RBE variation between fast neutron beams as a function of energy. Intercomparison involving 7 neutrontherapy facilities

Different dose rate-dependent responses of human melanoma cells and fibroblasts to low dose fast neutrons

PURPOSE

To analyze the dose rate influence in hyper-radiosensitivity (HRS) of human melanoma cells to very low doses of fast neutrons and to compare to the behaviour of normal human skin fibroblasts.

MATERIALS AND METHODS

We explored different neutron dose rates as well as possible implication of DNA double-strand breaks (DSB), apoptosis, and energy-provider adenosine-triphosphate (ATP) levels during HRS.

RESULTS

HRS in melanoma cells appears only at a very low dose rate (VLDR), while a high dose rate (HDR) induces an initial cell-radioresistance (ICRR). HRS does not seem to be due either to DSB or to apoptosis. Both phenomena (HRS and ICRR) appear to be related to ATP availability for triggering cell repair. Fibroblast survival after neutron irradiation is also dose rate-dependent but without HRS.

CONCLUSIONS

Melanoma cells or fibroblasts exert their own survival behaviour at very low doses of neutrons, suggesting that in some cases there is a differential between cancer and normal cells radiation responses. Only the survival of fibroblasts at HDR fits the linear no-threshold model. This new insight into human cell responses to very low doses of neutrons, concerns natural radiations, surroundings of accelerators, proton-therapy devices, flights at high altitude. Furthermore, ATP inhibitors could increase HRS during high-linear energy transfer (high-LET) irradiation.

MCNP6 model of the University of Washington clinical neutron therapy system (CNTS)

A MCNP6 dosimetry model is presented for the Clinical Neutron Therapy System (CNTS) at the University of Washington. In the CNTS, fast neutrons are generated by a 50.5 MeV proton beam incident on a 10.5 mm thick Be target. The production, scattering and absorption of neutrons, photons, and other particles are explicitly tracked throughout the key components of the CNTS, including the target, primary collimator, flattening filter, monitor unit ionization chamber, and multi-leaf collimator. Simulations of the open field tissue maximum ratio (TMR), percentage depth dose profiles, and lateral dose profiles in a 40 cm × 40 cm × 40 cm water phantom are in good agreement with ionization chamber measurements. For a nominal 10 × 10 field, the measured and calculated TMR values for depths of 1.5 cm, 5 cm, 10 cm, and 20 cm (compared to the dose at 1.7 cm) are within 0.22%, 2.23%, 4.30%, and 6.27%, respectively. For the three field sizes studied, 2.8 cm × 2.8 cm, 10.4 cm × 10.3 cm, and 28.8 cm × 28.8 cm, a gamma test comparing the measured and simulated percent depth dose curves have pass rates of 96.4%, 100.0%, and 78.6% (depth from 1.5 to 15 cm), respectively, using a 3% or 3 mm agreement criterion. At a representative depth of 10 cm, simulated lateral dose profiles have in-field (⩾ 10% of central axis dose) pass rates of 89.7% (2.8 cm × 2.8 cm), 89.6% (10.4 cm × 10.3 cm), and 100.0% (28.8 cm × 28.8 cm) using a 3% and 3 mm criterion. The MCNP6 model of the CNTS meets the minimum requirements for use as a quality assurance tool for treatment planning and provides useful insights and information to aid in the advancement of fast neutron therapy.

RBE of the MIT epithermal neutron beam for crypt cell regeneration in mice

The RBE of the new MIT fission converter epithermal neutron capture therapy (NCT) beam has been determined using intestinal crypt regeneration in mice as the reference biological system. Female BALB/c mice were positioned separately at depths of 2.5 and 9.7 cm in a Lucite phantom where the measured total absorbed dose rates were 0.45 and 0.17 Gy/ min, respectively, and irradiated to the whole body with no boron present. The gamma-ray (low-LET) contributions to the total absorbed dose (low- + high-LET dose components) were 77% (2.5 cm) and 90% (9.7 cm), respectively. Control irradiations were performed with the same batch of animals using 6 MV photons at a dose rate of 0.83 Gy/min as the reference radiation. The data were consistent with there being a single RBE for each NCT beam relative to the reference 6 MV photon beam. Fitting the data according to the LQ model, the RBEs of the NCT beams were estimated as 1.50 +/- 0.04 and 1.03 +/- 0.03 at depths of 2.5 and 9.7 cm, respectively. An alternative parameterization of the LQ model considering the proportion of the high- and low-LET dose components yielded RBE values at a survival level corresponding to 20 crypts (16.7%) of 5.2 +/- 0.6 and 4.0 +/- 0.7 for the high-LET component (neutrons) at 2.5 and 9.7 cm, respectively. The two estimates are significantly different (P = 0.016). There was also some evidence to suggest that the shapes of the curves do differ somewhat for the different radiation sources. These discrepancies could be ascribed to differences in the mechanism of action, to dose-rate effects, or, more likely, to differential sampling of a more complex dose-response relationship.

