Water Equivalent Thickness Research Articles

Material assignment for proton range prediction in Monte Carlo patient simulations using stopping-power datasets

Motivation and objective. For each institute, the selection and calibration of the most suitable approach to assign material properties for Monte Carlo (MC) patient simulation in proton therapy is a major challenge. Current conventional approaches based on computed tomography (CT) depend on CT acquisition and reconstruction settings. This study proposes a material assignment approach, referred to as MATA (MATerial Assignment), which is independent of CT scanner properties and, therefore, universally applicable by any institute. Materials and methods. The MATA approach assigns material properties to the physical quantity stopping-power ratio (SPR) using a set of 40 material compositions specified for human tissues and linearly determined mass density. The application of clinically available CT-number-to-SPR conversion avoids the need for any further calibration. The MATA approach was validated with homogeneous and heterogeneous SPR datasets by assessing the SPR accuracy after material assignment obtained either based on dose scoring or determination of water-equivalent thickness. Finally, MATA was applied on patient datasets to evaluate dose differences induced by different approaches for material assignment and SPR prediction. Results. The deviation between the SPR after material assignment and the input SPR was close to zero in homogeneous datasets and below 0.002 (0.2% relative to water) in heterogeneous datasets, which was within the systematic uncertainty in SPR estimation. The comparison of different material assignment approaches revealed relevant differences in dose distribution and SPR. The comparison between two SPR prediction approaches, a standard look-up table and direct SPR determination from dual-energy CT, resulted in patient-specific mean proton range shifts between 1.3 mm and 4.8 mm. Conclusion. MATA eliminates the need for institution-specific adaptations of the material assignment. It allows for using any SPR dataset and thus facilitates the implementation of more accurate SPR prediction approaches. Hence, MATA provides a universal solution for patient modeling in MC-based proton treatment planning.

A scintillator-based range telescope for particle therapy

The commissioning and operation of a particle therapy centre requires an extensive set of detectors for measuring various parameters of the treatment beam. Among the key devices are detectors for beam range quality assurance. In this work, a novel range telescope based on a plastic scintillator and read out by a large-scale CMOS sensor is presented. The detector is made of a stack of 49 plastic scintillator sheets with a thickness of 2–3 mm and an active area of 100 × 100 mm2, resulting in a total physical stack thickness of 124.2 mm. This compact design avoids optical artefacts that are common in other scintillation detectors. The range of a proton beam is reconstructed using a novel Bragg curve model that incorporates scintillator quenching effects. Measurements to characterise the performance of the detector were carried out at the Heidelberger Ionenstrahl-Therapiezentrum (HIT, Heidelberg, GER) and the Clatterbridge Cancer Centre (CCC, Bebington, UK). The maximum difference between the measured range and the reference range was found to be 0.41 mm at a proton beam range of 310 mm and was dominated by detector alignment uncertainties. With the new detector prototype, the water-equivalent thickness of PMMA degrader blocks has been reconstructed within ± 0.1 mm. An evaluation of the radiation hardness proves that the range reconstruction algorithm is robust following the deposition of 6,300 Gy peak dose into the detector. Furthermore, small variations in the beam spot size and transverse beam position are shown to have a negligible effect on the range reconstruction accuracy. The potential for range measurements of ion beams is also investigated.

Radiation Dose Reduction for Computed Tomography Localizer Radiography Using an Ag Additional Filter.

This study aimed to assess the potential of an Ag additional filter attached to the bow tie filter of a computed tomography (CT) scanner to reduce the radiation dose in CT localizer radiography. Radiation doses in CT localizer radiography with Cu and Ag additional filters were evaluated based on dose measurements and Monte Carlo simulations. Image quality evaluations of an adult torso phantom were performed, and the automatic exposure control performance was evaluated in terms of the water-equivalent thickness estimated from CT localizer radiographs. With the Ag additional filter, effective doses were approximately 72% to 75% lower than those with the Cu additional filter. The image quality and water-equivalent thickness with the Ag additional filter were similar to those with the Cu additional filter. The Ag additional filter helped significantly reduce radiation doses in CT localizer radiography while maintaining image quality and performance.

