Tissue-equivalent Ionization Chamber Research Articles

Effects of tissue heterogeneity and comparisons of collapsed cone and Monte Carlo fast neutron patient dosimetry using the University of Washington clinical neutron therapy system (CNTS)

Fast neutron therapy is a high linear energy transfer (LET) radiation treatment modality offering advantages over low LET radiations. Multileaf collimator technology reduces normal-tissue dose (toxicity) and makes neutron therapy more comparable to MV x-ray treatments. Published clinical-trial and other experiences with fast neutron therapy are reported. Early comparative studies failed to consider differences in target-dose spatial conformality between x-ray and neutron treatments, which is especially important for organs-at-risk close to tumor targets. Treatments planning systems (TPS) for high-energy neutrons lag behind TPS tools for MV x-rays, creating challenges for comparative studies of clinical outcomes. A previously published Monte Carlo model of the University of Washington (UW) Clinical Neutron Therapy System (CNTS) is refined and integrated with the RayStation TPS as an external dose planning/verification tool. The collapsed cone (CC) dose calculations in the TPS are based on measured dose profiles and output factors in water, with the absolute dose determined using a tissue-equivalent ionization chamber. For comparison, independent (external) Monte Carlo simulation computes dose on a voxel-by-voxel basis using an atlas that maps Hounsfield Unit (HU) numbers to elemental composition and density. Although the CC algorithm in the TPS accurately computes neutron dose to water compared to Monte Carlo calculations, calculated dose to water differs from bone or tissue depending largely on hydrogen content. Therefore, the elemental composition of tissue and bone, rather than the material or electron density, affects fast neutron dose. While the CC algorithm suffices for reproducible patient dosimetry in fast neutron therapy, adopting methods that consider tissue heterogeneity would enhance patient-specific neutron dose accuracy relative to national standards for other types of ionizing radiation. Corrections for tissue composition have a significant impact on absolute dose and the relative biological effectiveness (RBE) of neutron treatments compared to other radiation types (MV x-rays, protons, and carbon ions).

Traceable dosimetry for MeV ion beams

Standard dosimetry protocols exist for highly penetrating photon and particle beams used in the clinic and in research. However, these protocols cannot be directly applied to shallow penetration MeV-range ion beams. The Radiological Research Accelerator Facility has been using such beams for almost 50 years to irradiate cell monolayers, using self-developed dosimetry, based on tissue equivalent ionization chambers. To better align with the internationally accepted standards, we describe implementation of a commercial, NIST-traceable, air-filled ionization chamber for measurement of absorbed dose to water from low energy ions, using radiation quality correction factors calculated using TRS-398 recommendations. The reported dose does not depend on the ionization density in the range of 10–150 keV/μm.

In Memoriam: Marvin A. Van Dilla: 1919-2019.

© 2020 International Society for Advancement of Cytometry.

Experimental determination of the epithermal neutron sensitivity of a new ionization chamber

Introduction The resulting improvement in the accuracy of the determination of the dose from gamma rays and epithermal and fast neutrons is important in view of the required accuracy for dosimetry in accelerator based neuron sources. Purpose A new ionization chamber detector is being developed for the epithermal neutron dosimetry at accelerator based neuron sources. To obtain a high accuracy in the dosimetry of an epithermal neutron beam used for boron–neutron capture therapy (BNCT), the neutron sensitivity of dosimeters applied to determine the various dose components in polyethylene phantom has been investigated. Materials and methods Well-stabilized neutron filed of Neutron exposure Accelerator System for Biological Effect Experiments (NASBEE) in National Institute of Radiological Sciences (NIRS) was used in this study. The epithermal neutron sensitivity LBO ionization chamber, tissue equivalent ionization chamber, and optically stimulated luminescence (OSL) dosimeters has been experimentally determined in a neutron beam fields. Results The epithermal neutron values much higher than theoretically expected were obtained and a variation up to a factor of 1.8 was found between values for the epithermal neutron sensitivity. The ratio of a gamma dose and a thermal neutron dose was 23% in a depth of 30 mm. Conclusion The new epithermal ionization chamber response characteristic was evaluated at the accelerator based neutron sources field. Disclosure This work was supported by JSPS Kakenhi Grant Number 15K00544 .