Intestinal crypt regeneration in mice: a biological system for quality assurance in non-conventional radiation therapy

BACKGROUND AND PURPOSE

The Relative Biological Effectiveness (RBE) of 8 fast-neutron beams, 5 proton beams and 1 carbonion beam was determined using as biological criterion intestinal crypt regeneration in mice, i.e. an in vivo system. These beams are used or planned for clinical cancer therapy applications. In addition, the RBE of 6 epithermal neutron beams, used or planned for Boron Neutron Capture Therapy (BNCT), was determined; no boron was administered. The goal of the program was to improve the exchange of information between the centers, facilitate the interpretation of the results and increase the safety of the clinical applications.

MATERIALS AND METHODS

In all visited centers, the same technique was applied in the same conditions by the same radiobiology team. The number of regenerating crypts per circumference was scored 3.5 days after single fraction total body irradiation. The control irradiations were performed locally using cobalt-60 units. The mice were randomized according to radiation quality and dose level.

RESULTS

(1) For fast neutron beams, the RBE (Ref. cobalt-60 gamma rays) increases with decreasing energy (from approximately 1.7 for p(65)+Be neutrons to approximately 2.4 for d(14.5)+Be neutrons). In addition, it is specific to each facility and depends on the nuclear reaction (p or d + Be), target and collimation type. (2) For proton beams, the RBEs (Ref cobalt-60 gamma rays) at the reference position (middle of a 7-cm Spread Out Bragg Peak, SOBP) range between 1.08 and 1.18. They might differ by approximately 6-8% according to the mode of beam production or delivery. The RBEs at the end of the SOBP are always 5-10 % higher than at the middle of the SOBP. (3) For the carbon ion beam studied at NIRS in Chiba, Japan, the RBE significantly increases with depth. Relative to gamma rays, it ranges from 1.3 in the initial plateau, 1.6 at the beginning, 1.7 at the middle and 1.9 at the end of a 6-cm SOBP. 4) In BNCT beams, the radiation quality (in particular the relative contribution of the different dose components) varies rapidly with depth and depends strongly on the arrangement of the irradiation set-up (e.g. presence or not of back scattering material). Moreover, the (total) dose rates are highly variable (from 0.05 to approximately 0.5 Gy/min) according to the power of the reactors. Wide range of RBE values (Ref. gamma rays) was thus obtained (RBE = 1.4 - 2.2) at shallow depths of 1.5 - 2.5 cm.

DISCUSSION AND CONCLUSION

Intestinal crypt regeneration in mice is an in vivo system perfectly suitable to perform intercomparisons between centers applying different types of non-conventional radiation qualities. It was proven to be reproducible, reliable and accurate, and becomes progressively recognized worldwide as part of the Quality Control (QA) procedures for new beams. It should be stressed that the observed RBE for intestinal crypt cells after a single high dose provide some radiobiological characterization of the radiation quality but cannot be used as the RBE weighting factor in clinical prescriptions.

Relative biologic effectiveness determination in mouse intestine for scanning proton beam at Paul Scherrer Institute, Switzerland. Influence of motion

PURPOSE

To determine the relative biologic effectiveness (RBE) of the Paul Scherrer Institute (PSI) scanning proton beam in reference conditions and to evaluate the influence of intestine motion on the proton dose homogeneity.

