Depth In Phantom Research Articles

Determination of the dose enhancement exclusively in tumor tissue due to the presence of GNPs

PurposeThe purpose of this study is to investigate the differences between obtained percentage dose enhancements in areas around nanoparticles in GNPs (gold nanoparticles) enriched medium and percentage dose enhancements in the entire GNPs enriched medium including nanoparticles region. Methods and materialsTo verify the accuracy of Ir-192 source simulation, the obtained values of air kerma strength, dose rate constants, and radial dose functions were compared against previously published results. Then a 1 cm × 1 cm× 1 cm tumor volume loaded with different diameters of GNPs were considered at a source to the tumor center of 1 cm. Finally, dose enhancements were obtained for 50, 100 and 200 nm GNPs as a function of various concentrations of the radiosensitizer and depth in phantom. ResultsCalculations showed that dose enhancement could be customized by varying the size of nanoparticles, concentrations and radial distance from the source. The highest PDEGNP (The ratio of the increased deposited dose in the tumor region to the dose deposition in the no nano gold-containing structure) was acquired by 50 nm nanoparticles, 30 mg/g concentrations and in the highest distance from the source. (PDEGNP) and (PDEaround−GNP) due to the presence of 7–30 mg/g concentration of GNPs ranged from 3−18.19% and 3.45–21.13%, respectively. The results of this study revealed that the correlation is significant at the 0.01 level and there is a non-negligible difference (up to 3%) between (Daround−GNP)and (DGNP). ConclusionBy considering exclusively determination of dose enhancement in the just tumor tissue, calculating (Daround−GNP) Instead of DGNP may be a strategy for clinical use of nanoparticles in the radiation therapy. The results showed that with the increasing trend of dose enhancement in the GNPs loaded-tumor, dose enhancement decreases with an increase in the size of GNPs.

Microstructure changes in radiochromic films due to magnetic field and radiation

To correlate the dose response and changes in microscopic structures of the radiochromic films exposed to the clinical magnetic field in the range 1.5-3 T with standard and flattening filter-free (FFF) photon beams. The radiochromic film was cut into 5 × 5 cm2 sheets/samples from one batch. These samples were exposed to a 1.5-T and/or 3-T B-fields from an MRI scanner using an abdominal sequence for 7 min before and after irradiation with 6 MV and/or 6 MV FFF beams. Films were placed in a reference condition at 5 cm depth in a solid water phantom and exposed up to 20 Gy. The sample orientation was maintained the same during exposure, readout, and scanning electron microscopic (SEM) analysis. The samples were scanned with an Epson Expression 11000XL in a 48-bit RGB color mode at 300 dpi with red channel. Scanned images were processed in Image J and red channel mean intensity values were recorded. The samples were then coated with 6 nm gold and imaged by SEM Teneo (5 kV, 13 pA) under 2000, 2500, and 3000 magnifications for texture analysis. The changes in the microstructure of the films in magnetic fields (1.5- and 3.0-T) were dose dependent. The orientation and granular size of samples at higher doses were altered compared to the controls. Needle-shaped structures of the active layer were longer and aligned for samples exposed to higher doses and magnetic field. However, no significant changes in optical density due to the presence of a magnetic field pre/postirradiation up to 20 Gy were observed. Fine structures of the film represent the polymerization characteristics that are affected by the radiation dose in the magnetic field. Upon exposure to radiation, diacetylene monomers undergo polymerization that forms longer chains with a temporal response. Even though this study did not notice any significant changes in optical density due to the presence of magnetic field, this should be studied in simultaneous application of the magnetic field during treatment in a dedicated MR-linac unit.

Monte Carlo and 60 Co-based kilovoltage x-ray dosimetry methods.

