residual vector after the reconstruction process further emphasizes that the minimization criterion is efficient with minimal biases. Conclusions: The outcome is a novel synergism of an established stable reconstruction algorithm with a conventional regularization technique. It maintains the ability to recover high quality microwave tomographic images without the bias of a priori information while substantially improving image quality. The results are confirmed on both phantom experiments and patient exams.
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| 20. |
- Morén, Björn, 1987-, et al.
(författare)
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An extended dose-volume model in high dose-rate brachytherapy : Using mean-tail-dose to reduce tumor underdosage
- 2019
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Ingår i: Medical physics (Lancaster). - : Wiley-Blackwell Publishing Inc.. - 0094-2405 .- 2473-4209. ; 46:6, s. 2556-2566
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Tidskriftsartikel (refereegranskat)abstract
- Purpose High dose-rate brachytherapy is a method of radiotherapy for cancer treatment in which the radiation source is placed within the body. In addition to give a high enough dose to a tumor, it is also important to spare nearby healthy organs [organs at risk (OAR)]. Dose plans are commonly evaluated using the so-called dosimetric indices; for the tumor, the portion of the structure that receives a sufficiently high dose is calculated, while for OAR it is instead the portion of the structure that receives a sufficiently low dose that is of interest. Models that include dosimetric indices are referred to as dose-volume models (DVMs) and have received much interest recently. Such models do not take the dose to the coldest (least irradiated) volume of the tumor into account, which is a distinct weakness since research indicates that the treatment effect can be largely impaired by tumor underdosage even to small volumes. Therefore, our aim is to extend a DVM to also consider the dose to the coldest volume. Methods An improved DVM for dose planning is proposed. In addition to optimizing with respect to dosimetric indices, this model also takes mean dose to the coldest volume of the tumor into account. Results Our extended model has been evaluated against a standard DVM in ten prostate geometries. Our results show that the dose to the coldest volume could be increased, while also computing times for the dose planning were improved. Conclusion While the proposed model yields dose plans similar to other models in most aspects, it fulfils its purpose of increasing the dose to cold tumor volumes. An additional benefit is shorter solution times, and especially for clinically relevant times (of minutes) we show major improvements in tumour dosimetric indices.
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