METHODS AND MATERIALS

First, RBE was determined for crypt regeneration in mice after irradiation in a single fraction. Irradiation was performed at the middle of a 7-cm spread out Bragg peak (SOBP; reference position), as well as in the proximal part of the plateau and at the distal end of the SOBP. Control gamma-irradiation was randomized with proton irradiation and performed simultaneously. Second, motion of mouse intestine was determined by radiographs after copper wire markers had been placed on the jejunum and intestinal wall.

RESULTS

Proton RBE (reference (60)Co gamma) was equal to 1.16 for irradiation at the middle of the SOBP and to 1.11 and 1.21 for irradiation in the initial plateau and end of the SOBP, respectively. The confidence intervals for these RBE values were much larger than those obtained in the other proton beams we have tested so far. They exceeded +/-0.20 (compared with the usual value of +/-0.07), which resulted from the unusually large dispersion of the individual proton data. The instantaneous positions of the mice intestines varied by +/-2 mm in the course of irradiation.

CONCLUSION

The results of this study have shown that the RBE of the PSI proton beam is in total accordance with the RBE obtained at the other centers. This experiment has corroborated that proton RBE at the middle of the SOBP is slightly larger than the generic value of 1.10 and that there is a slight tendency for the RBE to increase close to the end of the SOBP. Also, excessive dispersion of individual proton data may be considered to result from intestine motion, taking into account that irradiation at the PSI is delivered dynamically by scanning the target volume with a pencil proton beam ("spot scanning"). Because 2-mm movements resulted in significant variations in local dose depositions, this should be considered for moving targets. Strategies to reduce this effect for the spot scanning technique have been developed at the PSI for radiotherapy of humans.

Design of a cylindrical brachytherapy implant applicator for the irradiation of an intestinal segment in mice

YingExperimental determination of the RBE of new isotopes for brachytherapy implants (e.g. iodine-125 and palladium-103) remains a very difficult problem, especially in small animals, where the seeds cannot be implanted easily in the planned geometry in a reproducible way. This technical note describes an original device that makes it possible to irradiate a segment of the intestine in mice for the purpose of determining the RBE for crypt regeneration. The device is a length of tube (3.4 mm and 7 mm internal and external diameter, respectively) whose external surface has been longitudinally grooved and into which the seeds can be squeezed (each groove holds either one or two seeds). The tube is composed of two sections. This seed container can be surgically positioned around an intestinal ansa while the mice are anesthetized. The mean dose rates in the intestine (for eight seeds) were found to be 86.3 +/- 5.9 and 79.0 +/- 5.4 cGy/h for 29.2 MBq (1 U) iodine and 28.6 MBq (1 U) palladium seeds, respectively. So far, more than 100 mice have been irradiated successfully. Full dose-effect relationships can be obtained using the same seeds and applying them successively in different groups of animals (which ensured the accuracy of the relative doses).

Proton RBE for early intestinal tolerance in mice after fractionated irradiation

BACKGROUND AND PURPOSE

To determine the influence of the number of fractions (or the dose per fraction) on the proton relative biological effectiveness (RBE).

MATERIALS AND METHODS

Intestinal crypt regeneration in mice was used as the biological endpoint. RBE was determined relative to cobalt-60 gamma rays for irradiations in one, three and ten fractions separated by a time interval of 3.5h. Proton irradiations were performed at the middle of a 7-cm Spread Out Bragg Peak (SOBP).

RESULTS

Proton RBEs (and corresponding gamma dose per fraction) at the level of 20 regenerated crypts per circumference were found equal to 1.15+/-0.04 (10.0 Gy), 1.15+/-0.05 (4.8 Gy) and 1.14+/-0.07 (1.7 Gy) for irradiations in one, three and ten fractions, respectively. Alpha/beta ratios as derived from direct analysis of the 'quantal radiation response data' were found to be 7.6 Gy for gamma rays and 8.2 Gy for protons. Additional proton irradiations in ten fractions at the end of the SOBP were found to be more effective than at the middle of the SOBP by a factor of 1.14 (1.05-1.23).

CONCLUSION

Proton RBE for crypt regeneration was found to be independent of fractionation up to ten fractions. One can expect that it remains unchanged for higher number of fractions as the lethalities for doses smaller than 3 Gy are exclusively due to direct lethal events. As a tendency for increased effectiveness at the end of the SOBP is reported in the majority of the studies, for clinical applications it would be advisable to allow for by arranging a sloping depth dose curve in the deeper part of the target volume. Finally, it must be noticed that most of in vitro and in vivo RBE values for protons are larger than the current clinical RBE (RBE=1.10).

