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.     

Mixed high energy photon and electron radiation fields for calibrating radiation protection dosemeters

According to ISO 4037-3, calibrations of radiation protection dosemeters with photon radiation of energies above 3 MeV are performed under conditions of charged particle equilibrium. No information is provided concerning how to determine the response of dosemeters to radiation fields in the more general case when these conditions are not fulfilled. This paper deals with the production of mixed high energy photon and electron fields characterised by a lack or an excess of charged particles relative to conditions of equilibrium and describes a new procedure for the dosimetry in such fields. Through variation of the charged particle fluence fraction with respect to a nearly constant photon fluence, Hp(10) and H'(10) values varied by up to a factor of 1.74. The above mentioned basic study was utilised in the recent IAEA intercomparison (Co-ordinated Research Project 1996-1998) and EURADOS 'trial performance test' (1996-1998) for individual monitoring of photon radiation in testing response characteristics of individual dosemeters in non-charged particle equilibrium conditions.     

医用电子束吸收剂量测量中的平行板电离室校准

基于平板电离室的结构和物理特性,比圆柱形电离室更适用于电子束的水中吸收剂量测量,特别是在能量相对较低的电子束测量中更具优越性.介绍了基于两种量传体系的平板电离室溯源方式,提出了在现场如何利用已知校准因子的参考电离室得到平板电离室的校准因子的方法.在国内尚未建立水吸收剂量量传体系时,借助该方法可获得平板电离室水吸收剂量校准因子,据此按照TRS-398报告开展高能电子束水中吸收剂量测量,进一步减小测量结果的不确定度.     

A transition radiation detector for electron identification in a high energy experiment

We have built a transition radiation detector consisting of four sets of multilayered polyethylene radiators each followed by a xenon-filled proportional chamber. This detector has been used in a hadron-proton scattering experiment at CERN SPS to discriminate electrons from hadrons in the final state. Using the technique of recording chamber signals of amplitude exceeding a fixed threshold we normally achieved 0.5% pion contamination at 80% electron efficiency.     

Cooling equilibrium and beam loss with internal targets in high energy storage rings

The beam cooling equilibrium with internal target interaction is analyzed for parameters relevant to the proposed High Energy Storage Ring (HESR). For the proposed experiments with anti-protons high luminosities together with low momentum spreads are required. Rate equations are used to predict the rms equilibrium beam parameters. The cooling and IBS rate coefficients are obtained from simplified models. Energy loss straggling in the target and the associated beam loss are analyzed analytically assuming a thin target. A longitudinal kinetic simulation code is used to study the evolution of the momentum distribution in coasting and bunched beams. Analytic expressions for the target induced momentum tail are found in good agreement with the simulation results.     

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.     

Neutron absorbed dose determination by calculations of recoil energy

The aim of this work is to calculate the absorbed dose to matter due to neutrons in the 5-150 MeV energy range. Materials involved in the calculations are Al2O3, CaSO4 and CaS, which may be used as dosemeters and have already been studied for their luminescent properties. The absorbed dose is assumed to be mainly due to the energy deposited by the recoils. Elastic reactions are treated with the ECIS code while for the non-elastic ones, a Monte Carlo code has been developed and allowed to follow the nucleus decay and to determine its characteristics (nature and energy). Finally, the calculations show that the absorbed dose is mainly due to non-elastic process and that above 20 MeV this dose decreases slightly with the neutron energy.     

Determination of the absorbed dose and the average LET of space radiation in dependence on shielding conditions

The HTR method, developed for determination of absorbed dose and average LET of mixed radiation fields in space, was applied during several space missions on space station MIR, space shuttles and satellites. The method utilises the changes of peak height ratios in the glow curves in dependence on the linear energy transfer LET. Due to the small size of the dosemeters the evaluation of the variation of absorbed dose and average LET in dependence on the position of the dosemeters inside the space station is possible. The dose and LET distribution was determined during the experiment ADLET where dosemeters were exposed in two positions with different shielding conditions and during two following experiments (MIR-95, MIR-96) using six positions inside the space station. The results were compared with the shielding conditions of the positions. Calculations of the absorbed dose were carried out for comparison. Results have shown that the average LET increases with increasing absorbing thickness while the absorbed dose decreases.     

Differential absorbed dose distributions in lineal energy for neutrons and gamma rays at the mono-energetic neutron calibration facility

Absorbed dose distributions in lineal energy for neutrons and gamma rays of mono-energetic neutron sources from 140 keV to 15 MeV were measured in the Fast Neutron Laboratory at Tohoku University. By using both a tissue-equivalent plastic walled counter and a graphite-walled low-pressure proportional counter, absorbed dose distributions in lineal energy for neutrons were obtained separately from those for gamma rays. This method needs no knowledge of energy spectra and dose distributions for gamma rays. The gamma-ray contribution in this neutron calibration field >1 MeV neutron was <3%, while for <550 keV it was >40%. The measured neutron absolute absorbed doses per unit neutron fluence agreed with the LA150 evaluated kerma factors. By using this method, absorbed dose distributions in lineal energy for neutrons and gamma rays in an unknown neutron field can be obtained separately.     

Determination of electron currents below 1 nA in the storage ring BESSY by measurement of the synchrotron radiation of single electrons

Since the electron storage ring BESSY is suitable for use as a calculable radiation standard of extremely low photon flux, currents below 1 nA have to be measured exactly. For this we installed two different photon detection systems for monitoring the stepwise decrease in the photon flux as the number of stored electrons is reduced one by one. The performances of a photon counter, accepting a large solid angle of the visible and near-UV synchrotron radiation, and an array of photodiodes, sensitive mostly in the soft X-ray region, were investigated for different operational conditions of the storage ring. With a computerized data analysis routinely up to 1000 electrons are counted unambiguously allowing the determination of electron currents up to 0.77 nA with an uncertainty of 3 × 10⁻⁷ of the measured current.     

Optimal removal of scattered particles at medium energy storage rings with internal target

We have studied at which places projectiles that have been scattered by an internal target out of the acceptance of a medium energy storage ring, can be dumped locally so that the effort for shielding can be minimized. General arguments are given where slits should be installed and their effect on the useful ring acceptance is discussed. In addition we have carried out Monte Carlo simulations for the dispersion-free mode of operation of COSY, which show that 86% of the lost beam can be removed locally with the use of only two sets of slits.     

Future balloon experiments for electron and positron measurements at high energy

A large interest has been triggered by the recent direct measurements of cosmic positrons and electrons. They include: the anomalous positron abundance reported by PAMELA; the spectral “feature” suggested by the analysis of the inclusive electron and positron data of the ATIC balloon experiment; the results of FERMI and H.E.S.S. experiments. Further accurate measurements, with smaller systematic errors, might clarify the present experimental scenario. While measurements of dipole asymmetries and of the electron spectrum above 1 TeV may require a space mission to provide a sufficient exposure, balloons can investigate the spectral shape below 1 TeV and contribute to the clarification of the, so-called, “ATIC anomaly”. A concise review will be presented of the main balloon missions that have been specifically designed to provide new electron measurements in the near future.     

Determination of absorbed dose in the vicinity of a neutron source

A computational method is presented that transfers the spectral neutron fluence from the measuring position to a reference point in a biological sample. The method is applied to the determination of the absorbed dose in adult blood irradiated with monoenergetic neutrons of 0.035 and 14.8 MeV.