Dose Buildup Research Articles

Refined formulation of the Taylor equation for calculating build-up factors of gamma rays in aluminium and anticipated radiation effects

Measuring the build-up of gamma rays within materials used as shields is of great importance to accurately assess radiation doses and design effective protective barriers. Taylor's formula was studied to calculate gamma-ray build-up factors, whose application is limited to a specific group of energies and a large number of parameters. The aim of this study is to improve Taylor's formula by developing mathematical formulas to calculate the parameters at each energy to include a wider range of energy and a small, fixed number of parameters. The results demonstrated that the difference between the gamma radiation dose build-up factors calculated according to the two empirical formulas (the previous traditional formula and the current improved formula) remains very small, and the values are more or less identical. In general, it is observed that the accumulation factor decreases with increasing gamma-ray energy and increases with increasing shield thickness. Researchers also explored the intricate impacts of gamma rays on aluminum shields and the subsequent reactions within the material. A number of potential interactions have been found in one of the world's leading laboratories. One of these reactions is selected in this study. It was found that increasing the gamma-ray build-up factor in aluminum is accompanied by an increase in its cross sections. This, in turn, indicates an increased probability of secondary reactions and subsequent gamma-ray emission.

Spectral composition of secondary electrons based on the Kiefer-Straaten ion track structure model.

The major part of energy deposition of ionizing radiation is caused by secondary electrons, independent of the primary radiation type. However, their spatial concentration and their spectral properties strongly depend on the primary radiation type and finally determine the pattern of molecular damage e.g. to biological targets as the DNA, and thus the final effect of the radiation exposure. To describe the physical and to predict the biological consequences of charged ion irradiation, amorphous track structure approaches have proven to be pragmatic and helpful. There, the local dose deposition in the ion track is equated by considering the emission and slowing down of the secondary electrons from the primary particle track. In the present work we exploit the model of Kiefer and Straaten and derive the spectral composition of secondary electrons as function of the distance to the track center. The spectral composition indicates differences to spectra of low linear energy transfer (LET) photon radiation, which we confirm by a comparison with Monte Carlo studies. We demonstrate that the amorphous track structure approach provides a simple tool for evaluating the spectral electron properties within the track structure. Predictions of the LET of electrons across the track structure as well as the electronic dose build-up effect are derived. Implications for biological effects and corresponding predicting models based on amorphous track structure are discussed.

New Deformity Outline on the Breast Radiation Therapy for diminishing Absorbed Dose Ratio

Breast cancer is one of the most common malignant diseases in women. After tumor mass surgery, radiation therapy is regularly taken into account the gold standard for the treatment. Kilo to Mega voltage photons have been suggested due to their characteristic depth dose build-up regime, reducing the dose to the breast skin to a fraction of the maximum dose exposure. During treatment, mean glandular dose is commonly used as a criterion for identifying radiation risks. Here, two outlines in cubic-rectangular (CR) and cylindrical- taper (CT) outlines were modelled together with corresponding assumptions using Monte-Carlo simulation and the recorded absorbed dose ratio (ADR) values were compared via defined a positron source by 511 keV energy. The results showed that the amounts of absorption and scattering cross-sections next to the ADR amount decreased as the height of the CR outline decreased. The average dose ratio amount in the CR outline was reduced by about 96% compared to that in the CT outline. By increasing the positron source distance from the nipple, the ADR amounts decreased for both outlines. The amount of accumulated dose ratio decreased harshly in the CR outline rather than in the CT outline. This study can be useful to examine breast tissue deformity in treatment planning.

