Linear Energy Transfer Measurements and Estimation of Relative Biological Effectiveness in Proton and Helium Ion Beams Using Fluorescent Nuclear Track Detectors

PURPOSE

Our objective was to develop a methodology for assessing the linear energy transfer (LET) and relative biological effectiveness (RBE) in clinical proton and helium ion beams using fluorescent nuclear track detectors (FNTDs).

METHODS AND MATERIALS

FNTDs were exposed behind solid water to proton and helium (4He) ion spread-out Bragg peaks. Detectors were imaged with a confocal microscope, and the LET spectra were derived from the fluorescence intensity. The track- and dose-averaged LET (LETF and LETD, respectively) were calculated from the LET spectra. LET measurements were used as input on RBE models to estimate the RBE. Human alveolar adenocarcinoma cells (A549) were exposed at the same positions as the FNTDs. The RBE was calculated from the resulting survival curves. All measurements were compared with Monte Carlo simulations.

RESULTS

For protons, average relative differences between measurements and simulations were 6% and 19% for LETF and LETD, respectively. For helium ions, the same differences were 11% for both quantities. The position of the experimental LET spectra primary peaks agreed with the simulations within 9% and 14% for protons and helium ions, respectively. For the RBE models using LETD as input, FNTD-based RBE values ranged from 1.02 ± 0.01 to 1.25 ± 0.04 and from 1.08 ± 0.09 to 2.68 ± 1.26 for protons and helium ions, respectively. The average relative differences between these values and simulations were 2% and 4%. For A549 cells, the RBE ranged from 1.05 ± 0.07 to 1.47 ± 0.09 and from 0.89 ± 0.06 to 3.28 ± 0.20 for protons and helium ions, respectively. Regarding the RBE-weighted dose (2.0 Gy at the spread-out Bragg peak), the differences between simulations and measurements were below 0.10 Gy.

CONCLUSIONS

This study demonstrates for the first time that FNTDs can be used to perform direct LET measurements and to estimate the RBE in clinical proton and helium ion beams.

Monte Carlo simulation for proton dosimetry in magnetic fields: Fano test and magnetic field correction factorskBfor Farmer-type ionization chambers

Objective. In this contribution we present a special Fano test for charged particles in presence of magnetic fields in the MC code TOol for PArticle Simulation (TOPAS), as well as the determination of magnetic field correction factorskBfor Farmer-type ionization chambers using proton beams.Approach. Customized C++ extensions for TOPAS were implemented to model the special Fano tests in presence of magnetic fields for electrons and protons. The Geant4-specific transport parameters,DRoverRandfinalRange,were investigated to optimize passing rate and computation time. ThekBwas determined for the Farmer-type PTW 30013 ionization chamber, and 5 custom built ionization chambers with same geometry but varying inner radius, testing magnetic flux density ranging from 0 to 1.0 T and two proton beam energies of 157.43 and 221.05 MeV.Main results. Using the investigated parameters, TOPAS passed the Fano test within 0.39 ± 0.15% and 0.82 ± 0.42%, respectively for electrons and protons. The chamber response (kB,M,Q) gives a maximum at different magnetic flux densities depending of the chamber size, 1.0043 at 1.0 T for the smallest chamber and 1.0051 at 0.2 T for the largest chamber. The local dose differencecBremained ≤ 0.1% for both tested energies. The magnetic field correction factorkB, for the chamber PTW 30013, varied from 0.9946 to 1.0036 for both tested energies.Significance. The developed extension for the special Fano test in TOPAS MC code with the adjusted transport parameters, can accurately transport electron and proton particles in magnetic field. This makes TOPAS a valuable tool for the determination ofkB. The ionization chambers we tested showed thatkBremains small (≤0.72%). To the best of our knowledge, this is the first calculations ofkBfor proton beams. This work represents a significant step forward in the development of MRgPT and protocols for proton dosimetry in presence of magnetic field.

Proton beam dosimetry in the presence of magnetic fields using Farmer-type ionization chambers of different radii

BACKGROUND

Magnetic resonance-guided proton therapy is promising, as it combines high-contrast imaging of soft tissue with highly conformal dose delivery. However, proton dosimetry in magnetic fields using ionization chambers is challenging since the dose distribution as well as the detector response are perturbed.

PURPOSE

This work investigates the effect of the magnetic field on the ionization chamber response, and on the polarity and ion recombination correction factors, which are essential for the implementation of a proton beam dosimetry protocol in the presence of magnetic fields.

