Solid Phantom Research Articles

Monte Carlo simulations and analysis of transmitted gamma ray spectra through various tissue phantoms

Studies of radiation interactions with tissue equivalent material find importance in efforts that seek to avoid unjustifiable dose to patients, also in ensuring quality control of for instance nuclear medicine imaging equipment. Use of the Monte Carlo (MC) simulation tool in such characterization processes allows for the avoidance of costly experiments involving transmitted X- and γ-ray spectrometry. Present work investigates MC simulations of γ-ray transmission through tissue equivalent solid phantoms. Use has been made of a range of radionuclide gamma ray sources, 99mTc, 131I, 137Cs, 60Co (offering photons in the energy range from a few keV up to low MeV), popularly applied in medicine and in some cases for gauging in industry, obtaining the transmission spectra following their interaction with various phantom materials and thicknesses. In validation of the model, the simulated values of mass attenuation coefficients (μ/ρ) for different phantom materials and thicknesses were found to be in good agreement with reference values (NIST, 2004) to within 1.1% for all material compositions. For all of the primary photon energies and medium thicknesses of interest herein, results show that multiple scattering peaks are generally located at energies lower than 100 keV, although for the larger phantom thicknesses it is more difficult to distinguish single, double and multiple scattering in the gamma spectra. Transmitted photon spectra investigated for water, soft tissue, breast, brain and lung tissue slab phantoms are demonstrated to be practically independent of the phantom material, while a significant difference is observed for the spectra transmitted through bone that was proved to be due to the density effect and not material composition.

Simulation of photon-counting detectors for conversion of dual-energy-subtracted computed tomography number to electron density.

For accurate tissue-inhomogeneity correction in radiotherapy treatment planning, the author previously proposed a conversion of the energy-subtracted computed tomography (CT) number to electron density (ΔHU-ρe conversion). The purpose of the present study was to provide a method for investigating the accuracy of a photon-counting detector (PCD) used in the ΔHU-ρe conversion by performing dual-energy CT image simulations of a PCD system with two energy bins. To optimize the tube voltage and threshold energy, the image noise and errors in ρe calibration were evaluated using three types of virtual phantoms: a 35-cm-diameter pure water phantom, 33-cm-diameter solid water surrogate phantom equipped with 16 inserts, and another solid water surrogate phantom with a 25-cm diameter. The third phantom was used to investigate the effect of the object's size on the ρe-calibration accuracy of PCDs. Two different scenarios for the PCD energy response were considered, corresponding to the ideal and realistic cases. In addition, a simple correction method for improving the spectral separation of the dual energies in a realistic PCD was proposed to compensate for its performance loss. In the realistic PCD case, there exists a trade-off between the image noise and ρe-calibration errors. Furthermore, the weakest image noise was nearly twice that for the ideal case, and the ρe-calibration error did not reach practical levels for any threshold energy. Nevertheless, the proposed correction method is likely to decrease the ρe-calibration errors of a realistic PCD to the level of the ideal case, yielding more accurate ρe values that are less affected by object size variation.

Clinical implementation of brass mesh bolus for chest wall postmastectomy radiotherapy and film dosimetry for surface dose estimates.

