Polyethylene Phantom Research Articles

Breakthrough electroneutron multi-response miniature dosimetry/spectrometry in medical accelerator

Breakthrough multi-response miniature dosimetry/spectrometry of electroneutrons (EN) was made on surface and in-depths of whole-body polyethylene phantom under 10 cm × 10 cm electron beam of 20 MV Varian Clinac 2100C electron medical accelerator commonly applied for prostate treatment. While dosimetry/spectrometry of photoneutrons (PN) has been well characterized for decades, those of ENs lagged behind due to very low EN reaction cross section and lack of sensitive neutron dosimeters/spectrometers meeting neutron dosimetry requirements. Recently, Sohrabi “miniature neutron dosimeter/spectrometer” and “Stripe polycarbonate dosimeter” have broken this barrier and determined seven EN ambient dose equivalent (ENDE) (µSv.Gy–1) responses from electron beam and from albedo ENs including beam thermal (21 ± 2.63), albedo thermal (43 ± 3.70), total thermal (64 ± 6.33), total epithermal (32 ± 3.90), total fast (112.00), total thermal + epithermal (l96 ± 10), and total thermal + epithermal + fast (208 ± 10.23) ENs. Having seven ENDE responses of this study and seven PNDE responses of previous study with the same accelerator obtained at identical conditions by the same principle author provided the opportunity to compare the two sets of responses. The PNDE (µSv.Gy–1) responses have comparatively higher values and 22.60 times at isocenter which provide for the first time breakthrough ENDE responses not yet reported in any studies before worldwide.

Secondary fragments of proton and helium ion beams in High-Density Polyethylene phantom: A Monte Carlo simulation study

In hadrontherapy, secondary fragments are generated by nuclear interactions of the incident heavy ion beam with the atomic nuclei of the target. It is important to determine the yield of production and the dose contribution of these secondary fragments in order to determine the radiobiological effectiveness more accurately. This work aims to fully identify the secondary fragments generated by nuclear interactions of proton and helium (4He) ion beams in a High-Density Polyethylene (HDPE) target and to investigate the dose contributions by secondary fragments. Incident protons with energies of 55.90 MeV and 105.20 MeV and helium ions with energies of 52.55 MeV/u and 103.50 MeV/u in the HDPE phantom have been investigated by the means of Geant4 Monte Carlo (MC) simulations. Simulated results were validated using NASA Space Radiation Laboratory (NSRL) Bragg curves experimental data. The results showed that the dose contribution of secondary fragments deriving from helium ion beams is three times higher than in the case of proton beams. This is due to a higher production of nuclear fragments in the case of helium ion beams. This work contributes to a better understanding of secondary fragments generated by protons and helium ions in the HDPE target.

Deep learning-based protoacoustic signal denoising for proton range verification

Proton therapy is a type of radiation therapy that can provide better dose distribution compared to photon therapy by delivering most of the energy at the end of range, which is called the Bragg peak (BP). The protoacoustic technique was developed to determine the BP locations in vivo, but it requires a large dose delivery to the tissue to obtain a high number of signal averaging (NSA) to achieve a sufficient signal-to-noise ratio (SNR), which is not suitable for clinical use. A novel deep learning-based technique has been proposed to denoise acoustic signals and reduce BP range uncertainty with much lower doses. Three accelerometers were placed on the distal surface of a cylindrical polyethylene (PE) phantom to collect protoacoustic signals. In total, 512 raw signals were collected at each device. Device-specific stack autoencoder (SAE) denoising models were trained to denoise the noise-containing input signals, which were generated by averaging only 1, 2, 4, 8, 16, or 24 raw signals (low NSA signals), while the clean signals were obtained by averaging 192 raw signals (high NSA). Both supervised and unsupervised training strategies were employed, and the evaluation of the models was based on mean squared error (MSE), SNR, and BP range uncertainty. Overall, the supervised SAEs outperformed the unsupervised SAEs in BP range verification. For the high accuracy detector, it achieved a BP range uncertainty of 0.20 ± 3.44 mm by averaging over 8 raw signals, while for the other two low accuracy detectors, they achieved the BP uncertainty of 1.44 ± 6.45 mm and −0.23 ± 4.88 mm by averaging 16 raw signals, respectively. This deep learning-based denoising method has shown promising results in enhancing the SNR of protoacoustic measurements and improving the accuracy in BP range verification. It greatly reduces the dose and time for potential clinical applications.