Experimental comparison of clinically used ion beams for imaging applications using a range telescope

In particle therapy, the x-ray based treatment planning converting photon attenuation values to relative stopping power ratio (RSP) introduces clinically relevant range uncertainties. Recently, novel imaging technologies using transmission ion beams have been investigated to directly assess the water equivalent thickness (WET) of tissue, showing improved accuracy in RSP reconstruction, while potentially reducing the imaging dose. Due to their greater availability, protons have been mostly used for ion imaging. To this end, in this work, the influence of three ion species (protons, helium and carbon ions) on the image quality of radiographic WET retrieval has been explored with a dedicated experimental setup and compared to Monte Carlo (MC) simulations. Three phantom setups with different tissue interfaces and features have been irradiated with clinically validated proton, helium and carbon ion pencil beams under comparable imaging dose and beam settings at the Heidelberg Ion-Beam Therapy Center. Ion radiographies (iRADs) were acquired with an integration mode detector, that functions as a range telescope with 61 parallel plate ionization chambers. For comparison, experiments were reproduced in-silico with FLUKA MC simulations. Carbon ions provide iRADs with highest image quality in terms of normalized root mean square error, followed by helium ions and protons. All ions show similar capabilities of resolving WET for the considered phantoms, as shown by the similar average relative error < 3%. Besides for the slab phantom, MC simulations yielded better results than the experiment, indicating potential improvement of the experimental setup. Our results showed that the ability to resolve the WET is similar for all particles, intrinsically limited by the granularity of the detector system. While carbon ions are best suited for acquiring iRADs with the investigated integration mode detector, helium ions are put forward as a less technical challenging alternative.

Application of Hyperspectral Imaging for Assessment of Tomato Leaf Water Status in Plant Factories

Irrigation management continues to be an important issue for tomato cultivation, especially in plant factories. Accurate and timely assessment of tomato leaf water status is a key factor in enabling appropriate irrigation, which can save nutrition solution and labor. In recent decades, hyperspectral imaging has been widely used as a nondestructive measurement method in agriculture to obtain plant biological information. The objective of this research was to establish an approach to obtain the tomato leaf water status—specifically, the relative water content (WC) and equivalent water thickness (MC)—for five different tomato cultivars in real time by using hyperspectral imaging. The normalized difference vegetation index (NDVI) and two-band vegetation index (TBI) analyses were performed on the tomato leaf raw relative reflection (RAW), the inversion-logarithm relative reflection (LOG), and the first derivative of relative reflection (DIFF) from wavelengths of 900 nm to 1700 nm. The best regression model for WC assessment was obtained by TBI regression using DIFF at wavelengths of 1410 nm and 1520 nm, and the best regression model for MC assessment was obtained by NDVI regression using RAW at wavelengths of 1300 nm and 1310 nm. Higher model performance was obtained with MC assessment than with WC assessment. The results will help improve our understanding of the relationship between hyperspectral reflectance and leaf water status.

