Dose Measurements at Provision Proton Therapy Center.

ABSTRACT: Proton therapy is an advanced method for treating cancerous tumors, and its adoption has expanded significantly in recent years. The production of high-energy protons, however, may result in the creation of secondary neutrons and gamma rays. Hence, ensuring radiation safety at proton therapy centers is crucial, with shielding playing a vital role. This study aimed to evaluate the efficacy of the shielding implemented at the Provision Proton Therapy center in Knoxville, TN, USA. For this purpose, we measured and compared gamma ray radiation levels within the treatment room and the facility's roof. These measurements were conducted using a NaI(Tl) scintillator detector. The PHITS Monte Carlo code was used to deconvolute the incident spectrum using detector response functions. Findings reveal that the facility's shielding effectively protects the general public from gamma ray radiation, with the effective dose within the treatment room being minimal and dose on the roof was comparable to background radiation levels. However, it is important to note that this study did not address the issue of secondary neutron radiation field, which is an important aspect of dose and radiation safety in proton therapy centers.

Advances in space radiation physics and transport at NASA.

The space radiation environment is a complex mixture of particle types and energies originating from sources inside and outside of the galaxy. These environments may be modified by the heliospheric and geomagnetic conditions as well as planetary bodies and vehicle or habitat mass shielding. In low Earth orbit (LEO), the geomagnetic field deflects a portion of the galactic cosmic rays (GCR) and all but the most intense solar particle events (SPE). There are also dynamic belts of trapped electrons and protons with low to medium energy and intense particle count rates. In deep space, the GCR exposure is more severe than in LEO and varies inversely with solar activity. Unpredictable solar storms also present an acute risk to astronauts if adequate shielding is not provided. Near planetary surfaces such as the Earth, moon or Mars, secondary particles are produced when the ambient deep space radiation environment interacts with these surfaces and/or atmospheres. These secondary particles further complicate the local radiation environment and modify the associated health risks. Characterizing the radiation fields in this vast array of scenarios and environments is a challenging task and is currently accomplished with a combination of computational models and dosimetry. The computational tools include models for the ambient space radiation environment, mass shielding geometry, and atomic and nuclear interaction parameters. These models are then coupled to a radiation transport code to describe the radiation field at the location of interest within a vehicle or habitat. Many new advances in these models have been made in the last decade, and the present review article focuses on the progress and contributions made by workers and collaborators at NASA Langley Research Center in the same time frame. Although great progress has been made, and models continue to improve, significant gaps remain and are discussed in the context of planned future missions. Of particular interest is the juxtaposition of various review committee findings regarding the accuracy and gaps of combined space radiation environment, physics, and transport models with the progress achieved over the past decade. While current models are now fully capable of characterizing radiation environments in the broad range of forecasted mission scenarios, it should be remembered that uncertainties still remain and need to be addressed.

Reactor production of promethium-147.

In this paper, we describe the 147Pm production yields and level of impurities from several targets that consisted of milligram quantities of highly enriched 146Nd oxide irradiated at the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory for durations ranging from 24 to 180 h. A comparison between theoretical and experimental data are also presented, and attempts were made to empirically evaluate the neutron capture cross-sections of 41.3-d 148mPm and 5.4-d 148gPm. For a one-cycle irradiation (~24 days), 147Pm yield reaches a maximum value of 101.8 MBq/mg (2.75 mCi/mg) at 60 days after the end of bombardment. Because of large neutron capture cross-sections of 147Pm, the yield of 147Pm does not significantly increase with longer irradiation. Our estimates of the thermal neutron capture cross-section and resonance integral for 146Nd at 1.48 ±â€¯0.05 b and 2.56 ±â€¯0.25 b, respectively, were consistent with the reported values. The effective neutron capture cross-section of 147Pm to 148mPm was 53.3 ±â€¯2.7 b-a factor of 2 lower than the 98.7 ±â€¯6.5 b calculated from reported cross-sections. The measured σeff to the ground state (5.37-d 148gPm) was 82.0 ±â€¯4.1 b; ~34% lower than the value of 139 ±â€¯10 b calculated from reported cross-sections. In this work, we also describe the development of a chemical process based on extraction and ion-exchange chromatography for separation of 147Pm from milligram quantities of 146Nd and other impurities. Sequential separation of Pm from the Nd target and from other radioisotopic impurities (153Gd and 154&155Eu, 192Ir, and 60Co) was achieved using a LN extraction resin in HCl media followed by further purification of Pm from 60Co and 192Ir using a low cross-linking cation exchange resin. Based on these data, we estimated that two rounds of purification under our experimental conditions can provide a mass separation factor of >104 between Pm and Nd. Our data indicate that curie quantities of 147Pm with suitable chemical and radioisotopic purity for applications in beta voltaic batteries can be produced by irradiating gram quantities of highly enriched 146Nd in the flux trap of HFIR for one cycle.