Commissioning of intensity modulated neutron radiotherapy (IMNRT)

Intensity modulated neutron radiotherapy (IMNRT) has been developed using inhouse treatment planning and delivery systems at the Karmanos Cancer Center∕Wayne State University Fast Neutron Therapy facility. The process of commissioning IMNRT for clinical use is presented here. Results of commissioning tests are provided including validation measurements using representative patient plans as well as those from the TG-119 test suite. IMNRT plans were created using the Varian Eclipse optimization algorithm and an inhouse planning system for calculation of neutron dose distributions. Tissue equivalent ionization chambers and an ionization chamber array were used for point dose and planar dose distribution comparisons with calculated values. Validation plans were delivered to water and virtual water phantoms using TG-119 measurement points and evaluation techniques. Photon and neutron doses were evaluated both inside and outside the target volume for a typical IMNRT plan to determine effects of intensity modulation on the photon dose component. Monitor unit linearity and effects of beam current and gantry angle on output were investigated, and an independent validation of neutron dosimetry was obtained. While IMNRT plan quality is superior to conventional fast neutron therapy plans for clinical sites such as prostate and head and neck, it is inferior to photon IMRT for most TG-119 planning goals, particularly for complex cases. This results significantly from current limitations on the number of segments. Measured and calculated doses for 11 representative plans (six prostate∕five head and neck) agreed to within -0.8 ± 1.4% and 5.0 ± 6.0% within and outside the target, respectively. Nearly all (22∕24) ion chamber point measurements in the two phantom arrangements were within the respective confidence intervals for the quantity [(measured-planned)∕prescription dose] derived in TG-119. Mean differences for all measurements were 0.5% (max = 7.0%) and 1.4% (max = 4.1%) in water and virtual water, respectively. The mean gamma pass rate for all cases was 92.8% (min = 88.6%). These pass rates are lower than typically achieved with photon IMRT, warranting development of a planar dosimetry system designed specifically for IMNRT and∕or the improvement of neutron beam modeling in the penumbral region. The fractional photon dose component did not change significantly in a typical IMNRT plan versus a conventional fast neutron therapy plan, and IMNRT delivery is not expected to significantly alter the RBE. All other commissioning results were considered satisfactory for clinical implementation of IMNRT, including the external neutron dose validation, which agreed with the predicted neutron dose to within 1%. IMNRT has been successfully commissioned for clinical use. While current plan quality is inferior to photon IMRT, it is superior to conventional fast neutron therapy. Ion chamber validation results for IMNRT commissioning are also comparable to those typically achieved with photon IMRT. Gamma pass rates for planar dose distributions are lower than typically observed for photon IMRT but may be improved with improved planar dosimetry equipment and beam modeling techniques. In the meantime, patient-specific quality assurance measurements should rely more heavily on point dose measurements with tissue equivalent ionization chambers. No significant technical impediments are anticipated in the clinical implementation of IMNRT as described here.

Dosimetry measurements at the fast neutron therapy facility in Seattle

Abstract The fast neutron therapy facility at the University of Washington has been in routine clinical use for 25 years. 50.5 MeV protons produce neutrons in a beryllium target mounted on an isocentric gantry. Beam shaping is accomplished with a 40-leaf collimator. Dosimetry measurements for treatment planning and calibration are performed with tissue equivalent ion chambers. A layered phantom of alternating Solid Water ® and Plastic Water ® slabs has been developed for rapid dose verification measurements. The neutron field in the room has been used for radiation testing of electronic components.

Simulation of the Charge Produced by Protons inside a Tissue Equivalent Ionization Chamber in a Mixed Neutron/Gamma Field in BNCT