This work seeks to investigate new methods to determine the absorbed dose to water from kilovoltage x rays. Current methods are based on measurements in air and rely on correction factors in order to account for differences between the photon spectrum in air and at depth in phantom, between the photon spectra of the calibration beam and the beam of interest, or in the radiation absorption properties of air and water. This work aims to determine the absorbed dose to water in the NIST-matched x-ray beams at the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UWADCL). This will facilitate the use of detectors in terms of dose to water, which will allow for a simpler determination of dose to water in clinical kilovoltage x-ray beams. A model of the moderately filtered x-ray beams at the UWADCL was created using the BEAMnrc user code of the EGSnrc Monte Carlo code system. This model was validated against measurements and the dose to water per unit air kerma was calculated in a custom built water tank. Using this value and the highly precise measurement of the air kerma made by the UWADCL, the dose to water was determined in the water tank for the x-ray beams of interest. The dose to water was also determined using the formalism defined in the report of AAPM Task Group 61 and using a method that makes use of a Co absorbed dose-to-water calibration coefficient and a beam quality correction factor to account for differences in beam quality between the Co calibration and kilovoltage x-ray beam of interest. The dose to water values as determined by these different methods was then compared. The BEAMnrc models used in this work produced simulations of transverse and depth dose profiles that agreed with measurements with a 2%/2 mm criteria gamma test. The dose to water as determined from the different methods used here agreed within 3.5% at the surface of the water tank and agreed within 1.8% at a depth of 2 cm in phantom. The dose-to-water values all agreed within the associated uncertainties of the methods used in this work. Both the Monte Carlo-based method and the Co-based method had a lower uncertainty than the TG-61 methodology for all of the x-ray beams used in this work. Two new dose determination methods were used to determine the dose to water in the NIST-matched x-ray beams at the UWADCL and they showed good agreement with previously established techniques. Due to the improved Monte Carlo calculation techniques used in this work, both of the methods have lower uncertainties compared to TG-61. The methods presented in this work compare favorably with calorimetry-based standards established at other institutions.

Assessing the Validity of Clinician Advice That Patients Avoid Use of Topical Agents Before Daily Radiotherapy Treatments

Radiation dermatitis is common and often treated with topical therapy. Patients are typically advised to avoid topical agents for several hours before daily radiotherapy (RT) out of concern that topical agents might increase the radiation dose to the skin. With modern RT's improved skin-sparing properties, this recommendation may be irrelevant. To assess whether applying either metallic or nonmetallic topical agents before radiation treatment alters the skin dose. A 24-question online survey of patients and clinicians was conducted from January 15, 2015, to March 15, 2017, to determine current practices regarding topical therapy use. In preclinical studies, dosimetric effect of the topical agents was evaluated by delivering 200 monitor units and measuring the dose at the surface and at 2-cm depth in a tissue-equivalent phantom with or without 2 common topical agents: a petroleum-based ointment (Aquaphor, petrolatum 41%) and silver sulfadiazine cream, 1%. Skin doses associated with various photon and electron energies, topical agent thicknesses, and beam incidence were assessed. Whether topical agents altered the skin dose was also evaluated in 24 C57BL/6 mice by using phosphorylated histone (γ-H2AX) immunofluorescent staining and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay. Preclinical studies took place at the University of Pennsylvania. Patient and clinician survey responses; surface radiation dose readings in tissue-equivalent phantom; and γ-H2AX and TUNEL intensity measured in mice. The 133 patients surveyed received RT for cancer and had a median (range) age of 60 (18-86) years; 117 (87.9%) were women. One hundred eight clinicians completed the survey with 105 reporting that they were involved in managing patient skin care during RT. One hundred eleven (83.4%) of the patients and 96 (91.4%) of the 105 clinicians received or gave the advice to avoid applying topical agents before RT treatments. Dosimetric measurements showed no difference in the delivered dose at either the surface or a 2-cm depth with or without a 1- to 2-mm application of either topical agent when using en face 6- or 15-megavoltage (MV) photons. The same application of topicals did not alter the surface dose as a function of beam incident angle from 15° to 60°, except for a 6% increase at 60° with the silver sulfadiazine cream. Surface dose for 6- and 15-MV beams were significantly increased with a thicker (≥3-mm) topical application. For 6 MV, the surface dose was 1.05 Gy with a thick layer of petroleum-based ointment and 1.02 Gy for silver sulfadiazine cream vs 0.88 Gy without topical agents. For 15 MV, the doses were 0.70 Gy for a thick layer of petroleum-based ointment and 0.60 Gy for silver sulfadiazine cream vs 0.52 Gy for the controls. With 6- and 9-MeV electrons, there was a 2% to 5% increase in surface dose with the use of the topical agents. There were no dose differences at 2-cm depth. Irradiated skin in mice showed no differences in γ-H2AX-positive foci or in TUNEL staining with or without topical agents of varying thickness. Thin or moderately applied topical agents, even if applied just before RT, may have minimal influence on skin dose regardless of beam energy or beam incidence. The findings of this study suggest that applying very thick amounts of a topical agent before RT may increase the surface dose and should be avoided.

