Dose quantities in radiation protection and their limitations

For more than 50 years the quantity absorbed dose has been the basic physical quantity in the medical applications of ionising radiation as well as radiological protection against harm from ionising radiation. In radiotherapy relatively high doses are applied (to a part of the human body) within a short period and the absorbed dose is mainly correlated with deterministic effects such as cell killing and tissue damage. In contrast, in radiological protection one is dealing with low doses and low dose rates and long-term stochastic effects in tissue such as cancer induction. The dose quantity (absorbed dose) is considered to be correlated with the probability of cancer incidence and thus risk induced by exposure. ICRP has developed specific dosimetric quantities for radiological protection that allow the extent of exposure to ionising radiation from whole and partial body external radiation as well as from intakes of radionuclides to be taken into account by one quantity. Moreover, radiological protection quantities are designed to provide a correlation with risk of radiation induced cancer. In addition, operational dose quantities have been defined for use in measurements of external radiation exposure and practical applications. The paper describes the concept and considerations underlying the actual system of dose quantities, and discusses the advantage as well as the limitations of applicability of such a system. For example, absorbed dose is a non-stochastic quantity defined at any point in matter. All dose quantities in use are based on an averaging procedure. Stochastic effects and microscopic biological and energy deposition structures are not considered in the definition. Absorbed dose is correlated to the initial very short phase of the radiation interaction with tissue while the radiation induced biological reactions of the tissue may last for minutes or hours or even longer. There are many parameters other than absorbed dose that influence the process of cancer induction, which may influence the consideration of cells and/or tissues at risk which are most important for radiological protection.     

Radiation protection concepts and quantities for the occupational exposure to cosmic radiation.

For the purposes of dose limitation and dose control, the harm, or detriment, of exposure to radiation is assessed by the quantity effective dose. Effective dose is evaluated by the application of factors to the averaged absorbed dose in the organs and tissues of the body. Radiation monitoring instruments are generally calibrated in terms of the quantity ambient dose equivalent which is defined in a simple spherical phantom. The relationship of these quantities is described. Requirements for the radiation protection of aircraft crew are given in the European Union Council Directive 96/29/EURATOM. There are requirements to assess the exposure of aircraft crew, to inform them of health risks, to reduce higher doses, and to control the dose to the fetus. There are no explicit dose limits, other than a dose objective to be applied to the exposure of the fetus, and no requirements for designation of areas or classification of workers. There are significant differences between the exposure condition of aircraft crew and workers in most other industries where there is occupational exposure to radiation. There are greater ranges of radiation types and energy, and there are different dose distributions and characteristics of the working populations. However, the field intensity is predictable and, with the exception of rare solar events, there is no risk of significant unexpected exposures. Dose assessment is anticipated to be by folding staff roster information with estimates of route doses, since there is little variability of dose rate within an aircraft. Route doses, which may be either an agreed average value for a given airport pairing and aircraft type, or be flight specific, will be closely linked to measured values. Requirements as to the accuracy of dose assessment should be applied which are broadly similar to those used in individual monitoring generally.     

Optical images of dose distributions in Gel-Fricke: dosimetric performances of the gel

The purpose of this work was to examine the dosimetric performances of the radiochromic Fricke-Agarose-Xylenol Orange gel by optical measurements in order to perform dose reconstructions, in view of a future development for 3-D maps. Optical images and dose-response curves of the gel were obtained by a CCD-based device, originally designed for reading radiochromic films, that was modified to meet the optical properties of the dosemeter. With a resolution of 0.18 x 0.18 mm the optimum range of doses in which per cent uncertainty is lower than 2% was 3-10 Gy. The minimum detectable dose, estimated as the absorbed dose corresponding to 3 SD above background, was 0.1 Gy. With a resolution of 1.98 x 1.98 mm the optimum range of doses in which per cent uncertainty is lower than 2% was 0.3-10 Gy. The minimum detectable dose, estimated as the absorbed dose corresponding to 3 SD above background, was 0.015 Gy. The comparison with alanine dosemeters in the dose range 7-10 Gy showed agreement within a few per cent and the same agreement was observed for the comparison with TLD in the range 1-3 Gy.     

