The incorporation of the concept of minimum RBE (RbEmin) into the linear-quadratic model and the potential for improved radiobiological analysis of high-LET treatments

PURPOSE: The formulation of relative biological effectiveness (RBE) for high linear energy transfer (high-LET) radiation treatments is revisited. The effects of changed production of sub-lethal damage with varying LET is now considered via the RBEmin concept, where RBEmin represents the lower limit to which RBE tends at high doses per fraction. MATERIALS AND METHODS: An existing linear-quadratic formulation for calculating RBE variations with fractional dose for high-LET radiations is modified to incorporate the twin concepts of RBEmax (which represents the value of RBE at an effective dose-per-fraction of 0 Gy) and RBEmin. RESULTS: Fits of the model to data showed RBEmin values in the range of 0.1- 2.27. In all cases the raw data was a better statistical fit to the model which included RBEmin, although this was only very highly significant in one case. In the case of the mouse oesophagus it is shown that, if change in the beta-radiosensitivity coefficient with LET is considered as trivial, an underestimation > 5% in RBE can be expected at X-ray doses of 2 Gy/fraction if RBEmin is not considered. To ensure that the results were not biased by the statistical method used to obtain the parameter values relevant to this analysis (i.e., using fraction-size effect or Fe-plots), an alternative method was used which provided very similar correlation with the data. CONCLUSIONS: If the production of sublethal damage is considered independent of LET, there will be a risk that non-corrected evaluation of RBE will lead to an over- or under-estimate of RBE at low doses per fractions (the clinically relevant region).     

Calculation of high-LET radiotherapy dose required for compensation of overall treatment time extensions

A method is presented that allows biological effective dose (BED) equations to be used to calculate compensatory doses for treatment time extensions when high-LET (linear energy transfer) radiotherapy schedules are used. The principles involved are the same as those for low-LET radiations, but incorporate two relative biological effectiveness (RBE) factors, RBE(max) and RBE(min), which represent the RBE at very low and very high fraction doses, respectively, with the actual RBE changing between these extremes. The method has the advantage that low-LET alpha/beta ratios and low-LET daily dose-equivalent repopulation factors are used in the calculations. The daily dose repopulation equivalents and increments in dose per fraction in the case of high LET radiotherapy are smaller than those for low LET.     

LET dependence of lethality of carbon ion irradiation to single tobacco cells

PURPOSE: To determine the radiation sensitivity and relationship between linear energy transfer (LET) and relative biological effectiveness (RBE) in single plant cells irradiated with heavy ions. MATERIALS AND METHODS: Single cells were isolated from the tobacco BY-2 cell line and irradiated with carbon ions (78.6-309 keV microm(-1)) and gamma-rays (0.2 keV microm(-1)). Two weeks after irradiation, colonies with 16 cells or more derived from the irradiated cells were counted as survivors. The surviving fraction was fitted using the single-hit, multitarget theory. RESULTS: The doses needed to reduce the surviving fraction of the cells to 0.1 (D10) of gamma-rays and carbon ions were 47.2 and 10.5-12.6 Gy, respectively. The RBE based on the D10 peaked at an LET of 247 keV microm(-1). The inactivation cross-section of carbon ions reached a plateau of 11.3 microm2 at an LET of 247 keV microm(-1). CONCLUSIONS: The radiation sensitivity of single tobacco cells was much lower than that of mammalian cells, although the mean number of base pairs per chromosome in the two cell types was similar. The RBE peak based on the D10 of carbon ions in single tobacco cells occurred at a higher LET than it does in other organisms.     

DNA DSB induction and rejoining in V79 cells irradiated with light ions: a constant field gel electrophoresis study

Purpose : To study the induction and the time-course of rejoining of DNA double strand breaks (DSB) in V79 cells irradiated with light ions with different linear energy transfer (LET). Materials and methods : V79 cells were irradiated in monolayer with monoenergetic proton, deuteron, helium-3 or helium-4 ion beams, each at two different energy values. Gamma rays were used as reference radiation. DSB have been measured by constant field gel electrophoresis (CFGE). Results : The initial yield depended little on the particle type and LET. The amount of DSB left unrejoined for up to 2 h incubation time could be roughly described by a decreasing exponential function with a final plateau, although more complex functions cannot be excluded. Radiation quality had little effect on the rejoining rate but affected the plateau. The amount of residual DSB after 2h was higher for densely than for sparsely ionizing radiation, and for the same particle was dependent on LET. The corresponding RBE ranged from 1.8 to 6.0. Conclusions : The results support the hypothesis that complex, less reparable DSB are induced in higher proportion by light ions with respect to gamma-rays and that, for the same ion, increasing LET leads to an increase in this proportion.     