METHODS

Three Farmer-type cylindrical ionization chambers, the 30013 with 3 mm inner radius (PTW, Freiburg, Germany) and two custom built chambers "R1" and "R6" with 1 and 6 mm inner radii respectively were placed at the center of an experimental electromagnet (Schwarzbeck Mess - Elektronik, Germany) 2 cm depth of an in-house developed 3D printed water phantom. The detector response was measured for a 3×10 cm² field of mono-energetic protons 221.05 MeV/u for the three chambers, and with an additional proton beam of 157.43 MeV/u for the chamber PTW 30013. The magnetic flux density was varied between 0.1 and 1.0 Tesla in steps of 0.1 Tesla.

RESULTS

At both energies, the ionization chamber PTW 30013 showed a non-linear response as a function of the magnetic field strength, with a decrease of the ionization chamber response of up to 0.27±0.06% (1 SD) at 0.2 Tesla, followed by a smaller effect at higher magnetic field strength. For the chamber R1, the response decreased slightly with the magnetic field strength up to 0.45±0.12% at 1 Tesla, and for the chamber R6, the response decreased up to 0.54±0.13% at 0.1 Tesla, followed by a plateau up to 0.3 Tesla, and a weaker effect at higher magnetic field strength. The dependence of the polarity and recombination correction factor on the magnetic field was ≤ $ \le $ 0.1% for the chamber PTW 30013.

CONCLUSIONS

The magnetic field has a small but significant effect on the chamber response in the low magnetic field region for the chamber PTW 30013 and for R6, and in the high magnetic field region for the chamber R1. Corrections may be necessary for ionization chamber measurements, depending on both the chamber volume and the magnetic flux density. No significant effect of the magnetic field on the polarity and recombination correction factor was detected in this work for the ionization chamber PTW 30013. This article is protected by copyright. All rights reserved.

Sensitivity correction of Fluorescent Nuclear Track Detectors using alpha particles: Determining LET spectra of light ions with enhanced accuracy

BACKGROUND

Radiation fields encountered in proton therapy (PT) and ion-beam therapy (IBT) are characterized by a variable linear energy transfer (LET), which lead to a variation of relative biological effectiveness and also affect the response of certain dosimeters. Therefore, reliable tools to measure LET are advantageous to predict and correct LET effects. Fluorescent nuclear track detectors (FNTDs) are suitable to measure LET spectra within the range of interest for PT and IBT, but so far the accuracy and precision have been challenged by sensitivity variations between individual crystals.

PURPOSE

To develop a novel methodology to correct changes in the fluorescent intensity due to sensitivity variations among FNTDs. This methodology is based on exposing FNTDs to alpha particles in order to derive a detector-specific correction factor. This will allow us to improve the accuracy and precision of LET spectra measurements with FNTDs.

METHODS

FNTDs were exposed to alpha particles. Afterward, the detectors were irradiated to monoenergetic protons, ⁴ He-, ¹² C-, and ¹⁶ O-ions. At each step, the detectors were imaged with a confocal laser scanning microscope. The tracks were reconstructed and analyzed using in-house developed tools. Alpha-particle tracks were used to derive a detector-specific sensitivity correction factor ( k s , i ${k_{s,i}}$ ). Proton, ⁴ He-, ¹² C-, and ¹⁶ O-ion tracks were used to establish a traceable calibration curve that relates the fluorescence intensity with the LET in water ( L E T H 2 O $LE{T_{{{\rm{H}}_2}{\rm{O}}}}$ ). FNTDs from a second batch were exposed and analyzed following the same procedures, to test if k s , i ${k_{s,i}}$ can be used to extend the applicability of the calibration curve to detectors from different batches. Finally, a set of blind tests was performed to assess the accuracy of the proposed methodology without user bias. Throughout all stages, the main sources of uncertainty were evaluated.