Effect of X-ray minibeam radiation therapy on clonogenic survival of glioma cells

The goal is to compare, in vitro, the efficiency of minibeam radiotherapy (MBRT) and standard RT in inducing clonogenic cell death in glioma cell lines. With this aim, we report on the first in vitro study performed in an X-ray Small Animal Radiation Research Platform (SARRP) modified for minibeam irradiations. F98 rat and U87 human glioma cells were irradiated with either an array of minibeams (MB) or with conventional homogeneous beams (broad beam, BB). A specially designed multislit collimator was used to generate the minibeams with a width of a center-to-center distance of 1465 (±10) μm, and a PVDR value of 12.4 (±2.3) measured at 1 cm depth in a water phantom. Cells were either replated for clonogenic assay directly (immediate plating, IP) or 24 h after irradiation (delayed plating, DP) to assess the effect of potentially lethal damage repair (PLDR) on cell survival. Our hypothesis is that with MBRT, a similar level of clonogenic cell death can be reached compared to standard RT, when using equal mean radiation doses. To prove this, we performed dose escalations to determine the minimum integrated dose needed to reach a similar level of clonogenic cell death for both treatments. We show that this minimum dose can vary per cell line: in F98 cells a dose of 19 Gy was needed to obtain similar levels of clonogenic survival, whereas in U87 cells there was still a slightly increased survival with MB compared to BB 19 Gy treatment.The results suggest also an impairment of DNA damage repair in F98 cells as there is no difference in clonogenic cell survival between immediately and delayed plated cells for each dose and irradiation mode. For U87 cells, a small IP-DP effect was observed in the case of BB irradiation up to a dose of 17 Gy. However, at 19 Gy BB, as well as for the complete dose range of MB irradiation, U87 cells did not show a difference in clonogenic survival between IP and DP. We therefore speculate that MBRT might influence PLDR. The current results show that X-ray MBRT is a promising method for treatment of gliomas: future preclinical and clinical studies should aim at reaching a minimum radiation (valley) dose for effective eradication of gliomas with increased sparing of normal tissues compared to standard RT.

OA249] Experimental validation of a Monte Carlo code for internal breast dosimetry in mammography

Purpose To validate Monte Carlo (MC) simulations in absolute terms at the local dose deposition level for breast dosimetry applications using new experimental methodologies. Methods Three types of dose measurement devices were used for this study; thermoluminescent dosimeters (TLD-100H), metal oxide semiconductor field-effect transistor dosimeters (MOSFET, model TN-1002RD), and GafChromicTM films (model XR-QA2). First, experimental methodologies were developed and the performance of each of the three dosimeter technologies tested using a monoenergetic synchrotron-based setup. For validation under clinical conditions, a clinical digital mammography system (Mammomat INSPIRATION, Siemens Healthcare, Forchheim, Germany) and two phantoms, a semi-cylindrical homogeneous 50% glandular density phantom, and a heterogeneous anthropomorphic breast phantom, were used. MC simulations reproducing the experimental conditions were performed using code based on the Geant4 toolkit. Experimental and simulated 2D-dose maps and dose profiles for different depths in the axial-phantom plane were compared. Results The dosimeters have an energy-dependent response (it increases by ∼ 48% for XR-QA2 films and by ∼ 37% for MOSFET in the energy interval 18–28 keV), resulting in the need for calibration at each phantom depth to be investigated to obtain accurate measurements. The developed experimental methodologies allow for dose measurements with uncertainty levels of ∼ 5% (one combined standard uncertainty) at the entrance surface, up to ∼ 9% 4 cm deep inside the phantom. This increase in uncertainty is due to the decrease in dose with increasing phantom depth, which is reduced by 93% after 4 cm of the homogeneous phantom. In a heterogeneous background, dosimeters are able to detect local dose gradients of up to 46% between the boundaries of two materials. The MC-based estimates of the dose matched, to within one combined standard uncertainty, with the experimental ones for all three dosimeters technologies and for all tested depths. Conclusions The developed experimental methodologies allow for internal breast dose estimation with good accuracy. These procedures could be successfully applied to validate MC code at the local dose level and in absolute terms. The validation in a heterogeneous, anthropomorphic breast phantom is crucial to the development of a new breast model and dosimetry method.

P179] Small field correction factors determination for the IBA razor nano chamber and the IBA razor chamber