DNA DSB induction and rejoining in V79 cells irradiated with light ions: a constant field gel electrophoresis study

PURPOSE: To study the induction and the time-course of rejoining of DNA double strand breaks (DSB) in V79 cells irradiated with light ions with different linear energy transfer (LET). MATERIALS AND METHODS: V79 cells were irradiated in monolayer with monoenergetic proton, deuteron, helium-3 or helium-4 ion beams, each at two different energy values. Gamma rays were used as reference radiation. DSB have been measured by constant field gel electrophoresis (CFGE). RESULTS: The initial yield depended little on the particle type and LET. The amount of DSB left unrejoined for up to 2 h incubation time could be roughly described by a decreasing exponential function with a final plateau, although more complex functions cannot be excluded. Radiation quality had little effect on the rejoining rate but affected the plateau. The amount of residual DSB after 2 h was higher for densely than for sparsely ionizing radiation, and for the same particle was dependent on LET. The corresponding RBE ranged from 1.8 to 6.0. CONCLUSIONS: The results support the hypothesis that complex, less reparable DSB are induced in higher proportion by light ions with respect to gamma-rays and that, for the same ion, increasing LET leads to an increase in this proportion.     

An illustrative comparison of the event-size distributions for gamma-rays and alpha -particles in the whole mammalian cell nucleus

Purpose: Recent laboratory studies of endpoints designated as due to radiation-induced genomic instability have cast doubt on the validity of the current theoretical framework. Under this framework extrapolations are made from directly determined risks of radiation-induced cancer to those circumstances for which no direct information exists, namely at low doses and dose rates at low LET and at low dose exposures to high LET radiations. Based upon an approach in which the 'state' of the genome, as exemplified by the pattern of gene expression, rather than the base sequence of the genomic DNA, is taken to be the origin of genomic stability, it is hypothesized that the critical factor determining the likelihood of destabilization by ionizing radiation is the dose to the whole cell nucleus. Calculations : The frequency distributions of event sizes from two qualities of radiation, low LET gamma -rays and 5MeV alpha -particles, are compared with 60Co gamma -rays being taken to be a low LET reference radiation in determining the RBE of other radiation qualities. In the absence of measured event-size distributions for 60Co gamma -rays in spheres of similar size to the human cell nucleus, the 4.5 mu m sphere has been chosen as illustrative. Frequency distributions for 5MeV alpha -particles are derived, based on the idealized situation of a parallel beam of constant LET, with all particles traversing the sphere. Results: When compared for a dose of 1mGy the event-size (dose) distributions of the two qualities do not intersect. It is estimated that only 0.4% of the energy from the alpha -particles falls in the range of event sizes that can be produced by 60Co gamma -rays. Conclusions: Contrary to belief over the past 50 years, there is, in this low dose range, no 'continuum' based upon quantities such as LET or lineal energy that would provide a basis for extrapolation from measured RBE values. RBE is thus seen to be purely empirical. In addition, the potential to induce effects in bystander cells is not considered when deriving weighting factors for alpha particles of the type that contribute significantly to public exposure to environmental radiation.     

The formation of strand breaks in DNA after high-LET irradiation: a comparison of data from in vitro and cellular systems

This paper presents a summary of our understanding to date of the formation of DNA strand breaks induced by highly energetic particle beams (high-LET radiation). We have compared our own recent data on the formation of strand breaks induced in DNA in an aqueous solution with our previous data and those of others available from the literature for similar lesions made in cellular DNA. When the strand break induction frequency, as number of breaks per Gy per unit DNA, is plotted against LET, a series of biological effect curves (one for each particle atomic number Z) is obtained. The frequency of the formation of single-strand breaks has an RBE of less than 1 for DNA in solution and for DNA in the cell; the frequency of the formation of double-strand breaks (dsb) also has an RBE of less than 1 for DNA in a solution containing low amounts of free radical scavenger(s), while the RBE can be greater than 1 in the 50-200 keV/microns range for cellular DNA. RBE values are with respect to X-rays or cobalt gamma-rays. In cells the level of unrejoined strand breaks is also highest in the 50-200 keV/microns range and may reach 25-35% of the initial break yield depending on particle energy and Z-value. These irreparable lesions include double-strand scissions and some form(s) of single-strand breaks. The data presented cover results obtained for helium to uranium particles, with an LET range of 16 to 160,000 keV/microns. When different biological end-points are compared a strong correlation is found between induction of dsb, chromosomal abnormalities and mutation induction.     

