Assessing the DNA Damaging Effectiveness of Ionizing Radiation Using Plasmid DNA.

Plasmid DNA is useful for investigating the DNA damaging effects of ionizing radiation. In this study, we have explored the feasibility of plasmid DNA-based detectors to assess the DNA damaging effectiveness of two radiotherapy X-ray beam qualities after undergoing return shipment of ~8000 km between two institutions. The detectors consisted of 18 µL of pBR322 DNA enclosed with an aluminum seal in nine cylindrical cavities drilled into polycarbonate blocks. We shipped them to Toronto, Canada for irradiation with either 100 kVp or 6 MV X-ray beams to doses of 10, 20, and 30 Gy in triplicate before being shipped back to San Diego, USA. The Toronto return shipment also included non-irradiated controls and we kept a separate set of controls in San Diego. In San Diego, we quantified DNA single strand breaks (SSBs), double strand breaks (DSBs), and applied Nth and Fpg enzymes to quantify oxidized base damage. The rate of DSBs/Gy/plasmid was 2.8±0.7 greater for the 100 kVp than the 6 MV irradiation. The 100 kVp irradiation also resulted in 5±2 times more DSBs/SSB than the 6 MV beam, demonstrating that the detector is sensitive enough to quantify relative DNA damage effectiveness, even after shipment over thousands of kilometers.

Safety and Efficacy of Stereotactic Body Radiation Therapy for Ultra-central Thoracic Tumors: A Single Center Retrospective Review.

PURPOSE: We sought to evaluate the toxicity and efficacy of stereotactic body radiation therapy (SBRT) for ultracentral thoracic tumors at our institution. METHODS AND MATERIALS: Patients with ultracentral lung tumors or nodes, defined as having the planning target volume (PTV) overlapping or abutting the central bronchial tree and/or esophagus, treated at our institution with SBRT between 2009 and 2019 were retrospectively reviewed. All SBRT plans were generated with the goal of creating homogenous dose distributions. The primary endpoint was incidence of SBRT-related grade ≥3 toxicity, defined using the Common Terminology Criteria for Adverse Events (V5.0). Secondary endpoints included local failure (LF), progression-free survival (PFS), and overall survival. Competing risk analysis was used to estimate incidence and identify predictors of severe toxicity and LF, while the Kaplan-Meier method was used to estimate PFS and OS. RESULTS: A total of 154 patients receiving 162 ultracentral courses of SBRT were included. The most common prescription was 50 Gy in 5 fractions (42%), with doses ranging from 30 to 55 Gy in 5 fractions (BED10 range, 48-115 Gy). The incidence of severe toxicity was 9.4% at 3 years. The most common severe toxicity was pneumonitis (n = 4). There was 1 possible treatment-related death from pneumonitis/pneumonia. Predictors of severe toxicity included increased PTV size, decreased PTV V95%, lung V5 Gy, and lung V20 Gy. The incidence of LF was 14% at 3 years. Predictors of LF included younger age and greater volume of overlap between the PTV and esophagus. The median PFS was 8.8 months, while the median overall survival was 44.0 months. CONCLUSIONS: In the largest case series of ultracentral thoracic SBRT to date, homogenously prescribed SBRT was associated with relatively low rates of severe toxicity and LF. Predictors of toxicity should be interpreted in the context of the heterogeneity in toxicities observed.

Monte Carlo optimization and experimental validation of a prototype ionization chamber for accurate magnetic resonance image guided radiation therapy (MRgRT) daily output constancy measurements in solid phantoms.

PURPOSE: To optimize the design, develop and test a prototype ionization chamber for accurate daily output constancy measurements in solid phantoms in clinical magnetic resonance-guided radiation therapy (MRgRT) radiotherapy beams. Up to 4% variations in response using commercial ionization chambers have been previously reported; the prototype ionization chamber developed here aims to minimize these variations. METHODS: Monte Carlo simulations with the EGSnrc code system are used to optimize an ionization chamber design by increasing the thickness of a brass (high-density, nonferromagnetic, easy-to-machine) wall until results consistent with no air gap are produced for simulations with a 1.5 T and 0.35 T magnetic field, with a 0.2 mm air gap and varying the placement of the chamber model within the air gap. Based on the results of these simulations, prototype ionization chambers are manufactured and tested in conventional linac beams and in a 7 MV Elekta Unity MR-linac. The chambers are rotated about their axes, both parallel and perpendicular to the 1.5 T magnetic field, through 360º in a plastic phantom with measurements made at each cardinal angle. This reveals any variation in chamber response by varying the thickness of the air gap between the chamber and the phantom. RESULTS: Monte Carlo simulations demonstrate that the optimal thickness of the chamber wall to mitigate the effect of an asymmetric air gap between the chamber and the plastic phantom is 1.1 mm of brass. With this thickness, the differences between simulations with and without an air gap and with asymmetric placement of the chamber within the air gap are less than 0.2%. A prototype chamber constructed with a 1.1 mm brass wall thickness exhibits less than 0.3% variation in response when rotated about its axis in the plastic phantom in a beam from an MR-linac, independent of whether its axis is parallel or perpendicular to the magnetic field. CONCLUSION: The optimized ionization chamber design and validated prototype for accurate MR-linac daily output constancy measurements allows utilization of conventional phantoms and procedures in MRgRT systems. This can minimize disruption to clinical workflow for MR-linac quality assurance measurements.