Monte Carlo simulation of fast neutron spectra: mean lineal energy estimation with an effectiveness function and correlation to RBE

PURPOSE

Intercomparisons of radiotherapy trials conducted at different fast neutron facilities are complicated by the dependence of the relative biologic effectiveness (RBE) of the different beams on the fast neutrons spectra. To obtain a better understanding of the influence of neutron energy on radiation quality, Monte Carlo simulations were performed to calculate fast neutron (FN) spectra at different irradiation positions. To allow for comparisons with experimental data, the positions were chosen to be the same as that used by other investigators to obtain microdosimetry readings and radiobiological data.

METHODS AND MATERIALS

The primary neutron yield for beryllium targets bombarded with protons at the National Accelerator Center, Louvain, Nice, and Orleans facilities were calculated using the FLUKA code. Neutron transport simulations were performed with MCNP-4A, giving FN spectra for various phantom depths, hardening filter thickness, and field sizes. Using an effectiveness function, FN energy groups were correlated with mean lineal energies (y*-values) obtained experimentally by other workers.

RESULTS

Calculations confirm earlier measurements that a decrease in beam quality by a hardening filter is the result of a reduction in the low-energy neutron component, i.e., neutrons below 3 MeV. Variations in RBE due to changes in field size and different phantom depths could also be explained by variations of neutrons with energies between 3-15 MeV. The effectiveness function allows one to calculate changes in y* observed for the NAC beam with great accuracy (R(2) = 0.99, p < 0.0001). Also, when this function is applied to beams with different neutron energies, y* calculated values show a very significant correlation with measured RBE values (R(2) = 0.98, p < 0.0001).

CONCLUSION

The effectiveness function appears to be suitable to predict changes in y*-values and variations in RBE, using FN spectra simulated for various neutron therapy facilities.

RBE, reference RBE and clinical RBE: applications of these concepts in hadron therapy

Introduction of heavy particles (hadrons) into radiation therapy aims at improving the physical selectivity of the irradiation (e.g. proton beams), or the radiobiological differential effect (e.g. fast neutrons), or both (e.g. heavy-ion beams). Each of these new therapy modalities requires several types of information before prescribing safely the doses to patients, as well as for recording and reporting the treatments: (i) absorbed dose measured in a homogeneous phantom in reference conditions; (ii) dose distribution computed at the level of the target volume(s) and the normal tissues at risk; (iii) radiation quality from which a RBE evaluation could be predicted and (iv) RBE measured on biological systems or derived from clinical observation. In hadron therapy, the RBE of the different beams raises specific problems. For fast neutrons, the RBE varies within wide limits (about 2 to 5) depending on the neutron energy spectrum, dose, and biological system. For protons, the RBE values range between smaller limits (about 1.0 to 1.2). A clinical benefit can thus not be expected from RBE differences. However, the proton RBE problem cannot be ignored since dose differences of about 5% can be detected clinically in some cases. The situation is most complex with heavy ions since RBE variations are at least as large as for fast neutrons, as a function of particle type and energy, dose and biological system. In addition, RBE varies with depth. Radiation quality thus has to be taken into account when prescribing and reporting a treatment. This can be done in different ways: (a) description of the method of beam production; (b) computed LET spectra and/or measured microdosimetric spectra at the points clinically relevant; (c) RBE determination. The most relevant RBE data are those obtained for late tolerance of normal tissues at 2 Gy per fraction ("reference RBE"). The "clinical RBE" selected by the radiation oncologist when prescribing the treatment will be close to the reference RBE, but other factors (such as heterogeneity in dose distribution) may influence the selection of the clinical RBE. Combination of microdosimetric data and experimental RBE values improves the confidence in both sets of data.