Purpose The aim of this study was to measure small beam output factors (OFs) using the IBA Razor Nano Chamber (RNC) and the IBA Razor Chamber (RC) and to determine correction factors for comparison with Gafchromic EBT3 films. Methods The detectors were irradiated with 6 MV photon beams using a Varian TrueBeam STx system equipped with a HD multileaf collimator (MLC). Fields ranging from 0.5 × 0.5 to 10 × 10 cm2 were defined by both the X-Y collimator jaws and the MLC. The microchambers were used in parallel configuration at a source-surface distance of 100 cm and at 10 cm depth in a Sun Nuclear 3D SCANNER water phantom. Polarity correction factors kpol were evaluated for both the chambers and used to correct the raw measurements. Films were irradiated inside a Solid Water phantom in the same geometry. Results Deviations from unity of kpol were observed for field size less than 1 cm and greater than 2 cm, ranging from 0.943 to 1.025 for the RNC and from 0.970 to 1.005 for the RC. The output factors corrected for the polarity showed an underestimation for the smallest beam for both the detectors with respect to films (k = 1.033 and 1.052 for the RNC and the RC respectively). Correction factors values for beams ranging from 8 to 30 mm were close to unity. Conclusions OFs corrected for the RNC and the RC were in agreement with films, although an underestimation has been observed for the smallest beam connected to a small volume averaging effect. The similar behavior of the two ionization chambers is probably due to the same size of the electrode diameter (2 mm). Both the detectors could be used for output factors measurements in small beam geometry, albeit the magnitude of the polarity effect suggests to correct the readings carried out with the two chambers in order to achieve a better accuracy.

OA237] The output factor measurements of cyberknife system using various detectors

Purpose In Stereotactic Radiosurgery, the extremely high radiation dose is delivered to the target in a single or few fractions using narrow radiation fields. The radiation fields are collimated with 12 fixed circular collimators ranging from 5 to 60 mm in diameter in CyberKnife which is a robotic radiosurgery system. The accuracy of the output factors directly affects the accuracy of treatment delivery to the patients. The output factor (OF) measurement of small fields is very problematic and the small field measurements are more sensitive to physical properties of detectors. The aim of the present study is to perform the relative OF measurements of 12 fixed collimators in CyberKnife system using different detectors. Methods In this study, the OF measurements of 12 fixed collimators were made with CyberKnife system at maximum dose depth in water equivalent RW3 slab phantom using PTW 60019 microDiamond, PTW 31014 0.015 cc PinPoint, PTW 31010 0.125 cc Semiflex, EBT3 Gafchromic film and GR-200A TLD. All detectors were positioned at the center of the radiation field with the point laser from treatment head. The measured data were compared with Monte Carlo (MC) factors. Results The OF values measured at 30 mm and greater collimator sizes were found similar for all detectors. The OF values decrease rapidly as the collimator size becomes smaller. The measured data with TLD and microDiamond detector at the smallest collimator sizes such as 7.5, 10, 12.5 and 15 mm were found very similar with MC factors. Conclusions 0.125 cc Semiflex ion chamber may lead to large uncertainties in the OF measurements for the smallest collimator sizes. TLD and microDiamond detector show similar results for all collimator sizes except 5 mm. TLD is an utilizable dosimetric tool in the small field OF measurements by means of its small active volume.

Internal breast dosimetry in mammography: Monte Carlo validation in homogeneous and anthropomorphic breast phantoms with a clinical mammography system.

PurposeTo validate Monte Carlo (MC)‐based breast dosimetry estimations using both a homogeneous and a 3D anthropomorphic breast phantom under polyenergetic irradiation for internal breast dosimetry purposes.MethodsExperimental measurements were performed with a clinical digital mammography system (Mammomat Inspiration, Siemens Healthcare), using the x‐ray spectrum selected by the automatic exposure control and a tube current‐exposure time product of 360 mAs. A homogeneous 50% glandular breast phantom and a 3D anthropomorphic breast phantom were used to investigate the dose at different depths (range 0–4 cm with 1 cm steps) for the homogeneous case and at a depth of 2.25 cm for the anthropomorphic case. Local dose deposition was measured using thermoluminescent dosimeters (TLD), metal oxide semiconductor field‐effect transistor dosimeters (MOSFET), and GafChromic™ films. A Geant4‐based MC simulation was modified to match the clinical experimental setup. Thirty sensitive volumes (3.2 × 3.2 × 0.38 mm3) on the axial‐phantom plane were included at each depth in the simulation to characterize the internal dose variation and compare it to the experimental TLD and MOSFET measurements. The experimental 2D dose maps obtained with the GafChromic™ films were compared to the simulated 2D dose distributions.ResultsDue to the energy dependence of the dosimeters and due to x‐ray beam hardening, dosimeters based on these three technologies have to be calibrated at each depth of the phantom. As expected, the dose was found to decrease with increasing phantom depth, with the reduction being ~93% after 4 cm for the homogeneous breast phantom. The 2D dose map showed nonuniformities in the dose distribution in the axial plane of the phantom. The mean combined standard uncertainty increased with phantom depth by up to 5.3% for TLD, 6.3% for MOSFET, and 9.6% for GafChromic™ film. In the case of a heterogeneous phantom, the dosimeters are able to detect local dose gradient variations. In particular, GafChromic™ film showed local dose variations of about 46% at the boundaries of two materials.ConclusionsResults showed a good agreement between experimental measurements (with TLD and MOSFET) and MC data for both homogeneous and anthropomorphic breast phantoms. Larger discrepancies are found when comparing the GafChromic™ dose values to the MC results due to the inherent less precise nature of the former.MC validations in a heterogeneous background at the level of local dose deposition and in absolute terms play a fundamental role in the development of an accurate method to estimate radiation dose. The potential introduction of a breast dosimetry model involving a nonhomogeneous glandular/adipose tissue composition makes the validation of internal dose distributions estimates crucial.