Efficacy of epinephrine rinse in allergen immunotherapy with aeroallergen and venom

Introduction In a suburban allergy outpatient office, the practice of epinephrine rinse of subcutaneous allergen immunotherapy (SCIT) syringes prior to injection has been employed for over 30 years with documented favorable clinical outcomes. A retrospective study was conducted to explore the effectiveness of epinephrine rinsing of SCIT syringes in managing local site reactions (LSR) in patients with allergen hypersensitivity. Methods Chart review of patients currently receiving SCITs was conducted using the clinical database at the outpatient office. 549 SCITs were reviewed, and 140 cases were traced to employ the use of epinephrine rinse to improve LSRs. The data was separated into allergic rhinitis and venom immunotherapy groups and was recorded for both build-up and maintenance phases. Results Out of 140 total patients requiring epinephrine rinse, 88.6% were in the allergic rhinitis group and 11.4% were in the venom group. In the allergic rhinitis group, 69.3% of patients and 87.5% in the venom group required epinephrine rinse during the build-up phase. In the allergic rhinitis group, 65.7% patients receiving SCIT at maintenance dose and 69.8% patients at build-up dose had resolution of LSR post-epinephrine rinse. In the venom group, 64.2% patients receiving SCITs at build-up dose improved LSR after epinephrine rinse. Conclusion Epinephrine rinse technique prior to injection showed favorable outcomes in over 60% of patients currently receiving treatment for allergen hypersensitivity. Epinephrine rinse is beneficial during the build-up phase of SCIT to manage LSR to effectively increase allergen dose.

Surface Dose and Skin Toxicity in Low-Magnetic Field MR-Linac

<h3>Purpose/Objective(s)</h3> Radiation induced skin toxicity is a common side effect that results in poor patient satisfaction and unplanned treatment breaks thus prolonging the overall treatment time with poor outcome. It is hypothesized that treatments on low-field MR-Linac will not produce higher surface dose which is investigated in this study. <h3>Materials/Methods</h3> Skin dose has always been an area of active clinical interest that is dependent on many parameters including beam energy, field size, material thickness and atomic number in the beam, material distance from surface, source to surface distance, beam angle (obliquity factor), concomitant chemotherapy etc. Along with these parameters, magnetic field is also a critical parameter that affect surface dose. With the development of the MRL system focus has shifted towards Lawrence force giving rise to electron return effect along with other treatment parameters affecting the skin dose. A Linac equipped with 6MV FFF photons with 90 cm source to axis distance is used. A Markus parallel plate ionization chamber is used in a solid water phantom with proper correction factor to investigate surface and buildup dose with six different gantry angles 15⁰ apart gantry (equivalent of standard Linac 0⁰, 15⁰, 30⁰, 45⁰, 60⁰, and 75⁰) for field sizes of 1.66 × 1.66 cm² (small), 9.96 × 9.9 cm² (reference) and 24.1 × 24.1 cm² (maximum). Measurements were made with increasing buildup thickness in steps of 1 mm to 20 mm of solid water in 0.35 T magnetic field. <h3>Results</h3> In traditional linear accelerators surface dose is 30-40% for a 6MV photon beam, without any modifier. This value increases significantly with beam angle. In MR-Linac, surface dose is relatively low, 12.6%, 14.9% and 16.7% for small, reference and maximum fields at zero-degree gantry angle, respectively. These values are 12% to 2% higher (at 0⁰ and 75⁰ respectively) for maximum field size compared to the reference field size. For lower field sizes minimum variation of 10% was observed at 75° for reference field size. The surface dose increases with increasing radiation field size but magnitude is still lower as the case with traditional lilacs. Table 1 shows the surface dose with field size and gantry angles. <h3>Conclusion</h3> Surface dose in low-field magnetic environment is relatively lower than traditional accelerators. It is concluded that low-field MR-Linac system may not add any additional skin toxicity compared to conventional Linac. However, the effect may be different for high-field MR-Linac system, and appropriate skin evaluation needs to be performed to differentiate between radiation induced toxicity specially for the oblique beams versus RF-thermal effect.

Assessment of Surface and Build-up Doses for a 6 MV Photon Beam using Parallel Plate Chamber, EBT3 Gafchromic Films, and PRIMO Monte Carlo Simulation Code.