CT dose estimations using the ICRU/AAPM phantom

A proposed method to obtain the dose for Computed Tomography was evaluated on long a phantom, made of polyethylene. This phantom is 600 mm large. Adopting an empirical characterized function , a tendency of the asymptotical grow of the dose by the increase of the scanning length can estimate the dose in an infinite scanning length for a single axial rotation of the x-ray tube. The curve of was estimated for different clinical protocols: tension, pitch, head/body, and center/peripheral measurements. The ultrahigh molecular weight polyethylene phantom of 600 mm length, named ICRU/AAPM phantom, was proposed by a partnership between ICRU and AAPM (Task Group 200). This length provides a higher scanner length then PMMA phantom for , whose measurement is 100 mm. The measures of ICRU/AAPM phantom were performed on General Electric, Toshiba, and Philips scanners. The measurement was performed on a thimble ion chamber inserted in the center or periphery of the phantom. Using the measurements, functions (equilibrium dose, equilibrium length and scatter fraction) were obtained. A single scan has been demonstrated to be sufficient to obtain the function.

Calorimetry in Computed Tomography Beams.

A portable calorimeter for direct realization of absorbed dose in medical computed tomography (CT) procedures was constructed and tested in a positron emission tomography (PET) CT scanner. The calorimeter consists of two small thermistors embedded in a polystyrene (PS) cylindrical "core" (1.5 cm diameter) that can be inserted into a cylindrical high-density polyethylene (HDPE) phantom (30 cm diameter). The cylindrical design of core and phantom allows coaxial alignment of the system with the scanner rotation axis, which is necessary to minimize variations in dose that would otherwise occur as the X-ray source is rotated during scanning operations. The core can be replaced by a cylindrical ionization chamber for comparing dose measurement results. Measurements using the core and a calibrated thimble ionization chamber were carried out in a beam of 6 MV X-rays from a clinical accelerator and in 120 kV X-rays from a CT scanner. Doses obtained from the calorimeter and chamber in the 6 MV beam exhibited good agreement over a range of dose rates from 0.8 Gy/min to 4 Gy/min, with negligible excess heat. For the CT beam, as anticipated for these X-ray energies, the calorimeter response was complicated by excess heat from device components. Analyses done in the frequency domain and time domain indicated that excess heat increased calorimetric temperature rise by a factor of about 15. The calorimeter's response was dominated by dose to the thermistor, which contains high-atomic-number elements. Therefore, for future construction of calorimeters for CT beams, lower-atomic-number temperature sensors will be needed. These results serve as a guide for future alternative design of calorimeters toward a calorimetry absorbed dose standard for diagnostic CT.

Whole-body photoneutron 360° angular distribution dosimetry by novel “Sohrabi neutron dosimetry methods”

PurposeWhole-body photoneutron (PN) energy-specific dosimetry on phantom surfaces and 360°angular fast PN distribution dosimetry on and around polyethylene (PE) phantom organs for prostate cancer in 10 cm × 10 cm and 20 cm × 20 cm field sizes of 18 MV X-rays of Varian Clinac 2100C linear accelerator. MethodsNovel “Sohrabi neutron dosimetry methods” including “miniature passive neutron dosimeter/spectrometer” and “strip polycarbonate neutron dosimeters” were applied. Energy-specific PN dose equivalents on surface as well as 360°fast PN dose equivalent angular distributions on and around PE phantom organs; head, neck, thorax, arms, pelvis, thighs and legs were determined. ResultsMatrix of surface (skin) energy-specific PN dose equivalents including total thermal, total epithermal, total fast, sum total thermal + epithermal and sum total thermal + epithermal + fast, and 360°angular fast PN dose equivalent responses were determined. The results indicate that data matrix of 20 cm × 20 cm field size provides higher PN dose equivalent values than those of 10 cm × 10 cm field size for surface points even remote from the central axis and for all 360°angular fast PN dose equivalent distributions on organs studied. ConclusionsMatrix of energy-specific PN dose equivalents was obtained demonstrating that the Sohrabi neutron dosimetry methods applied are unique for energy-specific PN dose equivalent studies as well as for 360°angular PN dose equivalent distribution data for PN-SPC risk estimation. The dosimetry methods can be specifically applied to many other exotic applications in health physics, medical physics, space flight dosimetry, and nuclear science and technology.