RESULTS

Based on a sample of 100 FNTDs, our findings show a high sensitivity heterogeneity between FNTDs, with k s , i ${k_{s,i}}$ having values between 0.57 and 2.55. The fitting quality of the calibration curve, characterized by the mean absolute percentage residuals and correlation coefficient, was improved when k s , i ${k_{s,i}}$ was considered. Results for detectors from the second batch show that, if the fluorescence signal is corrected by k s , i ${k_{s,i}}$ , the differences in the predicted L E T H 2 O $LE{T_{{{\rm{H}}_2}{\rm{O}}}}$ with respect to the reference set are reduced from 55%, 141%, 41%, and 186% to 4.2%, 6.5%, 5.0%, and 11.0%, for protons, ⁴ He-, ¹² C-, and ¹⁶ O-ions, respectively. The blind tests showed that it is possible to measure the track- and dose-average L E T H 2 O $LE{T_{{{\rm{H}}_2}{\rm{O}}}}$ with an accuracy of 0.3%, 16%, and 9.6% and 1.7%, 28%, and 30% for protons, ¹² C-ions and mixed beams, respectively. On average, the combined uncertainty of the measured L E T H 2 O $LE{T_{{{\rm{H}}_2}{\rm{O}}}}$ was 11%, 13%, 21%, and 26% for protons, ⁴ He-, ¹² C-, and ¹⁶ O-ions, respectively. These values were increased by a mean factor of 2.0 when k s , i ${k_{s,i}}$ was not applied.

CONCLUSIONS

We have demonstrated for the first time that alpha particles can be used to derive a detector-specific sensitivity correction factor. The proposed methodology allows us to measure LET spectra using FNTD-technology, with a degree of accuracy and precision unreachable before with sole experimental approaches.

Biosensor for deconvolution of individual cell fate in response to ion beam irradiation

Clonogenic survival assay constitutes the gold standard method for quantifying radiobiological effects. However, it neglects cellular radiation response variability and heterogeneous energy deposition by ion beams on the microscopic scale. We introduce "Cell-Fit-HD4D" a biosensor that enables a deconvolution of individual cell fate in response to the microscopic energy deposition as visualized by optical microscopy. Cell-Fit-HD4D enables single-cell dosimetry in clinically relevant complex radiation fields by correlating microscopic beam parameters with biological endpoints. Decrypting the ion beam's energy deposition and molecular effects at the single-cell level has the potential to improve our understanding of radiobiological dose concepts as well as radiobiological study approaches in general.

Triple channel analysis of Gafchromic EBT3 irradiated with clinical carbon-ion beams

Self-developing radiochromic film is widely used in radiotherapy QA procedures. To compensate for typical film inhomogeneities, the triple channel analysis method is commonly used for photon-irradiated film. We investigated the applicability of this method for Gafchromic^TMEBT3 (Ashland) film irradiated with a clinically used carbon-ion beam. Calibration curves were taken from EBT3 film specimens irradiated with monoenergetic carbon-ion beams of different doses. Measurements of the lateral field shape and homogeneity were performed in the middle of a passively modulated spread-out Bragg peak and compared to simultaneous characterization by means of a 2D ionization chamber array. Additional measurements to investigate the applicability of EBT3 for quality assurance (QA) measurement in carbon-ion beams were performed. The triple-channel analysis reduced the relative standard deviation of the doses in a uniform carbon ion field by 30% (from 1.9% to 1.3%) and reduced the maximum deviation by almost a factor of 3 (from 28.6% to 9.8%), demonstrating the elimination of film artifacts. The corrected film signal showed considerably improved image quality and quantitative agreement with the ionization chamber data, thus providing a clear rationale for the usage of the triple channel analysis in carbon-beam QA.

Dosimetry in magnetic fields with dedicated MR-compatible ionization chambers

MR-integrated radiotherapy requires suitable dosimetry detectors to be used in magnetic fields. This study investigates the feasibility of using dedicated MR-compatible ionization chambers at MR-integrated radiotherapy devices. MR-compatible ionization chambers (Exradin A19MR, A1SLMR, A26MR, A28MR) were precisely modeled and their relative response in a 6MV treatment beam in the presence of a magnetic field was simulated using EGSnrc. Monte Carlo simulations were carried out with the magnetic field in three orientations: the magnetic field aligned perpendicular to the chamber and beam axis (transverse orientation), the magnetic field parallel to the chamber as well as parallel to the beam axis. Monte Carlo simulation results were validated with measurements using an electromagnet with magnetic field strength upto 1.1 T with the chambers in transverse orientation. The measurements and simulation results were in good agreement, except for the A26MR ionization chamber in transverse orientation. The maximum increase in response of the ionization chambers observed was 8.6% for the transverse orientation. No appreciable change in chamber response due to the magnetic field was observed for the magnetic field parallel to the ionization chamber and parallel to the photon beam. Polarity and recombination correction factor were experimentally investigated in the transverse orientation. The polarity effect and recombination effect were not altered by a magnetic field. This study further investigates the response of the ionization chambers as a function of the chambers' rotation around their longitudinal axis. A variation in response was observed when the chamber was not rotationally symmetric, which was independent of the magnetic field.