This study presents the dosimetric data taken with radiochromic EBT3 film with brass mesh bolus using solid water and semi-breast phantoms, and its clinical implementation to analyze the surface dose estimates to the chest wall in postmastectomy radiotherapy (PMRT) patients. Water-equivalent thickness of brass bolus was estimated with solid water phantom under 6 megavoltage photon beam. Following measurements with film were taken with no bolus, 1, 2, and 3 layers of brass bolus: (a) surface doses on solid water phantom with normal incidence and on curved surface of a locally fabricated cylindrical semi-breast phantom for tangential field irradiation, (b) depth doses (in solid phantom), and (c) surface dose measurements around the scar area in six patients undergoing PMRT with prescribed dose of 50 Gy in 25 fractions. Water-equivalent thickness (per layer) of brass bolus 2.09 ± 0.13 mm was calculated. Surface dose measured by film under the bolus with solid water phantom increased from 25.2% ±0.9% without bolus to 62.5% ± 3.1%, 80.1% ± 1.5%, and 104.4% ± 1.7% with 1, 2, and 3 layers of bolus, respectively. Corresponding observations with semi-breast phantom were 32.6% ± 5.3% without bolus to 96.7% ± 9.1%, 107.3% ± 9.0%, and 110.2% ± 8.7%, respectively. A film measurement shows that the dose at depths of 3, 5, and 10 cm is nearly same with or without brass bolus and the percentage difference is <1.5% at these depths. Mean surface doses from 6 patients treated with brass bolus ranged from 79.5% to 84.9%. The bolus application was discontinued between 18th and 23rd fractions on the development of Grade 2 skin toxicity for different patients. The total skin dose to chest wall for a patient was 3699 cGy from overall treatment with and without bolus. Brass mesh bolus does not significantly change dose at depths, and the surface dose is increased. This may be used as a substitute for tissue-equivalent bolus to improve surface conformity in PMRT.

Therapeutic Proton Beam Range Measurement with EBT3 Film and Comparison with Tool for Particle Simulation

Purpose The advantages of ocular proton therapy are that it spares the optic nerve and delivers the minimal dose to normal surrounding tissues. In this study, it developed a solid eye phantom that enabled us to perform quality assurance (QA) to verify the dose and beam range for passive single scattering proton therapy using a single phantom. For this purpose, a new solid eye phantom with a polymethyl-methacrylate (PMMA) wedge was developed using film dosimetry and an ionization chamber. Methods The typical beam shape used for eye treatment is approximately 3 cm in diameter and the beam range is below 5 cm. Since proton therapy has a problem with beam range uncertainty due to differences in the stopping power of normal tissue, bone, air, etc, the beam range should be confirmed before treatment. A film can be placed on the slope of the phantom to evaluate the Spread-out Bragg Peak based on the water equivalent thickness value of PMMA on the film. In addition, an ionization chamber (Pin-point, PTW 31014) can be inserted into a hole in the phantom to measure the absolute dose. Results The eye phantom was used for independent patient-specific QA. The differences in the output and beam range between the measurement and the planned treatment were less than 1.5% and 0.1 cm, respectively. Conclusions An eye phantom was developed and the performance was successfully validated. The phantom can be employed to verify the output and beam range for ocular proton therapy.

Validation of the Gel &amp; Wax Boluses and Comparison of their Dosimetric Performance with Virtual Bolus

Background: In general, radiotherapy treatment planning is performed using the virtual bolus. It is necessary to investigate physical bolus in comparison to virtual one.Objectives: In the present study, first, radiological properties of superflab Gel bolus and Paraffin wax bolus was investigated in terms of their relative electron density. Then, dosimetric performance of both the bolus (i.e. Gel and Parafin wax) was compared with Virtual bolusMaterial and Methods: In This experimental study, the radiological property of Wax and Gel boluses was investigated using two methods. In one, the relative electron density of both the Gel and Wax boluses was calculated by measuring their linear attenuation coefficient where in another method relative electron density was calculated by recording their CT No directly from their CT scan. Later CT scan of solid water slab phantom (dimension 30x30x15 cm3), with physical boluses (i.e. Gel and Wax bolus) of appropriate thicknesses required to deliver a dose of 200 cGy at Dmax using 4 MV, 6 MV and 15 MV photon beams, was taken. These CT data sets were retrieved to TPS. A plan was done to deliver a dose of 200 cGy at Dmax using Single 4 MV, 6 MV and 15 MV photon beams. Dose at depths Dmax, 1 cm, 2 cm, 3 cm, 4 cm and 5 cm was recorded. Using this similar method, doses at depths viz Dmax, 1 cm, 2 cm, 3 cm, 4 cm and 5 cm was recorded for the Gel and Wax boluses. The differences in dose of gel and wax bolus from virtual bolus were recorded for comparison of their dosimetric performance.Results: The measured (calculated) relative electron density of wax and Gel bolus was found to be 0.958 (0.926) and 0.923 (0.907), respectively. Variation in dosimetric performance of Gel and Wax with reference to Virtual bolus was studied. However, on average, Gel bolus was more consistent with virtual bolus.Conclusion: To avoid any dose difference between, delivered (using physical bolus) and planned (using virtual bolus), the physical boluses should be investigated for their dosimetric performance in comparison to virtual bolus. The results obtained and methodology used in this study can be applied in routine radiotherapy practices