Miniaturized microdosimeters as LET monitors: First comparison of calculated and experimental data performed at the 62 MeV/u 12C beam of INFN-LNS with four different detectors

PurposeThe aim of this paper is to investigate the limits of LET monitoring of therapeutic carbon ion beams with miniaturized microdosimetric detectors. MethodsFour different miniaturized microdosimeters have been used at the 62 MeV/u 12C beam of INFN Southern National Laboratory (LNS) of Catania for this purpose, i.e. a mini-TEPC and a GEM-microdosimeter, both filled with propane gas, and a silicon and a diamond microdosimeter. The y-D (dose-mean lineal energy) values, measured at different depths in a PMMA phantom, have been compared withLET¯D (dose-mean LET) values in water, calculated at the same water-equivalent depth with a Monte Carlo simulation setup based on the GEANT4 toolkit. ResultsIn these first measurements, no detector was found to be significantly better than the others as a LET monitor. The y-D relative standard deviation has been assessed to be 13% for all the detectors. On average, the ratio between y-D and LET¯D values is 0.9 ± 0.3, spanning from 0.73 ± 0.08 (in the proximal edge and Bragg peak region) to 1.1 ± 0.3 at the distal edge. ConclusionsAll the four microdosimeters are able to monitor the dose-mean LET with the 11% precision up to the distal edge. In the distal edge region, the ratio of y-D to LET¯D changes. Such variability is possibly due to a dependence of the detector response on depth, since the particle mean-path length inside the detectors can vary, especially in the distal edge region.

Collimator scatter factor: Monte Carlo and in-air measurements approaches

BackgroundLinac output as a function of field sizes has a phantom and a head scatter component. This last term can be measured in-air with appropriate build-up ensuring a complete electron equilibrium and the absence of the contaminant electrons. Equilibrium conditions could be achieved using a build-up cap or a mini-phantom. Monte Carlo simulations in a virtual phantom mimicking a mini-phantom were analysed with the aim of better understanding the setup conditions for measuring the collimator scatter factor that is the head scatter component of the linac output factors.MethodsBeams of 6 and 15 MV from a TrueBeam, with size from 4 × 4 to 40 × 40 cm2 were simulated in cylindrical acrylic phantoms 20 cm long, of different diameters, from 0.5 to 4 cm, with the cylinder axis coincident with the beam central axis. The PRIMO package, based on PENELOPE Monte Carlo code, was used. The phase-space files for a Varian TrueBeam linac, provided by the linac vendor, were used for the linac head simulation. Depth dose curves were analysed, and collimator scatter factors estimated at different depth in the different phantom conditions.Additionally, in-air measurements using acyrilic and brass build-up caps, as well as acrylic mini-phantom were acquired for 6 and 18 MV beams from a Varian Clinac DHX.ResultsThe depth dose curves along the cylinders were compared, showing, in each phantom, very similar curves for all analysed field sizes, proving the correctness in estimating the collimator scatter factor in the mini-phantom, provided to position the detector to a sufficient depth to exclude electron contamination. The results were confirmed by the measurements, where the acrylic build-up cap showed to be inadequate to properly estimate the collimator scatter factors, while the mini-phantom and the brass caps gave reasonable measurements.ConclusionA better understanding of the beam characteristics inside a virtual mini-phantom through the analysis of depth dose curves, showed the critical points of using the acrylic build-up cap, and suggested the use of the mini-phantom for the collimator scatter factor measurements in the medium-large field size range.

On the impact of ICRU report 90 recommendations on kQ factors for high-energy photon beams.

