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RBE of the MIT clinical epithermal neutron beam


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RBE of the MIT clinical epithermal neutron beam

  1. 1. RADIATION RESEARCH 164, 805–809 (2005) 0033-7587/05 $15.00 2005 by Radiation Research Society. All rights of reproduction in any form reserved. RBE of the MIT Epithermal Neutron Beam for Crypt Cell Regeneration in Mice J. Gueulette,a,1 P. J. Binns,b B. M. De Coster,a X-Q. Lu,c S. A. Robertsd and K. J. Rileyb a Universite catholique de Louvain, Radiobiologie et Radioprotection (RBNT-5469), Brussels, Belgium; b Nuclear Reactor Laboratory, Massachusetts ´ Institute of Technology, Cambridge, Massachusetts 02139; c Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts 02215; and d Biostatistics Group, Division of Epidemiology and Health Sciences, University of Manchester, Manchester, M13 9PT, United Kingdom an element possessing a large cross section for thermal neu- Gueulette, J., Binns, P. J., De Coster, B. M., Lu, X-Q., Rob- tron absorption—in conjunction with beams of low- or in- erts, S. A. and Riley, K. J. RBE of the MIT Epithermal Neu- termediate-energy neutrons (1). The boron is nonradioac- tron Beam for Crypt Cell Regeneration in Mice. Radiat. Res. tive and is chemically bound to a delivery agent that is 164, 805–809 (2005). nontoxic and accumulates preferentially in the tumor during The RBE of the new MIT fission converter epithermal neu- its administration prior to irradiation. A therapeutic dose is tron capture therapy (NCT) beam has been determined using achieved only if the boron atoms in the tumor absorb suf- intestinal crypt regeneration in mice as the reference biolog- ficient thermal neutrons from the beam. Fission fragments ical system. Female BALB/c mice were positioned separately created by the 10B(n, )7 Li reaction possess a high linear at depths of 2.5 and 9.7 cm in a Lucite phantom where the energy transfer and have very short ranges in tissue that measured total absorbed dose rates were 0.45 and 0.17 Gy/ deposit dose adjacent to the target boron atoms on a scale min, respectively, and irradiated to the whole body with no approximately equivalent to the dimensions of a mamma- boron present. The -ray (low-LET) contributions to the total lian cell. absorbed dose (low- high-LET dose components) were 77% (2.5 cm) and 90% (9.7 cm), respectively. Control irradiations In BNCT, the absorbed dose at any point in irradiated were performed with the same batch of animals using 6 MV tissue results from the addition of four components: the - photons at a dose rate of 0.83 Gy/min as the reference radi- ray dose (both incident and induced), D ; the fast-neutron ation. The data were consistent with there being a single RBE dose arising mainly from interactions with hydrogen nuclei, for each NCT beam relative to the reference 6 MV photon DN; the thermal neutron dose, due principally to the beam. Fitting the data according to the LQ model, the RBEs 14 N(n,p) 14C reaction, DP; and the boron capture dose, DB, of the NCT beams were estimated as 1.50 0.04 and 1.03 which is directly proportional to the product of the boron 0.03 at depths of 2.5 and 9.7 cm, respectively. An alternative concentration and the thermal neutron flux in the tissue of parameterization of the LQ model considering the proportion interest. Each of these radiation types has a different rela- of the high- and low-LET dose components yielded RBE val- tive biological effectiveness (RBE), and the total effective ues 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 (neu- BNCT dose is expressed as the sum of the RBE-weighted trons) at 2.5 and 9.7 cm, respectively. The two estimates are components. Extensive preclinical research was performed significantly different (P 0.016). There was also some evi- to determine appropriate values for the individual RBEs of dence to suggest that the shapes of the curves do differ some- the various dose components, and these were applied by what for the different radiation sources. These discrepancies the clinical facilities where they were measured as well as could be ascribed to differences in the mechanism of action, by others in the specification of a treatment prescription (2– to dose-rate effects, or, more likely, to differential sampling 5). of a more complex dose–response relationship. 