Techniques for radiation measurements: microdosimetry and dosimetry

Experimental microdosimetry is concerned with the determination of radiation quality and how this can be specified in terms of the distribution of energy deposition arising from the interaction of a radiation field with a particular target site. This paper discusses various techniques that have been developed to measure radiation energy deposition over the three orders of magnitude of site-size; nanometer, micrometer and millimetre, which radiation biology suggests is required to fully account for radiation quality. Inevitably, much of the discussion will concern the use of tissue-equivalent proportional counters and variants of this device, but other technologies that have been studied, or are under development, for their potential in experimental microdosimetry are also covered. Through an examination of some of the quantities used in radiation metrology and dosimetry the natural link with microdosimetric techniques will be shown and the particular benefits of using microdosimetric methods for dosimetry illustrated.     

Two miniaturised TEPCS in a single detector for BNCT microdosimetry

Microdosimetry with tissue-equivalent proportional counters (TEPC) has proven to be an ideal dosimetry technique for mixed radiation fields as those ones used in boron neutron capture therapy (BNCT). A new counter, composed of two twin cylindrical mini TEPCs inserted in a slim titanium sleeve of 2.7 mm external diameter, has been constructed. The detector has been designed to perform dosimetry and microdosimetry in intense radiation fields. The two mini TEPCs work in gas flow mode. They have right cylinder sensitive volumes of 0.9 mm. In spite of gas line tiny sizes, the gas pressure inside the two counters is well established with <1% of uncertainty. The counter has been calibrated in a secondary standard photon fields. The mean of the effective sensitive volume sizes has been measured to be 0.86 mm. The twin TEPC acquisition system processes properly the signals up to about 30 kHz of counting rate. Therefore, twin TEPC can perform dosimetric measurements in photon field with intensities of some tens of Gy h(-1).     

Solid state microdosimetry in hadron therapy

A report of recent developments in silicon microdosimetry is presented. SOI based microdosemeters have shown promise as a viable alternative to traditional tissue-equivalent proportional counters. The application of these silicon microdosemeters to such radiation therapy modalities as boron neutron capture therapy (BNCT), boron neutron capture synovectomy (BNCS), proton therapy (PT), and fast neutron therapy (FNT) has been performed. Several shortcomings of the current silicon microdosemeter were identified and will be taken into account in the design of a second-generation device.     

Microdosimetric spectra of the THOR neutron beam for boron neutron capture therapy

A primary objective of the BNCT project in Taiwan, involving THOR (Tsing Hua Open Pool Reactor), was to examine the potential treatment of hepatoma. To characterise the epithermal neutron beam in THOR, the microdosimetry distributions in lineal energy were determined using paired tissue-equivalent proportional counters with and without boron microfoils. Microdosimetry results were obtained in free-air and at various depths in a PMMA phantom near the exit of the beam port. A biological weighting function, dependent on lineal energy, was used to estimate the relative biological effectiveness of the beam. An effective RBE of 2.7 was found at several depths in the phantom.     

Simulations of the mean chord length of a multi-element TEPC irradiated by monoenergetic neutrons

In recent years IRSN has developed tissue-equivalent proportional counters (TEPCs) for neutron monitoring. A detector with a multi-element geometry was studied for personal dosimetry purposes. The determination of the personal dose equivalent using a multi-element TEPC requires to calculate the mean chord length of the charged particles in the counter gas. This paper presents the results of the simulations using the MCNPX code and explains the influence of the gas parameters on the mean chord length and the consequences on the dose equivalent response.     

