PURPOSE:To characterize the biological effects of low-energy photons generated by a ∼60-keV quasimonoenergetic focused convergent-beam system (patented x-ray converging lens) compared with orthovoltage and 6-MV clinical photon beams. METHODS AND MATERIALS:Human (NCI-H460 and NCI-H2172) and murine (KLN-205 and LKR-13) lung cancer cell lines and a normal human bronchial epithelial cell line (Beas-2B) were irradiated with photons from a 6-MV linear accelerator, an 180-kVp orthovoltage unit (average energy ∼60 keV), or a system that generates 60-keV quasimonoenergetic convergent photons. Clonogenic cell survival was analyzed to assess dose-response metrics and relative biological effectiveness (RBE) in all cell lines. DNA damage and cell cycle analyses were assessed in human-derived lung cancer cell lines. A human lung cancer (NCI-H460) xenograft model was used to test tumor growth delay in vivo. RESULTS:Relative to 6-MV photons, the 60-keV quasimonoenergetic convergent and orthovoltage photons induced comparable cell kill effect and RBE for the LKR-13, KLN-205, and Beas-2B cell lines and higher cell kill effect and RBE for the NCI-H460 (significant) and NCI-H2172 (nonsignificant) cell lines. The enhanced biological response to orthovoltage and 60-keV quasimonoenergetic convergent photons in NCI-H460 cells was also evidenced by increased DNA damage signaling and cell cycle checkpoint activation. Radiosensitivity differed considerably among cell lines, even for the same radiation type. The 60-keV quasimonoenergetic convergent photons significantly delayed tumor growth, achieving effects comparable to 6-MV photons, in the NCI-H460 xenograft model. CONCLUSIONS:In vitro and in vivo evaluations confirmed that the biological effectiveness of 60-keV quasimonoenergetic convergent photons is similar to that of photons from an orthovoltage unit (average energy ∼60 keV) and either comparable (3 cell lines) or higher (2 cell lines, but 1 was not significant) than photons from a 6-MV linear accelerator.

Reflecting x-rays using Bragg diffraction with specialized crystalline materials is a powerful technique in applied physics and materials science that can enhance applications such as radiation therapy through converging x-ray beams. However, measuring convergent x-ray beams is challenging owing to their focused nature; innovative approaches are required for accurate characterization of the deposited radiation dose. Monte Carlo simulation is a well-established method of analyzing x-ray interactions with materials and would be a vital tool for modeling converging x-rays produced through Bragg diffraction. We developed a novel photon physics process within the Geant4 framework called G4BraggReflection to address the challenges associated with simulating Bragg reflection in a Monte Carlo framework. A hybrid methodology integrating particle and solid-state physics principles was employed. The G4BraggReflection process seamlessly integrates the Bragg reflection phenomenon into Geant4 simulations without toolkit recompilation. The G4BraggReflection process was validated through comparisons benchmarked data based on Zachariasen theory utilizing x-ray oriented programs (XOP) software as a benchmarking tool. The simulations covered a range of parameters, including mosaicity, particle energy, crystal atomic number, crystal absorption, and crystallite direction and thickness. The reflectivity profiles for aluminum crystals were evaluated under various conditions, comparing Geant4 Monte Carlo simulations (G4) with the XOP analytical method, and showed strong agreement across all scenarios. The reflectivity profile for a mosaic crystal obtained using Geant4 exhibited excellent agreement with XOP results, as indicated by a near-perfect for both 59.318 Kev ( ρ = 0.93, root mean square error (RMSE) = 0.007, CosSim = 1) and 8 keV ( ρ = 0.9, RMSE = 0.004, CosSim = 0.99). For perfect crystals, the agreement was also notable at 59.318 keV for absorbing ( ρ = 0.99, RMSE = 0.069, CosSim = 0.98) and for nonabsorbing crystals ( ρ = 0.61, RMSE = 0.106, CosSim = 0.96). The same result was observed for 8 keV in both absorbing ( ρ = 0.99, RMSE = 0.055, CosSim = 0.97) and nonabsorbing ( ρ = 0.53, RMSE = 0.109, CosSim = 0.92) perfect crystals. These findings validate the high accuracy of Geant4 in modeling reflectivity under diverse conditions. This work advances the simulation capabilities in Bragg reflection in a Monte Carlo framework, providing a reliable and comprehensive physics process for researchers. The forthcoming addition of specific data on different crystal properties will further illuminate the performance and applicability of the G4BraggReflection process in diverse scientific and medical contexts.