First-stage validation of a portable imageable MR-compatible water calorimeter.

PURPOSE: The purpose of this study is to design a water calorimeter with three goals in mind: (a) To be fully magnetic resonance (MR)-compatible; (b) To be imaged using kV cone beam computed tomography (CBCT), MV portal imaging or MRI for accurate positioning; (c) To accommodate both vertical and horizontal beam incidence, as well as volumetric deliveries or Gamma Knife®. Following this, the calorimeter performance will be measured using an accelerator-based high-energy photon beam. METHODS: A portable 4°C cooled stagnant water calorimeter was built using MR-compatible materials. The walls consist of layers of acrylic plastic, aerogel-based material acting as thermal insulation, as well as tubing for coolant to flow to keep the calorimeter temperature stable at 4°C. The lid contains additional pathways for coolant to flow through as well as two hydraulically driven stirrers. The water calorimeter was positioned in an Elekta Versa using kV CBCT imaging as well as orthogonal MV image pairs. Absolute absorbed dose to water was then determined under a 6 MV flattening filter-free (FFF) beam. This was compared against reference dosimetry results that were measured under identical conditions with an Exradin A1SL ionization chamber with a calibration coefficient directly traceable to the National Research Council Canada. RESULTS: The dose to water determined with the calorimeter (n = 30) agreed with the A1SL ionization chamber reference dose measurements (n = 15) to within 0.25%. The uncertainty associated with the water calorimeter absorbed dose measurement was estimated to be 0.54% (k = 1). CONCLUSIONS: An MR-compatible water calorimeter was successfully built and absolute absorbed dose to water under a conventional 6 MV FFF beam was determined successfully as a first-stage validation of the system.

Maximum RBE change in 192Ir, 125I, and 169Yb brachytherapy and the corresponding effect on treatment planning.

PURPOSE: The purpose of this study was to examine RBE variation as a function of distance from the radioactive source, and the potential impact of this variation on a realistic prostate brachytherapy treatment plan. METHODS: Three brachytherapy sources (125I, 192Ir, and 169Yb) were modelled in Geant4 Monte Carlo code, and the resulting electron energy spectrum in water in 3D space around these sources was scored (voxel size of 2 mm3). With this energy spectrum, microdosimetric techniques were used to calculate the maximum RBE, RBEM, as a function of distance from the source. RBEM of 125I relative to 192Ir was calculated in order to validate simulations against literature; all other RBEM calculations were done by normalizing electron fluence at various distances to the source position. In order to examine the impact of RBEM variation in treatment planning, a realistic 192Ir prostate plan was re-evaluated in terms of RBE instead of absorbed dose. RESULTS: The RBEM of 125I, 192Ir, and 169Yb at 8 cm away from the source was 0.994 (+/-0.002), 1.030 (+/-0.003), and 1.066 (+/-0.008), respectively. RBEM in the HDR prostate treatment plan exhibited several hot (+3.6% in RBEM) spots. CONCLUSIONS: The large increase RBEM observed in 169Yb has not yet been described in the literature. Despite the presence of radiobiological hotspots in the HDR treatment, these variations are likely nominal and clinically insignificant.

Water calorimetry in MR-linac: Direct measurement of absorbed dose and determination of chamber k Q mag.