Breakthrough whole body energy-specific and tissue-specific photoneutron dosimetry by novel miniature neutron dosimeter/spectrometer

Breakthrough whole body energy-specific photoneutron (PN) dosimetry was made in/out-of-field in polyethylene phantom organ surface/depths remote from isocenter of 10 × 10 cm2 field prostate cancer therapy in 18 MV X-rays Varian Clinac 2100C medical linear accelerator for PN tissue-specific second primary cancer (PN-SPC) risk estimation. A novel miniature neutron dosimeter/spectrometer with polycarbonate/10B/cadmium inserts was invented and applied. Each dosimeter determines seven tissue-specific dose equivalent (mSv)/Gy X-ray dose at each measurement point providing seven major energy-specific responses for beam thermal, albedo thermal, total thermal, total epithermal, total fast, sum of totals (thermal + epithermal) and sum of totals (thermal + epithermal + fast) PNs dose equivalents. The neutron dosimeter is simple, efficient, and unique with high spatial resolution and provides matrix of energy-specific PN dose equivalent (mSv)/Gy X-ray dose on surface and organ depths for tissue-specific PN-SPC risk estimation. The dosimeter also performs like a “miniature neutron spectrometer” and is unique for other applications in health physics in particular individual neutron dosimetry, medical physics, space flights, science and technology.

Technical Note: A simple and fast daily quality assurance solution for modulated scanning proton and carbon ion beams.

A typical ion beam treatment facility has multiple treatment rooms and may treat with more than one ion species, thus requiring a significant quality assurance (QA) effort. The goal of this work was to perform daily QA using a single irradiation per ion species to obtain the beam dosimetry parameters of dose per monitor unit (D/MU), range, and spot position. The X-ray alignment system should also be checked and the entire procedure performed by therapists. This goal was achieved by designing a jig for the Sun Nuclear Daily QA™ 3 device and combining it with specific brass boluses, a standard QA plan, and a cuboid polyethylene phantom for positioning/repositioning tests. The design of the plan used for each ion species delivery ensured that there was no interference between the tests of the various characteristics. The 1-year monitoring results showed the proposed daily QA procedure was reliable and able to reflect each of the specified QA items of the proton and carbon ion beams. To simplify the daily analysis, the tolerances for the D/MU, beam range, and spot position (±1.5%, ±0.3mm, ±1.5mm, respectively) are checked using only the detector readings without the need for additional data processing. The proposed daily QA procedure was clinically implemented in our facility in April 2019 and has run smoothly for the first 2 years of operation. The total daily QA time for the four-room facility decreased from 1 to 1.5h to 30 to 40 min and was achieved not by reducing QA tests but rather by implementing new technology and procedures permitting acquisition of multiple beam information.

A Monte Carlo simulation study of scatter fraction and the impact of patient BMI on scatter in long axial field-of-view PET scanners

The NEMA NU-2 standard describes a protocol for measurement of scatter fraction (SF) using an axially-aligned line source, offset at 45mm from the central axis, in a cylindrical polyethylene phantom. In this work, which is an extension of our preliminary results previuosly published in the Proceedings of IEEE NSS/MIC 2018 [1], we aim to evaluate the performance of the NEMA NU-2 SF protocol in a Siemens Biograph mCT PET/CT whole-body scanner and a long axial field-of-view (LAFOV) total-body PET scanner to determine whether modifications to the NEMA NU-2 SF protocol are needed for the characterisation of scatter in such scanners. In addition, we evaluate the impact of patient body mass index (BMI) on SF in a LAFOV scanner. The Siemens Biograph mCT and a typical LAFOV PET scanner were modelled in GATE. Monte Carlo simulations were performed to validate the mCT scanner model against published experimental results. SF was estimated using a modified NEMA NU-2 protocol with variable radial offsets on both scanners and compared to ground truth SF measurements obtained with a uniform-activity cylindrical phantom. Correlation between BMI and SF in the LAFOV scanner was evaluated by simulating anthropomorphic phantoms with different BMIs and realistic 18F-FDG distributions, together with uniformly-filled 200cm long cylindrical phantoms with equivalent effective diameters. The optimal offset was found to be either 60mm or 80mm, depending on the chosen optimality metric. We conclude that modifications to NEMA NU-2 are required for accurate SF characterisation in whole-body and LAFOV scanners. Finally, SF in anthropomorphic phantoms with realistic tissue concentrations of 18F-FDG was found to be strongly correlated with SF in an equivalent-volume cylindrical phantom for the LAFOV PET scanner; BMI was also found to strongly positively correlate with the SF.