The radiation environment on the surface of Mars - Summary of model calculations and comparison to RAD data.

The radiation environment at the Martian surface is, apart from occasional solar energetic particle events, dominated by galactic cosmic radiation, secondary particles produced in their interaction with the Martian atmosphere and albedo particles from the Martian regolith. The highly energetic primary cosmic radiation consists mainly of fully ionized nuclei creating a complex radiation field at the Martian surface. This complex field, its formation and its potential health risk posed to astronauts on future manned missions to Mars can only be fully understood using a combination of measurements and model calculations. In this work the outcome of a workshop held in June 2016 in Boulder, CO, USA is presented: experimental results from the Radiation Assessment Detector of the Mars Science Laboratory are compared to model results from GEANT4, HETC-HEDS, HZETRN, MCNP6, and PHITS. Charged and neutral particle spectra and dose rates measured between 15 November 2015 and 15 January 2016 and model results calculated for this time period are investigated.

Simulation of the GCR spectrum in the Mars curiosity rover's RAD detector using MCNP6.

The paper presents results from MCNP6 simulations of galactic cosmic ray (GCR) propagation down through the Martian atmosphere to the surface and comparison with RAD measurements made there. This effort is part of a collaborative modeling workshop for space radiation hosted by Southwest Research Institute (SwRI). All modeling teams were tasked with simulating the galactic cosmic ray (GCR) spectrum through the Martian atmosphere and the Radiation Assessment Detector (RAD) on-board the Curiosity rover. The detector had two separate particle acceptance angles, 4π and 30 ° off zenith. All ions with Z = 1 through Z = 28 were tracked in both scenarios while some additional secondary particles were only tracked in the 4π cases. The MCNP6 4π absorbed dose rate was 307.3 ± 1.3 µGy/day while RAD measured 233 µGy/day. Using the ICRP-60 dose equivalent conversion factors built into MCNP6, the simulated 4π dose equivalent rate was found to be 473.1 ± 2.4 µSv/day while RAD reported 710 µSv/day.

Galactic cosmic ray simulation at the NASA Space Radiation Laboratory.

Most accelerator-based space radiation experiments have been performed with single ion beams at fixed energies. However, the space radiation environment consists of a wide variety of ion species with a continuous range of energies. Due to recent developments in beam switching technology implemented at the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory (BNL), it is now possible to rapidly switch ion species and energies, allowing for the possibility to more realistically simulate the actual radiation environment found in space. The present paper discusses a variety of issues related to implementation of galactic cosmic ray (GCR) simulation at NSRL, especially for experiments in radiobiology. Advantages and disadvantages of different approaches to developing a GCR simulator are presented. In addition, issues common to both GCR simulation and single beam experiments are compared to issues unique to GCR simulation studies. A set of conclusions is presented as well as a discussion of the technical implementation of GCR simulation.

Neutron yields and effective doses produced by Galactic Cosmic Ray interactions in shielded environments in space.

In order to define the ranges of relevant neutron energies for the purposes of measurement and dosimetry in space, we have performed a series of Monte Carlo transport model calculations that predict the neutron field created by Galactic Cosmic Ray interactions inside a variety of simple shielding configurations. These predictions indicate that a significant fraction of the neutron fluence and neutron effective dose lies in the region above 20 MeV up to several hundred MeV. These results are consistent over thicknesses of shielding that range from very thin (2.7 g/cm(2)) to thick (54 g/cm(2)), and over both shielding materials considered (aluminum and water). In addition to these results, we have also investigated whether simplified Galactic Cosmic Ray source terms can yield predictions that are equivalent to simulations run with a full GCR source term. We found that a source using a GCR proton and helium spectrum together with a scaled oxygen spectrum yielded nearly identical results to a full GCR spectrum, and that the scaling factor used for the oxygen spectrum was independent of shielding material and thickness. Good results were also obtained using a GCR proton spectrum together with a scaled helium spectrum, with the helium scaling factor also independent of shielding material and thickness. Using a proton spectrum alone was unable to reproduce the full GCR results.

Quality factors for space radiation: A new approach.

