Eric Burgett

Secondary neutron spectra from modern Varian, Siemens, and Elekta linacs with multileaf collimators

Neutrons are a by-product of high-energy x-ray radiation therapy (threshold for [gamma,n] reactions in high-Z material -7 MeV). Neutron production varies depending on photon beam energy as well as on the manufacturer of the accelerator. Neutron production from modern linear accelerators (linacs) has not been extensively compared, particularly in terms of the differences in the strategies that various manufacturers have used to implement multileaf collimators (MLCs) into their linac designs. However, such information is necessary to determine neutron dose equivalents for different linacs and to calculate vault shielding requirements. The purpose of the current study, therefore, was to measure the neutron spectra from the most up-to-date linacs from three manufacturers: Varian 21EX operating at 15, 18, and 20 MV, Siemens ONCOR operating at 15 and 18 MV, and Elekta Precise operating at 15 and 18 MV. Neutron production was measured by means of gold foil activation in Bonner spheres. Based on the measurements, the authors determined neutron spectra and calculated the average energy, total neutron fluence, ambient dose equivalent, and neutron source strength. The shapes of the neutron spectra did not change significantly between accelerators or even as a function of treatment energy. However, the neutron fluence, and therefore the ambient dose equivalent, did vary, increasing with increasing treatment energy. For a given nominal treatment energy, these values were always highest for the Varian linac. The current study thus offers medical physicists extensive information about the neutron production of MLC-equipped linacs currently in operation and provides them information vital for accurate comparison and prediction of neutron dose equivalents and calculation of vault shielding requirements.

Effects of tertiary MLC configuration on secondary neutron spectra from 18 MV x-ray beams for the Varian 21EX linear accelerator

The effect of the jaw configuration and the presence and configuration of the tertiary multileaf collimator (MLC) on the secondary neutron spectra for an 18 MV Varian 21EX linear accelerator (linac) is investigated in detail. The authors report the measured spectra for four collimator (jaw-and-MLC) configurations. These configurations represent the extreme settings of the jaws and MLC and should therefore describe the range of possible fluence and spectra that may be encountered during use of this linac. In addition to measurements, a Monte Carlo model was used to simulate the four collimator configurations and calculate the energy spectra and fluence at the same location as it was measured. The Monte Carlo model was also used to calculate the sources of neutron production in the linac head for each collimator configuration. They found that photoneutron production in the linac treatment head is dominated by the order in which the primary photon beam intercepts the high-Z material. The primary collimator, which has the highest position in the linac head (in a fixed location), is the largest source of secondary neutrons. Thereafter, the collimator configuration plays a role in where the neutrons originate. For instance, if the jaws are closed, they intercept the beam and contribute substantially to the secondary neutron production. Conversely, if the jaws are open, the MLC plays a larger role in neutron production (assuming, of course, that it intercepts the beam). They found that different collimator configurations make up to a factor of 2 difference in the ambient dose equivalent.

Liposome-based delivery of a boron-containing cholesteryl ester for high-LET particle-induced damage of prostate cancer cells: A boron neutron capture therapy study

Abstract Purpose: The efficacy of a boron-containing cholesteryl ester compound (BCH) as a boron neutron capture therapy (BNCT) agent for the targeted irradiation of PC-3 human prostate cancer cells was examined. Materials and methods: Liposome-based delivery of BCH was quantified with inductively coupled plasma-mass spectrometry (ICP-MS) and high-performance liquid chromatography (HPLC). Cytotoxicity of the BCH-containing liposomes was evaluated with neutral red, 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS), and lactate dehydrogenase assays. Colony formation assays were utilized to evaluate the decrease in cell survival due to high-linear energy transfer (LET) particles resulting from 10B thermal neutron capture. Results: BCH delivery by means of encapsulation in a lipid bilayer resulted in a boron uptake of 35.2 ± 4.3 μg/109 cells, with minimal cytotoxic effects. PC-3 cells treated with BCH and exposed to a 9.4 × 1011 n/cm2 thermal neutron fluence yielded a 20–25% decrease in clonogenic capacity. The decreased survival is attributed to the generation of high-LET α particles and 7Li nuclei that deposit energy in densely ionizing radiation tracks. Conclusion: Liposome-based delivery of BCH is capable of introducing sufficient boron to PC-3 cells for BNCT. High-LET α particles and 7Li nuclei generated from 10B thermal neutron capture significantly decrease colony formation ability in the targeted PC-3 cells.