Dose reconstruction from PET images in carbon ion therapy: a deconvolution approach

Dose and range verification have become important tools to bring carbon ion therapy to a higher level of confidence in clinical applications. Positron emission tomography is among the most commonly used approaches for this purpose and relies on the creation of positron emitting nuclei in nuclear interactions of the primary ions with tissue. Predictions of these positron emitter distributions are usually obtained from time-consuming Monte Carlo simulations or measurements from previous treatment fractions, and their comparison to the current, measured image allows for treatment verification. Still, a direct comparison of planned and delivered dose would be highly desirable, since the dose is the quantity of interest in radiation therapy and its confirmation improves quality assurance in carbon ion therapy.In this work, we present a deconvolution approach to predict dose distributions from PET images in carbon ion therapy. Under the assumption that the one-dimensional PET distribution is described by a convolution of the depth dose distribution and a filter kernel, an evolutionary algorithm is introduced to perform the reverse step and predict the depth dose distribution from a measured PET distribution. Filter kernels are obtained from either a library or are created for any given situation on-the-fly, using predictions of the -decay and depth dose distributions, and the very same evolutionary algorithm.The applicability of this approach is demonstrated for monoenergetic and polyenergetic carbon ion irradiation of homogeneous and heterogeneous solid phantoms as well as a patient computed tomography image, using Monte Carlo simulated distributions and measured in-beam PET data. Carbon ion ranges are predicted within less than 0.5 mm and 1 mm deviation for simulated and measured distributions, respectively.

Optical parameters detection with multi-frequency modulation based on NIR DPDW

For measuring the optical parameters of biological tissues accurately and rapidly, this study built an optical parameters measurement system based on near-infrared (NIR) Diffuse Photon Density Waves (DPDW) technology using frequency sweep modulation. The transmitted swept optical amplitude and phase can be nonlinearly fitted according to the physical diffusive approximation equation then to obtain the absorption coefficient and reduced scattering coefficient of the tissue phantoms, respectively. The experimental results of the 20 Intralipid solutions show that the average absolute errors of μa and μs′ are 0.066 cm−1 and 0.716 cm−1, respectively, and the correlation coefficients between the measured and true values were 0.965 and 0.989, respectively. The experimental results of the 3 solid phantoms show that the average absolute errors of μa and μs′ are 0.1140 cm−1 and 0.0023 cm−1, respectively. Therefore, the DPDW technology with frequency sweep modulation can detect the optical parameters of the tissue phantoms accurately and quickly, and it would be optimized for tissue tomography imaging and blood components noninvasively detection in vivo.