The neutron and gamma dose in boron neutron capture therapy (BNCT) can be determined by using ionization chambers of different materials. However, inexplicable results, such as negative doses, are sometimes obtained. Computer simulations using MCNPX can help one to understand the behavior of ionization chambers. This paper deals with a part of this investigation: the contribution of protons to the total measured charge in a tissue equivalent (TE) ionization chamber that is flushed with methane-based TE gas. The inherent problem is that the Monte Carlo code MCNPX cannot track protons below 1 MeV.A custom-made program, called Proton Produced Ionization Chamber Charge (PPICC), calculates the deposited energy and thus the charge in the TE gas per proton. For this, it uses the stopping powers for protons in TE plastic and gas. MCNPX provides the total number of protons produced by all neutron interactions near the gas. To check this new procedure, measurements and simulations have been performed using a validated mixed beam of neutrons and gammas. The neutron fluence consists of 12% fast neutrons and 87% epithermal neutrons. In one setup the chamber is free-in-air (epithermal/fast neutron field) and in the other is in a cubic polymethylmethacrylate phantom at 25 mm depth (thermal/epithermal neutron field).The total charge is the sum of the charges due to electrons, originating from primary and neutron-induced gammas, and protons from 1H(n,n)1H and 14N(n,p)14C reactions. The total measured and calculated charges in the two setups have acceptable uncertainties and are in good agreement. The charge collected in a TE ionization chamber can be simulated in a mixed field of neutrons and gammas. The charge resulting from proton recoil in the gas is unexpectedly large.

Stability of A-150 plastic ionisation chamber response over a 30-y period

At the Northern Illinois University Institute for Neutron Therapy at Fermilab, the clinical tissue-equivalent ionisation chamber response is measured every treatment day using a cesium source that was configured to match readings obtained at the National Bureau of Standards. Daily measurements are performed in air using the air-to-tissue dose conversion factors given in AAPM Report #7. The measured exposure calibration factors have been tabulated and graphed as a function of time from 1978 to present. For A-150 plastic ionisation chambers, these factors exhibit a sinusoidal variation with a period of approximately 1 y and amplitude of +/- 1%. This variation, attributable to the hygroscopic nature of A-150 plastic, is correlated with the relative humidity of the facility, and is greater than the humidity corrections for gas described in the literature. The data suggest that chamber calibration should be performed at least weekly to accommodate these variations.

High-energy neutron depth-dose distribution experiment

A unique set of high-energy neutron depth-dose benchmark experiments were performed at the Los Alamos Neutron Science Center/Weapons Neutron Research (LANSCE/WNR) complex. The experiments consisted of filtered neutron beams with energies up to 800 MeV impinging on a 30 x 30 x 30 cm3 liquid, tissue-equivalent phantom. The absorbed dose was measured in the phantom at various depths with tissue-equivalent ion chambers. This experiment is intended to serve as a benchmark experiment for the testing of high-energy radiation transport codes for the international radiation protection community.

Study of the dosimetric characteristics of cosmic radiation at civil aviation altitudes.

The dependence of the doses on solar activity for intermediate levels of the solar modulation parameter has been studied by means of simulations carried out by the Monte Carlo transport code FLUKA. The vertical cut-off rigidities investigated lie between 0.4 and 6.1 GV. The calculated results show that the linear dependence proposed in a previous work, for the effective dose rate as a function of the solar modulation parameter, can be considered as an acceptable approximation. In addition, some dosimetric characteristics of cosmic radiation and some properties of the dosemeters in use for monitoring in the cosmic ray environment have been analysed with a view to simplifying measurements. The depth-dose curves in the ICRU sphere and the response of a tissue-equivalent ionisation chamber have been determined by the FLUKA code for a number of cosmic ray spectra On the basis of the calculated results, it is concluded that a value of the depth. d, which would make the ambient dose equivalent a conservative predictor of the effective dose, cannot be specified for cosmic radiation. However, the operational quantity can be useful in order to verify the predictions of Monte Carlo calculations. It is demonstrated that a crude approximation of the ambient dose equivalent could be obtained by multiplying by 2 the absorbed dose measured by a tissue-equivalent ionisation chamber with wall thickness of 10 mm.

Simulation of neutron interactions at the single-cell level.

We recently reported that the exposure of cancer cells to 14 MeV neutrons at a very low dose rate (0.8 mGy min(-1)) produced a marked increase in cell killing at 5 cGy, followed by a plateau in survival and chromosomal damage. Simulation of the energy deposition events in irradiated cells may help to explain these unusual cell responses. We describe here a Monte Carlo simulation code, Energy Deposition in Cells Irradiated by Neutrons (EDCIN). The procedure considered the experimental setup and a hemispheric cell model. The simulation data fitted the dosimetric measurements performed using tissue-equivalent ionization chambers, Geiger-Müller counters, fission chambers, and silicon diodes. The simulation showed that 80% of the energy deposited in a single cell came from the interactions of neutrons outside the cell and only 20% came from neutron interactions inside the cell. Thus the "external" interactions that result in the production of recoil protons and secondary electrons may induce most of the biological damage, which may be repaired efficiently at low dose rate. The repair process may be triggered from a threshold level of damage, which would explain an initial increase cell death due to unrepaired sublethal damage, and then may compensate for induced damage, resulting in the plateaus.

