Comprehensive investigation of lateral dose profile and output factor measurements in small proton fields from different delivery techniques

Background and purpose: As a part of the commissioning and quality assurance in proton beam therapy, lateral dose profiles and output factors have to be acquired. Such measurements can be performed with point detectors and are especially challenging in small fields or steep lateral penumbra regions as the detector's volume effect may lead to perturbations. To address this issue, this work aims to quantify and correct for such perturbations of six point detectors in small proton fields created via three different delivery techniques. Methods: Lateral dose profile and output measurements of three proton beam delivery techniques (pencil beam scanning, pencil beam scanning combined with collimators, passive scattering with collimators) were performed using high-resolution EBT3 films, a PinPoint 3D 31022 ionization chamber, a microSilicon diode 60023 and a microDiamond detector 60019 (all PTW Freiburg, Germany). Detector specific lateral dose response functions K(x,y) acting as the convolution kernel transforming the undisturbed dose distribution D(x,y) into the measured signal profiles M(x,y) were applied to quantify perturbations of the six investigated detectors in the proton fields and correct the measurements. A signal theoretical analysis in Fourier space of the dose distributions and detector's K(x,y) was performed to aid the understanding of the measurement process with regard to the combination of detector choice and delivery technique. Results: Quantification of the lateral penumbra broadening and signal reduction at the fields center revealed that measurements in the pencil beam scanning fields are only compromised slightly even by large volume ionization chambers with maximum differences in the lateral penumbra of 0.25 mm and 4% signal reduction at the field center. In contrast, radiation techniques with collimation are not accurately represented by the investigated detectors as indicated by a penumbra broadening up to 1.6 mm for passive scattering with collimators and 2.2 mm for pencil beam scanning with collimators. For a 3 mm diameter collimator field, a signal reduction at field center between 7.6% and 60.7% was asserted. Lateral dose profile measurements have been corrected via deconvolution with the corresponding K(x,y) to obtain the undisturbed D(x,y). Corrected output ratios of the passively scattered collimated fields obtained for the microDiamond, microSilicon and PinPoint 3D show agreement better than 0.9% (one standard deviation) for the smallest field size of 3 mm. Conclusion: Point detector perturbations in small proton fields created with three delivery techniques were quantified and found to be especially pronounced for collimated small proton fields with steep dose gradients. Among all investigated detectors, the microSilicon diode showed the smallest perturbations. The correction strategies based on detector's K(x,y) were found suitable for obtaining unperturbed lateral dose profiles and output factors. Approximation of K(x,y) by considering only the geometrical averaging effect has been shown to provide reasonable prediction of the detector's volume effect. The findings of this work may be used to guide the choice of point detectors in various proton fields and to contribute toward the development of a code of practice for small field proton dosimetry.</p

Validating a double Gaussian source model for small proton fields in a commercial Monte-Carlo dose calculation engine

Purpose: The primary fluence of a proton pencil beam exiting the accelerator is enveloped by a region of secondaries, commonly called “spray”. Although small in magnitude, this spray may affect dose distributions in pencil beam scanning mode e.g., in the calculation of the small field output, if not modelled properly in a treatment planning system (TPS). The purpose of this study was to dosimetrically benchmark the Monte Carlo (MC) dose engine of the RayStation TPS (v.10A) in small proton fields and systematically compare single Gaussian (SG) and double Gaussian (DG) modeling of initial proton fluence providing a more accurate representation of the nozzle spray. Methods: The initial proton fluence distribution for SG/DG beam modeling was deduced from two-dimensional measurements in air with a scintillation screen with electronic readout. The DG model was either based on direct fits of the two Gaussians to the measured profiles, or by an iterative optimization procedure, which uses the measured profiles to mimic in-air scan-field factor (SF) measurements. To validate the DG beam models SFs, i.e. relative doses to a 10 × 10 cm2 field, were measured in water for three different initial proton energies (100MeV, 160MeV, 226.7MeV) and square field sizes from 1×1cm2 to 10×10cm2 using a small field ionization chamber (IBA CC01) and an IBA ProteusPlus system (universal nozzle). Furthermore, the dose to the center of spherical target volumes (diameters: 1cm to 10cm) was determined using the same small volume ionization chamber (IC). A comprehensive uncertainty analysis was performed, including estimates of influence factors typical for small field dosimetry deduced from a simple two-dimensional analytical model of the relative fluence distribution. Measurements were compared to the predictions of the RayStation TPS. Results: SFs deviated by more than 2% from TPS predictions in all fields <4×4cm2 with a maximum deviation of 5.8% for SG modeling. In contrast, deviations were smaller than 2% for all field-sizes and proton energies when using the directly fitted DG model. The optimized DG model performed similarly except for slightly larger deviations in the 1×1cm2 scan-fields. The uncertainty estimates showed a significant impact of pencil beam size variations (±5%) resulting in up to 5.0% standard uncertainty. The point doses within spherical irradiation volumes deviated from calculations by up to 3.3% for the SG model and 2.0% for the DG model. Conclusion: Properly representing nozzle spray in RayStation's MC-based dose engine using a DG beam model was found to reduce the deviation to measurements in small spherical targets to below 2%. A thorough uncertainty analysis shows a similar magnitude for the combined standard uncertainty of such measurements

Die Bedeutung des Adjektivs geschäftsmäßig im juristischen Fach- und massenmedialen Gemeinsprachgebrauch. Eine rechtslinguistische Korpusstudie als Beispiel für computergestützte Bedeutungsanalyse im Recht