Measurement of High-Energy Neutron Spectra with a Bonner Sphere Extension System

Abstract Above 20 MeV the standard spheres of a Bonner sphere spectrometer (BSS) have similar responses, both in shape and sensitivity. The responses of the standard set also exhibit strongly diminishing sensitivities above 20 MeV. In the current work the Monte Carlo N-Particle eXtended (MCNPX) code was used to investigate different design modifications to increase the high-energy neutron response of a BSS. The cost-effective system expands upon the design of an existing, commercially available BSS system by adding concentric shells of copper, tungsten, and lead. These shells are used in various combinations with the existing spheres. The design, referred to as the Bonner sphere extension (BSE), incorporates both passive and active detection techniques including activation foils and the standard 6LiI(Eu) scintillator. Detailed models in MCNPX were used to create fine-group neutron responses from thermal to 1000 MeV. Measurements were performed with the BSE at Los Alamos Neutron Science Center, and the data were unfolded using the MXD-FC33 code and the calculated BSE response matrix. The resulting spectrum demonstrated the BSE system provided improvement in the measurement of the neutron spectra in the energy regions above 20 MeV when compared to the standard Bonner sphere system. The BSE system extends the sensitivity of the system to more than ten decades in energy while maintaining a nearly isotropic angular response.

Using handheld plastic scintillator detectors to triage individuals exposed to a radiological dispersal device

After a radiological dispersal device (RDD) event, people could become internally contaminated by inhaling dispersed radioactive particles. A rapid method to screen individuals who are internally contaminated is desirable. Such initial screening can help in prompt identification of those who are highly contaminated and in prioritising individuals for further and more definitive evaluation such as laboratory testing. The use of handheld plastic scintillators to rapidly screen those exposed to an RDD with gamma-emitting radionuclides was investigated in this study. The Monte Carlo N-Particle transport code was used to model two commercially available plastic scintillation detectors in conjunction with anthropomorphic phantom models to determine the detector response to inhaled radionuclides. Biokinetic models were used to simulate an inhaled radionuclide and its progression through the anthropomorphic phantoms up to 30 d after intake. The objective of the study was to see if internal contamination levels equivalent to 250 mSv committed effective dose equivalent could be detected using these instruments. Five radionuclides were examined: (60)Co, (137)Cs, (192)Ir, (131)I and (241)Am. The results demonstrate that all of the radionuclides except (241)Am could be detected when placing either one of the two plastic scintillator detector systems on the posterior right torso of the contaminated individuals.

Guidance on the use of handheld survey meters for radiological triage: time-dependent detector count rates corresponding to 50, 250, and 500 mSV effective dose for adult males and adult females.

AbstractIn June 2006, the Radiation Studies Branch of the Centers for Disease Control and Prevention held a workshop to explore rapid methods of facilitating radiological triage of large numbers of potentially contaminated individuals following detonation of a radiological dispersal device. Two options were discussed. The first was the use of traditional gamma cameras in nuclear medicine departments operated as makeshift whole-body counters. Guidance on this approach is currently available from the CDC. This approach would be feasible if a manageable number of individuals were involved, transportation to the relevant hospitals was quickly provided, and the medical staff at each facility had been previously trained in this non-traditional use of their radiopharmaceutical imaging devices. If, however, substantially larger numbers of individuals (100’s to 1,000’s) needed radiological screening, other options must be given to first responders, first receivers, and health physicists providing medical management. In this study, the second option of the workshop was investigated—the use of commercially available portable survey meters (either NaI or GM based) for assessing potential ranges of effective dose (<50, 50–250, 250–500, and >500 mSv). Two hybrid computational phantoms were used to model an adult male and an adult female subject internally contaminated with 241Am, 60Cs, 137Cs, 131I, or 192Ir following an acute inhalation or ingestion intake. As a function of time following the exposure, the net count rates corresponding to committed effective doses of 50, 250, and 500 mSv were estimated via Monte Carlo radiation transport simulation for each of four different detector types, positions, and screening distances. Measured net count rates can be compared to these values, and an assignment of one of four possible effective dose ranges could be made. The method implicitly assumes that all external contamination has been removed prior to screening and that the measurements be conducted in a low background, and possibly mobile, facility positioned at the triage location. Net count rate data are provided in both tabular and graphical format within a series of eight handbooks available at the CDC website (http://www.bt.cdc.gov/radiation/clinicians/evaluation).