Measurement of absorbed dose distributions in a plastic phantom irradiated by 40- and 65-MeV quasi-monoenergetic neutrons

In order to validate dose evaluation methods for radiation shielding design of accelerator facilities, absorbed dose distributions in a plastic phantom of 30×30×30 cm 3 slab were measured for 40- and 65-MeV quasi-monoenergetic neutrons with a tissue-equivalent proportional counter and a tissue-equivalent ionization chamber. The measured distributions were compared with the calculated ones from the energy deposition of charged particles and neutron energy spectra by using the Monte Carlo codes, HETC-3STEP and MORSE-CG/KFA, respectively. As a result, it was revealed that the calculation method used in the present work was applicable to absorbed dose evaluation for intermediate energy neutrons with high accuracy.

Modification of the University of Washington Neutron Radiotherapy Facility for optimization of neutron capture enhanced fast-neutron therapy.

A modified neutron production target assembly has been developed to provide improved performance of the proton-cyclotron-based neutron radiotherapy facility at the University of Washington for applications involving neutron capture enhanced fast-neutron therapy. The new target produces a neutron beam that yields essentially the same fast-neutron physical depth-dose distribution as is produced by the current UW clinical system, but that also has an increased fraction of BNCT enhancement relative to the total therapeutic dose. The modified target is composed of a 5-millimeter layer of beryllium, followed by a 2.5-millimeter layer of tungsten, with a water-cooled copper backing. Measurements of the free-field neutron spectrum of the beam produced by the new target were performed using activation foils with a direct spectral unfolding technique. Water phantom measurements were performed using a tissue-equivalent ion chamber to characterize the fast-neutron depth-dose curve and sodium activation in soda-lime glass beads to characterize the thermal-neutron flux (and thus the expected neutron capture dose enhancement) as a function of depth. The results of the various measurements were quite consistent with expectations based on the design calculations for the modified target. The spectrum of the neutron beam produced by the new target features an enhanced low-energy flux component relative to the spectrum of the beam produced by the standard UW target. However, it has essentially the same high-energy neutron flux, with a reduced flux component in the mid-range of the energy spectrum. As a result, the measured physical depth-dose curve in a large water phantom has the same shape compared to the case of the standard UW clinical beam, but approximately twice the level of BNCT enhancement per unit background neutron dose at depths of clinical interest. In-vivo clinical testing of BNCT-enhanced fast-neutron therapy for canine lung tumors using the new beam was recently initiated.

Relative Neutron Sensitivity of Tissue-Equivalent Ionisation Chambers in an Epithermal Neutron Beam for Boron Neutron Capture Therapy

In recent years, boron neutron capture therapy has received renewed interest as new promising boron compounds and epithermal neutron beams have become available. For establishment of the different dose components in epithermal neutron beams, various techniques are applied, including the use of tissue-equivalent (TE) ionisation chambers for the determination of the intermediate and fast neutron dose. This paper is directed towards the calculation of the relative neutron sensitivity of TE ionisation chambers, kT, for the free-in-air spectrum at the HB11 epithermal beam facility of the High Flux Reactor in Petten. The kT value obtained from the present calculations is 0.87. This value is about 10% lower than data currently applied for similar conditions.

The Neutron Response of AL2O3:C, 7LiF:Mg,Cu,P and 7LiF:Mg,Ti TLDs

The high photon sensitivity demonstrated by the newly available phosphors, Al 2 O 3 :C and 7 LiF:Mg,Cu,P, makes them attractive for use in personnel and environmental dosimetry. For potential application in mixed neutron-gamma fields, the relative neutron:gamma sensitivity, k u , of these thermoluminescence dosemeters (TLDs) was measured, along with that of the widely used TLD-700 ( 7 LiF:Mg,Ti) for comparison. Narrow spectra neutrons of energies from 0.3 to 14 MeV produced by a Van de Gruff accelerator were used to irradiate pairs of TLDs. The neutron and gamma doses were measured independently using a tissue-equivalent ionisation chamber in combination with a compensated Geiger-Muller detector. For neutron energies from 0.3 to 6.0 MeV, the k u of Al 2 O 3 °C was about one tenth of that of TLD-700. The k u of 7 LiF:Mg,Cu,P was about one third that of TLD-700 for the same neutron energy range. All phosphors showed greater response to 14 MeV neutrons, but the relative neutron:gamma sensitivity of the newer phosphors was one third to one fifth that of TLD-700.