Gegenstand ist eine vergleichende empirische Korpusstudie zur Bedeutung des Ausdrucks geschäftsmäßig im (bundesdeutschen) Gemeinsprach- und juristischen Fachsprachgebrauch. Die Studie illustriert an einem aktuellen Fall strittiger Wortdeutung (hier zu § 217 StGB) die Möglichkeiten computergestützter Sprachgebrauchsanalyse für die Auslegung vor Gericht und die Normtextprognose in der Rechtsetzung

Magnetization Transfer Ratio of Peripheral Nerve and Skeletal Muscle

Purpose!#!To assess the correlation of peripheral nerve and skeletal muscle magnetization transfer ratio (MTR) with demographic variables.!##!Methods!#!In this study 59 healthy adults evenly distributed across 6 decades (mean age 50.5 years ±17.1, 29 women) underwent magnetization transfer imaging and high-resolution T2-weighted imaging of the sciatic nerve at 3 T. Mean sciatic nerve MTR as well as MTR of biceps femoris and vastus lateralis muscles were calculated based on manual segmentation on six representative slices. Correlations of MTR with age, body height, body weight, and body mass index (BMI) were expressed by Pearson coefficients. Best predictors for nerve and muscle MTR were determined using a multiple linear regression model with forward variable selection and fivefold cross-validation.!##!Results!#!Sciatic nerve MTR showed significant negative correlations with age (r = -0.47, p &amp;lt; 0.001), BMI (r = -0.44, p &amp;lt; 0.001), and body weight (r = -0.36, p = 0.006) but not with body height (p = 0.55). The multiple linear regression model determined age and BMI as best predictors for nerve MTR (R!##!Conclusion!#!Peripheral nerve MTR decreases with higher age and BMI. Studies that assess peripheral nerve MTR should consider age and BMI effects. Skeletal muscle MTR is primarily associated with BMI but overall less dependent on demographic variables

Quantitative MR Neurography in Multifocal Motor Neuropathy and Amyotrophic Lateral Sclerosis

Background: The aim of this study was to assess the phenotype of multifocal motor neuropathy (MMN) and amyotrophic lateral sclerosis (ALS) in quantitative MR neurography. Methods: In this prospective study, 22 patients with ALS, 8 patients with MMN, and 10 healthy volunteers were examined with 3T MR neurography, using a high-resolution fat-saturated T2-weighted sequence, diffusion-tensor imaging (DTI), and a multi-echo T2-relaxometry sequence. The quantitative biomarkers fractional anisotropy (FA), radial and axial diffusivity (RD, AD), mean diffusivity (MD), cross-sectional area (CSA), T2-relaxation time, and proton spin density (PSD) were measured in the tibial nerve at the thigh and calf, and in the median, radial, and ulnar nerves at the mid-upper arm. Results: MMN showed a characteristic imaging pattern of decreased FA (p = 0.018), increased RD (p = 0.014), increased CSA (p p p = 0.025) in the upper arm nerves compared to ALS and controls. ALS patients did not differ from controls in any imaging marker, nor were there any group differences in the tibial nerve (p > 0.05). Conclusions: MMN shows a characteristic pattern of quantitative DTI and T2-relaxometry parameters in the upper-arm nerves, primarily indicating demyelination. Peripheral nerve changes in ALS seem to be below the detection level of current state-of-the-art quantitative MR neurography

Validating a double Gaussian source model for small proton fields in a commercial Monte-Carlo dose calculation engine

Purpose: The primary fluence of a proton pencil beam exiting the accelerator is enveloped by a region of secondaries, commonly called “spray”. Although small in magnitude, this spray may affect dose distributions in pencil beam scanning mode e.g., in the calculation of the small field output, if not modelled properly in a treatment planning system (TPS). The purpose of this study was to dosimetrically benchmark the Monte Carlo (MC) dose engine of the RayStation TPS (v.10A) in small proton fields and systematically compare single Gaussian (SG) and double Gaussian (DG) modeling of initial proton fluence providing a more accurate representation of the nozzle spray. Methods: The initial proton fluence distribution for SG/DG beam modeling was deduced from two-dimensional measurements in air with a scintillation screen with electronic readout. The DG model was either based on direct fits of the two Gaussians to the measured profiles, or by an iterative optimization procedure, which uses the measured profiles to mimic in-air scan-field factor (SF) measurements. To validate the DG beam models SFs, i.e. relative doses to a 10 × 10 cm2 field, were measured in water for three different initial proton energies (100MeV, 160MeV, 226.7MeV) and square field sizes from 1×1cm2 to 10×10cm2 using a small field ionization chamber (IBA CC01) and an IBA ProteusPlus system (universal nozzle). Furthermore, the dose to the center of spherical target volumes (diameters: 1cm to 10cm) was determined using the same small volume ionization chamber (IC). A comprehensive uncertainty analysis was performed, including estimates of influence factors typical for small field dosimetry deduced from a simple two-dimensional analytical model of the relative fluence distribution. Measurements were compared to the predictions of the RayStation TPS. Results: SFs deviated by more than 2% from TPS predictions in all fields <4×4cm2 with a maximum deviation of 5.8% for SG modeling. In contrast, deviations were smaller than 2% for all field-sizes and proton energies when using the directly fitted DG model. The optimized DG model performed similarly except for slightly larger deviations in the 1×1cm2 scan-fields. The uncertainty estimates showed a significant impact of pencil beam size variations (±5%) resulting in up to 5.0% standard uncertainty. The point doses within spherical irradiation volumes deviated from calculations by up to 3.3% for the SG model and 2.0% for the DG model. Conclusion: Properly representing nozzle spray in RayStation's MC-based dose engine using a DG beam model was found to reduce the deviation to measurements in small spherical targets to below 2%. A thorough uncertainty analysis shows a similar magnitude for the combined standard uncertainty of such measurements