Relative biological effectiveness of neutrons for cancer induction and other late effects: a review of radiobiological data

The risk of secondary cancer induction after a therapeutic irradiation with conventional photon beams is well recognised and documented. However, in general, it is totally overwhelmed by the benefit of the treatment. The same is true to a large extent for the combinations of radiation and drug therapy. After fast neutron therapy, the risk of secondary cancer induction is greater than after photon therapy. This can be expected from the whole set of radiobiological data, accumulated so far, which shows systematically a greater relative biological effectiveness (RBE) for neutrons for all the biological systems which have been investigated. Furthermore, the neutron RBE increases with decreasing dose and there is extensive evidence that neutron RBE is greater for cancer induction and for other late effects relevant in radiation protection than for cell killing at high doses as used in therapy. Almost no reliable human epidemiological data are available so far, and the aim of this work is to derive the best risks estimate for cancer induction after neutron irradiation and in particular fast neutron therapy. Animal data on RBE for tumour induction are analysed. In addition, other biological effects are reviewed, such as life shortening, malignant cell transformation in vitro, chromosome aberrations, genetic effects. These effects can be related, directly or indirectly, to cancer induction to the extent that they express a "genomic" lesion. Since neutron RBE depends on the energy spectrum, the radiation quality has to be carefully specified. Therefore, the microdosimetric spectra are reported each time they are available. Lastly, since heavy-ion beam therapy is being developed at several centres worldwide, the available data on RBE at low doses are reviewed. It can be concluded from this review that the risk of induction of a secondary cancer after fast neutron therapy should not be greater than 10-20 times the risk after photon beam therapy. For heavy ions, and in particular for carbon ions, the risk estimate should be divided by a factor of about 3 due to the reduced integral dose. The risk has to be balanced against the expected improvement in cure rate when the indication for high-LET therapy has been correctly evaluated in well-selected patient groups.

Specification of radiation quality in fast neutron therapy: microdosimetric and radiobiological approach

Specification of radiation quality is an important issue in fast neutron therapy since the biological effectiveness of the beams varies to a large extent with neutron energy. It must meet specific criteria, mainly derived from the accuracy requirement for absorbed dose delivery. A first approach to this problem consists in identifying physical parameters that can be related to Relative Biological Effectiveness (RBE) and which describe the beam production technique (e.g. neutron-producing reaction, p + Be or d + Be, energy of the incident particle). A second is based on microdosimetry, which provides a description of the secondary radiation components to which the biological consequences of irradiations are more directly correlated. A third approach consists in experimental RBE determinations in reference conditions: intestinal crypt regeneration in mice after irradiation to the whole body with single doses is proposed as a standard biological system for radiobiological calibrations of clinical fast neutron beams. Dosimetric, microdosimetric and radiobiological intercomparisons are encouraged since they provide a homogeneous set of data which facilitate the exchange of clinical information. They also constitute a basis for the clinical RBE approach and an overall check of the irradiation procedure. Therefore they should be recommended in every non-conventional radiation therapy facility.

RBE variation as a function of depth in the 200-MeV proton beam produced at the National Accelerator Centre in Faure (South Africa)

BACKGROUND AND PURPOSE

Thorough knowledge of the RBE of clinical proton beams is indispensable for exploiting their full ballistic advantage. Therefore, the RBE of the 200-MeV clinical proton beam produced at the National Accelerator Centre of Faure (South Africa) was measured at different critical points of the depth-dose distribution.

MATERIAL AND METHODS

RBEs were determined at the initial plateau of the unmodulated and modulated beam (depth in Perspex = 43.5 mm), and at the beginning, middle and end of a 7-cm spread-out Bragg peak (SOBP) (depths in Perspex = 144.5, 165.5 and 191.5 mm, respectively). The biological system was the regeneration of intestinal crypts in mice after irradiation with a single fraction.

RESULTS

Using 60Co gamma-rays as the reference, the RBE values (for a gamma-dose of 14.38 Gy corresponding to 10 regenerated crypts) were found equal to 1.16 +/- 0.04, 1.10 +/- 0.03, 1.18 +/- 0.04, 1.12 +/- 0.03 and 1.23 +/- 0.03, respectively. At all depths, RBEs were found to increase slightly (about 4%) with decreasing dose, in the investigated dose range (12-17 Gy). No significant RBE variation with depth was observed, although RBEs in the SOBP were found to average a higher value (1.18 +/- 0.06) than in the entrance plateau (1.13 +/- 0.04).

CONCLUSION

An RBE value slightly larger than the current value of 1.10 should be adopted for clinical application with a 200-MeV proton beam.