Image quality of list-mode proton imaging without front trackers

List mode proton imaging relies on accurate reconstruction of the proton most likely path (MLP) through the patient. This typically requires two sets of position sensitive detector systems, one upstream (front) and one downstream (rear) of the patient. However, for a clinical implementation it can be preferable to omit the front trackers (single-sided proton imaging). For such a system, the MLP can be computed from information available through the beam delivery system and the remaining rear tracker set. In this work, we use Monte Carlo simulations to compare a conventional double-sided (using both front and rear detector systems) with a single-sided system (only rear detector system) by evaluating the spatial resolution of proton radiographs (pRad) and proton CT images (pCT) acquired with these set-ups. Both the pencil beam spot size, as well as the spacing between spots was also adjusted to identify the impact of these beam parameters on the image quality.Relying only on the pencil beam central position for computing the MLP resulted in severe image artifacts both in pRad and pCT. Using the recently extended-MLP formalism that incorporate pencil beam uncertainty removed these image artifacts. However, using a more focused pencil beam with this algorithm induced image artifacts when the spot spacing was the same as the beam spot size. The spatial resolution tested with a sharp edge gradient technique was reduced by 40% for single-sided (MTF10% = 3.0 lp/cm) compared to double-sided (MTF10% = 4.9 lp/cm) pRad with ideal tracking detectors. Using realistic trackers the difference decreased to 30%, with MTF10% of 4.0 lp/cm for the realistic double-sided and 2.7 lp/cm for the realistic single-sided setup. When studying an anthropomorphic paediatric head phantom both single- and double-sided set-ups performed similarly where the difference in water equivalent thickness (WET) between the two set-ups were less than 0.01 mm in homogeneous areas of the head. Larger discrepancies between the two set-ups were visible in high density gradients like the facial structures. A complete CT reconstruction of a Catphan module was performed. Assuming ideal detectors, the obtained spatial resolution was 5.1 lp/cm for double-sided and 3.8 lp/cm for the single-sided setup. Double- and single-sided pRad with realistic tracker properties returned a spatial resolution of 3.8 lp/cm and 3.2 lp/cm, respectively. Future studies should investigate the development of dedicated reconstruction algorithms targeted for single-sided particle imaging.

Theoretical considerations on the spatial resolution limit of single-event particle radiography

Particle radiography (pRad) has been proposed and investigated as a promising tool for particle therapy as it provides a water equivalent thickness (WET) image of the patient. In single-event particle imaging, for each measured particle, the related most likely path (MLP) through the object is estimated to account for multiple Coulomb scattering (MCS). In previous studies, the accuracy limit of the MLP has been used to determine the spatial resolution limit. In this work, we investigate the limit of the spatial resolution achievable with different pRad algorithms based on a theoretical model of the particle scattering for an ideal beam and detector. First, we investigate binning the particles in a plane seated at the depth of the object of interest (plane-of-interest binning; PIB) and extend existing theoretical considerations also to objects not located in the binning plane. We use this to model the spatial resolution in case of binning the particles directly at the front or rear tracker (FTB and RTB, respectively). Further, we investigate evenly distributing the particles’ WET along their trajectory into pixel channels and creating the pRad image as channel mean (along-path-binning; APB). Monte Carlo simulations are used to qualitatively investigate the different algorithms and to validate the theoretical predictions. We show that projecting the scattered particle paths onto a single image will inevitably result in a limited spatial resolution lower than expected from only the MLP uncertainty. Only in the case where the depth of a feature is known and used as binning depth for PIB, the spatial resolution of that feature is equal to the path estimation accuracy. For the APB algorithm the spatial resolution decreases with increasing depth in the object, especially if the true particle path through the object would be known. The derived theoretical models will be useful for future development of improved pRad reconstruction algorithms.

Calculation of water equivalent diameter based on anteroposterior localizer CT images

ObjectiveTo explore a method for calculating water equivalent diameter (Dw) based on localizer CT images for calculation of the size specific dose estimates (SSDE).MethodGE Revolution CT and LightSpeed VCT were used to scan CT dose index phantoms 16 cm and 32 cm in diameter at the tube voltages of 80, 100 and 120 kV to obtain the axial image and anteroposterior localizer radiograph. According to the definition of CT Hounsfield unit, the axial images were used to calculate the conversion factors that convert the phantom thickness to water equivalent thickness. The gray value of the localizer radiograph and the water equivalent thickness were calibrated with a linear equation, and the parameters of the calibration were used to calculate the water equivalent thickness. The method was verified using 2 CT dose index phantoms and in 22 patients undergoing chest and abdominal CT examination.ResultComparison of the water equivalent diameter (Dw) based on the localizer radiograph and axial image of the 2 phantoms showed that the percentage difference between Dw from the axial images and from the localizer radiograph was below 3%. The trend of Dw variations with location in the two methods was sonsistent. The difference in Dw in intermediate region of interest between the axial image and the localizer radiograph from the 22 patients was below 6.6%. With the mean Dw in the ROI, the maximum percentage difference was 7.5%.ConclusionCalibration of the gray value of the localizer radiograph and the water equivalent thickness using the axial image and localizer radiograph of CT dose index phantoms allows quick calculation of the SSDE based on the parameters of calibration.