2005 by Radiation While comparison of both the dosimetry and the physical Research Society characteristics of different epithermal neutron beams has begun (6–8), little information has been reported on the radiobiological properties of the different beams now used INTRODUCTION in BNCT research. Physical and radiobiological beam com- parisons are required if experimental results derived at one Boron neutron capture therapy (BNCT) is a binary form institute are to be generalized and applied at other facilities. of radiotherapy that uses a compound containing boron— Moreover, if preclinical and clinical data from different cen- 1 Address for correspondence: UCL-RBNT 5469, Cliniques Universi- ters are to be compared or combined, some degree of uni- taire St-Luc, 54, avenue Hippocrate, 1200 Bruxelles, Belgium; e-mail: formity in specifying the biological quality of the different beams must be established. 805
  2. 2. 806 GUEULETTE ET AL. As an extension of the previous successful studies of the RBE of different clinical fast-neutron beams using the crypt cell colony regeneration assay in mice (9), this assay is now applied to the radiobiological characterization of the new fission converter beam (FCB) at the Massachusetts Institute of Technology (MIT) (10). To distinguish the characteristics of this neutron capture therapy (NCT) beam from those related to the capture agent, the assay was performed with- out the infusion of a boron compound. MATERIALS AND METHODS The Biological System Intestinal crypt regeneration in mice is an in vivo end point that is a precursor to the intestinal syndrome and that can be scored before the occurrence of the full functional effects (11). Early response scoring re- FIG. 1. Irradiation set-up (see the text). Irradiations were performed duces the potential influence of environmental factors, which, together separately with the mice at a depth of either 2.5 or 9.7 cm in a solid with the steepness of the dose–effect relationship, makes the system suit- Lucite phantom. The thickness of each mouse (cross section) is 1.5 cm. able for RBE determinations. The system also has the advantage that the The ‘‘inner’’ pair of mice (in black on the front view) and the ‘‘outer’’ animals may be given whole-body irradiation. This allows the use of large pair (in gray) are displaced 1.8 cm and 5.3 cm from the central axis of radiation fields whose dose distributions are considerably more easily the beam, respectively. measured and prescribed. Experimental Procedure beam. The 9.7-cm depth was chosen to provide an absorbed dose mixture Conventional female BALB/c mice (11–13 weeks old) were supplied comprised predominantly of rays with only a small contribution from by a commercial vendor (Charles River Laboratories) the week prior to thermal neutrons. Neutron flux for each irradiation was monitored using irradiation and were kept in quarantine facilities at the animal care facility the four fission counters housed in the collimator. These were positioned of MIT. Mice were randomly assigned to beam type (control photon or in the periphery of the beam and formed part of the automated dose epithermal neutron beam) and nominal dose levels. Mice were housed in delivery system for clinical irradiations. This monitoring system was cal- groups of six in small plastic cages; each animal was identified by a color ibrated against dosimetry measurements performed at each of the mouse code. The mice were killed humanely 84 1 h after the end of the locations using individual phantoms constructed from nylon (type 6) that irradiations, and a 3-cm jejunal section was taken 1 cm below the pylorus. each had a mass of approximately 24 g, equivalent to that of the irradiated The samples were immediately immersed in Bouin Hollande fixative. mice. Thermal neutron doses were determined from measured differences They were then embedded in paraffin, transversely sliced into 4- m-thick in the activation of bare and cadmium-covered gold foils while -ray and sections, and stained according to the classical trichrome technique. This fast-neutron absorbed doses were measured with separate graphite and study and its technical procedures were approved by the Committee on A-150 plastic-walled ionization chambers (13, 14). Total absorbed doses Animal Care at MIT, for which animal welfare assurance number A-3125- are given for A-150 plastic with an uncertainty, expressed as one standard 01 was issued. deviation, that changed with depth, decreasing from 7% at 2.5 cm to 4% The reported number of regenerated crypts per circumference is the at 