Objective. This work aimed to evaluate the dosimetric characteristics of the new HyperScintTM Research Platform 200 system (HS-RP200), with orthovoltage beams and a novel kilovoltage converging beam, converging radiotherapy and radiosurgery (CRnR) beam. Approach. Several dosimetric properties of the HS-RP200 detector with several orthovoltage beams were studied in air. Relative dosimetry of the HS-RP200 detector was also studied for use on the converging x-ray beam. Energy correction factors were compared between measurement, Monte Carlo, and analytical methods. Main results. The signal-to-noise ratio for the detector increased as the frame exposure time increased, and repeated measurements had less uncertainty when measurements were performed without stem-effect removal (WOSER). The short-term stability study of the detector's signal showed an acceptable variance of its readings to within 1% during 1 h measurements. The detector exhibited angular independence within the measurement uncertainty of 1%. The detector exhibited excellent dose linearity response with the orthovoltage beams. During an 80 d period, repeated measurements determined that the detector possesses long-term stability within a root mean square (rms) of 2.3% and 0.6% when measurements were performed with stem effect removal and WOSER, respectively. Calculated energy correction factors implied significant fluctuations of the HS-RP200 detector with the beam quality of orthovoltage beams, ranging from 3% to 66%, depending on the divergence of the effective energy of the beam from reference quality. Monte Carlo-simulated energy correction factors agreed with measurements with a maximum difference of 5.4% when measurements were performed WOSER. Lastly, the HS-RP200 detector system exhibited good relative dosimetric characteristics with the CRnR beam. A maximum rms value of 0.019 was calculated for the normalized profiles when measurements from the HS-RP200 detector were compared to measurements from film. Significance. The HS-RP200 detector could be used as an alternative to other detectors with orthovoltage beams, including the CRnR beam.

Background: Generalizability and domain dependency are critical challenges in developing predictive models for healthcare, particularly in medical diagnostics and radiation oncology. Predictive models designed to assess tumor recurrence rely on comprehensive and high-quality datasets, encompassing treatment planning parameters, imaging protocols, and patient-specific data. However, domain dependency, arising from variations in dose calculation algorithms, computed tomography (CT) density conversion curves, imaging modalities, and institutional protocols, can significantly undermine model reliability and clinical utility. Methods: This study evaluated dose calculation differences in the head and neck cancer treatment plans of 19 patients using two treatment planning systems, Pinnacle 9.10 and RayStation 11, with similar dose calculation algorithms. Variations in the dose grid size and CT density conversion curves were assessed for their impact on domain dependency. Results: Results showed that dose grid size differences had a more significant influence within RayStation than Pinnacle, while CT curve variations introduced potential domain discrepancies. The findings underscore the critical role of precise and standardized treatment planning in enhancing the reliability of predictive modeling for tumor recurrence assessment. Conclusions: Incorporating treatment planning parameters, such as dose distribution and target volumes, as explicit features in model training can mitigate the impact of domain dependency and enhance prediction accuracy. Solutions such as multi-institutional data harmonization and domain adaptation techniques are essential to improve model generalizability and robustness. These strategies support the better integration of predictive modeling into clinical workflows, ultimately optimizing patient outcomes and personalized treatment strategies.

BACKGROUND:Carbon nanotube-based cold cathode technology has revolutionized the miniaturization of X-ray tubes. However, current applications of these devices required optimization for large, uniform fields with low intensity.PURPOSE:This work investigated the feasibility and radiological characteristics of a novel conical X-ray target optimized for high intensity and high directionality to be used in a compact X-ray tube.METHODS:The proposed device uses an ultrathin, conical tungsten-diamond target that exhibits significant heat loading while maintaining a small focal spot size and promoting forward-directedness of the X-ray field through preferential attenuation of oblique-angled photons. The electrostatic and thermal properties of the theoretical tube were calculated and analyzed using COMSOL Multiphysics software. The production, transport, and calculation of radiological properties associated with the resultant X-ray field were performed using the Geant4 toolkit via its wrapper, TOPAS.RESULTS:Heat transfer analysis of this X-ray tube demonstrated the feasibility of a 200-kV electron beam bombarding the proposed target at a maximum current of 100 mA using a 1-ms symmetric duty cycle. The cathode of the X-ray tube was designed to be segmented into nine switchable electrical segments for modulation of the focal spot size from 0.4- to 10.8-mm. After importing the COMSOL-derived electron beam into TOPAS for X-ray production simulations, radiological analysis of the resultant field demonstrated high levels of intrinsic beam collimation while maintaining high intensity. A maximum dose rate of 17,887 cGy/min was calculated for 1-mm depth in water at 7-cm distance.CONCLUSIONS:The proposed X-ray tube design can create highly directional X-ray fields with superior fluence compared to that of current commercial X-ray tubes of comparable size.