Towards a generalised development of synthetic CT images and assessment of their dosimetric accuracy

Background: The interest in generating "synthetic computed tomography (CT) images" from magnetic resonance (MR) images has been increasing over the past years due to advances in MR guidance for radiotherapy. A variety of methods for synthetic CT creation have been developed, from simple bulk density assignment to complex machine learning algorithms.Material and methods: In this study, we present a general method to determine simplistic synthetic CTs and evaluate them according to their dosimetric accuracy. It separates the requirements on the MR image and the associated calculation effort to generate a synthetic CT. To evaluate the significance of the dosimetric accuracy under realistic conditions, clinically common uncertainties including position shifts and Hounsfield lookup table (HLUT) errors were simulated. To illustrate our approach, we first translated CT images from a test set of six pelvic cancer patients to relative electron density (ED) via a clinical HLUT. For each patient, seven simplified ED images (simED) were generated at different levels of complexity, ranging from one to four tissue classes. Then, dose distributions optimised on the reference ED image and the simEDs were compared to each other in terms of gamma pass rates (2 mm/2% criteria) and dose volume metrics.Results: For our test set, best results were obtained for simEDs with four tissue classes representing fat, soft tissue, air, and bone. For this simED, gamma pass rates of 99.95% (range: 99.72-100%) were achieved. The decrease in accuracy from ED simplification was smaller in this case than the influence of the uncertainty scenarios on the reference image, both for gamma pass rates and dose volume metrics.Conclusions: The presented workflow helps to determine the required complexity of synthetic CTs with respect to their dosimetric accuracy. The investigated cases showed potential simplifications, based on which the synthetic CT generation could be faster and more reproducible.

Isolation of time-dependent DNA damage induced by energetic carbon ions and their fragments using fluorescent nuclear track detectors

PURPOSE

High energetic carbon (C-) ion beams undergo nuclear interactions with tissue, producing secondary nuclear fragments. Thus, at depth, C-ion beams are composed of a mixture of different particles with different linear energy transfer (LET) values. We developed a technique to enable isolation of DNA damage response (DDR) in mixed radiation fields using beam line microscopy coupled with fluorescence nuclear track detectors (FNTDs).

METHODS

We imaged live cells on a coverslip made of FNTDs right after C-ion, proton or photon irradiation using an in-house built confocal microscope placed in the beam path. We used the FNTD to link track traversals with DNA damage and separated DNA damage induced by primary particles from fragments.

RESULTS

We were able to spatially link physical parameters of radiation tracks to DDR in live cells to investigate spatiotemporal DDR in multi-ion radiation fields in real time, which was previously not possible. We demonstrated that the response of lesions produced by the high-LET primary particles associates most strongly with cell death in a multi-LET radiation field, and that this association is not seen when analyzing radiation induced foci in aggregate without primary/fragment classification.

CONCLUSIONS

We report a new method that uses confocal microscopy in combination with FNTDs to provide submicrometer spatial-resolution measurements of radiation tracks in live cells. Our method facilitates expansion of the radiation-induced DDR research because it can be used in any particle beam line including particle therapy beam lines.

CATEGORY

Biological Physics and Response Prediction.

The characterization of a large multi-axis ionization chamber array in a 1.5 T MRI linac

By combining magnetic resonance imaging (MRI) scanners and radiotherapy treatment units the need arises for new radiation measurement equipment that can be used in the magnetic field of the MRI. This study describes the investigation of the influence of the 1.5 T magnetic field from an MRI linac on the STARCHECK^{MAXI MR}, a large 2D ionization chamber detector panel. Measurements were performed on an MRI linac and a conventional linac to investigate the behaviour of the detector panel with and without the 1.5 T magnetic field. We measured reproducibility, linearity, warm-up effect, saturation/recombination and chamber orientation. A comparison with gafchromic film was performed and the effect of motion of the panel during measurements inside a magnetic field was investigated. The reproducibility, linearity, warm-up effect, saturation/recombination show no significant deviations with or without magnetic field. An absolute difference in reading of 2.1% was found between off-axis chambers on different axes. The comparison with film shows good agreement. Spurious readings are induced while the panel is undergoing a motion in the magnetic field during measurements. The STARCHECK^{MAXI MR} is suited for use in a 1.5 T MRI linac. Care must be taken when comparing un-normalized profiles from different axes of the detector panel and when the panel is undergoing motion during measurements.