9.7 cm. In the epithermal neutron beam, the mice received absorbed average from two sets of three consecutive slices taken approximately 1 doses ranging from 2.6 to 12.3 Gy, for which the irradiation times were mm on either side of the middle of the intestine segment, i.e. from six between 7 and 62 min. This dose range was determined after a pilot study intestinal cross sections. No differences in crypt size between control performed using 10 mice at five different dose increments 1 month prior photon and NCT beam samples were observed. Given this, and since this to the main experiment. work was not intended to determine the detailed shape of the crypt sur- Control photon irradiations were performed in a 6 MV clinical linear vival curves or to make any inferences regarding crypt or cellular param- accelerator. As for epithermal neutrons, the mice were irradiated during eters, no correction factor was applied to the number of regenerated the day, with half of the batch of animals being irradiated before and the crypts (12). other half the day after neutron irradiations. Mice were placed in the same jig as for NCT irradiations and were irradiated to the whole body at the depth of the peak dose (8-cm Lucite backscatter). The dose rate was 0.83 Irradiation Conditions and Dosimetry Gy/min. Twelve dose levels (four mice per dose) were used. Absorbed Irradiations were performed in the fission converter beam (FCB) at the doses were specified in water and measured according to the ICRU pro- MITR-II reactor (10). The converter was operated at approximately 78 tocol for irradiation (15). kW with the reactor at 4.7 MW to provide the nominal maximum beam intensity for this experiment. The mice were irradiated simultaneously in Statistical Analysis groups of four, held side by side in a custom-made jig that formed part of a solid Lucite phantom, shown schematically in Fig. 1. The phantom Assuming a single RBE, s, for each source, s, and using the conven- was positioned on the treatment couch with its front face at the (16-cm- tional linear-quadratic form for crypt survival with m clonogens, we rep- diameter) circular collimator aperture. The height of the couch was ad- resent the three crypt survival curves by the following formula: [ ]] m justed to align the abdomen of each mouse with the central axis of the beam. Whole-body irradiations were carried out with the mice at depths of 2.5 and 9.7 cm measured from the midline of the abdomen to the front S(D) 1 1 exp s [ D1 s D , (1) surface of the phantom. The shallow depth was chosen as a reference where D is the dose and and / are the parameters of the LQ model. point since this is the approximate depth of the dose maximum in the This formula works well providing the range of killing is not too great
  3. 3. RBE OF THE MIT EPITHERMAL NEUTRON BEAM 807 TABLE 1 Absorbed Dose Rates Measured in Nylon (type 6) Mouse Phantoms Placed in the Custom-Made Jig Forming Part of a Lucite Cube Used for Irradiating Mice with the Reactor Operating at a Power of 4.7 MW Absorbed dose rate (Gy/min) 2.5 cm 9.7 cm Depth/location Photons Neutrons a Photons Neutronsa Inner positions 0.38 (77%)b 0.11 (23%) 0.16 (90%) 0.02 (10%) Total: 0.49 Gy/min Total: 0.18 Gy/min Outer positions 0.32 (77%) 0.10 (23%) 0.14 (90%) 0.02 (10%) Total: 0.42 Gy/min Total: 0.16 Gy/min All positions Mean total dose rate: 0.45 Gy/min Mean total dose rate: 0.17 Gy/min a The proton recoil dose from nitrogen capture of thermal neutrons in A-150 plastic has been included with the fast-neutron dose to give a combined or total neutron dose. b Percentage of each component relative to the total measured dose. (16). The functional formula (Eq. 1) is fitted to the data for all three shown in Table 1, where the data for the inner and outer sources simultaneously using direct maximization of the binomial quasi- pairs of mice (see Fig. 1) have been averaged. The contri- likelihood, which explicitly allows for overdispersion between animals. The binomial denominators were based on the measured number of crypts bution from thermal neutrons to the total absorbed dose per circumference (120) in control animals multiplied by the number of rates measured in A-150 plastic was determined from foil sections scored (six sections). The parameter m was reparameterized in activation. At a depth of 2.5 cm, the absorbed dose rates terms of its logarithm to improve numerical stability. from thermal neutrons were 0.092 0.004 and 0.083 Alternatively, the