Background: Accurate assessment of surface and build-up doses has a key role in radiotherapy, especially for the superficial lesions with uncertainties involved while performing measurements in the build-up region.Objective: This study aimed to assess surface and build-up doses for 6 MV photon beam from linear accelerator using parallel plate ionization chamber, EBT3 Gafchromic films, and PRIMO Monte Carlo (MC) simulation code.Material and Methods: In this experimental study, parallel plate chamber (PPC05) and EBT3 Gafchromic films were used to measure doses in a build-up region for 6 MV beam from the linear accelerator for different field sizes at various depths ranging from 0 to 2 cm from the surface with 100 cm source to surface distance (SSD) in a solid water phantom. Measured results were compared with Monte Carlo simulated results using PENELOPE-based PRIMO simulation code for the same setup conditions. Effect of gantry angle incidence and SSD were also analyzed for depth doses at the surface and build-up regions using PPC05 ion chamber and EBT3 Gafchromic films.Results: Doses measured at the surface were 14.78%, 19.87%, 25.83%, and 31.54% for field sizes of 5×5, 10×10, 15×15, and 20×20 cm2, respectively for a 6 MV photon beam with a parallel plate chamber and 14.20%, 19.14%, 25.149%, and 30.90%, respectively for EBT3 Gafchromic films. Both measurement sets were in good agreement with corresponding simulated results from the PRIMO MC simulation code; doses increase with the increase in field sizes.Conclusion: Good agreement was observed between the measured depth doses using parallel plate ionization chamber, EBT3 Gafchromic films, and the simulated depth doses using PRIMO Monte Carlo simulation code.

Novel multi jet fusion 3D-printed patient immobilization for radiation therapy.

Thermoplastic immobilizers are used routinely in radiation therapy to achieve positioning accuracy. These devices are variable in quality as they are dependent on the skill of the human fabricator. We examine the potential multi jet fusion (MJF) 3D printing for the production immobilizers with a focus on the surface dosimetry of several MJF-printed PA12-based material candidates. Materials are compared with the goal of minimizing surface dose with comparison to standard thermoplastic. We introduce a novel metamaterial design for the shell of the immobilizer, with the aims of mechanical robustness and low-dose buildup. We demonstrate first examples of adult and pediatric cranial and head-and-neck immobilizers. Three different PA12 materials were examined and compared to fused deposition modeling-printed polylactic acid (PLA), PLA with density lowered by adding hollow glass microspheres, and to perforated or perforated/stretched and solid status quo thermoplastic samples. Build-up dose measurements were made using a parallel plate chamber. A metamaterial design was established based on a packed hexagonal geometry. Radiochromic film dosimetry was performed to determine the dependence of surface dose on the metamaterial design. Full cranial and head-and-neck prototype immobilizers were designed, printed, and assessed with regard to dimensional accuracy. Build-up dose measurements demonstrated the superiority of the PA12 material with a light fusing agent, which yielded a ∼15% dose reduction compared to other MJF materials. Metamaterial samples provided dose reductions ranging from 11% to 40% compared to stretched thermoplastic. MJF-printed immobilizers were produced reliably, demonstrated the versatility of digital design, and showed dimensional accuracy with 97% of sampled points within ±2mm. MJF is a promising technology for an automated fabrication of patient immobilizers. Material selection and metamaterial design can be leveraged to yield surface dose reduction of up to 40%. Immobilizer design is highly customizable, and the first examples of MJF-printed immobilizers demonstrate excellent dimensional accuracy.

Dosimetric Evaluation of Commercially Available Flat vs. Self-Produced 3D-Conformal Silicone Boluses for the Head and Neck Region.