Application of TEPC microdosimetry to boron neutron capture therapy

Boron neutron capture therapy (BNCT) is a bimodal radiation therapy used primarily for highly malignant gliomas. Tissue-equivalent proportional counter (TEPC) microdosimetry has proven an ideal dosimetry technique for BNCT, facilitating accurate separation of the photon and neutron absorbed dose components, assessment of radiation quality and measurement of the BNC dose. A miniature dual-TEPC system has been constructed to facilitate microdosimetry measurements with excellent spatial resolution in high-flux clinical neutron capture therapy beams. A 10B-loaded TEPC allows direct measurement of the secondary charged particle spectrum resulting from the BNC reaction. A matching TEPC fabricated from brain-tissue-equivalent plastic allows evaluation of secondary charged particle spectra from photon and neutron interactions in normal brain tissue. Microdosimetric measurements performed in clinical BNCT beams using these novel miniature TEPCs are presented, and the advantages of this technique for such applications are discussed.     

Measurements of radiation fields around high-energy proton accelerators

Monitoring of ionising radiation around high-energy particle accelerators is a difficult task due to the complexity of the radiation field, which is made up of neutrons, charged hadrons, muons, photons and electrons, with energy spectra extending over a wide energy range. The dose-equivalent outside a thick shield is mainly owing to neutrons, with some contribution from photons and, to a minor extent, the other particles. Neutron dosimetry and spectrometry are thus of primary importance to correctly evaluate the exposure of personnel. This paper reviews the relevant techniques and instrumentation employed for monitoring radiation fields around high-energy proton accelerators, with particular emphasis on the recent development to increase the response of neutron measuring devices > 20 MeV. Rem-counters, pressurised ionisation chambers, superheated emulsions, tissue-equivalent proportional counters and Bonner sphere spectrometers are discussed.     

Active neutron dosemeters based on microdosimetric principles: research studies

Over the past few years, the Institute for Radiation Protection and Nuclear Safety (IRSN) has been studying a personal electronic neutron dosemeter and an ambient electronic neutron dosemeter based on experimental microdosimetric principles using low pressure proportional counters. The results obtained in 2000 and in 2001 with the cylindrical tissue-equivalent proportional counter developed for use in radiation protection and filled with a low pressure tissue-equivalent gas (propane based) are presented here.     

Radiation dosimetry using three-dimensional optical random access memories

This and twelve previous Symposia reflect the evolution of microdosimetry, a field of research that has determined major new developments in radiation research, radiation protection, and radiology during the past four decades. The concepts of microdosimetry and its techniques were developed almost single handedly by H. H. Rossi. This memorial lecture outlines some of the ideas and some of the work of Harald Rossi that led to microdosimetry. It describes its major impact on radiobiology and, especially, its impact on studies with fast neutrons and on risk assessment. Microdosimetry was primarily designed as a tool for the elucidation of basic mechanisms of radiation action, but it has found its most important applications in the dosimetric measurement techniques that have become indispensable in radiation protection and in the dosimetry for radiation therapy. The advances of molecular biology are now providing new possibilities for a quantitative application of microdosimetry to radiobiology along the lines that Harald Rossi defined.     

Microdosimetry distributions for 40-200 MeV protons

Theoretical calculations have been performed to obtain microdosimetrical characteristics for protons in energy range from 40 to 200 MeV. This energy range is a representative of proton energies in tissue during radiation therapy and it also represents a large portion of the proton fluency in the South Atlantic Anomaly. Distributions of deposited energy calculated using Monte Carlo track structure code TRIOL and own-made programs were compared with experimental data obtained using spherical tissue-equivalent proportional counter. A good agreement between calculated and experimentally obtained microdosimetry spectra has been found.     

Study of a solid state microdosemeter based on a monolithic silicon telescope: irradiations with low-energy neutrons and direct comparison with a cylindrical TEPC