Reduction of survival and induction of chromosome aberrations in tobacco irradiated by carbon ions with different linear energy transfers

PURPOSE: To determine the relationship between linear energy transfer (LET) and the relative biological effectiveness (RBE) for survival reduction and chromosome aberration induction in plants. MATERIALS AND METHODS: Tobacco seeds were exposed to carbon ions having LET ranging from 92 to 260 keV microm(-1). Survival ratc was determined at 7 weeks after sowing. Chromosome aberrations were observed when the root length reached about 0.5 mm (immediately after radicle emergence), 3 and 10 mm. RESULTS: The RBE for both endpoints increased with increasing LET and showed the highest value at 230 keV um(-1). The highest RBE was 65.0 for survival reduction and 52.5 for chromosome aberration induction. The types and yield ratio of chromosome aberrations such as fragments and bridges were not affected by radiation type at 0.5mm root length. As the roots elongated from 0.5 to 10 mm, the frequency of aberrant cells gradually decreased. The number of cells with fragments decreased faster than the number of cells with bridges. The decrement of chromosome aberrations appeared to be slower in roots irradiated by carbon ions than in roots irradiated by gamma-rays. CONCLUSIONS: The results show a close relationship between survival reduction and chromosome aberration induction in plants. The types and yield ratio of initial chromosome aberrations did not differ among gamma-rays and carbon ions with different LET.     

Reduction of survival and induction of chromosome aberrations in tobacco irradiated by carbon ions with different linear energy transfers

Purpose : To determine the relationship between linear energy transfer (LET) and the relative biological effectiveness (RBE) for survival reduction and chromosome aberration induction in plants. Materials and methods : Tobacco seeds were exposed to carbon ions having LET ranging from 92 to 260 keV μ m -1. Survival rate was determined at 7 weeks after sowing. Chromosome aberrations were observed when the root length reached about 0.5mm (immediately after radicle emergence), 3 and 10 mm. Results : The RBE for both endpoints increased with increasing LET and showed the highest value at 230 keV μ m -1. The highest RBE was 65.0 for survival reduction and 52.5 for chromosome aberration induction. The types and yield ratio of chromosome aberrations such as fragments and bridges were not affected by radiation type at 0.5mm root length. As the roots elongated from 0.5 to 10 mm, the frequency of aberrant cells gradually decreased. The number of cells with fragments decreased faster than the number of cells with bridges. The decrement of chromosome aberrations appeared to be slower in roots irradiated by carbon ions than in roots irradiated by γ -rays. Conclusions : The results show a close relationship between survival reduction and chromosome aberration induction in plants. The types and yield ratio of initial chromosome aberrations did not differ among γ -rays and carbon ions with different LET.     

Inactivation of human normal and tumour cells irradiated with low energy protons

PURPOSE: To analyse the cell inactivation frequencies induced by low energy protons in human cells with different sensitivity to photon radiation. MATERIALS AND METHODS: Four human cell lines with various sensitivities to photon irradiation were used: the SCC25 and SQ20B derived from human epithelium tumours of the tongue and larynx, respectively, and the normal lines M/10, derived from human mammary epithelium, and HF19 derived from a lung fibroblast. The cells were irradiated with y-rays and proton beams with linear energy transfer (LET) from 7 to 33 keV/microm. Clonogenic survival was assessed. RESULTS: Survival curves are reported for each cell line following irradiation with gamma-rays and with various proton LETs. The surviving fraction after 2 Gy of gamma-rays was 0.72 for SQ20B cells, and 0.28-0.35 for the other cell lines. The maximum LET proton effectiveness was generally greater than that of gamma-rays. In particular there was a marked increase in beam effectiveness with increasing LET for the most resistant cells (SQ20B) whose 2 Gy-survival varied from 0.72 with gamma-radiation down to 0.37 with 30 keV/microm protons. The relative biological effectiveness (RBE(2 Gy gamma)) with the 30 keV/microm beam, evaluated as the ratio of 2 Gy to the proton dose producing the same inactivation level as that given by 2 Gy of gamma-rays, was 3.2, 1.8, 1.3 and 0.8 for SQ20B, M/10, SCC25, and HF19, respectively. CONCLUSIONS: RBE for inactivation with high-LET protons increased with the cellular radioresistance to gamma-rays. The cell line with the greatest resistance to gamma-rays was the most responsive to the highest LET proton beam. A similar trend has also been found in studies reported in the literature with He, C, N ions with LET in the range 20-125 keV/microm on human tumour cell lines.     