BackgroundBoluses are routinely used in radiotherapy to modify surface doses. Nevertheless, considerable dose discrepancies may occur in some cases due to fit inaccuracy of commercially available standard flat boluses. Moreover, due to the simple geometric design of conventional boluses, also surrounding healthy skin areas may be unintentionally covered, resulting in the unwanted dose buildup. With the fused deposition modeling (FDM) technique, there is a simple and possibly cost-effective way to solve these problems in routine clinical practice. This paper presents a procedure of self-manufacturing bespoke patient-specific silicone boluses and the evaluation of buildup and fit accuracy in comparison to standard rectangular commercially available silicone boluses.Methods3D-conformal silicone boluses were custom-built to cover the surgical scar region of 25 patients who received adjuvant radiotherapy of head and neck cancer at the University Hospital Würzburg. During a standard CT-based planning procedure, a 5-mm-thick 3D bolus contour was generated to cover the radiopaque marked surgical scar with an additional safety margin. From these digital contours, molds were 3D printed and poured with silicone. Dose measurements for both types of boluses were performed with radiochromic films (EBT3) at three points per patient—at least one aimed to be in the high-dose area (scar) and one in the lower-dose area (spared healthy skin). Surface–bolus distance, which ideally should not be present, was determined from cone-beam CT performed for positioning control. The dosimetric influence of surface–bolus distance was also determined on slab phantom for different field sizes. The trial was performed with hardware that may be routinely available in every radiotherapy department, with the exception of the 3D printer. The required number of patients was determined based on the results of preparatory measurements with the help of the statistical consultancy of the University of Würzburg. The number of measuring points represents the total number of patients.ResultsIn the high-dose area of the scar, there was a significantly better intended dose buildup of 2.45% (95%CI 0.0014–0.0477, p = 0.038, N = 30) in favor of a 3D-conformal bolus. Median distances between the body surface and bolus differed significantly between 3D-conformal and commercially available boluses (3.5 vs. 7.9 mm, p = 0.001). The surface dose at the slab phantom did not differ between commercially available and 3D-conformal boluses. Increasing the surface–bolus distance from 5 to 10 mm decreased the surface dose by approximately 2% and 11% in the 6 × 6- and 3 × 3-cm2 fields, respectively. In comparison to the commercially available bolus, an unintended dose buildup in the healthy skin areas was reduced by 25.9% (95%CI 19.5–32.3, p < 0.01, N = 37) using the 3D-conformal bolus limited to the region surrounding the surgical scar.ConclusionsUsing 3D-conformal boluses allows a comparison to the commercially available boluses’ dose buildup in the covered areas. Smaller field size is prone to a larger surface–bolus distance effect. Higher conformity of 3D-conformal boluses reduces this effect. This may be especially relevant for volumetric modulated arc therapy (VMAT) and intensity-modulated radiotherapy (IMRT) techniques with a huge number of smaller fields. High conformity of 3D-conformal boluses reduces an unintended dose buildup in healthy skin. The limiting factor in the conformity of 3D-conformal boluses in our setting was the immobilization mask, which was produced primarily for the 3D boluses. The mask itself limited tight contact of subsequently produced 3D-conformal boluses to the mask-covered body areas. In this respect, bolus adjustment before mask fabrication will be done in the future setting.

Effect of block tray on Build-up dose in γ- ray radiotherapy

Effect of block tray on Build-up dose in γ- ray radiotherapy

Dosimetry and radioprotection evaluations of very high energy electron beams

Very high energy electrons (VHEEs) represent a promising alternative for the treatment of deep-seated tumors over conventional radiotherapy (RT), owing to their favourable dosimetric characteristics. Given the high energy of the electrons, one of the concerns has been the production of photoneutrons. In this article we explore the consequence, in terms of neutron yield in a water phantom, of using a typical electron applicator in conjunction with a 2 GeV and 200 MeV VHEE beam. Additionally, we evaluate the resulting ambient neutron dose equivalent at various locations between the phantom and a concrete wall. Through Monte Carlo (MC) simulations it was found that an applicator acts to reduce the depth of the dose build-up region, giving rise to lower exit doses but higher entrance doses. Furthermore, neutrons are injected into the entrance region of the phantom. The highest dose equivalent found was approximately 1.7 mSv/Gy in the vicinity of the concrete wall. Nevertheless, we concluded that configurations of VHEEs studied in this article are similar to conventional proton therapy treatments in terms of their neutron yield and ambient dose equivalent. Therefore, a clinical implementation of VHEEs would likely not warrant additional radioprotection safeguards compared to conventional RT treatments.