To assess the impact of the ICRU report 90 recommendations on the beam-quality conversion factor, kQ , used for clinical reference dosimetry of megavoltage linac photon beams. The absorbed dose to water and the absorbed dose to the air in ionization chambers representative of those typically used for linac photon reference dosimetry are calculated at the reference depth in a water phantom using Monte Carlo simulations. Depth-dose calculations in water are also performed to investigate changes in beam quality specifiers. The calculations are performed in a cobalt-60 beam and MV photon beams with nominal energy between 6MV and 25MV using the EGSnrc simulation toolkit. Inputs to the calculations use stopping-power data for graphite and water from the original ICRU-37 report and the new proposed values from the recently published ICRU-90 report. Calculated kQ factors are compared using the two different recommendations for key dosimetry data and measured kQ factors. Less than about 0.1% effects from ICRU-90 recommendations on the beam quality specifiers, the photon component of the percentage depth-dose at 10cm, %dd(10)x , and the tissue-phantom ratio at 20cm and 10cm, TPR1020, are observed. Although using different recommendations for key dosimetric data impact water-to-air stopping-power ratios and ion chamber perturbation corrections by up to 0.54% and 0.40%, respectively, we observe little difference (≤0.14%) in calculated kQ factors. This is contradictory to the predictions in ICRU-90 that suggest differences up to 0.5% in high-energy photon beams. A slightly better agreement with experimental values is obtained when using ICRU-90 recommendations. Users of the addendum to the TG-51 protocol for reference dosimetry of high-energy photon beams, which recommends Monte Carlo calculated kQ factors, can rest assured that the recommendations of ICRU report 90 on basic data have little impact on this central dosimetric parameter.

Evaluation of in-field neutron production for medical LINACs with and without flattening filter for various beam parameters - Experiment and Monte Carlo simulation

A comprehensive study on the evaluation of in-field photo-neutron and electro-neutron dose equivalent (H∗(10)), neutron fluence and energy spectra was carried out using passive detectors such as PADC and TLDs and supplemented with Monte Carlo simulation. The studies were carried out for VARIAN LINAC models NovalisTx and TrueBeam. Measurements on the influence of beam parameters such as field size, wedge angle, depth in phantom, photon and electron energies and variation of neutron dose equivalent with distance from isocentre were studied. Further, influence of beam flattening filter for 10 MV photon beam in TrueBeam was included. Simulation using FLUKA Monte Carlo code for the generation of neutron lethargy spectra and spectra weighted H*(10) at different depths of phantom was performed. Influence of different components of LINAC head on neutron spectra was also simulated for 15 MV photon beam with and without flattening filter.

The relative biological effectiveness for carbon, nitrogen, and oxygen ion beams using passive and scanning techniques evaluated with fully 3D silicon microdosimeters.

The aim of this study was to measure the microdosimetric distributions of a carbon pencil beam scanning (PBS) and passive scattering system as well as to evaluate the relative biological effectiveness (RBE) of different ions, namely 12 C, 14 N, and 16 O, using a silicon-on-insulator (SOI) microdosimeter with well-defined 3D-sensitive volumes (SV). Geant4 simulations were performed with the same experimental setup and results were compared to the experimental results for benchmarking. Two different silicon microdosimeters with rectangular parallelepiped and cylindrical shaped SVs, both 10 μm in thickness were used in this study. The microdosimeters were connected to low noise electronics which allowed for the detection of lineal energies as low as 0.15 keV/μm in tissue. The silicon microdosimeters provide extremely high spatial resolution and can be used for in-field and out-of-field measurements in both passive scattering and PBS deliveries. The response of the microdosimeters was studied in 290 MeV/u 12 C, 180 MeV/u 14 N, 400 MeV/u 16 O passive ion beams, and 290 MeV/u 12 C scanning carbon therapy beam at heavy ion medical accelerator in Chiba (HIMAC) and Gunma University Heavy Ion Medical Center (GHMC), Japan, respectively. The microdosimeters were placed at various depths in a water phantom along the central axis of the ion beam, and at the distal part of the Spread Out Bragg Peak (SOBP) in 0.5 mm increments. The RBE values of the pristine Bragg peak (BP) and SOBP were derived using the microdosimetric lineal energy spectra and the modified microdosimetric kinetic model (MKM), using MKM input parameters corresponding to human salivary gland (HSG) tumor cells. Geant4 simulations were performed in order to verify the calculated depth-dose distribution from the treatment planning system (TPS) and to compare the simulated dose-mean lineal energy to the experimental results. For a 180 MeV/u 14 N pristine BP, the dose-mean lineal energy yD¯ obtained with two types of silicon microdosimeters started from approximately 29 keV/μm at the entrance to 92 keV/μm at the BP, with a maximum value in the range of 412 to 438 keV/μm at the distal edge. For 400 MeV/u 16 O ions, the dose-mean lineal energy yD¯ started from about 24 keV/μm at the entrance to 106 keV/μm at the BP, with a maximum value of approximately 381 keV/μm at the distal edge. The maximum derived RBE10 values for 14 N and 16 O ions were found to be 3.10 ± 0.47 and 2.93 ± 0.45, respectively. Silicon microdosimetry measurements using pencilbeam scanning 12 C ions were also compared to the passive scattering beam. These SOI microdosimeters with well-defined three-dimensional (3D) SVs have applicability in characterizing heavy ion radiation fields and measuring lineal energy deposition with sub-millimeter spatial resolution. It has been shown that the dose-mean lineal energy increased significantly at the distal part of the BP and SOBP due to very high LET particles. Good agreement was observed for the experimental and simulation results obtained with silicon microdosimeters in 14 N and 16 O ion beams, confirming the potential application of SOI microdosimeter with 3D SV for quality assurance in charged particle therapy.