Evaluation of Additional Track Parameters from Fluorescent Nuclear Track Detectors to Determine the LET of Individual Ions

The measurement of single-track intensity in fluorescence nuclear track detectors can yield relative linear energy transfer (LET)-spectra with small line-width. The absolute determination of LET is, however, currently hampered by the inter-detector variability of crystal coloration and hence detector sensitivity. We therefore investigated the LET response of three additional quantities (average width and the variation of intensity and width along single tracks) using detectors irradiated with mono-energetic ion beams with LETs from 1.5 to 150 keV/μm in alumina. All quantities showed in fact smaller inter-detector variability, but at the same time larger line-width and limited dynamic range as the average intensity along a track. The additional quantities might therefore serve as a helpful complement, but not as a replacement for the current approach.

On the equivalence of image-based dual-energy CT methods for the determination of electron density and effective atomic number in radiotherapy

Dual-energy computed tomography enables the determination of relative electron density and effective atomic number. As this can increase accuracy in radiotherapy treatment planning, a substantial number of algorithms for the determination of the two quantities has been suggested – most of them based on reconstructed CT images. We show that many of these methods share a common theoretical framework. Equations can be transformed from one method to the other by re-definition of the calibration parameters. We suggest that further work should be spent on practical calibration and the reliability of CT numbers rather than on the theoretical framework.

Experimental verification of stopping-power prediction from single- and dual-energy computed tomography in biological tissues

An experimental setup for consecutive measurement of ion and x-ray absorption in tissue or other materials is introduced. With this setup using a 3D-printed sample container, the reference stopping-power ratio (SPR) of materials can be measured with an uncertainty of below 0.1%. A total of 65 porcine and bovine tissue samples were prepared for measurement, comprising five samples each of 13 tissue types representing about 80% of the total body mass (three different muscle and fatty tissues, liver, kidney, brain, heart, blood, lung and bone). Using a standard stoichiometric calibration for single-energy CT (SECT) as well as a state-of-the-art dual-energy CT (DECT) approach, SPR was predicted for all tissues and then compared to the measured reference. With the SECT approach, the SPRs of all tissues were predicted with a mean error of (-0.84  ±  0.12)% and a mean absolute error of (1.27  ±  0.12)%. In contrast, the DECT-based SPR predictions were overall consistent with the measured reference with a mean error of (-0.02  ±  0.15)% and a mean absolute error of (0.10  ±  0.15)%. Thus, in this study, the potential of DECT to decrease range uncertainty could be confirmed in biological tissue.

Dual-energy CT based proton range prediction in head and pelvic tumor patients

BACKGROUND AND PURPOSE

To reduce range uncertainty in particle therapy, an accurate computation of stopping-power ratios (SPRs) based on computed tomography (CT) is crucial. Here, we assess range differences between the state-of-the-art CT-number-to-SPR conversion using a generic Hounsfield look-up table (HLUT) and a direct patient-specific SPR prediction (RhoSigma) based on dual-energy CT (DECT) in 100 proton treatment fields.

MATERIAL AND METHODS

For 25 head-tumor and 25 prostate-cancer patients, the clinically applied treatment plan, optimized using a HLUT, was recalculated with RhoSigma as CT-number-to-SPR conversion. Depth-dose curves in beam direction were extracted for both dose distributions in a regular grid and range deviations were determined and correlated to SPR differences within the irradiated volume.

RESULTS

Absolute (relative) mean water-equivalent range shifts of 1.1mm (1.2%) and 4.1mm (1.7%) were observed in the head-tumor and prostate-cancer cohort, respectively. Due to the case dependency of a generic HLUT, range deviations within treatment fields strongly depend on the tissues traversed leading to a larger variation within one patient than between patients.

CONCLUSIONS

The magnitude of patient-specific range deviations between HLUT and the more accurate DECT-based SPR prediction is clinically relevant. A clinical application of the latter seems feasible as demonstrated in this study using medically approved systems from CT acquisition to treatment planning.