Detailed tooth models for ICRP mesh-type reference computational phantoms

For use in electron paramagnetic resonance dosimetry with tooth enamel, in the present study, very detailed mesh-type tooth models composed of 198 individual tooth models (i.e. newborn: 20; 1 year: 28; 5 years: 48; 10 years: 38; 15 years: 32; and adult: 32) were developed for each sex. The developed tooth models were then implanted in the International Commission on Radiological Protection pediatric and adult mesh-type reference computational phantoms and used to calculate tooth enamel doses, by Monte Carlo simulations with Geant4, for external photon exposures in several idealized irradiation geometries. The calculated dose values were then compared to investigate the dependency of the enamel dose on the age and sex of the phantom and the sites of the teeth. The results of the present study generally show that, if the photon energy is low (i.e. <0.1 MeV), the enamel dose is significantly affected by the age and sex of the phantom and also the sites of the teeth used for dose calculation; the differences are frequently greater than a few times or even orders of magnitude. However, with a few exceptions, the enamel dose was hardly affected by these parameters for energies between 0.1 and 3 MeV. For energies >3 MeV, moderate differences were observed (i.e., up to a factor of two), due to the existence of dose build-up in the head of the phantom for high-energy photons. The calculated dose values were also compared with those of the previous studies where voxel and mathematical models were used to calculate the enamel doses. The results again show significant differences at low energies, e.g., up to ∼3500 times at 0.015 MeV, which are mainly due to the differences in the level of tooth-modeling detailedness.

Development of Patient Specific Conformal 3D-Printed Devices for Dose Verification in Radiotherapy

In radiation therapy, a bolus is used to improve dose distribution in superficial tumors; however, commercial boluses lack conformity to patient surface leading to the formation of an air gap between the bolus and patient surface and suboptimal tumor control. The aim of this study was to explore 3D-printing technology for the development of patient-specific conformal 3D-printed devices, which can be used for the radiation treatment of superficial head and neck cancer (HNC). Two 3D boluses (0.5 and 1.0 cm thick) for surface dose build-up and patient-specific 3D phantom were printed based on reconstruction of computed tomography (CT) images of a patient with HNC. The 3D-printed patient-specific phantom indicated good tissue equivalency (HU = −32) and geometric accuracy (DSC = 0.957). Both boluses indicated high conformity to the irregular skin surface with minimal air gaps (0.4–2.1 mm for 0.5 cm bolus and 0.6–2.2 mm for 1.0 cm bolus). The performed dose assessment showed that boluses of both thicknesses have comparable effectiveness, increasing the dose that covers 99% of the target volume by 52% and 26% for single field and intensity modulated fields, respectively, when compared with no bolus case. The performed investigation showed the potential of 3D printing in development of cost effective, patient specific and patient friendly conformal devices for dose verification in radiotherapy.

Transmission study of the Abdominal Compression plate (BodyFIX Diaphragm Control) for abdominal and stereotactic body radiotherapy.

PurposeAbdominal Compression is one of the methods available to minimize breathing motion during stereotactic body radiotherapy (SBRT), particularly for abdominal malignancies. It might be necessary to treat some tumors with radiation entering through the compression device. One clinically available compression plate device (Elekta BodyFIX Diaphragm Control) was evaluated to understand its impact on dosimetry during clinical treatments.MethodsThe BodyFIX compression device was CT scanned following departmental stereo scanning protocols. Treatment planning system (TPS) calculations were used to determine attenuation ratios through each section of the compression device: the outer frame, compression plate, and higher density couch fixation points and compression screw. TPS calculated skin doses where the compression plate will come in contact with the skin were recorded. All attenuation ratio fields were measured on an Elekta Versa HD linear accelerator. Where differences in attenuation were observed, TPS density overrides were found to bring calculated doses into agreement with measurement.ResultsThe compression plate and frame showed low dose attenuation (3%–4%). Only minor density overrides for the frame were required due to artefacts from the limited CT field‐of‐view. The high‐density materials in the couch fixation points resulted in higher attenuation (14%–20%). Similarly, the compression screw recorded very high attenuation (44%–65%), depending on the length of screw used. Skin doses assessed from the TPS calculations showed dose build‐up under the compression plate that would result in skin receiving the maximum dose.ConclusionCompression devices can cause significant dose attenuation. Density overrides for TPS calculations are recommended for correcting attenuation in some sections of the device. High‐density structures like the fixation screw and frame fixation points create high levels of dosimetric uncertainty, and beam entry through those areas has been disallowed.