Cytogenetic analysis in human lymphocytes after exposure to simulated cosmic radiation which reflects the inflight radiation environment.

PURPOSE: To determine the relative biological effectiveness (RBE) of a mixed neutron-gamma-radiation field and its high LET component on the induction of chromosome aberrations in human lymphocytes. MATERIALS AND METHODS: Human lymphocyte cultures were exposed in vitro to low doses of simulated cosmic radiation (2.39-5.81 mGy) at low dose rates (0.04-0.15 mGy/h). Chromosome aberrations, micronuclei, and sister chromatid exchanges (SCE) were analysed. The RBE for dicentric chromosomes was given in comparison to 60Co gamma-rays. RESULTS: For the induction of dicentric chromosomes by simulated cosmic radiation the RBE was up to 64, and up to 113 when calculating only the high LET component. The investigation of micronuclei and SCE showed no significant differences between controls and irradiated samples. CONCLUSIONS: Preliminary data indicate a high biological effectiveness of cosmic radiation and its neutron component in comparison with 60Co gamma-radiation.     

Evidence of DNA double strand breaks formation in Escherichia coli bacteria exposed to alpha particles of different LET assessed by the SOS response

Ionizing radiation produces a plethora of lesion upon DNA which sometimes is generated among a relatively small region due to clustered energy deposition events, the so called locally multiply damaged sites that could change to DSB. Such clustered damages are more likely to occur in high LET radiation exposures. The effect of alpha particles of different LET was evaluated on the bacterium Escherichia coli either by survival properties or the SOS response activity. Alpha radiation and LET distribution was controlled by means of Nuclear Track Detectors. The results suggest that alpha particles produce two types of lesion: lethal lesions and SOS inducing-mutagenic, a proportion that varies depending on the LET values. The SOS response as a sensitive parameter to assess RBE is mentioned.     

Radiobiological dose-response relationships by monitoring the ATP-concentration in L 1210 cells using 31P-NMR-spectroscopy

In conventional radiobiology the survival fraction of cell cultures in the exponential phase is observed by the colony-forming ability, and the linear-quadratic model represents an adequate frame of the numerical adaptation. The measurement of the ATP-concentration of L 1210 cells by 31P-NMR-spectroscopy in dependence of the applied radiation dose can also be used to evaluate dose-response relationships. In particular, the beta-peak of the ATP is suitable to seize the chronological behavior of the cellular ATP-concentration. An access to RBE of a radiation quality is obtained by monitoring the ATP-concentration, and the combination treatment (irradiation and incubation of cisPlatinum) has also been studied with regard to the question of synergistic interactions.     

Microdosimetric specification of radiation quality in neutron radiation therapy

The neutron beams used by various radiotherapy centres are of widely differing energies, and differences of up to 50 per cent in the relative biological effectiveness (RBE) between different beams have been found in radiobiological experiments. Moreover, at some facilities RBE variations have been observed with increasing depth in a phantom. In spite of this evidence, there is no quantitative and uniquely accepted specification of radiation quality used in practice. The urgency of an adequate solution of this problem is illustrated by the fact that in radiation therapy the usual accuracy requirement for the quantity of radiation, i.e. the absorbed dose to be delivered to the tumour, is 3.5 per cent (1 SD). In this paper a pragmatic solution for the specification of radiation quality for fast neutron therapy is proposed. It is based on empirical RBE versus lineal energy response or weighting functions. These were established by using existing radiobiological data and microdosimetric spectra measured under identical irradiation conditions at several European neutron irradiation units.     