Purpose/Objective(s) Perform Monte Carlo simulations to understand the dosimetric performance of a novel converging beam small animal irradiator using a theoretical compact X-ray tube previously reported in the literature by Insley et al. (Medical Physics, 2023). Materials/Methods The X-ray tube comprises a compact, carbon nanotube-based 200-kV tube optimized for high intensity and directionality using an ultrathin conical transmission diamond-tungsten target. We have designed a multi-source, converging irradiator platform to place 52 of these sources uniformly around a spherical collimating shell to generate conformal and arbitrary dose distributions within small animals for preclinical research. A simulation toolkit/TOPAS Monte Carlo codes were employed to simulate the dosimetric performance of the platform for various collimating configurations and source settings (including focal spot size setting and source to axis distance). Dose rate distributions within a 4-cm diameter sphere were calculated for 110 different combinations of X-ray tube focal spot size, source to axis distance, collimator geometry, and field size. Results Spherical dose spots were calculated with a large range of dose rates, field sizes, and penumbrae. Maximum dose rates >80 Gy/s (pulse averaged) were calculated for the largest focal spot size at field sizes greater than 4 cm. FLASH dose rates (>40 Gy/s) were calculated for field sizes down to 2.5 cm FWHM. Conventional dose rates (600 cGy/min minimum) were calculated for 1-mm field sizes, and penumbrae as low as 18% of the FWHM were demonstrated. Additionally, preliminary KERMA simulations demonstrate the feasibility of a 0.1-mm field size. Leakage and scatter simulations report a minimum shielding thickness of 8 mm of lead in an 80-cm cube based on workload estimates from SARRP. This platform design features a large parameter space, offering 52 individual X-ray sources with variable focal spot size (9 different choices), tube current (up to a maximum of 100 mA for the largest focal spot size), source to axis distance (between 7 and 20 cm), field size (10, 1, or 0.1 mm), and collimation geometry (3 choices of collimators for each focal spot size and field size, with selections weighted towards either high intensity, high precision, or high field flatness). The theoretical platform requires a peak power of 520 kW, which is between the average power of a CT and a low-power LINAC. Conclusion Monte Carlo simulations of this multi-source, converging beam small animal irradiator demonstrate the potential to greatly expand the reach of preclinical research into new explorations of FLASH therapy, highly conformal SBRT, and microbeam therapy. With future work aimed towards prototyping the device, creating dose calculation, dose painting, and inverse planning software, designing on-board imaging for precise target localization, and developing proper QA protocols, this platform has significant potential to surpass the current state-of-the-art in small animal radiotherapy research.

Objective. A novel x-ray field produced by an ultrathin conical target is described in the literature. However, the optimal design for an associated collimator remains ambiguous. Current optimization methods using Monte Carlo calculations restrict the efficiency and robustness of the design process. A more generic optimization method that reduces parameter constraints while minimizing computational load is necessary. A numerical method for optimizing the longitudinal collimator hole geometry for a cylindrically-symmetrical x-ray tube is demonstrated and compared to Monte Carlo calculations. Approach. The x-ray phase space was modelled as a four-dimensional histogram differential in photon initial position, final position, and photon energy. The collimator was modeled as a stack of thin washers with varying inner radii. Simulated annealing was employed to optimize this set of inner radii according to various objective functions calculated on the photon flux at a specified plane. Main results. The analytical transport model used for optimization was validated against Monte Carlo calculations using Geant4 via its wrapper, TOPAS. Optimized collimators and the resulting photon flux profiles are presented for three focal spot sizes and five positions of the source. Optimizations were performed with multiple objective functions based on various weightings of precision, intensity, and field flatness metrics. Finally, a select set of these optimized collimators, plus a parallel-hole collimator for comparison, were modeled in TOPAS. The evolution of the radiation field profiles are presented for various positions of the source for each collimator. Significance. This novel optimization strategy proved consistent and robust across the range of x-ray tube settings regardless of the optimization starting point. Common collimator geometries were re-derived using this algorithm while simultaneously optimizing geometry-specific parameters. The advantages of this strategy over iterative Monte Carlo-based techniques, including computational efficiency, radiation source-specificity, and solution flexibility, make it a desirable optimization method for complex irradiation geometries.