Radiation dosimetry effect evaluation of a carbon fiber couch on novel uRT-linac 506c accelerator

Recently, a diagnostic helical CT is integrated into a linear accelerator, called uRT-linac 506c, whose CT scanning dataset can be directly used to do simulation. This novel structure provides a possibility for online adaptive radiotherapy. For adaptive radiotherapy, the carbon fiber couch is an essential external device for supporting and positioning patients. And the effect on dose attenuation and distribution caused by a couch is inevitable and vital for precise treatment. In this research, the couch equipped with uRT-linac 506c was evaluated on the radiation dosimetry effect. The treatment couch equipped on the uRT-linac 506c accelerator was evaluated, and its effect on the attenuation, surface dose and dose buildup were measured for different phantom positions (offset = 0 cm, offset = + 10 cm and offset = − 10 cm, respectively) and different gantry angles. Since uRT-linac 506c is exclusively capable to provide diagnostic CT scanning data with real relative electron density (RED), this CT scanning data of the couch can be used directly in uRT-TPS to design plans. This scanned couch dataset was designated as the model A. The model B was a dummy structure of a treatment couch inserted with artificially preset RED. The dose calculation accuracy of these two models was compared using PB, CC, and MC on uRT-TPS. With the effect of carbon fiber couch, the surface dose was increased at least 97.94% for 25 × 25 cm2 field and 188.83% for 10 × 10 cm2 field, compared with those without. At different phantom positions (offset = 0, + 10, − 10 cm), the attenuation for 6 MV photon beam at gantry angle 180° were 4.4%, 4.4%, and 4.3%, respectively, and varied with changes of gantry angle. There do exists dose deviation between measurement and TPS calculation with the involvement of treatment couch, among the three algorithms, MC presented the least deviation, and the model A made less and steadier deviation than the model B, showing promising superiority. The attenuation, surface dose, and buildup effects of the carbon fiber couch in this study were measured similarly to most counterparts. The dose deviation calculated based on the couch dataset scanned by the diagnostic helical CT was smaller than those based on a dummy couch. This result suggests that an accelerator equipped with a diagnostic CT, which can help reduce the dose deviation of the carbon fiber couch, is a promising platform for online adaptive radiotherapy.

Surface and the Build-up Dose Distribution Due to Applying Thermoplastic Masks in External Radiotherapy

Background: Thermoplastic immobilization devices are used to position the patient on the table in order to correctly reposition the patient during treatment courses. Objective: The Purpose of this work is investigating the degradation of surface dose and the dose distribution in the build-up region for photon beams associated with immobilization devices using Gafchromic films. Materials and Methods: After heating, these masks are stretched and fitted over the considered location of body before treatment simulation for insuring the reproducibility of patient position during treatment fractions. In this research, dosimetry was carried out using Gafchromic EBT3 film and three kinds of thermoplastic masks (Orfit with thickness 2.2mm, holes diameter 2.5mm, Orfit with thickness 2mm, holes diameter 1mm, and Klarity mask, thickness 2mm, holes diameter 3mm). Measurements were made with and without the mask materials on the surface of the Perspex phantom for 6 and 15 MV X-ray beams of a LINAC machine. Results: The results showed that surface dose increases 2.1 to 6.7 times and 2 to 3.9 times than the surface dose in the open field for 15 MV and 6 MV photons, respectively. According to the obtained results from the Analyses of Variances (ANOVA) test , it is defined that there is a significant difference in surface dose among three kind of thermoplastic masks (χ2=49.78 and df=3 and P&lt;0.0001). The surface dose in Klarity has a significant difference in comparison of other masks according to PostHoc exams and there is no significant difference among two other masks (P&gt;0.05). Conclusion: According to the results, Klarity mask is more acceptable immobilization device when compared with other masks in the test.