A Monte Carlo feasibility study on quantitative laser-driven proton radiography

Laser-accelerated proton bunches with kinetic energies up to several tens of MeV and at repetition rates in the order of Hz are nowadays achievable at several research centres housing high-power laser system. The unique features of such ultra-short bunches are also arousing interest in the field of radiological and biomedical applications. For many of these applications, accurate positioning of the biological target is crucial, raising the need for on-site imaging. One convenient option is proton radiography, which can exploit the polyenergetic spectrum of laser-accelerated proton bunches. We present a Monte Carlo (MC) feasibility study to assess the applicability and potential of laser-driven proton radiography of millimetre to centimetre sized objects. Our radiography setup consists of a thin time-of-flight spectrometer operated in transmission prior to the object and a pixelated silicon detector for imaging. Proton bunches with kinetic energies up to 20MeV and up to 100MeV were investigated. The water equivalent thickness (WET) of the traversed material is calculated from the energy deposition inside an imaging detector, using an online generated calibration curve that is based on a MC generated look-up table and the reconstructed proton energy distribution. With a dose of 43mGy for a 1mm thin object imaged with protons up to 20MeV, the reconstructed WET of defined regions-of-interest was within 1.5% of the ground truth values. The spatial resolution, which strongly depends on the gap between object and imaging detector, was 2.5lpmm−1 for a realistic distance of 5mm. Due to this relatively high imaging dose, our proposed setup for laser-driven proton radiography is currently limited to objects with low radio-sensitivity, but possibilities for further dose reduction are presented and discussed.

Multi-modality bedding platform for combined imaging and irradiation of mice

Preclinical imaging and irradiation yields valuable insights into clinically relevant research topics. While complementary imaging methods such as computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET) can be combined within single devices, this is technically demanding and cost-intensive. Similarly, bedding and setup solutions are often specific to certain devices and research questions. We present a bedding platform for mice that is compatible with various preclinical imaging modalities (combined PET/MRI, cone beam CT) and irradiation with photons and protons. It consists of a 3D-printed bedding unit (acrylonitrile butadiene styrene, ABS) holding the animal and features an inhalation anesthesia mask, jaw fixation, ear pins, and immobilization for the hind leg. It can be embedded on mounting adaptors for multi-modal imaging and into a transport box (polymethyl methacrylate, PMMA) for experiments outside dedicated animal facilities while maintaining the animal’s hygiene status. A vital support unit provides heating, inhalation anesthesia, and a respiration monitor. We dosimetrically evaluated used materials in order to assess their interaction with incident irradiation. Proof-of-concept multi-modal imaging protocols were used on phantoms and mice. The measured attenuation of the bedding unit for 40/60/80/200 kV X-rays was less than 3%. The measured stopping-power-ratio of ABS was 0.951, the combined water-equivalent thickness of bedding unit and transport box was 4.2 mm for proton energies of 150 MeV and 200 MeV. Proof-of-concept imaging showed no loss of image quality. Imaging data of individual mice from different imaging modalities could be aligned rigidly. The presented bed aims to provide a platform for experiments related to both multi-modal imaging and irradiation, thus offering the possibility for image-guided irradiation which relies on precise imaging and positioning. The usage as a self-contained, stand-alone unit outside dedicated animal facilities represents an advantage over setups designed for specific devices.

Design of a focused collimator for proton therapy spot scanning using Monte Carlo methods.