222. Optimization of pretreatment IMRT dose distribution verification: Bidimensional gaussian kernel implementation

Purpose Intensity modulated radiation therapy (IMRT) is a widely used technique delivering a highly conformal dose to the target together with a better sparing of organs at risk with respect to conventional irradiation techniques. Quality assurance (QA) of the generated IMRT treatment plan is of most importance to ensure that the patient receives the planned dose; a two-dimensional array of ionization chambers in a solid phantom can be used for the verification. Different methods exist for comparison of dose distributions and the gamma evaluation is the most commonly used. The aim of this work is to convolve the calculated dose distributions in order to optimize the comparison with the measured data obtained with the MatriXX ionization chamber detector arrays (IBA Dosimetry, Germany). Methods In this study a bidimensional gaussian kernel, which addresses the ionization chamber volume averaging effect, has been implemented to optimize the comparison of dose distributions for pretreatment patient-specific IMRT QA. It was determined by scanning a single chamber with a slit beam. A point spread function was reconstructed with a Gaussian function obtained with a MATLAB script. The kernel has been tested on head&neck and prostate treatment plans. Results It has been found that the convolution of the kernel with the treatment planning system calculated dose distribution can improve the comparison respect to the conventional method of IMRT verification. The gamma index passing rate values found with convolution method are higher in all considered clinical cases. Conclusions Intensity modulated radiation therapy (IMRT) is a widely used technique delivering a highly conformal dose to the target together with a better sparing of organs at risk with respect to conventional irradiation techniques. Quality assurance (QA) of the generated IMRT treatment plan is of most importance to ensure that the patient receives the planned dose; a two-dimensional array of ionization chambers in a solid phantom can be used for the verification. Different methods exist for comparison of dose distributions and the gamma evaluation is the most commonly used. The aim of this work is to convolve the calculated dose distributions in order to optimize the comparison with the measured data obtained with the MatriXX ionization chamber detector arrays (IBA Dosimetry, Germany). In this study a bidimensional gaussian kernel, which addresses the ionization chamber volume averaging effect, has been implemented to optimize the comparison of dose distributions for pretreatment patient-specific IMRT QA. It was determined by scanning a single chamber with a slit beam. A point spread function was reconstructed with a Gaussian function obtained with a MATLAB script. The kernel has been tested on head&neck and prostate treatment plans. It has been found that the convolution of the kernel with the treatment planning system calculated dose distribution can improve the comparison respect to the conventional method of IMRT verification. The gamma index passing rate values found with convolution method are higher in all considered clinical cases.

Can Gafchromic EBT3 films effectively characterize small fields of 6 MV unflattened photon beams of Cyberknife system?

Abstract Shielded silicon diodes are commonly employed in commissioning of Cyberknife 6 MV photon beams. This study aims to measure output factors, off centered ratio (OCR), percentage depth dose (PDD) of 6 MV photons using shielded and unshielded diodes and to compare with Gafchromic EBT3 film measurements to investigate whether EBT3 could effectively characterize small 6 MV photon beams. Output factors, OCR and PDD were measured with shielded and unshielded silicon detectors in a radiation field analyzer system at reference condition. Water equivalent solid phantom were used while irradiating EBT3 films. From multiuser data, diodes underestimated output factor by 3% for collimator fields ≤ 10 mm, while EBT3 underestimated the output factor by 3.9% for 5 mm collimator. 1D Gamma analysis of OCR between diode and film, results in gamma ≤ 1 for all measured points with 1 mm distance to agreement (DTA) and 1% relative dose difference (DD). Dose at surface is overestimated with diodes compared to EBT3. PDD results were within 2% relative dose values between diode and EBT3 except for 5 mm collimator. Except for small collimator fields of up to 10 mm, results of output factor, OCR, PDD of all detectors used in this study exhibited similar results. Relative dose measurements with Gafchromic EBT3 in this work show that EBT3 films can be used effectively as an independent tool to verify commissioning beam data of small fields only after careful verification of methodology for any systematic errors with appropriate readout procedure.

Dose calibration uncertainty and plan-specific dose calibration for IMRT QA

Purpose: This study was to investigate IMRT QA dose calibration uncertainty that has not been realized before, and develop plan-specific calibration methods to reduce such uncertainty. Methods: IMRT plans were created on the Varian Eclipse treatment planning system. IMRT QA used a PTW Octavius phantom with an Octavius 2D array ionization chamber detector and was also tested for a solid water cube with a Farmer chamber. Dose calibration for a solid phantom was analyzed using the AAPM TG-51/TG-21 formulism. The reference fields used for IMRT dose calibration included squared fields and IMRT plan-specific fields that included geometric equivalent fields, and dose-equivalent fields defined by the 50% isodose distributions. Results: Thirteen IMRT QA cases including three VMAT cases were used for this study. For the single field calibration on the PTW Octavius device, the calibration factors varied with field size by up to 5.6%, and varied with gantry angle by up to 4.9%. Those variations changed IMRT QA passing rates by up to 16.2%. Using the plan-specific calibration, the Gamma passing rate was increased by up to 27.7%. The VMAT passing rates varied by up to 30%, and the equivalent field dose calibration gave the highest passing rate. For absolute point dose measurement using a Farmer chamber, calibration factors varied by up to 3.7% with different field sizes and different gantry angles. The dose equivalent field calibration achieved the best IMRT QA results. Conclusion: Different fields can have different dose calibration factors. Conventional dose calibration methods for IMRT QA using simple reference fields include significant uncertainty that would affect IMRT QA results. Dose calibration uncertainty is caused by the complex composition of phantom materials and complex design and characteristics of the detectors. The plan-specific dose calibration can reduce dose calibration uncertainty and improve IMRT QA accuracy.