The apparent increase in the β-parameter of the linear quadratic model with increased linear energy transfer during fast neutron irradiation

The issue of whether the β-parameter of the linear quadratic model changes with linear energy transfer (LET) remains controversial. Retrospective analysis of UK fast neutron experimental data using human cell lines at Clatterbridge shows that the β-parameter of the linear quadratic model probably does increase with LET during neutron irradiation. For cells without a deficiency in DNA damage repair and for experiments in which β-parameter estimates were considered to be unreliably low, a provisional relationship of β_H = 1.82 β_L was found (where the suffixes refer to high and low LET exposures, respectively). This implies that √β increases by around 1.35 in the specific case of 62.5 MeV neutrons relative to 4 MeV X-rays. Increments in the β-parameter with LET influence the relative biological effect (RBE), especially at high doses per fraction. Large fractions are being used in experimental carbon ion therapy, in which broadly similar RBE values to fast neutrons are found. These interesting findings after fast neutron exposure need to be studied further for applications in charged particle beam therapy using light ions, which is presently undergoing a worldwide expansion.Charged particle radiotherapy offers considerable promise because of the excellent dose distribution achieved owing to the Bragg peak effect [1–4]. However, it is important to take into account the relative biological effect (RBE) when the ionisation events at the microscopic level are especially close, i.e. if the linear energy transfer (LET) is high compared with the megavoltage X-rays.RBE is defined as the ratio of dose of a test radiation to a control radiation, such as cobalt γ-rays or megavoltage X-rays, required to achieve the same bio-effect. RBE varies with dose per fraction in radiotherapy, such that, at very low doses, RBE approaches a maximum value (R_max). This is expressed, in terms of the linear quadratic model of radiation effect, as the ratio of the α-parameters, whereas, at very high doses, it approaches R_min, the ratio of the square roots of the β-parameters (where the suffixes L and H refer to low and high LET radiation, respectively) as follows [5, 6]:(1)At all intermediate doses per fraction, the RBE has an intermediate value between these limits.There is some controversy regarding the theoretical prediction from microdosimetry considerations by Goodhead [7] that both α- and β-parameters increase with LET in order to fit experimental data. It is clear that the largest effect with LET is an increment in α [8]; longstanding in vivo experience has shown an increase in the α/β ratio or reduced fractionation sensitivity with the use of high-LET radiations, such as neutrons [9]. Some experiments have failed to show an increment in β with LET, e.g. Chapman et al [10, 11] showed no increase in √β with LET in plateau-arrested Chinese hamster ovary (CHO) cells. In contrast, experiments in Japan showed a probable increase in the β-parameter [12]. Further evidence from fast neutron studies in vivo has shown that, at the higher dose ranges needed for bio-effects in normal tissues, the value of RBE tends to reach an asymptotic lower (minimum) level, which exceeds unity in the higher dose range [7]. This provides an apparent justification for β increasing with LET. However, historical animal experiments can be criticised because dose cannot be increased further than certain biological limits (associated with death or severe radiation effects), and also because variations in experimental techniques, such as the use of priming doses, could influence the measured RBE.One alternative method for investigating this enigma is to use previously published experimental data of human cell line irradiation using megavoltage X-rays and fast neutrons on the Clatterbridge cyclotron [8]. These data were used to determine α and β values and the RBE, but are amenable to further analysis.     

Microdosimetric Specification of Radiation Quality in Neutron Radiation Therapy

Summary

The neutron beams used by various radiotherapy centres are of widely differing energies, and differences of up to 50 per cent in the relative biological effectiveness (RBE) between different beams have been found in radiobiological experiments. Moreover, at some facilities RBE variations have been observed with increasing depth in a phantom. In spite of this evidence, there is no quantitative and uniquely accepted specification of radiation quality used in practice. The urgency of an adequate solution of this problem is illustrated by the fact that in radiation therapy the usual accuracy requirement for the quantity of radiation, i.e. the absorbed dose to be delivered to the tumour, is 3.5 per cent (1SD). In this paper a pragmatic solution for the specification of radiation quality for fast neutron therapy is proposed. It is based on empirical RBE versus lineal energy response or weighting functions. These were established by using existing radiobiological data and microdosimetric spectra measured under identical irradiation conditions at several European neutron irradiation units.     