A silicon device based on the monolithic silicon telescope technology coupled to a tissue-equivalent converter was proposed and investigated for solid state microdosimetry. The detector is constituted by a ΔE stage about 2 μm in thickness geometrically segmented in a matrix of micrometric diodes and a residual-energy measurement stage E about 500 μm in thickness. Each thin diode has a cylindrical sensitive volume 9 μm in nominal diameter, similar to that of a cylindrical tissue-equivalent proportional counter (TEPC). The silicon device and a cylindrical TEPC were irradiated in the same experimental conditions with quasi-monoenergetic neutrons of energy between 0.64 and 2.3 MeV at the INFN-Legnaro National Laboratories (LNL-INFN, Legnaro, Italy). The aim was to study the capability of the silicon-based system of reproducing microdosimetric spectra similar to those measured by a reference microdosemeter. The TEPC was set in order to simulate a tissue site about 2 μm in diameter. The spectra of the energy imparted to the ?E stage of the silicon telescope were corrected for tissue-equivalence through an optimized procedure that exploits the information from the residual energy measurement stage E. A geometrical correction based on parametric criteria for shape-equivalence was also applied. The agreement between the dose distributions of lineal energy and the corresponding mean values is satisfactory at each neutron energy considered.     

Studies of UV induced phototransferred thermoluminescence in CaSO4:Dy pellets

Single event spectra of a clinical carbon beam have been measured by an ultra-miniature tissue-equivalent proportional counter (UMC). In order to cover the energy range of the Bragg peak, the incident energy of the carbon beam was degraded by aluminium plates. Single event spectra for carbon-events incident to the UMC were analysed and selected at several carbon energies using thin scintillation counters. It was found that the dose weighted lineal energy distributions have a doublet peak structure due to incident carbon beam and fragment contributions.     

Silicon microdosimetry

Silicon detectors are being studied as microdosemeters since they can provide sensitive volumes of micrometric dimensions. They can be applied for assessing single-event effects in electronic instrumentation exposed to complex fields around high-energy accelerators or in space missions. When coupled to tissue-equivalent converters, they can be used for measuring the quality of radiation therapy beams or for dosimetry. The use of micrometric volumes avoids the contribution of wall effects to the measured spectra. Further advantages of such detectors are their compactness, cheapness, transportability and a low sensitivity to vibrations. The following problems need to be solved when silicon devices are used for microdosimetry: (i) the sensitive volume has to be confined in a region of well-known dimensions; (ii) the electric noise limits the minimum detectable energy; (iii) corrections for tissue-equivalency should be made; (iv) corrections for shape equivalency should be made when referring to a spherical simulated site of tissue; (v) the angular response should be evaluated carefully; (vi) the efficiency of a single detector of micrometric dimensions is very poor and detector arrays should be considered. Several devices have been proposed as silicon microdosemeters, based on different technologies (telescope detectors, silicon on insulator detectors and arrays of cylindrical p-n junctions with internal amplification), in order to satisfy the issues mentioned above.     

First microdosimetric measurements with a TEPC based on a GEM

A new type of mini multi-element tissue-equivalent proportional counter (TEPC) based on a gas electron multiplier (GEM) has been designed and constructed. This counter is in particular suitable to be constructed with a small sensitive volume so that it can be used for microdosimetry in intense pulsed radiation fields to measure the microdosimetric spectrum in the beam of, for instance, a clinical linear accelerator. The concept lends itself also for a mini multi-element version of the counter to be used for applications in which a high sensitivity is required. In this paper, we present the first microdosimetric measurements of this novel counter exposed to a 14 MeV monoenergetic neutron beam and a californium (252Cf) source for a counter cavity diameter of 1.8 mm simulating 1.0 microm tissue site size. The measured spectra showed an excellent agreement with spectra from the literature. The specific advantages of the TEPC-GEM are discussed.     

Miniature semiconductor detectors for in vivo dosimetry

Silicon mini-semiconductor detectors are found in wide applications for in vivo personal dosimetry and dosimetry and microdosimetry of different radiation oncology modalities. These applications are based on integral and spectroscopy modes of metal oxide semiconductor field effect transistor and silicon p-n junction detectors. The advantages and limitations of each are discussed.     