An investigation of clinical treatment field delivery verification using cherenkov imaging: IMRT positioning shifts and field matching.

Cherenkov light emission has been shown to correlate with ionizing radiation dose delivery in solid tissue. An important clinical application of Cherenkov light is the real-time verification of radiation treatment delivery invivo. To test the feasibility of treatment field verification, Cherenkov light images were acquired concurrent with radiation beam delivery to standard and anthropomorphic phantoms. Specifically, we tested two clinical treatment scenarios: (a) Observation of field overlaps or gaps in matched 3D fields and (b) Patient positioning shifts during intensity modulated radiation therapy (IMRT) field delivery. Further development of this technique would allow real-time detection of treatment delivery errors on the order of millimeters so that patient safety and treatment quality can be improved. Cherenkov light emission was captured using a PI-MAX4 intensified charge coupled device (ICCD) system (Princeton Instruments). All radiation delivery was performed using a Varian Trilogy linear accelerator (linac) operated at 6MV or 18MV for photon and 6MeV or 16MeV for electron studies. Field matching studies were conducted with photon and electron beams at gantry angles of 0°, 15°, and 45°. For each modality and gantry angle, a total of three data sets were acquired. Overlap and gap distances of 0, 2, 5, and 10mm were tested and delivered to solid phantom material of 30×30×5cm3 . Phantom materials used were white plastic water and brown solid water. Tests were additionally performed on an anthropomorphic phantom with an irregular surface. Positioning shift studies were performed using IMRT fields delivered to a thoracic anthropomorphic phantom. For thoracic phantom measurements, the camera was placed laterally to observe the entire right side of the phantom. Fields were delivered with known translational patient positioning shifts in four directions. Changes in the Cherenkov fluence were evaluated through the generation of difference maps from unshifted Cherenkov images. All images were evaluated using ImageJ, Python, and MATLAB software packages. For matched fields, Cherenkov images were able to quantitate matched field separations with discrepancies between 2 and 4mm, depending on gantry angle and beam energy or modality. For all photon and electron beams delivered at a gantry angle of 0°, image analysis indicated average discrepancies of less than 2mm for all field gaps and overlaps, with 83% of matched fields exhibiting discrepancies less than 1mm. Beams delivered obliquely to the phantom surface exhibited average discrepancies as high as 4mm for electron beams delivered at large oblique angles. Finally, for IMRT field delivery, vertical and lateral patient positioning shifts of 2mm were detected in some cases, indicating the potential detectability threshold of using this technique alone. Our study indicates that Cherenkov imaging can be used to support and bolster current treatment delivery verification techniques, improving our ability to recognize and rectify millimeter-scale delivery and positioning errors.

Design of a Dual-Polarized Wideband Conformal Loop Antenna for Capsule Endoscopy Systems

This paper presents a dual-polarized conformal loop antenna for capsule endoscopy systems. The antenna is designed on the capsule shell, with two orthogonal counterparts to generate dual-polarized radiation. This design exhibits broad impedance bandwidths of 271 and 247 MHz for each port, covering the entire Medical Device Radiocommunications Service band as well as the 433.05–434.79 MHz Industrial Scientific Medical band. Due to the highly orthogonal current distributions of the proposed structure, an isolation of higher than 33 dB is achieved among the operating frequency bands in measurement. The dual-polarized antenna also shows an omnidirectional radiation property and a low cross-polarization level of around −40 dB, which helps to enhance communication quality, regardless of position and orientation. Whether other electronics and biocompatible coatings are included or an electromagnetic interference shielding is considered, the antenna still presents a robust performance. The cubic tissue phantom model is utilized for the initial design and optimization of the antenna structure. Moreover, the CST voxel human model is applied to study the conformal antenna in a realistic environment. Measurements are implemented using a solid tissue-mimicking phantom, and performances consistent with theoretical predictions are achieved. Finally, the wireless biotelemetry property is evaluated via the calculation of the communication link budget with an external antenna.