MR-Compatible Haptic Display of Membrane Puncture in Robot-Assisted Needle Procedures.

Multilayer electroactive polymer films actuate a small hand-held device that can display tool tip forces during MR-guided interventions. The display produces localized skin stretch at the thumb and index fingertips. Tests confirm that the device does not significantly affect MR imaging and produces detectable stimuli in response to forces measured by a biopsy needle instrumented with optical fibers. Tests with human subjects explored robotic and teleoperated paradigms to detect when the needle contacted a membrane embedded at variable depth in a tissue phantom that approximated the properties of porcine liver. In the first case, naive users detected membranes with a 98.9% success rate as the needle was driven at fixed speed. In the second case, users with experience in needle-based procedures controlled the needle insertion and detected membranes embedded in tissue phantoms with a 98% success rate. In the second experiment, some users detected membranes with very light contact forces, but there was greater subject-to-subject variation.

Source to Skin Distance (SSD) Characteristics from Varian CX Linear Accelerator

This study aims to describe the characteristics of the source to skin distance (SSD) of Varian CX linear accelerator (LINAC) using the X-ray beam of 6 MV and 10 MV. The variation of the source to the SSD are 90, 100 and 110 cms; the depth of the water phantom used are 5, 10, 15, 20, and 25 cms, respectively. The depth of the water phantom was created for analysis of percentage depth dose (PDD) and profile dose. It can be concluded from the tests that from the measured SSD, SSD of 110 cm with the depth water phantom of 20-25 cm for energy beam of 6 MV and at all levels of depth for 10 MV energy corresponding tolerance limits to be used in clinical radiotherapy. For the SSD 90 and 100, the values beam symmetry and flatness obtained slightly beyond the limits of tolerance.

Spatially fractionated (GRID) radiation therapy using proton pencil beam scanning (PBS): Feasibility study and clinical implementation.

GRID therapy is an effective treatment for bulky tumors. Linear accelerator (Linac)-produced photon beams collimated through blocks or multileaf collimators (MLCs) are the most common methods used to deliver this therapy. Utilizing the newest proton delivery method of pencil beam scanning (PBS) can further improve the efficacy of GRID therapy. In this study, we developed a method of delivering GRID therapy using proton PBS, evaluated the dosimetry of this novel technique and applied this method in two clinical cases. In the feasibility study phase, a single PBS proton beam was optimized to heterogeneously irradiate a shallow 20×20×12cm3 target volume centered at a 6cm depth in a water phantom. The beam was constrained to have an identical spot pattern in all layers, creating a "beamlet" at each spot position. Another GRID treatment using PBS was also performed on a deep 15×15×8cm3 target volume centered at a 14cm depth in a water phantom. Dosimetric parameters of both PBS dose distributions were compared with typical photon GRID dose distributions. In the next phase, four patients have been treated at our center with this proton GRID technique. The planning, dosimetry, and measurements for two representative patients are reported. For the shallow phantom target, the depth-dose curve of the PBS plan was uniform within the target (variation <5%) and dropped quickly beyond the target (50% at 12.9cm and 0.5% at 14cm). The lateral profiles of the PBS plan were comparable to those of photon GRID in terms of valley-to-peak ratios. For the deep phantom target, the PBS plan provided smaller valley-to-peak ratios than the photon GRID technique. Pretreatment dose verification QA showed close agreement between the measurements and the plan (pass rate >95% with a gamma index criterion of 3%/3mm). Patients tolerated the treatment well without significant skin toxicity (radiation dermatitis grade ≤1). Proton GRID therapy using a PBS delivery method was successfully developed and implemented clinically. Proton GRID therapy offers many advantages over photon GRID techniques. The use of protons provides a more uniform beamlet dose within the tumor and spares normal tissues located beyond the tumor. This new PBS method will also reduce the dose to proximal organs when treating a deep-seated tumor.

Internal breast dosimetry in mammography: Experimental methods and Monte Carlo validation with a monoenergetic x-ray beam.