Case-based reasoning (CBR) means adopting previous experiences (i.e, data from previous patients) to meet new demands (new patients). In this scenario, we developed an application named OpGen to generate optimization objective values for head and neck radiotherapy inverse treatment planning, using the previously treated patient with an acceptable outcome. OpGen will be helpful in radiotherapy treatment planning automation and efficiently reducing the required time to reach the target goals and improving the quality and safety of the treatment.OpGen has been developed in Python and has four main modules: 1- Analyzing radiotherapy treatment planning parameters of the previous patients in the database. We have developed (a) a powerful graphical user interface (GUI) that allows quantitative and qualitative analysis of the previous knowledge, including the type of frequently used objective parameter (Min Dose, Max Dose, Min DVH dose, etc.); (b) The range of values used for each specific objective type, and (c) Histogram of the number of patients vs. values used for a specific objective type. 2- Importing patients from treatment planning and retrieving a user-defined number of treatment planning information from the previously treated patients in the database corresponding to the given patient using the user-defined quantitative features (geometrical, distance or geo-distance) and retrieval algorithms (k-Nearest Neighbor and Random Forest). 3- Generating optimization objective values based on the following available options: (a) The most similar patient in the database; (b)- Average or median value over the number of patients retrieved from the database; and (c) Deep learning generated values using a model trained over 100 similar cases retrieved from the database. 4- Generate a portable database of the objective values and patient geometric information for any inter- or intra-institutional usage of the algorithm. OpGen is compatible with RayStation (RaySearch Laboratories™) treatment planning and can retrieve, analyze and generate treatment planning optimization parameters in less than 3 minutes. OpGen can potentially increase the quality and safety of radiotherapy treatment planning by improving the efficiency of the optimization process. Citation Format: Reza Reiazi, Surendra Prajapati, Abdallah Sherif Mohamed, Clifton David Fuller, Mohammad Salehpour. Optimization generator app (OpGen) for radiotherapy treatment planning using case-based reasoning. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5398.

X-ray diffraction from a solid crystal shows the wave nature of photons. It is an important electromagnetic (EM) physics process when X-ray photons interact with a crystal. Bragg diffraction, often called Bragg reflection, is a special case of the general form of X-ray diffraction, known as Laue diffraction. When the Bragg's law is met, the incident photon beam is reflected from the crystal plane behaving as a specular reflection at the Bragg angle. However, the Bragg reflection physical process has not been integrated into the general-purpose Monte Carlo simulation toolkit Geant4 for particle physics. In the current study, we developed a new EM physical process class "G4CrystalBraggReflection" and a new EM physical model class "G4DarwinDynamicalModel" for modeling the Bragg reflection physical process within a crystal. We added the Bragg reflection physical process to the EM physics category of Geant4. The preliminary results of photon tracking in a silicon crystal slab have shown the feasibility of simulating the Bragg reflection process in addition to the standard EM processes in the framework of Geant4.

BackgroundMonte Carlo particle simulation has become the primary tool for designing low-energy miniature x-ray tubes due to the difficulties of physically prototyping these devices and characterizing their radiation fields. Accurate simulation of electronic interactions within their targets is necessary for modeling both photon production and heat transfer. Voxel-averaging can conceal hot spots in the target heat deposition profile that can threaten the integrity of the tube. PurposeThis research seeks a computationally-efficient method of estimating voxel-averaging error in energy deposition simulations of electron beams penetrating thin targets to inform the appropriate scoring resolution for a desired accuracy level. MethodsAn analytical model to estimate voxel-averaging along the target depth was developed and compared to results from Geant4 via its wrapper, TOPAS. A 200 keV planar electron beam was simulated to impinge tungsten targets of thicknesses between 1.5- and 12.5-mu m${{\umu {\rm m}}}$. For each target, the model was used to calculate the energy deposition ratio between voxels of varying sizes centered on the longitudinal midpoint of the target. Model-calculated ratios were compared to simulation outputs to gauge the model's accuracy. Then, the model was used to approximate the error between the point value of electron energy deposition and a voxel-based measurement. ResultsThe model underestimates error to within 5% for targets less than 7.5-mu m${{\umu {\rm m}}}$ in thickness with increasing error for greater thicknesses. For the 1.5-mu m${{\umu {\rm m}}}$ target, calculations of the point-vs.-voxel energy deposition show an 11% averaging effect between the midpoint and a 1.5-mu m${{\umu {\rm m}}}$ voxel. Energy deposition profiles along the target depth were also calculated in the Monte Carlo for reference. ConclusionA simple analytical model was developed with reasonable accuracy to guide Monte Carlo users in estimating the appropriate depth-voxel size for thin-target x-ray tube simulations. This methodology can be adapted for other radiological contexts to increase robustness in point-value estimations.