Microdosimetry and radiation quality determinations in radiation protection and radiation therapy

Beams of different radiation qualities may, for equal absorbed dose, lead to important differences in the degree of harm for a specific biological endpoint. In many practical situations absorbed dose is then not a sufficient measure when for instance the same treatment result or risk level is the focus of attention. In radiation protection, the absorbed dose may be different by a factor of 20 between the most and least effective radiation qualities. In radiation therapy the corresponding factor is approximately 3. Two physical quantities related to the charged particle track structure, LET, and lineal energy, y, are used to characterise radiation quality. Their values are dependent on whether focus is on targets in the micrometer range (chromosomes, cell nucleus, etc.) or in the nanometre range (DNA structures). The two quantities, LET, and y, have important differences, which emphasise different characteristics of a track. Applications will be discussed.     

An accelerator-based neutron microbeam system for studies of radiation effects

A novel neutron microbeam is being developed at the Radiological Research Accelerator Facility (RARAF) of Columbia University. The RARAF microbeam facility has been used for studies of radiation bystander effects in mammalian cells for many years. Now a prototype neutron microbeam is being developed that can be used for bystander effect studies. The neutron microbeam design here is based on the existing charged particle microbeam technology at the RARAF. The principle of the neutron microbeam is to use the proton beam with a micrometre-sized diameter impinging on a very thin lithium fluoride target system. From the kinematics of the ?Li(p,n)?Be reaction near the threshold of 1.881 MeV, the neutron beam is confined within a narrow, forward solid angle. Calculations show that the neutron spot using a target with a 17-μm thick gold backing foil will be <20 μm in diameter for cells attached to a 3.8-μm thick propylene-bottomed cell dish in contact with the target backing. The neutron flux will roughly be 2000 per second based on the current beam setup at the RARAF singleton accelerator. The dose rate will be about 200 mGy min?1. The principle of this neutron microbeam system has been preliminarily tested at the RARAF using a collimated proton beam. The imaging of the neutron beam was performed using novel fluorescent nuclear track detector technology based on Mg-doped luminescent aluminum oxide single crystals and confocal laser scanning fluorescent microscopy.     

Measurement and simulation of lineal energy distribution at the CERN high energy facility with a tissue equivalent proportional counter

The response of a tissue equivalent proportional counter (TEPC) in a mixed radiation field with a neutron energy distribution similar to the radiation field at commercial flight altitudes has been studied. The measurements have been done at the CERN-EU High-Energy Reference Field (CERF) facility where a well-characterised radiation field is available for intercomparison. The TEPC instrument used by the ARC Seibersdorf Research is filled with pure propane gas at low pressure and can be used to determine the lineal energy distribution of the energy deposition in a mass of gas equivalent to a 2 mum diameter volume of unit density tissue, of similar size to the nuclei of biological cells. The linearity of the detector response was checked both in term of dose and dose rate. The effect of dead-time has been corrected. The influence of the detector exposure location and orientation in the radiation field on the dose distribution was also studied as a function of the total dose. The microdosimetric distribution of the absorbed dose as a function of the lineal energy has been obtained and compared with the same distribution simulated with the FLUKA Monte Carlo transport code. The dose equivalent was calculated by folding this distribution with the quality factor as a function of linear energy transfer. The comparison between the measured and simulated distributions show that they are in good agreement. As a result of this study the detector is well characterised, thanks also to the numerical simulations the instrument response is well understood, and it's currently being used onboard the aircrafts to evaluate the dose to aircraft crew caused by cosmic radiation.     