To investigate the performance, such as energy dependence and sensitivity, of thermoluminescent dosimeters (TLD), metal oxide semiconductor field-effect transistor dosimeters (MOSFET), and GafChromic™ films, and to validate the estimates of local dose deposition of a Monte Carlo (MC) simulation for breast dosimetry applications. Experimental measurements were performed using a monoenergetic beam at the ELETTRA synchrotron radiation light source (Trieste, Italy). The three types of dosimeters were irradiated in a plane transversal to the beam axis and calibrated in terms of air kerma. The sensitivity of MOSFET dosimeters and GafChromic™ films was evaluated in the range of 18-28 keV. Three different calibration curves for the GafChromic™ films were tested (logarithmic, rational, and exponential functions) to evaluate the best-fit curve in the dose range of 1-20 mGy. Internal phantom dose measurements were performed at 20 keV for four different depths (range 0-3 cm, with 1 cm steps) using a homogeneous 50% glandular breast phantom. A GEANT4 MC simulation was modified to match the experimental setup. Thirty sensitive volumes, on the axial-phantom plane were included at each depth in the simulation to characterize the internal dose variation and compare it to the experimental TLD and MOSFET measurements. Experimental 2D dose maps were obtained with the GafChromic™ films and compared to the simulated 2D dose distributions estimated with the MC simulations. The sensitivity of the MOSFET dosimeters and GafChromic™ films increased with x-ray energy, by up to 37% and 48%, respectively. Dose-response curves for the GafChromic™ film result in an uncertainty lower than 5% above 6 mGy, when a logarithmic relationship is used in the dose range of 1-10 mGy. All experimental values fall within the experimental uncertainty and a good agreement (within 5%) is found against the MC simulation. The dose decreased with increasing phantom depth, with the reduction being ~80% after 3 cm. The uncertainty of the empirical measurements makes the experimental values compatible with a flat behavior across the phantom slab for all the investigated depths, while the MC points to a dose profile with a maximum toward the center of the phantom. The calibration procedures and the experimental methodologies proposed lead to good accuracy for internal breast dose estimation. In addition, these procedures can be successfully applied to validate MC codes for breast dosimetry at the local dose level. The agreement among the experimental and MC results not only shows the correctness of the empirical procedures used but also of the simulation parameters.

Theoretical calculation and measurement accuracy of Cerenkov optic-fiber dosimeter under electron and photon radiation therapies

This work aimed to study the mechanism and understand the influencing factors of measurement accuracy for the Cerenkov optic-fiber dosimeter under electron and photon radiation therapies. Through the Geant4 calculation, we determined responses between Cerenkov photon numbers recorded by the measurement device and the dose deposited in the fiber and found responses differed for electron and photon beam irradiations. For electron beams, the relative Cerenkov photon number recorded by the device agreed well with the relative dose for the depths after the maximum dose depth, but differed before that depth. For the photon beams, the relative Cerenkov photon number showed good agreement with the relative dose for all depths. Considering the transmission efficiency of the Cerenkov radiation in the fiber, energy spectra and angular distributions of electrons were analyzed to explain the response difference. For the photon beams, energy spectra and angular distributions of electrons changed less in various phantom depth than electron beams, therefore, better response was observed for photon beams. Besides, the influences of fiber parameters (i.e. diameter, refractive index, and material) were investigated. With the increase of the core diameter, the transmission efficiency of Cerenkov radiation changed slightly but the Cerenkov photon numbers produced in the fiber increased by square. With smaller ratio of cladding/core refractive index or using PMMA as the core materials, Cerenkov photon numbers recorded by the device could also be increased. This study could provide better understanding of the Cerenkov optic-fiber dosimeter and promote its development.

CPU time optimization and precise adjustment of the Geant4 physics parameters for a VARIAN 2100 C/D gamma radiotherapy linear accelerator simulation using GAMOS

We have verified the GAMOS/Geant4 simulation model of a 6 MV VARIAN Clinac 2100 C/D linear accelerator by the procedure of adjusting the initial beam parameters to fit the percentage depth dose and cross-profile dose experimental data at different depths in a water phantom. Thanks to the use of a wide range of field sizes, from 2 × 2 cm2 to 40 × 40 cm2, a small phantom voxel size and high statistics, fine precision in the determination of the beam parameters has been achieved. This precision has allowed us to make a thorough study of the different physics models and parameters that Geant4 offers. The three Geant4 electromagnetic physics sets of models, i.e. Standard, Livermore and Penelope, have been compared to the experiment, testing the four different models of angular bremsstrahlung distributions as well as the three available multiple-scattering models, and optimizing the most relevant Geant4 electromagnetic physics parameters. Before the fitting, a comprehensive CPU time optimization has been done, using several of the Geant4 efficiency improvement techniques plus a few more developed in GAMOS.