When designing a collimation system for pencil beam spot scanning proton therapy, a decision must be made whether or not to rotate, or focus, the collimator to match beamlet deflection as a function of off-axis distance. If the collimator is not focused, the beamlet shape and fluence will vary as a function of off-axis distance due to partial transmission through the collimator. In this work, we quantify the magnitude of these effects and propose a focused dynamic collimation system (DCS) for use in proton therapy spot scanning. This study was done in silico using a model of the Miami Cancer Institute's (MCI) IBA Proteus Plus system created in Geant4-based TOPAS. The DCS utilizes rectangular nickel trimmers mounted on rotating sliders that move in synchrony with the pencil beam to provide focused collimation at the edge of the target. Using a simplified setup of the DCS, simulations were performed at various off-axis locations corresponding to beam deflection angles ranging from 0° to 2.5°. At each off-axis location, focused (trimmer rotated) and unfocused (trimmer not rotated) simulations were performed. In all simulations, a 4cm water equivalent thickness range shifter was placed upstream of the collimator, and a voxelized water phantom that scored dose was placed downstream, each with 4cm airgaps. Increasing the beam deflection angle for an unfocused trimmer caused the collimated edge of the beamlet profile to shift 0.08-0.61mm from the baseline 0° simulation. There was also an increase in low-dose regions on the collimated edge ranging from 14.6% to 192.4%. Lastly, the maximum dose, , was 0-5% higher for the unfocused simulations. With a focused trimmer design, the profile shift and dose increases were all eliminated. We have shown that focusing a collimator in spot scanning proton therapy reduces dose at the collimated edge compared to conventional, unfocused collimation devices and presented a simple, mechanical design for achieving focusing for a range of source-to-collimator distances.

A technique for spatial resolution improvement in helium-beam radiography.

Ion-beam radiography exhibits a significantly lower spatial resolution (SR) compared to x-ray radiography. This is mostly due the multiple Coulomb scattering (MCS) that the ions undergo in the imaged object. In this work, a novel technique to improve the spatial resolution in helium-beam radiography was developed. Increasing helium-beam energies were exploited in order to decrease the MCS, and therefore increase the SR. The experimental investigation was carried out with a dedicated ion-tracking imaging system fully composed of thin, pixelated silicon detectors (Timepix). Four helium beams with increasing energies (from 168.8 to 220.5MeV/u) were used to image a homogeneous 160mm PMMA phantom with a 2mm air gap at middle depth. An energy degrader (ED) was placed between the rear tracking system and the energy-deposition detector to compensate for the longer range associated with more energetic ions. The SR was measured for each beam energy. To take into account the overall impact on the image quality, the contrast-to-noise ratio (CNR), the single-ion water equivalent thickness (WET) precision and the absorbed dose in the phantom were also evaluated as a function of the initial beam energy. FLUKA Monte Carlo simulations were used to support the conceptual design of the experimental setup and for dose estimation. In the investigated energy interval, a total SR increase by around 30% was measured with increasing beam energy, reaching a maximum value of 0.69lp/mm. For radiographs generated with 350μGy of absorbed dose and 220μm pixel size, a CNR decrease of 32% was found as the beam energy increases. For 1mm pixel size, the CNR decreases only by 22%. The CNR of the images was always above 6. The single-ion WET precision was found to be in a range between 1.2% and 1.5%. We have experimentally shown and quantified the possibility of improving SR in helium-beam radiography by using increasing beam energies in combination with an ED. A significant SR increase was measured with an acceptable decrease of CNR. Furthermore, we have shown that an ED can be a valuable tool to exploit increasing beam energies to generate energy-deposition radiographs.

Exploring radiation hardness of PEPITES, a new transparent charged particle beam profiler

PEPITES is a new type of transparent beam profiler (<10 µm water-equivalent thickness (WET)) under development that will equip the beam line of the ARRONAX cyclotron. Segmented electrodes consisting of nanometric thick gold strips deposited on a thin polymer membrane make the active part. In order to characterize the damages induced in the polymer, UV–Vis spectroscopy and scanning electron microscope have been used on 8 μm thick Kapton polyimide membranes irradiated with 68 MeV proton beams up to 7 × 1015 H+/cm2. Dynamic and permanent damage was observed only at a high fluence level, thus ensuring long-term operation of the PEPITES detector in its nominal working environment.