Radiation dosimetry in magnetic fields with Farmer-type ionization chambers: determination of magnetic field correction factors for different magnetic field strengths and field orientations

The aim of this work was to determine magnetic field correction factors that are needed for dosimetry in hybrid devices for MR-guided radiotherapy for Farmer-type ionization chambers for different magnetic field strengths and field orientations. The response of six custom-built Farmer-type chambers irradiated at a 6 MV linac was measured in a water tank positioned in a magnet with magnetic field strengths between 0.0 T and 1.1 T. Chamber axis, beam and magnetic field were perpendicular to each other and both magnetic field directions were investigated. EGSnrc Monte Carlo simulations were compared to the measurements and simulations with different field orientations were performed. For all geometries, magnetic field correction factors, [Formula: see text], and perturbation factors were calculated. A maximum increase of 8.8% in chamber response was measured for the magnetic field perpendicular to chamber and beam axis. The measured chamber response could be reproduced by adjusting the dead volume layer near the chamber stem in the Monte Carlo simulations. For the magnetic field parallel to the chamber axis or parallel to the beam, the simulated response increased by 1.1% at maximum for field strengths up to 1.1 T. A complex dependence of the response was found on chamber radius, magnetic field strength and orientation of beam, chamber axis and magnetic field direction. Especially for magnetic fields perpendicular to beam and chamber axis, the exact sensitive volume has to be considered in the simulations. To minimize magnetic field correction factors and the influence of dead volumes on the response of Farmer chambers, a measurement set-up with the magnetic field parallel to the chamber axis or parallel to the beam is recommended for dosimetry.

Methodological accuracy of image-based electron density assessment using dual-energy computed tomography

PURPOSE

Electron density is the most important tissue property influencing photon and ion dose distributions in radiotherapy patients. Dual-energy computed tomography (DECT) enables the determination of electron density by combining the information on photon attenuation obtained at two different effective x-ray energy spectra. Most algorithms suggested so far use the CT numbers provided after image reconstruction as input parameters, i.e., are imaged-based. To explore the accuracy that can be achieved with these approaches, we quantify the intrinsic methodological and calibration uncertainty of the seemingly simplest approach.

METHODS

In the studied approach, electron density is calculated with a one-parametric linear superposition ('alpha blending') of the two DECT images, which is shown to be equivalent to an affine relation between the photon attenuation cross sections of the two x-ray energy spectra. We propose to use the latter relation for empirical calibration of the spectrum-dependent blending parameter. For a conclusive assessment of the electron density uncertainty, we chose to isolate the purely methodological uncertainty component from CT-related effects such as noise and beam hardening.

RESULTS

Analyzing calculated spectrally weighted attenuation coefficients, we find universal applicability of the investigated approach to arbitrary mixtures of human tissue with an upper limit of the methodological uncertainty component of 0.2%, excluding high-Z elements such as iodine. The proposed calibration procedure is bias-free and straightforward to perform using standard equipment. Testing the calibration on five published data sets, we obtain very small differences in the calibration result in spite of different experimental setups and CT protocols used. Employing a general calibration per scanner type and voltage combination is thus conceivable.

CONCLUSION

Given the high suitability for clinical application of the alpha-blending approach in combination with a very small methodological uncertainty, we conclude that further refinement of image-based DECT-algorithms for electron density assessment is not advisable.

STED microscopy visualizes energy deposition of single ions in a solid-state detector beyond diffraction limit

Fluorescent nuclear track detectors (FNTDs) allow for visualization of single-particle traversal in clinical ion beams. The point spread function of the confocal readout has so far hindered a more detailed characterization of the track spots-the ion's characteristic signature left in the FNTD. Here we report on the readout of the FNTD by optical nanoscopy, namely stimulated emission depletion microscopy. It was firstly possible to visualize the track spots of carbon ions and protons beyond the diffraction limit of conventional light microscopy with a resolving power of approximately 80 nm (confocal: 320 nm). A clear discrimination of the spatial width, defined by the full width half maximum of track spots from particles (proton and carbon ions), with a linear energy transfer (LET) ranging from approximately 2-1016 keV µm^-1 was possible. Results suggest that the width depends on LET but not on particle charge within the uncertainties. A discrimination of particle type by width thus does not seem possible (as well as with confocal microscopy). The increased resolution, however, could allow for refined determination of the cross-sectional area facing substantial energy deposition. This work could pave the way towards development of optical nanoscopy-based analysis of radiation-induced cellular response using cell-fluorescent ion track hybrid detectors.