Stable tissue-simulating phantoms with various water and lipid contents for diffuse optical spectroscopy

We introduced a method for producing solid phantoms with various water-to-lipid ratios that can simulate the absorption, and to some extent the scattering characteristics of human breast tissue. We also achieved phantom stability for a minimum of one month by solidifying the emulsion phantoms. The characteristics of the phantoms were evaluated using the six-wavelength time-domain diffuse optical spectroscopy (TD-DOS) system we developed to measure water and lipid contents and hemoglobin concentration. The TD-DOS measurements were validated with a magnetic resonance imaging system.

Compact ultrasound-guided diffuse optical tomography system for breast cancer imaging.

Near-infrared diffuse optical tomography (DOT) has demonstrated a great potential as an adjunct modality for differentiation of malignant and benign breast lesions and for monitoring treatment response in patients with locally advanced breast cancers. The path toward commercialization of DOT techniques depends upon the improvement of robustness and user-friendliness of this technique in hardware and software. In this study, we introduce our recently developed ultrasound-guided DOT system, which has been improved in system compactness, robustness, and user-friendliness by custom-designed electronics, automated data preprocessing, and implementation of a new two-step reconstruction algorithm. The system performance has been tested with several sets of solid and blood phantoms and the results show accuracy in reconstructed absorption coefficients as well as blood oxygen saturation. A clinical example of a breast cancer patient, who was undergoing neoadjuvant chemotherapy, is given to demonstrate the system performance.

Quality assurance of linear accelerator: a comprehensive system using electronic portal imaging device

AbstractAimThe Electronic Portal Imaging Device (EPID), primarily used for patient setup during radiotherapy sessions is also used for dosimetric measurements. In the present study, the feasibility of EPID in both machine and patient-specific quality assurance (QA) are investigated. We have developed a comprehensive software tool for effective utilisation of EPID in our institutional QA protocol.Materials and methodsPortal Vision aS1000, amorphous silicon portal detector attached to Clinac iX—Linear Accelerator (LINAC) was used to measure daily profile and output constancy, various Multi-Leaf Collimator (MLC) checks and patient plan verification. Different QA plans were generated with the help of Eclipse Treatment Planning System (TPS) and MLC shaper software. The indigenously developed MATLAB programs were used for image analysis. Flatness, symmetry, output constancy, Field Width at Half Maximum (FWHM) and fluence comparison were studied from images obtained from TPS and EPID dosimetry.ResultsThe 3 years institutional data of profile constancy and patient-specific QA measured using EPID were found within the acceptable limits. The daily output of photon beam correlated with the output obtained through solid phantom measurements. The Pearson correlation coefficients are 0.941 (p = 0.0001), 0.888 (p = 0.0188) and 0.917 (p = 0.0007) for the years of 2014, 2015 and 2016, respectively. The accuracy of MLC for shaping complex treatment fields was studied in terms of FWHM at different portions of various fields, showed good agreement between TPS-generated and EPID-measured MLC positions. The comparison of selected patient plans in EPID with an independent 2D array detector system showed statistically significant correlation between these two systems. Percentage difference between TPS computed and EPID measured fluence maps calculated for number of patients using MATLAB code also exhibited the validity of those plans for treatment.