A Beam-Angle-Selection Method to Improve Inter-Fraction Motion Robustness for Lung Tumor Irradiation With Passive Proton Scattering.

In terms of dose distribution, protons are more sensitive to range variations than photons due to their unique properties. The aim of this study was to develop a method to identify patient-specific robust proton beam angles for lung tumor irradiation by investigating the association between water equivalent thickness (WET) variation and inter-fraction motion-induced target dose degradation. Using 3-dimensional computed tomography (3D-CT) images, the impact of WET variations on the target dose coverage of a series of coplanar proton beams was evaluated for 4 patients with lung cancer. Using ray tracing, WET maps, or WET baseline, were estimated for the internal target volume (ITV) at every 5° gantry interval in the axial plane. After calculating the WET baseline, the planning CT was shifted 5 mm in each anterior-posterior (AP), superior-inferior (SI), and left-right (LR) direction, yielding a total of 6 shifted CTs, and differential WET maps between the planning CT and each shifted CT were calculated. Target dose differences were associated with the average WET change between the original planning CT and the shifted CTs for all 360° gantry rotation beams. Target and OAR dose metrics in the ΔWET-guided plans were compared with those of the clinical plans. The WET variation maps showed areas of both high and low WET variations, with overall similar patterns yet individual differences reflecting tumor position differences. For all 4 patients investigated in this study, the coplanar plans demonstrated a strong correlation between WET changes and ITV dose reductions. Target dose coverage was more stable with the ΔWET-guided plan while OAR doses were comparable to the clinical plan. The WET variation maps have been used in this pilot study to identify proton beam angles that are either sensitive or robust to WET changes in proton passive scattering. This work demonstrates the feasibility of using WET variation maps to assist the planner in inter-fraction motion-robust proton beam angle selection.

Design of a compact proton beam energy modulator for imaging

Proton beam therapy is the current state of the art for irradiation of cancerous tumors near vital organs. Proton imaging is a new technology that images tumors with the same particle interaction as proton therapy, promising more accurate treatment preparation compared to traditional computerized tomography scans. A specific proton imaging technique in development at Massachusetts General Hospital requires an energy modulator that can fit inside the gantry nozzle. This work presents the design of a compact proton beam energy modulator required for such imaging applications. A combination of steel and lead wedges was used to create a balanced modulation wheel, with an axis of rotation perpendicular to the incident beam. The mechanical design also allows interchangeable disks and precise position control. The modulator design was verified by testing on a proton beam line. The final device achieved a wide range of energy modulation (21 cm water-equivalent thickness) while maintaining a constant exiting beam angular spread meeting device requirements. This compact design facilitates proton imaging to be practically incorporated into future gantry systems, which will advance proton treatment for tumors near vital organs.

Sparse deconvolution of proton radiography data to estimate water equivalent thickness maps.

In proton therapy, the conversion of the planning computed tomography (CT) into proton stopping powers is tainted by uncertainties which may jeopardize dose conformity. Proton radiography provides a direct information on the energy reduction of protons in the patient. However, it is currently limited by the degradation ("blurring") of the one-dimensional depth-dose deposition profiles which constitute the pixels. An iterative algorithm is implemented to extract high-resolution water equivalent thickness (WET) maps from the measurements of depth-dose profiles acquired with a multilayer ionization chamber. The method relies on the assumption that those curves are a function of the WET, which can benefit from a sparse representation. When used without relying on any prior knowledge derived from the planning CT, the method already outperforms the published one in terms of accuracy. We also propose a variant which integrates the planning CT in a robust fashion to further improve the deconvolution result and reach an accuracy of 1.5mm on the estimated WET. The methods are applied to both synthetic data and actual proton radiography acquisitions on phantoms. Besides the increase in accuracy achieved in the estimation of WET maps from proton radiography data, we demonstrate that the proposed deconvolution algorithm is also more robust with respect to confounding factors such as residual setup errors or changes in the anatomy. Therefore, proton radiography using a range probe provides both the required accuracy to assess and reduce range uncertainty in proton therapy and the simplicity of integrated-mode proton radiography.