MOSFET dose measurements for proton SOBP beam.

The aim of this work was to develop a computational scheme for the correction of the LET dependence on the MOSFET response in water phantom dose measurements for a spread-out Bragg peak (SOBP) proton beam. The LET dependence of MOSFET was attributed to the stopping power ratio of SiO2 to H2O and to the fractional hole yield in the SiO2 layer. Using literature values for the stopping powers of the continuous slowing down approximation and measured fractional hole yields vs. electric field and LET, formulas were derived for the computation of a dose-weighted correction factor of a SOBP beam. Dose-weighted correction factors were computed for a clinical 190-MeV proton SOBP beam in a high-density polyethylene phantom. By applying correction factors to the SOBP beam, which consisted of weighted monoenergetic Bragg peaks, the MOSFET outputs were predicted and agreed well with the measured MOSFET responses. By applying LET dependent correction factors to MOSFET data, quality assurance of dose verification based on MOSFET measurements becomes possible for proton therapy.

Derivative NMR Spectroscopy for J-Coupled Multiplet Resonances using Short Time Signals (0.5KB) Encoded at Low Magnetic Field Strengths (1.5T). Part II: Water Unsuppressed

The topic of this work is on reliable resolving of J-coupled resonances in spectral envelopes from proton nuclear magnetic resonance (NMR) spectroscopy. These resonances appear as multiplets that none of the conventional nonderivative shape estimators can disentangle. However, the recently formulated nonconventional shape estimator, the derivative fast Padé transform (dFPT), has a chance to meet this challenge. In the preceding article with a polyethylene phantom, using the time signals encoded with water suppressed, the nonparametric dFPT was shown to be able to split apart the compound resonances that contain the known J-coupled multiplets. In the present work, we address the same proton NMR theme, but with sharply different initial conditions from encodings. The goal within the nonparametric dFPT is again to accurately resolve the J-coupled resonances with the same polyethylene phantom, but using raw time signals encoded without water suppression. The parallel work on the same problem employing two startlingly unequal time signals, encoded with and without water suppression in the preceding and the current articles, respectively, can offer an answer to a question of utmost practical significance. How much does water suppression during encoding time signals actually perturb the resonances near and farther away from the dominant water peak? This is why it is important to apply the same dFPT estimator to the time signals encoded without water suppression to complement the findings with water suppression. A notable practical side of this inquiry is in challenging the common wisdom, which invariably takes for granted that it is absolutely necessary to subtract water from the encoded time signals in order to extract meaningful information by way of NMR spectroscopy.

Simulation of absorbed doses distribution in a polyethylene phantom for BNCT application at the Dalat research reactor

In this paper, the distribution of absorbed dose components in a polyethylene phantom for BNCT application at Dalat Nuclear Research Reactor (DNRR) were calculated using the MCNP5 code. The configuration of horizontal neutron channel No.2 of the DNRR, which contains a cylindrical collimator with neutron filters of 20-cm Si and 3-cm Bi, was simulated. The results show that the gamma dose along the central axis of the phantom has the maximum value of 1.82×10-6 Gy at the 0.5-cm depth, and reduces to 9.05×10-7 Gy at the 3-cm depth. The main contribution to gamma dose is due to the interaction of thermal neutron with hydrogen in the phantom via the 1H(n,γ)2H reaction, and its value is much smaller than thermal neutron dose. The total absorbed dose along the central axis of the phantom has the maximum value of 7.87×10-5 Gy at the 0.5-cm depth, and decreases rapidly to 1.52×10-5 Gy at the 3-cm position, and mainly depends on the boron and thermal neutron doses caused by the 10B(n, α)7Li and 14N(n, p)14C reactions, respectively.