Dosimetry characteristics of four fast neutron generators involved in rtog interinstitutional clinical trials

Purpose : To demonstrate the consistency of dosimetry data used for four fast neutron generators involved in interinstitutional clinical trials. Methods and Materials : Central-axis dosimetry characteristics (field size dependence, percentage depth dose, beam modifier factors) at four institutions were measured by independent physicists from the Radiological Physics Center. These measurements were made in water with a single set of dosimetry equipment using tissue equivalent ionization chambers. Measurements were made with the chamber cavities filled with stationary air and flowing tissue-equivalent gas. All measurements were performed using techniques developed by the Radiological Physics Center for conventional radiotherapy equipment. The results of our measurements were compared to the data used by the institution to determine the consistency of patient doses reported to the Radiation Therapy Oncology Group. Results : The agreement of the Radiological Physics Center and institution data is similar to what is typically found with conventional radiotherapy equipment. Conclusions : The doses reported by these institutions to the Fast Neutron Working Group of the Radiation Therapy Oncology Group are consistent, within accepted clinical criteria.

Variance-Covariance Measurements in Therapeutic and Diagnostic Radiation Beams

Abstract Cylindrical tissue-equivalent twin ionisation chambers have been developed based on experience with proportional counters for application in microdosimetric variance-covariance measurements of the dose mean lineal energy, yD, in therapeutic and diagnostic radiation beam qualities. It is concluded that the method is applicable but the size of the detector signals has to be optimised very carefully in respect to noise reduction and the total relative variance. It is proposed that results from the proportional mode and the ionisation mode are compared to obtain variance. It is proposed that results from the proportional mode and the ionisation mode are compared to obtain reliable values for the microdosimetric beam quality characterisation necessary to assess clinically relevant differences.

Measurement of the restricted LET of orthovoltage and low-energy X-rays in tissue-equivalent plastic

Systematic measurements of the dose mean restricted LET (L500,D, Delta =500 eV) in air and water are reported for X-rays ranging from 10 to 300 kVp. L500,D has been measured with a specially designed high-pressure tissue-equivalent (TE) ionization chamber. The water measurements are performed as a function of depth, field size and distance from the field edge. The experimental methods and results are described.

Code of practice for clinical proton dosimetry

The objective of this document is to make recommendations for the determination of absorbed dose to tissue for clinical proton beams and to achieve uniformity in proton dosimetry. A Code of Practice has been chosen, providing specific guidelines for the choice of the detector and the method of determination of absorbed dose for proton beams only. This Code of Practice is confined specifically to the determination of absorbed dose and is not concerned with the biological effects of proton beams. It is recommended that dosimeters be calibrated by comparison with a calorimeter. If this is not available, a Faraday cup, or alternatively, an ionization chamber, with a 60Co calibration factor should be used. Physical parameters for determining the dose from tissue-equivalent ionization chamber measurements are given together with a worksheet. It is recommended that calibrations be carried out in water at the centre of the spread-out-Bragg-peak and that dose distributions be measured in a water phantom. It is estimated that the error in the calibrations will be less than ± 5% (1 S.D.) in all cases. Adoption and implementation of this Code of Practice will facilitate the exchange of clinical information.

A comparison of two methods of in vivo dosimetry for a high energy neutron beam

Two methods of in vivo dosimetry have been compared in a high energy neutron beam. These were activation dosimetry and thermoluminescence dosimetry (TLD). Their suitability was determined by comparison with estimates of total dose, obtained using a tissue equivalent ionization chamber. Measurements were made on the central axis and a profile of a 10 x 10 cm square field and also behind a shielding block in order to simulate conditions of clinical use. The TLD system was found to provide the best estimate of total dose.