A prospective feasibility study of a 1-mm bolus for postmastectomy radiotherapy

BackgroundThe optimal chest wall bolus regimen for postmastectomy radiotherapy (PMRT) remains unknown. We aimed to prospectively evaluate the use of a 1-mm-thick daily tissue-equivalent bolus in patients who received PMRT using thermoluminescent dosimeters (TLDs) and skin toxicity assessment.MethodsPatients with a 1-mm-thick daily bolus during PMRT were prospectively enrolled at The Juntendo University Hospital. The surface dose was measured in vivo under the 1-mm-thick bolus on the chest wall. We assessed the acute skin toxicity weekly during PMRT, and 1, 2, 4, and 12 weeks after the completion of PMRT.ResultsA total of 19 patients aged 32–79 years old received PMRT from July 2019 to January 2020. All patients completed the protocol treatment without interruptions, and the median follow-up was 32 weeks. In vivo dosimetry analysis revealed surface doses between 77 and 113% of the prescribed dose, with a mean of 92% of the prescribed radiation dose, and a standard deviation of 7% being delivered. Grade 2 dermatitis was found in 10 patients (53%), and Grade 3 dermatitis was found in one patient (5%). All cases of Grade 2 and 3 dermatitis were improved 4 weeks after PMRT. There were no cases of Grade 4 dermatitis and no chest wall recurrences during the treatment or follow-up period.ConclusionsResults confirmed the feasibility of using a 1-mm-thick daily bolus for PMRT, exhibiting an appropriate dose buildup and acceptable skin toxicity without treatment interruptions.Trial registrationThe University Hospital Medical Information Network Clinical Trials Registry, UMIN000035773. Registered 1 July 2019.

All-MOCVD-grown gallium nitride diodes with ultra-low resistance tunnel junctions

We carefully investigate three important effects including postgrowth activation annealing, delta (δ) dose and magnesium (Mg) buildup delay as well as experimentally demonstrate their influence on the electrical properties of GaN homojunction p–n diodes with a tunnel junction (TJ). The diodes were monolithically grown by metalorganic chemical vapor deposition (MOCVD) in a single growth step. By optimizing the annealing parameters for Mg activation, δ-dose for both donors and acceptors at TJ interfaces, and p +-GaN layer thickness, a significant improvement in tunneling properties is achieved. For the TJs embedded within the continuously-grown, all-MOCVD GaN diode structures, ultra-low voltage penalties of 158 mV and 490 mV are obtained at current densities of 20 A cm−2 and 100 A cm−2, respectively. The diodes with the engineered TJs show a record-low differential resistivity of 1.6 × 10−4 Ω cm2 at 5 kA cm−2.

DeepMC: a deep learning method for efficient Monte Carlo beamlet dose calculation by predictive denoising in magnetic resonance-guided radiotherapy

Emerging magnetic resonance (MR) guided radiotherapy affords significantly improved anatomy visualization and, subsequently, more effective personalized treatment. The new therapy paradigm imposes significant demands on radiation dose calculation quality and speed, creating an unmet need for the acceleration of Monte Carlo (MC) dose calculation. Existing deep learning approaches to denoise the final plan MC dose fail to achieve the accuracy and speed requirements of large-scale beamlet dose calculation in the presence of a strong magnetic field for online adaptive radiotherapy planning. Our deep learning dose calculation method, DeepMC, addresses these needs by predicting low-noise dose from extremely noisy (but fast) MC-simulated dose and anatomical inputs, thus enabling significant acceleration. DeepMC simultaneously reduces MC sampling noise and predicts corrupted dose buildup at tissue-air material interfaces resulting from MR-field induced electron return effects. Here we demonstrate our model’s ability to accelerate dose calculation for daily treatment planning by a factor of 38 over traditional low-noise MC simulation with clinically meaningful accuracy in deliverable dose and treatment delivery parameters. As a post-processing approach, DeepMC provides compounded acceleration of large-scale dose calculation when used alongside established MC acceleration techniques in variance reduction and graphics processing unit-based MC simulation.

