Relative biological effectiveness (RBE) compares the severity of damage induced by a radiation under test at a dose D relative to the reference radiation D(x) for the same biological endpoint. RBE is an important parameter in estimation of risk from exposure to ionizing radiation (IR). The present work provides a review of the recently published data and the knowledge of the RBE of low energy electrons and photons. The review presents RBE values derived from experimental data and model calculations including cell inactivation, chromosome aberration, cell transformation, micronuclei formation and induction of double-strand breaks. Biophysical models, including physical features of radiation track, and microdosimetry parameters are presented, analysed and compared with experimental data. The biological effects of low energy electrons and photons are of particular interest in radiation biology as these are strongly absorbed in micrometer and sub-micrometer layers of tissue. RBE values not only depend on the electron and photon energies but also on the irradiation condition, cell type and experimental conditions.

Microdosimetry is a recommended method for characterizing radiation quality in situations when the biological effectiveness under test is not well known. In such situations, the radiation beams are described by their lineal energy probability distributions. Results from radiobiological investigations in the beams are then used to establish response functions that relate the lineal energy to the relative biological effectiveness (RBE). In this paper we present the influence of the size of the simulated volume on the relation to the clinical RBE values (or weighting factors). A single event probability distribution of the lineal energy is approximated by its dose average lineal energy (y[overline](D)) which can be measured or calculated for volumes from a few micrometres down to a few nanometres. The clinical RBE values were approximated as the ratio of the α-values derived from the LQ-relation. Model calculations are presented and discussed for the SOBP of a (12)C ion (290 MeV u(-1)) and the reference (60)Co γ therapy beam. Results were compared with those for a conventional x-ray therapy beam, a 290 MeV proton beam and a neutron therapy beam. It is concluded that for a simulated volume of about 10 nm, the α-ratio increases approximately linearly with the y[overline](D)-ratio for all the investigated beams. The correlation between y and α provides the evidence to characterize a radiation therapy beam by the lineal energy when, for instance, weighting factors are to be estimated.

The aim of the working group has been to bring together, in particular from European research groups, the available, preferably published, experimental data and results of calculations, together with detailed descriptions of the methods of measurement and calculation. The purpose is to provide a dataset for all European Union Member States for the assessment of individual doses and/or to assess the validity of different approaches, and to provide an input to technical recommendations by the Article 31 group of experts and the European Commission. The radiation protection quantity of interest is effective dose, E (ISO), but the comparison of measurement results obtained by different methods or groups, and comparison of measurement results and the results of calculations, is done in terms of the operational quantity ambient dose equivalent, H*(10). The final report giving the results of the investigations will be published by the European Commission Directorate General Transport and Energy. This paper gives a preview of the report.

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The present investigation provides an insight into differences in energy depositions in subcellular-size volumes when irradiated by proton and carbon ion beams. The results are useful for characterizing ion beams of practical importance for biophysical modeling of radiation-induced DNA damage response and repair in the depth profiles of protons and carbon ions used in radiotherapy.

Nanodosimetric single-event distributions or their mean values may contribute to a better understanding of how radiation induced biological damages are produced. They may also provide means for radiation quality characterization in therapy beams. Experimental nanodosimetry is however technically challenging and Monte Carlo simulations are valuable as a complementary tool for such investigations. The dose-mean lineal energy was determined in a therapeutic p(65)+Be neutron beam and in a (60)Co gamma beam using low-pressure gas detectors and the variance-covariance method. The neutron beam was simulated using the condensed history Monte Carlo codes MCNPX and SHIELD-HIT. The dose-mean lineal energy was calculated using the simulated dose and fluence spectra together with published data from track-structure simulations. A comparison between simulated and measured results revealed some systematic differences and different dependencies on the simulated object size. The results show that both experimental and theoretical approaches are needed for an accurate dosimetry in the nanometer region. In line with previously reported results, the dose-mean lineal energy determined at 10 nm was shown to be related to clinical RBE values in the neutron beam and in a simulated 175 MeV proton beam as well.

The procedure recommended by different dosimetry protocols for the determination of the absorbed dose to air chamber factor, ND,pp, of plane-parallel chambers, comparing absorbed dose determinations in a high-energy electron beam with a reference cylindrical chamber having a known ND,cyl factor, has been investigated. Attention has been focused on the case that the chamber serving as reference has a solid aluminium central electrode. It has been found that using a wide spread Farmer-type chamber (NE 2571), together with recommendations which specifically take into account central electrode corrections for electron beam dosimetry, kcelpcel = pcel-global(IAEA) = 1.008, yields inconsistent results compared with those obtained from a fully homogeneous ionization chamber; for the NE 2571 chamber, a value kcelpcel = pcel-global(IAEA) congruent to 1.0 has been obtained. Analytical calculations of kmkatt for Farmer-type cylindrical chambers and experimental determinations of the product kmkattkcelpcel in electron beams agree within experimental uncertainties, with no evidence of statistical significance for the commonly used assumption pcel = 1, which yields a 0.8% correction (due to kcel only) for the effect of the NE 2571 aluminium electrode in electron beam dosimetry. The use of a 'NACP-chamber' specific factor (kpp or kmkatt) to obtain ND,pp from NK,pp in NACP plane-parallel chambers has been found unsatisfactory, and direct experimental determinations of ND,pp are recommended instead. It is suggested that Standard Dosimetry Laboratories provide ND,pp calibration factors in 60Co beams.

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