PURPOSE: To use a portable 4°C cooled MR-compatible water calorimeter to measure absorbed dose in a magnetic resonance-guided radiation therapy (MRgRT) system. Furthermore, to use the calorimetric dose results and direct cross-calibration to experimentally measure the combined beam quality and magnetic field correction factor ( k Q mag ) of a clinically used reference-class ionization chamber placed under the same radiation field. METHODS: An Elekta Unity MR-linac (7 MV FFF, B = 1.5 T) was used in this study. Measurements were taken using the in-house designed and built water calorimeter. Following preparation and cooling of the system, the MR-compatible calorimeter was positioned using a combination of MR and EPID imaging and the dose to water was measured by monitoring the radiation-induced temperature change. Immediately after the calorimetric measurements, an A1SL ionization chamber was placed inside the calorimeter for direct cross-calibration. The results allowed for a direct and absolute experimental measurement of k Q mag for this chamber and comparison against existing Monte Carlo values. RESULTS: The calorimeter was successfully positioned using imaging in under an hour. The 1-hour setup time is from the time the calorimeter leaves storage to the first calorimetric measurement. Absorbed dose was successfully measured with a relative combined standard uncertainty of 0.71 % (k = 1). Through a cross-calibration, the k Q mag for an Exradin A1SL ionization chamber, set up perpendicular to the incident photon beam and opposite to the direction of the Lorentz force, was directly determined in water in absolute terms to be 0.977 ± 0.010. The currently published k Q mag results, obtained via Monte Carlo calculations, agree with experimental measurements in this work within combined uncertainties. CONCLUSIONS: A novel design of an MR-compatible water calorimeter was successfully used to measure absorbed dose in an MR-linac and determine an experimental value of k Q mag for a clinically used ionization chamber.

Quantifying the impact of lead doping on plastic scintillator response to radiation.

PURPOSE: Through the addition of high-Z dopants, the sensitivity of plastic scintillators to low-energy radiation can be increased. This study quantifies this change in sensitivity as a function of dopant concentration. METHODS: Measurements were conducted using four different lead-doped scintillators (0%, 1%, 1.5%, and 5% Pb) in high-energy electrons (6 to 15 MeV) and low-energy photon (100 to 300 kVp) radiation fields. High-energy and low-energy irradiations were done using a clinical linear accelerator and an orthovoltage unit, respectively. Light emitted by the scintillator was quantified using a photosensor module. The experimental setup was replicated in Geant4.10.3 Monte Carlo and scintillator parameters (Quenching parameter: kB and the light yield: L0 ) were varied until agreement between measured and simulated results was reached. Monoenergetic electrons were used to simulate the high-energy electron beam while a spectrum generated using SpekCalc® software was used in the low-energy simulations. Light produced by the scintillator was quantified using a flux scorer sensitive only to photons in the visible wavelength range. In order to compare measured and simulated results, the light produced by the scintillator was normalized to the absorbed dose-to-water at the point of measurement. RESULTS: At high lead dopant concentrations, the scintillator's sensitivity to the 100 kVp beam increased by 474% relative to the 15 MeV electron beam; the scintillator's kB parameter increased from 0.126 to 0.27 mm/MeV. A model quantifying the change in kB and L0 as a function of Zeff was derived; presenting a modified Birks' Law for metal-doped plastic scintillators. CONCLUSION: The impact of high-Z doping on plastic scintillator response was quantified; this can allow for the controlled induction of energy dependence in plastic scintillator detectors.

A molecular characterization of the agonist binding site of a nematode cys-loop GABA receptor.

BACKGROUND AND PURPOSE: Cys-loop GABA receptors represent important targets for human chemotherapeutics and insecticides and are potential targets for novel anthelmintics (nematicides). However, compared with insect and mammalian receptors, little is known regarding the pharmacological characteristics of nematode Cys-loop GABA receptors. Here we have investigated the agonist binding site of the Cys-loop GABA receptor UNC-49 (Hco-UNC-49) from the parasitic nematode Haemonchus contortus. EXPERIMENTAL APPROACH: We used two-electrode voltage-clamp electrophysiology to measure channel activation by classical GABA receptor agonists on Hco-UNC-49 expressed in Xenopus laevis oocytes, along with site-directed mutagenesis and in silico homology modelling. KEY RESULTS: The sulphonated molecules P4S and taurine had no effect on Hco-UNC-49. Other classical Cys-loop GABAA receptor agonists tested on the Hco-UNC-49B/C heteromeric channel had a rank order efficacy of GABA > trans-4-aminocrotonic acid > isoguvacine > imidazole-4-acetic acid (IMA) > (R)-(-)-4-amino-3-hydroxybutyric acid [R(-)-GABOB] > (S)-(+)-4-amino-3-hydroxybutyric acid [S(+)-GABOB] > guanidinoacetic acid > isonipecotic acid > 5-aminovaleric acid (DAVA) (partial agonist) > ß-alanine (partial agonist). In silico ligand docking revealed some variation in binding between agonists. Mutagenesis of a key serine residue in binding loop C to threonine had minimal effects on GABA and IMA but significantly increased the maximal response to DAVA and decreased twofold the EC50 for R(-)- and S(+)-GABOB. CONCLUSIONS AND IMPLICATIONS: The pharmacological profile of Hco-UNC-49 differed from that of vertebrate Cys-loop GABA receptors and insect resistance to dieldrin receptors, suggesting differences in the agonist binding pocket. These findings could be exploited to develop new drugs that specifically target GABA receptors of parasitic nematodes.