Measured Neutron Spectra and Dose Equivalents From a Mevion Single-Room, Passively Scattered Proton System Used for Craniospinal Irradiation

PURPOSE To measure, in the setting of typical passively scattered proton craniospinal irradiation (CSI) treatment, the secondary neutron spectra, and use these spectra to calculate dose equivalents for both internal and external neutrons delivered via a Mevion single-room compact proton system. METHODS AND MATERIALS Secondary neutron spectra were measured using extended-range Bonner spheres for whole brain, upper spine, and lower spine proton fields. The detector used can discriminate neutrons over the entire range of the energy spectrum encountered in proton therapy. To separately assess internally and externally generated neutrons, each of the fields was delivered with and without a phantom. Average neutron energy, total neutron fluence, and ambient dose equivalent [H* (10)] were calculated for each spectrum. Neutron dose equivalents as a function of depth were estimated by applying published neutron depth-dose data to in-air H* (10) values. RESULTS For CSI fields, neutron spectra were similar, with a high-energy direct neutron peak, an evaporation peak, a thermal peak, and an intermediate continuum between the evaporation and thermal peaks. Neutrons in the evaporation peak made the largest contribution to dose equivalent. Internal neutrons had a very low to negligible contribution to dose equivalent compared with external neutrons, largely attributed to the measurement location being far outside the primary proton beam. Average energies ranged from 8.6 to 14.5 MeV, whereas fluences ranged from 6.91 × 10(6) to 1.04 × 10(7) n/cm(2)/Gy, and H* (10) ranged from 2.27 to 3.92 mSv/Gy. CONCLUSIONS For CSI treatments delivered with a Mevion single-gantry proton therapy system, we found measured neutron dose was consistent with dose equivalents reported for CSI with other proton beamlines.

Energy Response and Angular Dependence of a Bonner Sphere Extension

A Bonner Sphere Spectrometer (BSS) is a widely used neutron spectrometer in the health physics and research arenas. It provides the means to measure neutron spectra over many orders of magnitude in energy with a single instrument, but has decreased response above 20-MeV. To increase the sensitivity of BSS at higher energies, a low cost, easily fabricated Bonner Sphere Extension (BSE) has been designed and constructed. The BSE extends the sensitivity of the BSS to higher energies and utilizes either an active (LiI(Eu) scintillator) or passive (gold foil activation) detector. Two factors which affect the uncertainties in unfolded neutron spectra are the energy dependence arising from the cross sections used to generate the response functions and the directional dependence of the measurement system. We assessed the energy dependence of the BSE by calculating the response functions using two different cross section libraries (ENDF B. VI and ACTL). These response functions were then used to unfold data measured with the BSE at the WNR facility at Los Alamos Neutron Science Center (LANSCE). The spectrum unfolded with the ENDF B.VI response functions was in better agreement with the LANSCE time of flight (TOF) spectrum. We evaluated the angular response of the BSE by unfolding data which were measured with the detector parallel, perpendicular, and at a 45deg angle to the incident neutron beam using response functions computed in three directional orientations (parallel, perpendicular, and at a 45deg angle). Directional dependence was found to be more significant for the passive detector, especially when used with small moderating spheres.

Experimental validation of the new nanodosimetry-based cell survival model for mixed neutron and gamma-ray irradiation.

The new nanodosimetry-based linear-quadratic (LQ) formula has been reviewed for mixed-LET irradiation. V-79 Chinese hamster cells have been irradiated with a mixed-LET field of fission neutrons and gamma rays at the University of Maryland Training Reactor (MUTR). The results show that the experimental survival curve agrees well with that predicted by the new nanodosimetry-based LQ model. The experimental study described in this note, therefore, serves as a validation for the new model to be used for mixed-LET radiotherapies, e.g. 252Cf brachytherapy.

GaN as a neutron detection material

GaN is a promising material for neutron detection applications, with advantages over Si and GaAs. GaN films doped with Gd have been grown by MOCVD and investigated for their feasibility for neutron detection. The films were structurally and electrically characterized through HRXRD and Hall effect measurements. Alpha particle luminescence of both doped and undoped films was used to investigate gamma discrimination properties.