Assessment of the homogeneous efficacy of carbon ions in the spread-out Bragg peak for human lung cancer cell lines

Purpose The aim of this study was to assess the radiosensitivities and homogeneous efficacy in the spread-out Bragg peak (SOBP) for lung cancer cell lines exposed to carbon ions. Materials and methods The dose-dependent survival rates of seven cell lines exposed to carbon ions, fast neutrons, and photons were obtained using colony-forming assays in vitro. The relative biological effectiveness (RBE) of carbon ions and fast neutrons to photons was determined by comparing the doses at the 10% and 1% survival levels. Results The RBEs at 13, 40, 50, and 80 keV/μm were 1.20–1.29, 1.55–1.80, 1.57–2.00, and 1.69–2.58, respectively, at the 10% survival level. The RBE of 290 MeV carbon ions increased with increasing linear energy transfer. The biological dose (relative physical dose × RBE) distributions in the SOBP did not statistically differ at the proximal, mid, or distal points at the 10% (p = 0.945) and 1% (p = 0.211) survival levels, respectively; however, deviation of the biological dose at 10% and 1% survival were 3%–16% and 6%–24%, respectively. Furthermore, 290 MeV carbon ions at 80 keV/μm in the SOBP were nearly equivalent to 30 MeV fast neutrons. Conclusion Our results demonstrate nearly homogeneous effectiveness in the SOBP, although we are aware of the deviation in some cell lines.     

Influence of the sampling time on chromosomal aberrations at G2 phase in Syrian hamster embryonic cells irradiated with different types of radiation

PURPOSE: To investigate the time-course of chromosomal aberrations following radiations of differing LET. MATERIALS AND METHODS: Syrian hamster embryonic cells were irradiated with nitrogen ions (LET(infinity) = 530 keV/microm) and helium-ions (LET(infinity) = 36 and 77 keV/microm), also 137Cs gamma-rays as a reference radiation. The frequency of chromatid-type aberrations was determined after 0-6 h incubation in a 5% CO2 incubator at 37 degrees C. RESULTS: The amount of chromosomal damage per cell for nitrogen ions detected immediately after irradiation was lower than induced by 137Cs gamma-rays. In contrast, helium ions were more effective than gamma rays in inducing chromatid type damage. The RBE values for the nitrogen-ion beams were 0.45 for gaps, 0.43 for deletions and 0.20 for exchanges. For helium-ion beams, the RBE values for the 36 keV/microm beams and the 77 keV/microm beams were 1.2 and 1.5 for gaps, 1.3 and 2.1 for deletions, and 1.5 and 1.9 for exchanges, respectively. The frequency of cells with chromosomal damage following exposure to gamma-rays and helium-ion beams showed a downward trend with increasing incubation period. In contrast, in the case of nitrogen-ion beams, there was an increase with the incubation period. CONCLUSIONS: The results show that it is possible to underestimate chromosomal damage for different types of radiation by scoring aberrations at a single fixed sampling time.     

Monte Carlo simulations of therapeutic proton beams for relative biological effectiveness of double-strand break

Abstract

Purpose: The relative biological effectiveness (RBE) values relative to ⁶⁰Co for the induction of double-strand breaks (DSB) were calculated for therapeutic proton beams. RBE-weighted absorbed doses were determined at different depths in a water phantom for proton beams.

Materials and methods: The depth-dose distributions and the fluence spectra for primary protons and secondary particles were calculated using the FLUKA (FLUktuierende KAskade) MC (Monte Carlo) transport code. These spectra were combined with the MCDS (Monte Carlo damage simulation) code to simulate the spectrum-averaged yields of clustered DNA lesions. RBE for the induction of DSB were then determined at different depths in a water phantom for the unmodulated and modulated proton beams.

Results: The maximum RBE for the induction of DSB at 1 Gy absorbed dose was found about 1.5 at 0.5 cm distal to the Bragg peak maximum for an unmodulated 160 MeV proton beam. The RBE-weighted absorbed dose extended the biologically effective range of the proton beam by 1.9 mm. The corresponding maximum RBE value was inversely proportional to the proton beam energy, reaching a value of about 1.9 for 70 MeV proton beam. For a modulated 160 MeV proton beam, the RBE weightings were more pronounced near the spread-out Bragg peak (SOBP) distal edge.

Conclusions: It was demonstrated that a fast MCDS code could be used to simulate the DNA damage yield for therapeutic proton beams. Simulated RBE for the induction of DSB were comparable to RBE measured in vitro and in vivo. Depth dependent RBE values in the SOBP region might have to be considered in certain treatment situations.