Microdosimetric evaluation of the neutron field for BNCT at Kyoto University reactor by using the PHITS code

In this study, microdosimetric energy distributions of secondary charged particles from the (10)B(n,α)(7)Li reaction in boron-neutron capture therapy (BNCT) field were calculated using the Particle and Heavy Ion Transport code System (PHITS). The PHITS simulation was performed to reproduce the geometrical set-up of an experiment that measured the microdosimetric energy distributions at the Kyoto University Reactor where two types of tissue-equivalent proportional counters were used, one with A-150 wall alone and another with a 50-ppm-boron-loaded A-150 wall. It was found that the PHITS code is a useful tool for the simulation of the energy deposited in tissue in BNCT based on the comparisons with experimental results.     

Secondary exposure for 73 and 200 MeV proton therapy

Following modifications on the beam line at the Orsay Protontherapy Center, dose measurements were performed in order to make a dose map in the treatment rooms and in the delimited radiation-controlled area around beam line. Measurements were performed using tissue-equivalent proportional counters and rem-counters. Analysis of TEPC single event measurements showed that high LET components (>10 keV.mum(-1)) represent 90 to 99% of total dose equivalent in the treatment rooms and 50 to 90% in the controlled area and quality factors range, respectively between 2 and 15. A fast neutron component was identified in the treatment rooms, where dose equivalent rate varied between few muSv.h(-1) to some dozen of mSv.h(-1). In high-energy radiation field rem-counters underestimated TEPC values for neutron component. The variation between instruments response according to the location is linked to energetic spectrum variations and instrument characteristics.     

Modification of ROSPEC to cover neutrons from thermal to 18 MeV

Rotating Spectrometer (ROSPEC) is a neutron spectrometer designed to measure neutron energy distributions, and provide accurate neutron dosimetry. It is a completely self-contained unit and measures neutron energy via recoiling protons in gas proportional counters. Each of the four original gas counters is dedicated to a particular neutron energy range dictated by sensitivity to gamma rays at the low energy end of the spectrum and by proton collisions with the counter walls at the high energy end. Introduced originally in 1992, ROSPEC has a proven operational record with a program of continued upgrades. The operating range of the original ROSPEC spans 50 keV-4.5 MeV. The range of the ROSPEC has now been extended down to include epithermal and thermal neutrons by adding two 2 in. (3)He counters. Also, an optional simple scintillation spectrometer was designed to extend the upper limit of ROSPEC up to 18 MeV.     

Energy deposition stochastics and track structure: what about the target?

The broad field of microdosimetry, as reflected in the proceedings of the 13 previous symposia in this series, has been largely concerned with the microscopic stochastics of energy deposition from ionising radiations of different qualities, the ways in which these can be described and the information that they can provide towards mechanistic understanding of the biological effects of radiation and for practical applications. Directions of the research have been strongly influenced by technical developments at particular times, most notably the tissue-equivalent proportional counter and later Monte Carlo track-structure simulation methods. Essential to the research have been evolving notions as to characteristics of the relevant biological targets, and in particular their sizes and structures in relation to the microscopic features of the radiation. Over the decades since the first Symposium on Microdosimetry, in 1967, emphasis has fluctuated from key targets being assumed to be of nanometre dimensions, then up to one micrometer, ten micrometers, and then back again to a few nanometres. Some of these historical threads are traced through the successive symposia, culminating in current emphasis on the predominant importance of clustered damage in DNA, first revealed by track-structure simulations, but tempered by recognition also of the contribution that novel 'non-targeted' effects may play in the overall biological consequences of radiation.     

Fast neutron spectrometry and dosimetry using a spherical moderator with position-sensitive detectors

A neutron spectrometry and dosimetry measurement system has been developed based on a different design of the divided regions for a sphere, with three position-sensitive counters. The characteristics of the measurement system have been investigated in the reference radiation fields of Am-Be and (252)Cf sources. When realistic input spectra are used for the unfolding, the overall deviations of the calculated results for four dosimetric quantities are less than +/-10%. The results of other input spectra are also discussed in this report.