Computational model for detector timing effects in Compton-camera based prompt-gamma imaging for proton radiotherapy

This paper describes a realistic simulation of a Compton-camera (CC) based prompt-gamma (PG) imaging system for proton range verification for a range of clinical dose rates, and its comparison to PG measured data with a pre-clinical CC. We used a Monte Carlo plus Detector Effects (MCDE) model to simulate the production of prompt gamma-rays (PG) and their energy depositions in the CC. With Monte Carlo, we simulated PG emission resulting from irradiation of a high density polyethylene phantom with a 150 MeV proton pencil beam at dose rates of 5.0 × 108, 2.6 × 109, and 4.6 × 109 p+ s−1. Realistic detector timing effects (e.g. delayed triggering time, event-coincidence, dead time, etc,) were added in post-processing to allow for flexible count rate variations. We acquired PG emission measurements with our pre-clinical CC during irradiation with a clinical 150 MeV proton pencil beam at the same dose rates. For simulations and measurements, three primary changes could be seen in the PG emission data as the dose rate increased: (1) reduction in the total number of detected events due to increased dead-time percentage; (2) increase in false-coincidence events (i.e. multiple PGs interacting, rather than a single PG scatter); and (3) loss of distinct PG emission peaks in the energy spectrum. We used the MCDE model to estimate the quality of our measured PG data, primarily with regards to true and false double-scatters and triple-scatters recorded by the CC. The simulation results showed that of the recorded double-scatter PG interactions 22%, 57%, and 70% were false double-scatters and for triple-scatter interactions 3%, 21%, and 35% were false events at 5.0 × 108, 2.6 × 109, and 4.6 × 109 p+ s−1, respectively. These false scatter events represent noise in the data, and the high percentage of these events in the data represents a major limitation in our ability to produce usable PG images with our prototype CC.

Novel air-to-tissue conversion factors for fast, epithermal and thermal photoneutrons in a Siemens ONCOR dual energy 18 MV X-ray medical linear accelerator

A novel “photoneutron air-to-tissue conversion factor” determination method was developed with provision of extensive data matrix for converting photoneutron dose equivalent at a position in air to that of a similar position in a phantom. The conversion factors were determined along the central axis as well as on surface and at depths of a multi-layer polyethylene phantom in a 10 cm × 10 cm field of an 18 MV X-ray Siemens ONCOR medical accelerator. Polycarbonate/10B neutron dosimeters (with/without cadmium covers) were applied. Fast-photoneutron-induced recoil tracks and thermal/epithermal-photoneutron-induced alpha tracks from 10B (nth,α)7Li reactions in polycarbonate were processed by ECE method, counted and converted to photoneutron dose equivalents. Fast, epithermal and thermal photoneutron air-to-tissue conversion factors along central axis as well as at different positions from the central axis on the surface and at 12 phantom depths were determined. Air-to-tissue conversion factors for fast, epithermal and thermal photoneutrons along the central axis follow the trends of depth dose equivalent responses. The air-to-tissue conversion factor responses determined at different depths on transverse axis are flat. Air-to-tissue conversion factors determined at the surface of the phantom are 1.89 ± 0.24 for sum of fast, epithermal and thermal; 3.52 ± 0.25 for sum of epithermal and thermal; 1.57 ± 0.13 for fast; 4.40 ± 0.33 for epithermal; and 3.35 ± 0.18 for thermal photoneutrons. In conclusion, novel “photoneutron air-to-tissue conversion factors” versus distance from the central axis were determined for the first time in data profile groups at 12 depths from the phantom surface. These factors are highly instrumental for converting a photoneutron dose equivalent value in air to that of tissue at the same position.