Feasibility study of a plastic scintillating plate-based treatment beam fluence monitoring system for use in pencil beam scanning proton therapy.

The purpose of this study was to describe a plastic scintillating plate-based gantry-attachable dosimetry system for pencil beam scanning proton therapy to monitor entrance proton beam fluence, and to evaluate the dosimetric characteristics of this system and its feasibility for clinical use. The dosimetry system, consisting of a plastic scintillating plate and a CMOS camera, was attached to a dedicated scanning nozzle and scintillation during proton beam irradiation was recorded. Dose distribution was calculated from the accumulated recorded frames. The dosimetric characteristics (energy dependency, dose linearity, dose rate dependency, and reproducibility) of the gantry-attachable dosimetry system for use with therapeutic proton beams were measured, and the feasibility of this system during clinical use was evaluated by determining selected quality assurance items at our institution. The scintillating plate shortened the range of the proton beam by the water-equivalent thickness of the plate and broadened the spatial profile of the single proton spot by 11% at 70MeV. The developed system functioned independently of the beam energy (<1.3%) and showed dose linearity, and also functioned independently of the dose rate. The feasibility of the system for clinical use was evaluated by comparing the measured quality assurance dose distribution to that of the treatment planning system. The gamma passing rate with a criterion of 3%/3mm was 97.58%. This study evaluated the dosimetric characteristics of a plastic scintillating plate-based dosimetry system for use with scanning proton beams. The ability to account for the interference of the dosimetry system on the therapeutic beam enabled offline monitoring of the entrance beam fluence of the pencil beam scanning proton therapy independent of the treatment system with high resolution and in a cost-effective manner.

Theoretical tumor edge detection technique using multiple Bragg peak decomposition in carbon ion therapy

Purpose: The range precision in carbon ion therapy is extremely sensitive to tissue density variations. A high energy carbon beam, after crossing a high-gradient edge parallel to the beam direction, suffers from range mixing leading to the detection of multiple Bragg peaks (BPs) varying intensity and water equivalent thickness (WET). The purpose of this work was to introduce a model that determines the position of a high-gradient edge based on information acquired from carbon transmission imaging. Methods: A model was derived to determine the lateral distance between the irradiation beam propagation axis and the edge position. To validate it, carbon beams were simulated and propagated through two parametric phantoms: (1) a bone cube in a water tank and (2) a semi-cylindrical bone insert in a water tank. The method was tested in a lung tumor case where range mixing led to more than two BPs being detected, requiring an iterative BP decomposition to determine the fraction of carbon ions crossing the materials surrounding the edge of interest. Results: The theoretical model predicted the edge position relative to the beam position with an error ≤1 mm for all studied cases with a maximum dose delivered of 24 μGy. Conclusions: The method presented here is a proof of principle. It does not take into account clinical uncertainties. However, this approach provided promising results suggesting that future extension to assess the impact of clinical uncertainties should be performed.

A Treatment Planning Study of Moderately Hypofractionated Intensity-Modulated Proton Therapy (IMPT) to the Prostate and Pelvic Combined with Extremely Hypofractionated IMPT Boost to Prostate with Urethral Dose Reduction for High-Risk Prostate Cancer