PURPOSE:Using evidence-based radiation therapy to direct care for patients with breast cancer is critical to standardize practice, improve safety, and optimize outcomes. To address this need, the Veterans Affairs (VA) National Radiation Oncology Program (NROP) established the VA Radiation Oncology Quality Surveillance Program to develop clinical quality measures (QMs). The VA NROP contracted with the American Society for Radiation Oncology to commission 5 Blue Ribbon Panels for breast, lung, prostate, rectal, and head and neck cancers. METHODS AND MATERIALS:The Breast Cancer Blue Ribbon Panel experts worked collaboratively with the NROP to develop consensus QMs for use throughout the VA system, establishing a set of QMs for patients in several areas, including consultation and work-up; simulation, treatment planning, and treatment; and follow-up care. As part of this initiative, consensus dose-volume histogram (DVH) constraints were outlined. RESULTS:In total, 36 QMs were established. Herein, we review the process used to develop QMs and final consensus QMs pertaining to all aspects of radiation patient care, as well as DVH constraints. CONCLUSIONS:The QMs and expert consensus DVH constraints are intended for ongoing quality surveillance within the VA system and centers providing community care for Veterans. They are also available for use by greater non-VA community measures of quality care for patients with breast cancer receiving radiation.

Objective . A novel treatment modality is currently being developed that produces converging monoenergetic x-rays. Conventional application of dosimetric calibration as presented in protocol TG61 is not applicable. Furthermore, the dosimetry of the focal point of the converging beam is on the order of a few millimeters, requiring a high-resolution dosimeter. Here we present a procedure to calibrate radiochromic film for narrow-beam monoenergetic 60 keV photons as well as absolute dosimetry of monoenergetic focused x-rays. A study of the focal spot dose rate after passing through a bone-equivalent material was also done to quantify the effects of heterogeneous materials. Approach. This was accomplished by configuring a polyenergetic beam of equivalent energy using a clinical orthovoltage machine. Calibrated films were then used to perform absolute dosimetry of the converging beam by measuring the beam profile at various depths in water . Main Results. A method for calibrating radiochromic film has been developed and detailed that allows absolute dosimetry of a monoenergetic photon beam. Absolute dosimetry of a focused, mono-energetic beam resulted in a focal spot dose rate of ∼30 cGy min −1 at a depth of 5 cm in water. Significance. This work serves to establish a dosimetry protocol for mono-energetic beam absolute dosimetry as well as the use of such a method for measurement of a novel teletherapy modality.

We describe the development and analysis of a new teletherapy modality that, through a novel approach to targeted radiation delivery, has the potential to provide greater conformality than conventional photon-based treatments. The proposed system uses an X-ray lens to reflect photons from a conventional X-ray tube toward a focal spot. The resulting dose distributions have a highly localized peak dose, with lower doses in the converging radiation cone. Physical principles governing the design of this system are presented, along with a series of measurements analyzing various characteristics of the converging beam. The beam was designed to be nearly monoenergetic (~ 59 keV), with an energy bandwidth of approximately 10 keV allowing for treatment energies lower than conventional therapies. The focal spot was measured to be approximately 2.5 cm long and 4 mm wide. Mounting the proposed X-ray delivery system on a robotic arm would allow sub-millimeter accuracy in focal spot positioning, resulting in highly conformal dose distribution via the optimal placement of individual focal spots within the target volume. Aspects of this novel radiation beam are discussed considering their possible clinical application as a treatment approach that takes maximum advantage of the unique properties afforded by converging X-ray beam therapy.

PURPOSE:To provide a series of suggestions for other Medical Physics practices to follow in order to provide effective radiation therapy treatments during the COVID-19 pandemic.METHODS AND MATERIALS:We reviewed our entire Radiation Oncology infrastructure to identify a series of workflows and policy changes that we implemented during the pandemic that yielded more effective practices during this time.RESULTS:We identified a structured list of several suggestions that can help other Medical Physics practices overcome the challenges involved in delivering high quality radiotherapy services during this pandemic.CONCLUSIONS:Our facility encompasses 4 smaller Houston Area Locations (HALs), a main campus with 8 distinct services based on treatment site (ie. Thoracic, Head and Neck, Breast, Gastrointestinal, Gynecology, Genitourinary, Hematologic Malignancies, Melanoma and Sarcoma and Central Nervous System/Pediatrics), a Proton Center facility, an MR-Linac, a Gamma Knife clinic and an array of brachytherapy services. Due to the scope of our services, we have gained experience in dealing with the rapidly changing pandemic effects on our clinical practice. Our paper provides a resource to other Medical Physics practices in search of workflows that have been resilient during these challenging times.