Space radiation measurements on-board ISS--the DOSMAP experiment

The experiment 'Dosimetric Mapping' conducted as part of the science program of NASA's Human Research Facility (HRF) between March and August 2001 was designed to measure integrated total absorbed doses (ionising radiation and neutrons), heavy ion fluxes and its energy, mass and linear energy transfer (LET) spectra, time-dependent count rates of charged particles and their corresponding dose rates at different locations inside the US Lab at the International Space Station. Owing to the variety of particles and energies, a dosimetry package consisting of thermoluminescence dosemeter (TLD) chips and nuclear track detectors with and without converters (NTDPs), a silicon dosimetry telescope (DOSTEL), four mobile silicon detector units (MDUs) and a TLD reader unit (PILLE) with 12 TLD bulbs as dosemeters was used. Dose rates of the ionising part of the radiation field measured with TLD bulbs applying the PILLE readout system at different locations varied between 153 and 231 microGy d(-1). The dose rate received by the active devices fits excellent to the TLD measurements and is significantly lower compared with measurements for the Shuttle (STS) to MIR missions. The comparison of the absorbed doses from passive and active devices showed an agreement within +/- 10%. The DOSTEL measurements in the HRF location yielded a mean dose equivalent rate of 535 microSv d(-1). DOSTEL measurements were also obtained during the Solar Particle Event on 15 April 2001.     

用于放疗量传的照射量、空气比释动能和水吸收剂量量值体系比较

用照射量和空气比释动能校准的电离室进行剂量测量时,依据IAEA TRS 277报告,需要经历四级量值转换过程,不确定度也较大,但目前仍是国内使用的量值体系.我国正在建立60Co γ射线以及高能光子下的水吸收剂量基准装置并进行国际比对,之后将拥有水吸收剂量的量值复现的能力.在60Co γ射线参考辐射场和加速器高能X射线辐射场下,使用NE2571和NE2570/1A、PTW TW30013和PTW UNIDOS两套电离室剂量仪,分别按照277和398报告的要求计算并比较2种方法计算出的水吸收剂量值,从而验证了277报告和398报告的一致性.     

A chamber for determining the conventionally true value of Hp(10) and H*(10) needed by calibration laboratories

A secondary standard chamber for photon radiation developed for measuring directly the conventionally true value of the personal dose equivalent, Hp(10), in a slab phantom is now commercially available. In addition, this chamber can be used for determining the true value of the ambient dose equivalent, H*(10), in monodirectional radiation fields; for example, photon fields generated by X ray facilities. Once the chamber has been calibrated at the facility of the calibration laboratory, the true value of Hp(10) or H*(10) can be measured at other facilities without applying any conversion coefficients. For low energy photon fields the conversion coefficients are strongly dependent on the spectral distribution. For nominally the same radiation quality small spectral differences, caused, for example, by use of different X ray facilities, may lead to differences between the spectrum-averaged conversion coefficients from Ka to Hp(10) and H*(10), respectively, of up to several tens per cent. For this reason, tabulated conversion coefficients for low energy radiation fields cannot be used for calibration purposes, if the standard uncertainty is to be 2-5%. Direct measurement by the secondary standard chamber overcomes this problem.     

Biological weighting of absorbed dose in radiation therapy

Absorbed dose is a quantity which is scientifically rigorously defined and used to quantify the exposure of biological objects, including humans, to ionising radiation. There is, however, no unique relationship between absorbed dose and induced biological effects. The effects induced by a given absorbed dose to a given biological object depend also on radiation quality and temporal distribution of the irradiation. In radiation therapy, empirical approaches are still used today to account for these dependencies in practice. In hadron therapy (neutrons, protons, ions), radiation quality is accounted for with a diversity of (almost hospital specific) methods. The necessity to account for temporal aspects is well known in external beam therapy and in high dose rate brachytherapy. The paper reviews the approaches for weighting the absorbed dose in radiation therapy, and focusses on the clinical aspects of these approaches, in particular the accuracy requirements.     