Moderately hypofractionated radiotherapy (RT) can be considered for all prostate cancer (PCa) patients while extremely hypofractionated RT is only recommended for low-intermediate risk PCa. Dose escalation is associated with improved biochemical outcomes for patients with high risk PCa. Currently intensity modulated proton therapy (IMPT) holds the potential to improve dose delivery and spare nontarget tissues. The purpose of this study was to explore the feasibility of moderately hypofractionated IMPT combined with extremely hypofractionated IMPT boost to prostate with urethral dose reduction for high risk PCa. We retrospectively reviewed the planning CT images of 3 PCa patients with a urethral catheter (4.0 mm dia.) treated with photon therapy and made the IMPT plan for these patients. The contours of the prostate were defined as the CTVp. CTVsv included proximal 2cm of the seminal vesicles. CTVpelvic included the pelvic regional lymph nodes. PTV was determined by a 3D expansion of the CTV with the addition of a 5-mm margin except for posterior direction to avoid the expansion to rectum. The urethra as an OAR was contoured in 2 mm diameter. A margin of 2 mm was applied to the urethra to determine the planning organ at risk volume (PRV). In the first course of the plan, A 8GyE boost was given in one fraction to PTVp subtract urethralPRV(PTVp-u). In the second course, the electron density of urethral was defined as same as prostate. A total of 67.5 GyE in 25 fractions (2.7 GyE/fraction) were delivered to the PTVp(whole prostate gland including the urethra). Prescription dose for PTVsv and PTVpelvic were 60GyE (2.4GyE/fraction) and 47.5 GyE(1.9GyE/fraction) in 25 fractions, respectively. A dose constraint of V70Gy < 10% was applied to the urethralPRV (6 mm dia.). Patients were treated with 2 lateral-opposed (LAT) fields. An additional 3.5% of proximal and distal water equivalent thickness plus 2 mm was added to the PTV to account for beam range uncertainty. In the combined plan, target volume coverage of 70GyE to the PTVp was 95.7±1.3%. Coverage of dose prescription of the PTVsv and PTVpelvic all reached 95%. In addition, a total dose of 78GyE covered more than 95% of the external region of 1cm expansion of the urethralPRV for all these patients. Meanwhile, the V70Gy of the urethralPRV was as low as 1.7±1.3%. The V70Gy, V60Gy, V50Gy of the rectum were 2.9±1.7%, 11.27±2.5% and 8.1±0.9%. The V50Gy of the bladder was only 8.1±0.9%. The V50Gy of the left and right femoral were 1.5±0.8% and 1.4±0.8%, respectively. For high risk PCa who received prostate and pelvic irradiation, moderately hypofractionated irradiation combined with extremely hypofractionated IMPT boost to prostate using IMPT technique could bring higher biological equivalent dose and excellently protect the urethral.

Technical Note: Quality assurance of proton central axis pencil‐beam spread‐out Bragg peak using large‐diameter multilayer ionization chambers

The aim of this work is to describe a method of machine quality assurance (QA) by measuring proton spread-out Bragg peak constructed by the integrated depth dose via a large-diameter (12-cm) multilayer ionization chamber (LD-MLIC). Two types of contours are used to create the nominal plan. The final nominal plan is composed of mixed-energy proton pencil-beam spots located close to the central axis. The integrated depth dose (IDD) curve contains a flat SOBP region. The LD-MLIC-measured IDD was compared to the IDD curve exported from the treatment planning system (TPS). In addition, three plans with intentionally modified energy layers to simulate wrong-delivered energy layers were created and measured by the LD-MLIC. The water equivalent thickness (WET) difference between the inserted and replaced energies was 0.2cm. Six weeks of measurements were analyzed. A low-pass filter was introduced to mitigate the high-frequency noise in the IDD signal ratios. The filtered IDD signal ratios between the modified plans in different weeks and the baseline were used to check the energy accuracy. The differences between the LD-MLIC-measured and TPS-exported IDDs of the nominal plan were within 2% in most parts of the curve. Bumps/dips (~1%) were noted in the filtered IDD ratio between the modified plans and the baseline. The LD-MLIC can be used to check the accuracy of multiple energies Bragg peak locations quickly in proton machine QA. The LD-MLIC was sensitive in identifying an erroneous energy with 0.2cm in WET.