Clinical Implementation of Dual-energy CT for Proton Treatment Planning on Pseudo-monoenergetic CT scans

PURPOSE

To determine whether a standardized clinical application of dual-energy computed tomography (DECT) for proton treatment planning based on pseudomonoenergetic CT scans (MonoCTs) is feasible and increases the precision of proton therapy in comparison with single-energy CT (SECT).

METHODS AND MATERIALS

To define an optimized DECT protocol, CT scan settings were analyzed experimentally concerning beam hardening, image quality, and influence on the heuristic conversion of CT numbers into stopping-power ratios (SPRs) and were compared with SECT scans with identical CT dose. Differences in range prediction and dose distribution between SECT and MonoCT were quantified for phantoms and a patient.

RESULTS

Dose distributions planned on SECT and MonoCT datasets revealed mean range deviations of 0.3 mm, γ passing rates (1%, 1 mm) greater than 99.9%, and no clinically relevant changes in dose-volume histograms. However, image noise and CT-related uncertainties could be reduced by MonoCT compared with SECT, which resulted in a slightly decreased dependence of SPR prediction on beam hardening. Consequently, DECT was clinically implemented at the University Proton Therapy Dresden in 2015. Until October 2016, 150 patients were treated based on MonoCTs, and more than 950 DECT scans of 351 patients were acquired during radiation therapy.

CONCLUSIONS

A standardized clinical use of MonoCT for treatment planning is feasible, leads to improved image quality and SPR prediction, extends diagnostic variety, and enables a stepwise clinical implementation of DECT toward a physics-based, patient-specific, nonheuristic SPR determination. Further reductions of CT-related uncertainties, as expected from such SPR approaches, can be evaluated on the resulting DECT patient database.

Registration procedure for spatial correlation of physical energy deposition of particle irradiation and cellular response utilizing cell-fluorescent ion track hybrid detectors

The hybrid technology cell-fluorescent ion track hybrid detector (Cell-Fit-HD) enables the investigation of radiation-related cellular events along single ion tracks on the subcellular scale in clinical ion beams. The Cell-Fit-HD comprises a fluorescent nuclear track detector (FNTD, the physical compartment), a device for individual particle detection and a substrate for viable cell-coating, i.e. the biological compartment. To date both compartments have been imaged sequentially in situ by confocal laser scanning microscopy (CLSM). This is yet in conflict with a functional read-out of the Cell-Fit-HD utilizing a fast live-cell imaging of the biological compartment with low phototoxicity on greater time scales. The read-out of the biological from the physical compartment was uncoupled. A read-out procedure was developed to image the cell layer by conventional widefield microscopy whereas the FNTD was imaged by CLSM. Point mapping registration of the confocal and widefield imaging data was performed. Non-fluorescent crystal defects (spinels) visible in both read-outs were used as control point pairs. The accuracy achieved was on the sub-µm scale. The read-out procedure by widefield microscopy does not impair the unique ability of spatial correlation by the Cell-Fit-HD. The uncoupling will enlarge the application potential of the hybrid technology significantly. The registration allows for an ultimate correlation of microscopic physical beam parameters and cell kinetics on greater time scales. The method reported herein will be instrumental for the introduction of a novel generation of compact detectors facilitating biodosimetric research towards high-throughput analysis.

Correlation of Particle Traversals with Clonogenic Survival Using Cell-Fluorescent Ion Track Hybrid Detector

Development of novel approaches linking the physical characteristics of particles with biological responses are of high relevance for the field of particle therapy. In radiobiology, the clonogenic survival of cells is considered the gold standard assay for the assessment of cellular sensitivity to ionizing radiation. Toward further development of next generation biodosimeters in particle therapy, cell-fluorescent ion track hybrid detector (Cell-FIT-HD) was recently engineered by our group and successfully employed to study physical particle track information in correlation with irradiation-induced DNA damage in cell nuclei. In this work, we investigated the feasibility of Cell-FIT-HD as a tool to study the effects of clinical beams on cellular clonogenic survival. Tumor cells were grown on the fluorescent nuclear track detector as cell culture, mimicking the standard procedures for clonogenic assay. Cell-FIT-HD was used to detect the spatial distribution of particle tracks within colony-initiating cells. The physical data were associated with radiation-induced foci as surrogates for DNA double-strand breaks, the hallmark of radiation-induced cell lethality. Long-term cell fate was monitored to determine the ability of cells to form colonies. We report the first successful detection of particle traversal within colony-initiating cells at subcellular resolution using Cell-FIT-HD.