Fracture toughness evaluation of ductile polymeric films

The energy for complete fracture in double edge-notched tension test specimens has been measured for a wide range of polymer films. Results indicated that the variation of the total specific work of fracture, wT, with ligament length, L, can be described by two straight lines, both of the form wT = we + β wpL, thus giving upper and lower intercept values at zero ligament length (i.e. we) for each film. The first term, we, is the energy absorbed per unit area of fracture, whereas the second term, wp, is the energy absorbed per unit volume of plastic deformation remote from the fracture surface. The lower we value was obtained from the extrapolation of the data within the mixed mode region (plane-stress/plane-strain) where the maximum net-section stress exceeded 1.15 times that of the tensile yield stress, σy, of the material, and the upper value was ascertained by extrapolating the data within the plane stress region where the net-section stress was 1.15 σy. It appears that the transition from plane stress to plane strain mode of fracture in thin films occurs at a ligament length much greater than 5B, where B is the specimen thickness. Moreover, it was found that the linearity of the data within the plane-stress region was not affected when ligament length values exceeded the plastic zone size. Moreover, variation of the extension to break with ligament length, for both pure plane stress and the mixed mode regions, was also linear; and the extrapolation values at zero ligament length were identified as crack opening displacements. Essential work estimated from the crack opening displacement agreed reasonably well with the extrapolated values. This revised version was published online in November 2006 with corrections to the Cover Date.     

吸收剂量基准石墨量热计基本性能分析

作为吸收剂量基准的石墨量热计,对于量值复现有很高要求。分析说明了NIM的石墨量热计系统的主要技术性能,包括:温度控制,电能（功率）测量,电能-辐射能量转换关系,以及吸收剂量基准值复现测量,并给出得到的结果 。对有关的技术性能指标进行说明,同时给出其随时间的变化（稳定性）。作为基准绝对测量旁证检验,对石墨比热容进行测定,与BIPM测定的结果进行比较,偏差在0.1%以内。     

Quantities and units in radiation protection

In its Publication No. 60, the International Commission on Radiological Protection (ICRP) introduced a new quantity, the equivalent dose in tissue, HT = sigma R WR DT.R where DT.R is the absorbed dose averaged over the tissue or organ, T, and WR is the radiation weighting factor. The latter depends on the incident radiation, i.e., the type and energy, but is independent of the tissue. On the other hand, a customarily defined quantity, the 'dose equivalent' is maintained (in particular the operational dose-equivalent quantities introduced in ICRU Report 39), but the relationship between the quality factor Q(L) and the linear energy transfer, L, has been redefined. In the case of neutrons, this procedure gives rise to an increase of the corresponding fluence to dose-equivalent conversion factors. The reasons for introducing these and the new tissue-weighting factor changes are discussed.     

Relation between absorbed dose, charged particle equilibrium and nuclear transformations: a non-equilibrium thermodynamics point of view

We present a discussion to show that the absorbed dose D is a time-dependent function. This time dependence is demonstrated based on the concepts of charged particle equilibrium and on radiation equilibrium within the context of thermodynamic non-equilibrium. In the latter, the time dependence is due to changes of the rest mass energy of the nuclei and elementary particles involved in the terms summation operator Q and Q that appear in the definitions of energy imparted epsilon and energy deposit epsilon(i), respectively. In fact, nothing is said about the averaging operation of the non-stochastic quantity mean energy imparted epsilon, which is used in the definition of D according to ICRU 60. It is shown in this research that the averaging operation necessary to define the epsilon employed to get D cannot be performed with an equilibrium statistical operator rho(r) as could be expected. Rather, the operation has to be defined with a time-dependent non-equilibrium statistical operator rho(r, t); therefore, D is a time-dependent function D(r,t).     

Review of reconstruction of radiation incident air kerma by measurement of absorbed dose in tooth enamel with EPR

Electron paramagnetic resonance dosimetry with tooth enamel has been proved to be a reliable method to determine retrospectively exposures from photon fields with minimal detectable doses of 100 mGy or lower, which is lower than achievable with cytogenetic dose reconstruction methods. For risk assessment or validating dosimetry systems for specific radiation incidents, the relevant dose from the incident has to be calculated from the total absorbed dose in enamel by subtracting additional dose contributions from the radionuclide content in teeth, natural external background radiation and medical exposures. For calculating organ doses or evaluating dosimetry systems the absorbed dose in enamel from a radiation incident has to be converted to air kerma using dose conversion factors depending on the photon energy spectrum and geometry of the exposure scenario. This paper outlines the approach to assess individual dose contributions to absorbed dose in enamel and calculate individual air kerma of a radiation incident from the absorbed dose in tooth enamel.     