Calibration and test of a 6LiF-diamond detector for the HCPB mock-up experiment at JET

One relevant experiment selected for the DT phase at JET is the irradiation of the HCPB-TBM mock-up of ITER which will be located in front of the horizontal port of Octant-8. The neutronics performances of the HCPB-TBM will be studied through independent measurement techniques and the experimental quantities compared to the results of Monte Carlo simulation. Of paramount importance is the measurement of the tritium production rate (TPR) to be performed by using a single crystal diamond detector covered with a thin layer of 6LiF (LiDia detector). This requires the knowledge of the 6Li mass. In this work the 6Li mass calibration of the selected LiDia detector was first performed in a well characterized thermal neutron flux spectrum. Furthermore, a performance test of the detector was carried out by locating it inside a polyethylene phantom irradiated with 14 MeV neutrons at the Frascati Neutron generator. The experimental set-up was accurately simulated by using the MCNP5 code and the TPR compared to the measured one. The goal was to point out and assess the issues with the measurement method.

Synchrotron radiation external beam rotational radiotherapy of breast cancer: proof of principle.

The principle of rotational summation of the absorbed dose for breast cancer treatment with orthovoltage X-ray beams was proposed by J. Boone in 2012. Here, use of X-ray synchrotron radiation for image guided external beam rotational radiotherapy treatment of breast cancer is proposed. Tumor irradiation occurs with the patient in the prone position hosted on a rotating bed, with her breast hanging from a hole in the bed, which rotates around a vertical axis passing through the tumor site. Horizontal collimation of the X-ray beam provides for whole breast or partial breast irradiation, while vertical translation of the bed and successive rotations allow for irradiation of the full tumor volume, with dose rates which permit also hypofractionated treatments. In this work, which follows a previous preliminary report, results are shown of a full series of measurements on polyethylene and acrylic cylindrical phantoms carried out at the Australian Synchrotron, confirmed by Geant4 Monte Carlo simulations, intended to demonstrate the proof of principle of the technique. Dose measurements were carried out with calibrated ion chambers, radiochromic films and thermoluminescence dosimeters. The photon energy investigated was 60 keV. Image guidance may occur with the transmitted beam for contrast-enhanced breast computed tomography. For a horizontal beam collimation of 1.5 cm and rotation around the central axis of a 14 cm-diameter polyethylene phantom, a periphery-to-center dose ratio of 14% was measured. The simulations showed that under the same conditions the dose ratio decreases with increasing photon energy down to 10% at 175 keV. These values are comparable with those achievable with conventional megavoltage radiotherapy of breast cancer with a medical linear accelerator. Dose painting was demonstrated with two off-center `cancer foci' with 1.3 Gy and 0.6 Gy target doses. The use of a radiosensitizing agent for dose enhancement isforeseen.

Subpixel x-ray imaging with an energy-resolving detector.

The detector pixel size can be a severe limitation in x-ray imaging of fine details in the human body. We demonstrate a method of using spectral x-ray measurements to image the spatial distribution of the linear attenuation coefficient on a length scale smaller than one pixel, based on the fact that interfaces parallel to the x-ray beam have a unique spectral response, which distinguishes them from homogeneous materials. We evaluate the method in a simulation study by simulating projection imaging of the border of an iodine insert with [Formula: see text] in a soft tissue phantom. The results show that the projected iodine profile can be recovered with an RMS resolution of 5% to 34% of the pixel size, using an ideal energy-resolving detector. We also validate this method in an experimental study by imaging an iodine insert in a polyethylene phantom using a photon-counting silicon-strip detector. The results show that abrupt and gradual transitions can be distinguished based on the transmitted x-ray spectrum, in good agreement with simulations. The demonstrated method may potentially be used for improving visualization of blood vessel boundaries, e.g., in acute stroke care.

Abstract ID: 198 Monte Carlo simulation and simple model of dose distribution in synchrotron radiation rotational radiotherapy of breast cancer: An experimental phantom study