NASA has derived new models for radiological risk assessment based on epidemiological data and radiation biology including differences in Relative Biological Effectiveness for leukemia and solid tumors. Comprehensive approaches were used to develop new risk cross sections and the extension of these into recommendations for risk assessment during space missions. The methodology relies on published data generated and the extensive research initiative managed by the NASA Human Research Program (HRP) and reviewed by the National Academy of Sciences. This resulted in recommendations for revised specifications of quality factors, QNASA(Z,ß) in terms of track structure concepts that extend beyond LET alone. The new paradigm for quality factors placed demands on radiation monitoring procedures that are not satisfied by existing dosimetry systems or particle spectrometers that are practical for space exploration where mass, volume, band width and power consumption are highly constrained. We have proposed a new definition of quality factors that relaxes the requirements for identifying charge, Z, and velocity, ß, of the incident radiation while still preserving the functional form of the inherent risk functions. The departure from the exact description of QNASA(Z,ß) is that the revised values are new functions of LET for solid cancers and leukemia. We present the motivation and process for developing the revised quality factors. We describe results of extensive simulations using GCR distributions in free space as well as the resulting spectra of primary and secondary particles behind aluminum shields and penetration through water. In all cases the revised dose averaged quality factors agreed with those based on the values obtained using QNASA(Z,ß). This provides confidence that emerging technologies for space radiation dosimetry can provide real time measurements of dose and dose equivalent while satisfying constraints on size, mass, power and bandwidth. The revised quality factors are sufficiently generalized to be applicable to radiation protection practices beyond space exploration.

The response of a spherical tissue-equivalent proportional counter to different heavy ions having similar velocities.

A tissue-equivalent proportional counter (TEPC) has been used as a dosimeter in mixed radiation fields. Since it does not measure LET directly, the response function must be characterized in order to estimate quality factor and thus equivalent dose for the incident radiation. The objectives of this study were to measure the response of a spherical TEPC for different high-energy heavy ions (HZE) having similar velocity and to determine how quality factors can be determined. Data were obtained at the HIMAC heavy ion accelerator for (4)He and (12)C at 220 +/- 5 MeV/nucleon (beta = 0.59) and (12)C, (16)O, (28)Si and (56)Fe at 376 +/- 15 MeV/nucleon (beta = 0.70). A particle spectrometer recorded the charge and position of each incident beam particle. Events with low energy deposition were observed for particles that passed through the wall of the TEPC but not through the sensitive volume. The frequency averaged lineal energy, y(f), was always less than the LET of the incident particles. The dose averaged lineal energy, y(D), was approximately equal to LET for particles with LET greater than 10 keV/mum, whereas y(D) was larger than LET for the lighter particles with lower LET. Part of this effect is due to detector resolution and energy straggling that increases the variance of the response function. Although the TEPC is not a LET spectrometer, it can provide real time measurements of dose and provide estimates of quality factors for HZE particles using averaged values of lineal energy.

The response of a spherical tissue-equivalent proportional counter to different ions having similar linear energy transfer.

The response of a tissue-equivalent proportional counter (TEPC) to different ions having a similar linear energy transfer (LET) has been studied. Three ions, 14N, 20Ne and 28Si, were investigated using the HIMAC accelerator at the National Institute of Radiological Sciences at Chiba, Japan. The calculated linear energy transfer (LET( infinity )) of all ions was 44 +/- 2 keV/microm at the sensitive volume of the TEPC. A particle spectrometer was used to record the charge and position of each incident beam particle. This enabled reconstruction of the location of the track as it passed though the TEPC and ensured that the particle survived without fragmentation. The spectrum of energy deposition events in the TEPC could be evaluated as a function of trajectory through the TEPC. The data indicated that there are many events from particles that did not pass through the sensitive volume. The fraction of these events increased as the energy of the particle increased due to changes in the maximum energy of the delta rays. Even though the LET of the incident particles was nearly identical, the frequency-averaged lineal energy, y(F), as well as the dose-averaged lineal energy, y(D), varied with the velocity of the incident particle. However, both values were within 15% of LET in all cases.

Spatial distribution and yield of DNA double-strand breaks induced by 3-7 MeV helium ions in human fibroblasts.

Accelerated helium ions with mean energies at the target location of 3-7 MeV were used to simulate alpha-particle radiation from radon daughters. The experimental setup and calibration procedure allowed determination of the helium-ion energy distribution and dose in the nuclei of irradiated cells. Using this system, the induction of DNA double-strand breaks and their spatial distributions along DNA were studied in irradiated human fibroblasts. It was found that the apparent number of double-strand breaks as measured by a standard pulsed-field gel assay (FAR assay) decreased with increasing LET in the range 67-120 keV/microm (corresponding to the energy of 7-3 MeV). On the other hand, the generation of small and intermediate-size DNA fragments (0.1-100 kbp) increased with LET, indicating an increased intratrack long-range clustering of breaks. The fragment size distribution was measured in several size classes down to the smallest class of 0.1-2 kbp. When the clustering was taken into account, the actual number of DNA double-strand breaks (separated by at least 0.1 kbp) could be calculated and was found to be in the range 0.010-0.012 breaks/Mbp Gy(-1). This is two- to threefold higher than the apparent yield obtained by the FAR assay. The measured yield of double-strand breaks as a function of LET is compared with theoretical Monte Carlo calculations that simulate the track structure of energy depositions from helium ions as they interact with the 30-nm chromatin fiber. When the calculation is performed to include fragments larger than 0.1 kbp (to correspond to the experimental measurements), there is good agreement between experiment and theory.