Theoretical treatment of the Cherenkov emission contribution to biological effects induced by gamma-rays in microorganisms

The contribution of Cherenkov emission in the formation of photoreactivatable damage (pyrimidine dimers) in E. coli cells has been analyzed. The mean quantity of Cherenkov photons in the wavelength range (200–600) nm produced in a suspension volume unit per absorbed dose unit was calculated by a Monte Carlo method for a point isotropic gamma-ray source with energy up to 30 MeV. The Cherenkov emission spectrum and the dose dependence on gamma-ray energy and the linear dimension irradiated suspension volumes were also obtained. On the basis of this data the magnitude of the photoreactivation effect as a function of gamma-ray energy and suspension volume have been predicted and are compared with experimental results. The role of direct electronic excitation of DNA in the formation of photoactivatable damage in E. coli cells is also discussed.     

Impact of release dynamics of laser-irradiated polymer micropallets on the viability of selected adherent cells

We use time-resolved interferometry, fluorescence assays and computational fluid dynamics (CFD) simulations to examine the viability of confluent adherent cell monolayers to selection via laser microbeam release of photoresist polymer micropallets. We demonstrate the importance of laser microbeam pulse energy and focal volume position relative to the glass–pallet interface in governing the threshold energies for pallet release as well as the pallet release dynamics. Measurements using time-resolved interferometry show that increases in laser pulse energy result in increasing pallet release velocities that can approach 10 m s⁻¹ through aqueous media. CFD simulations reveal that the pallet motion results in cellular exposure to transient hydrodynamic shear stress amplitudes that can exceed 100 kPa on microsecond timescales, and which produces reduced cell viability. Moreover, CFD simulation results show that the maximum shear stress on the pallet surface varies spatially, with the largest shear stresses occurring on the pallet periphery. Cell viability of confluent cell monolayers on the pallet surface confirms that the use of larger pulse energies results in increased rates of necrosis for those cells situated away from the pallet centre, while cells situated at the pallet centre remain viable. Nevertheless, experiments that examine the viability of these cell monolayers following pallet release show that proper choices for laser microbeam pulse energy and focal volume position lead to the routine achievement of cell viability in excess of 90 per cent. These laser microbeam parameters result in maximum pallet release velocities below 6 m s⁻¹ and cellular exposure of transient hydrodynamic shear stresses below 20 kPa. Collectively, these results provide a mechanistic understanding that relates pallet release dynamics and associated transient shear stresses with subsequent cellular viability. This provides a quantitative, mechanistic basis for determining optimal operating conditions for laser microbeam-based pallet release systems for the isolation and selection of adherent cells.     

Use of composites of topaz-glass as TSEE and TL dosemeters

The properties of the thermally stimulated exoelectron emission (TSEE) and thermoluminescent (TL) emission of topaz-glass composites were studied with the aim of using them as solid-state dosemeters. The TSEE response was studied as a function of radiation energy and as a function of absorbed dose. Topaz-glass composites presented a linear TL and TSEE response to dose within a range of 0.01-1 Gy. The topaz-glass composites presented higher TSEE peaks than topaz-Teflon pellets. In the dosimetry of radiotherapic fields normally the responses of the topaz-glass dosemeters are comparable to topaz-Teflon pellets. The results confirmed that these new dosemeters can be useful in monitoring the quality of the radiation sources. This dose mapping technique is particularly useful in investigating dose distribution throughout a planned target volume.     

The ICRU (International Commission on Radiation Units and Measurements): its contribution to dosimetry in diagnostic and interventional radiology

The ICRU (International Commission on Radiation Units and Measurements was created to develop a coherent system of quantities and units, universally accepted in all fields where ionizing radiation is used. Although the accuracy of dose or kerma may be low for most radiological applications, the quantity which is measured must be clearly specified. Radiological dosimetry instruments are generally calibrated free-in-air in terms of air kerma. However, to estimate the probability of harm at low dose, the mean absorbed dose for organs is used. In contrast, at high doses, the likelihood of harm is related to the absorbed dose at the site receiving the highest dose. Therefore, to assess the risk of deterministic and stochastic effects, a detailed knowledge of absorbed dose distribution, organ doses, patient age and gender is required. For interventional radiology, where the avoidance of deterministic effects becomes important, dose conversion coefficients are generally not yet developed.