External beam rotational radiotherapy of breast cancer with kilovoltage photons (kV-EBRT), with the patient in prone position, has been proposed [1] as a possible alternative to conventional radiotherapy with two tangential 6-MV X-ray beams produced by a medical linac with a supine patient. kV-EBRT uses a dedicated setup initially developed for breast CT, with a 320-kVp beam from an orthovoltage X-ray tube rotating in full circles around the breast; CT for tumour localization can be performed by using the same setup. Though dose build-up is not present, rotational summation of dose delivery allows a skin sparing comparable to conventional radiotherapy. We proposed the use of a monoenergetic synchrotron radiation (SR) collimated beam for image-guided rotational radiotherapy for the pendant breast (SR3T) [2] . The high flux of SR permits dose rates even greater to that of conventional radiotherapy, while the optimal photon energy can be selected for the treatment. In this work, we present Monte Carlo simulation and a simple model for evaluation of the 3D dose distribution in SR3T. The MC code was based on GEANT4 toolkit ver.10.00. We validated these simulations via measurements on breast phantoms performed at the Imaging and Medical Beamline of the Australian Synchrotron. We measured the dose distribution in cylindrical PMMA and polyethylene phantoms at 60 keV, using an ionization chamber, thermoluminescent dosimeters and radiochromic films. This study indicated that for a tumour localized at the centre of a mid-sized breast (14-cm diameter at chest wall), a ”skin-to-tumour” dose ratio of 14% can be reached at 60 keV with a collimated SR beam. Non uniform dose distribution to the target (dose painting) can be performed with multiple rotations around the organ. Kilovoltage SR3T with suitable radiosensitizers may exploit the dose enhancement factor due to increased photoelectric absorption, at SR beam energies in the 60–110 keV range.

Towards breast cancer rotational radiotherapy with synchrotron radiation

PurposeWe performed the first investigations, via measurements and Monte Carlo simulations on phantoms, of the feasibility of a new technique for synchrotron radiation rotational radiotherapy for breast cancer (SR3T). MethodsA Monte Carlo (MC) code based on Geant4 toolkit was developed in order to simulate the irradiation with the SR3T technique and to evaluate the skin sparing effect in terms of centre-to-periphery dose ratio at different energies in the range 60–175keV. Preliminary measurements were performed at the Australian Synchrotron facility. Radial dose profiles in a 14-cm diameter polyethylene phantom were measured with a 100-mm pencil ionization chamber for different beam sizes and compared with the results of MC simulations. Finally, the dose painting feasibility was demonstrated with measurements with EBT3 radiochromic films in a phantom and collimating the SR beam at 1.5cm in the horizontal direction. ResultsMC simulations showed that the SR3T technique assures a tumour-to-skin absorbed dose ratio from about 7:1 (at 60keV photon energy) to about 10:1 (at 175keV), sufficient for skin sparing during radiotherapy. The comparison between the results of MC simulations and measurements showed an agreement within 5%. Two off-centre foci were irradiated shifting the rotation centre in the horizontal direction. ConclusionsThe SR3T technique permits to obtain different dose distributions in the target with multiple rotations and can be guided via synchrotron radiation breast computed tomography imaging, in propagation based phase-contrast conditions. Use of contrast agents like iodinated solutions or gold nanoparticles for dose enhancement (DE-SR3T) is foreseen and will be investigated in future work.

Fast, epithermal and thermal photoneutron dosimetry in air and in tissue equivalent phantom for a high-energy X-ray medical accelerator

Photoneutron (PN) dosimetry in fast, epithermal and thermal energy ranges originated from the beam and albedo neutrons in high-energy X-ray medical accelerators is highly important from scientific, technical, radiation protection and medical physics points of view. Detailed dose equivalents in the fast, epithermal and thermal PN energy ranges in air up to 2m as well as at 35 positions from the central axis of 12 cross sections of the phantom at different depths were determined in 18MV X-ray beams of a Siemens ONCOR accelerator. A novel dosimetry method based on polycarbonate track dosimeters (PCTD)/10B (with/without cadmium cover) was used to determine and separate different PN dose equivalents in air and in a multilayer polyethylene phantom. Dose equivalent distributions of PNs, as originated from the main beam and/or albedo PNs, on cross-plane, in-plane and diagonal axes in 10cm×10cm fields are reported. PN dose equivalent distributions on the 3 axes have their maxima at the isocenter. Epithermal and thermal PN depth dose equivalent distributions in the phantom for different positions studied peak at ∼3cm depth. The neutron dosimeters used for the first time in such studies are highly effective for separating dose equivalents of PNs in the studied energy ranges (beam and/or albedo). The PN dose equivalent data matrix made available in this paper is highly essential for detailed patient dosimetry in general and for estimating secondary cancer risks in particular.