Evaluation of a separate-receive coil by magnetic particle imaging of a solid phantom

Abstract To aid the optimization and development of Magnetic particle imaging (MPI), an accurate and translatable apparatus and a methodology for the characterisation of imaging quality are required. This requires magnetic nanoparticles (MNP) as imaging tracer for MPI measurements in a long-term stable, defined state, since MNP in liquid suspension might tend to change their magnetic properties over time, e.g. as a result of particle aggregation, sedimentation, or disintegration. To this end, we have developed solid phantoms containing freeze-dried perimag in mannitol for the comparison of resolution capabilities between different MPI scanners. Freeze drying of MNP was determined to be the optimal immobilisation method for maintaining the dynamic magnetic properties of MNP as compared with polymer or starch techniques. Freeze-drying preserves the particle distribution, preventing the particles from aggregation or precipitation. Two different types of phantoms were developed, one (cross shaped) to seek for artefacts within reconstructed images, and one (coil shaped) to test the resolution limit. The resolution limit is determined from the coil phantom by measuring the smallest resolvable gap between the two adjacent tracer regions. Our resolution phantom contained a 2 mm wide channel filled with tracer. By using this phantom, the imaging performance of the preclinical MPI system and a prototype gradiometric separate receive coil installed within the same MPI scanner was determined. We found 2.1–2.2 mm resolution limit using the conventional set-up and 0.5–0.6 mm for the separate receive coil resulting in an improvement by factor four. Additionally, the resolution dependency on the channel wide was tested by a second resolution phantom contained a 1 mm wide channel. The long-term stability of the phantoms will be continuously monitored and improved.

RapidBrachyMCTPS: a Monte Carlo-based treatment planning system for brachytherapy applications

Despite being considered the gold standard for brachytherapy dosimetry, Monte Carlo (MC) has yet to be implemented into a software for brachytherapy treatment planning. The purpose of this work is to present RapidBrachyMCTPS, a novel treatment planning system (TPS) for brachytherapy applications equipped with a graphical user interface (GUI), optimization tools and a Geant4-based MC dose calculation engine, RapidBrachyMC. Brachytherapy sources and applicators were implemented in RapidBrachyMC and made available to the user via a source and applicator library in the GUI. To benchmark RapidBrachyMC, TG-43 parameters were calculated for the microSelectron v2 (192Ir) and SelectSeed (125I) source models and were compared against previously validated MC brachytherapy codes. The performance of RapidBrachyMC was evaluated for a prostate high dose rate case. To assess the accuracy of RapidBrachyMC in a heterogeneous setup, dose distributions with a cylindrical shielded/unshielded applicator were validated against film measurements in a Solid WaterTM phantom. TG-43 parameters calculated using RapidBrachyMC generally agreed within 1%–2% of the results obtained in previously published work. For the prostate case, clinical dosimetric indices showed general agreement with Oncentra TPS within 1%. Simulation times were on the order of minutes on a single core to achieve uncertainties below 2% in voxels within the prostate. The calculation time was decreased further using the multithreading features of Geant4. In the comparison between MC-calculated and film-measured dose distributions, at least 95% of points passed the 3%/3 mm gamma index criteria in all but one case. RapidBrachyMCTPS can be used as a post-implant dosimetry toolkit, as well as for MC-based brachytherapy treatment planning. This software is especially well suited for the development of new source and applicator models.

Evaluation of the 4D RADPOS dosimetry system for dose and position quality assurance of CyberKnife.