Surface and build-up dose comparison between Elekta 6MV flattening filter and flattening-filter-free beams using an advanced Markus ionization chamber and a solid water-equivalent phantom.

Using a plane‐parallel advanced Markus ionization chamber and a stack of water‐equivalent solid phantom blocks, percentage surface and build‐up doses of Elekta 6 MV flattening filter (FF) and flattening‐filter‐free (FFF) beams were measured as a function of the phantom depth for field sizes ranging from 2 × 2 to 10 × 10 cm2. It was found that the dose difference between the FF and the FFF beams was relatively small. The maximum dose difference between the FF and the FFF beams was 4.4% at a depth of 1 mm for a field size of 2 × 2 cm2. The dose difference was gradually decreased while the field size was increased up to 10 × 10 cm2. The measured data were also compared to published Varian FF and FFF data, suggesting that the percentage surface and build‐up doses as well as the percentage dose difference between FF and FFF beams by our Elekta linac were smaller than those by the Varian linac.

Quantitative Analysis of the Treatment Set Up Reliability of a Prone Breast Boost for Early Stage Breast Cancer

Adjuvant whole-breast radiation and a tumor bed boost for early-stage breast cancer can be delivered in the supine or prone position. Delivering the boost in the prone position has the advantage of removing the supine simulation CT and the capability of obtaining the total treatment dose distribution. However, the reliability of daily prone boost set up is largely unknown, and imaging is used for treatment verification. Here we evaluate the reliability of a daily clinical setup protocol for a prone mini-tangent boost, using daily cone beam CT (CBCT) to verify the adequacy of target coverage. We retrospectively reviewed 13 patients treated with a prone mini-tangent boost. The target (CTV) was the tumor bed without further expansion. Distance from CTV to block edge was 1.5cm. The boost prescription was 10Gy in 5 fractions. Coverage goals were the treating physician’s discretion, However, in all cases the planned dose to 90% of the CTV (D90) > 90% of prescription. Patients were positioned daily using a clinical setup with visual verification. Port films were taken to verify day 1 setup and when there was concern about the daily clinical setup. A CBCT was then acquired. If needed, additional shifts were made based on the CBCT, often followed by port films. Retrospectively, the simulation CT was independently registered to the daily CBCTs by a physician, and a daily tumor bed contour was generated. The treatment plan was transferred to the daily CBCT and delivered dose was calculated. To assess the reliability of clinical setup prior to CBCT acquisition, we assessed how often the daily D90 was < 90% of prescription dose (D90 < 90%) and how often the daily volume receiving full prescription dose (V100daily) did not receive the planned V100 (V100plan) by > 10% (V100plan-V100daily > 10%). Twelve patients with full imaging details were analyzed. Of 60 analyzable daily fractions, D90 < 90% for 3/60 fractions (5%), and V100plan-V100daily > 10% for 11/60 fractions (18%). Post-CBCT shifts were made for 1 fraction with D90 < 90% and for 2 fractions with V100plan-V100daily > 10%. Fractions for which coverage did not meet reliability metrics most commonly involved superficial tumor beds that extended into the region of subcutaneous dose build-up. In all such cases, allowing a portion of the target to receive less than prescription dose was deemed acceptable by the treating physician. We found that a daily clinical treatment setup for a prone boost was reliable in most cases. Addition of a CBCT was helpful to identify clinical setup variability. Treatment of a superficial tumor bed may be more sensitive to small variations in daily treatment setup due to the dose gradient across the target. When superficial target coverage is critical, supine boost positioning may be considered.