The Synchrony respiratory motion tracking of the CyberKnife system purports to provide real-time tumor motion compensation during robotic radiosurgery. Such a complex delivery system requires thorough quality assurance. In this work, RADPOS applicability as a dose and position quality assurance tool for CyberKnife treatments is assessed quantitatively for different phantom types and breathing motions, which increase in complexity to more closely resemble clinical situations. Two radiotherapy treatment experiments were performed where dose and position were measured with the RADPOS probe housed within a Solid Water phantom. For the first experiment, a Solid Water breast phantom was irradiated using isocentric beam delivery while stationary or moving sinusoidally in the anterior/posterior direction. For the second experiment, a phantom consisting of a Solid Water tumor in lung equivalent material was irradiated using isocentric and non-isocentric beam delivery while either stationary or moving. The phantom movement was either sinusoidal or based on a real patient's breathing waveform. For each experiment, RADPOS dose measurements were compared to EBT3 GafChromic film dose measurements and the CyberKnife treatment planning system's (TPS) Monte Carlo and ray-tracing dose calculation algorithms. RADPOS position measurements were compared to measurements made by the CyberKnife system and to the predicted breathing motion models used by the Synchrony respiratory motion compensation. For the static and dynamic (i.e., sinusoidal motion) cases of the breast experiment, RADPOS, film and the TPS agreed at the 2.0% level within 1.1 σ of estimated combined uncertainties. RADPOS position measurements were in good agreement with LED and fiducial position measurements, where the average standard deviation (SD) of the differences between any two of the three position datasets was ≤0.5mm for all directions. For the 10mm peak to peak amplitude sinusoidal motion of the breast experiment, the average Synchrony correlation errors were ≤0.2mm, indicative of an accurate predictive model. For all the cases of the lung experiment, RADPOS and film measurements agreed with each other at the 2.0% level within 1.5 σ of estimated experimental uncertainties provided that the measurements were corrected for imaging dose. The measured dose for RADPOS and film were 4.0% and 3.4% higher, respectively, than the TPS for the most complex dynamic cases (i.e., irregular motion) considered for the lung experiment. Assessment of the Synchrony correlation models by RADPOS showed that model accuracy declined as motion complexity increased; the SD of the differences between RADPOS and model position data measurements was ≤0.8mm for sinusoidal motion but increased to ≤2.6mm for irregular patient waveform motion. These results agreed with the Synchrony correlation errors reported by the CyberKnife system. RADPOS is an accurate and precise QA tool for dose and position measurements for CyberKnife deliveries with respiratory motion compensation.

Quantification accuracy for dynamic contrast enhanced (DCE) CT imaging: phantom and quality assurance framework

PurposeStandardization and protocol optimization is essential for quantification of Dynamic Contrast Enhanced CT as an imaging biomarker. Currently, no commercially available quality assurance (QA) phantoms can provide for testing a complete set of imaging parameters pertaining to routine quality control for contrast-enhanced (CE) CT, as well as spatiotemporal accuracy. The purpose of this work was, therefore: (a) developing a solid calibration phantom for routine CE CT quality assurance; (b) investigating the sensitivity of CECT to organ motion, and (c) characterizing a volumetric CT scanner for CECT. MethodsCECT calibration phantom consisting of an acrylic uniform cylinder containing multiple capsules of varying diameters and orientations was designed and built. The capsules contain different solid density materials mimicking iodine contrast enhancement. Sensitivity and accuracy of CECT measurements on all capsules was performed using a 320-slice CT scanner for a range of scan parameters both with and without phantom motion along the transaxial axis of the scanner. ResultsRoutine commissioning tests such as uniformity, spatial resolution and image noise were successfully determined using the CECT phantom. Partial volume effect and motion blurring both contribute to a general decrease in contrast enhancement and this was further dependent on capsule orientation (least pronounced for the transaxial orientation). Scanning with a rotation time of less than 0.5 s, the effect of blurring is less than 3% for all orientations and phantom speeds. ConclusionA new robust contrast calibration phantom was developed and used to evaluate the performance of a 320-slice volumetric CT scanner for DCE-CT.

Hemoglobin-Laden Microcapsules for Simulating Oxygen Dynamics of Biological Tissue.

We propose one-step synthesis of hemoglobin-laden microcapsules (HbMs) by the liquid-driven coaxial flow focusing (LDCFF) method, in which purified hemoglobin (Hb) is fully encapsulated inside photocurable resin with a solid core-shell structure. The polymeric shell constitutes a rigid semipermeable membrane which prevents the leakage of Hb and allows oxygen transport. Gas-binding capacity and oxygen affinity of HbMs are comparable to the purified Hb. Solid HbMs phantoms (SHMPs) are also fabricated using resin-based HbMs homogeneously mixed with oil-soluble silicone. The spectral characteristics of SHMPs closely resemble to that of oxy-hemoglobin, and its long-term stability is more than 25 weeks. HbM dispersion with certain absorption and scattering properties circulating through the dialysis tube simulates tissue oxygen dynamics compared with the post occlusive reactive hyperemia (PORH) test on the healthy volunteer. The occlusion test indicates that HbMs dispersion solution can substitute conventional blood phantom. Our studies have shown that microencapsulation of Hb effectively protects it from the external environment, which makes HbMs promising for optical tissue-simulating phantom in biomedical fields.