model was parameterized in terms of the specific - 0.004 Gy/min for the inner and outer pairs of mice, re- ray and neutron components of the mixed beams, with proportions and n, and RBEs, and n. Assuming a multitarget formulation, the spectively, and at 9.7 cm depth they were 0.013 0.001 survival function then becomes and 0.011 0.001 Gy/min. The contribution of the -ray component to the total absorbed dose increased from 77 to S(D) 1 [1 P(D)] m 90% when moving from a depth of 2.5 to 9.7 cm. P(D) exp [ D n n D 2 2 D2 Dose–Effect Curves n 2 n 2 n D2 2 n D2 . ] (2) The dose–effect curves for intestinal crypt regeneration after epithermal neutron or 6 MV photon irradiation are There are three quadratic terms, ( / )n, ( / ) and ( / ) n, representing presented in Fig. 2. A total of 144 mice were irradiated. ‘‘dual-hit’’ terms for pairs of neutrons, pairs of rays, and -ray–neutron Due to the distortion of the field profiles of the NCT beams, interactions, respectively. Given the proportions of the dose contributed by the two radiation types, and n, and defining the RBE of the the doses to the pairs of ‘‘inner’’ and ‘‘outer’’ mice (see rays ( ) as 1, the reparameterized form was fitted to the data in the same Fig. 1) differed by about 17% and 13% at depths of 2.5 way, and the apparent RBE of the neutron component ( n) was deter- and 9.7 cm, respectively. The data were fitted using the LQ mined. Since this formula has a large number of parameters, and since model (Eq. 1). Since there was no evidence that the / there was no evidence for a non-zero interaction term, ( / ) n , this in- ratios differed for the three beams, a model with a common teraction term was set to zero. The RBE of the -ray component, , is defined as 1. As the / ratios of the two components are a priori dif- / ratio was fitted to the data, allowing a single RBE ferent, the RBE will be dependent on dose, and the parameter estimate, estimate to be obtained; the parameters of this model are n, represents the extrapolation of the RBE to zero dose (100% survival). shown in Table 2. The RBEs of the NCT beams were es- The RBE at specific survival levels was obtained trivially from the pa- timated as 1.50 0.04 and 1.03 0.03 at depths of 2.5 rameter estimates, and the standard errors of these estimates were ob- and 9.7 cm, respectively. These estimates are little affected tained from these and the parameter covariance matrix using a parametric bootstrap method. We present the RBE at a survival level corresponding if the shoulder region is excluded. However, note that the to 20 crypts per circumference (i.e. a surviving fraction of 0.167). Com- combination of biological and dosimetry uncertainties de- parisons between models and inferences concerning differences between creases the precision of the RBE estimates, potentially in- parameters were performed using quasi-likelihood deviance-ratio F tests, creasing the standard errors by a factor of two. comparing the relevant model fits. DISCUSSION RESULTS Estimates of the RBE of the MIT epithermal neutron Dosimetry beam have been obtained at two depths. The determination The relative fractions of the total neutron and -ray dose of the RBE estimates is dependent on the fit to the LQ components did not vary appreciably across the field, as model. Visually, the fits presented in Fig. 2 seem reason-
  4. 4. 808 GUEULETTE ET AL. TABLE 2 Fitted Parameters of the LQ Model for the Three Mixed Beams (Common Estimates of the Parameters) and Estimates for the RBE ( ) Relative to the Reference 6 MV Photon Beam Parameter Estimate (SE) (Gy) 0.022 (0.006) / (Gy) 1.03 (0.27) log (m) 1.07 (0.08) RBE ( ), at depth of 2.5 cm 1.50 (0.04) RBE ( ), at depth of 9.7 cm 1.03 (0.03) eters fitted to this model are shown in Table 3. At a survival level corresponding to 20 crypts per circumference (16.7% survival), the RBE estimates are 5.2 0.6 at a depth of 2.5 cm and 4.0 0.7 at a depth of 9.7 cm. The neutron component of the dose at 9.7 cm is small (10% of total dose), and the dose rate is low (0.17 Gy/min) compared to the control photon (0.83 Gy/min) irradiation. The neutron FIG. 2. Dose–effect curves for intestinal crypt regeneration in mice RBE estimates differ significantly (P 0.016), again sug- after photon (open circles, broken line) or epithermal neutron irradiations at depths of 2.5 cm (open triangles, lower solid line) and 9.7 cm (closed gesting that not all the radiobiological mechanisms are ac- triangles, upper solid line). The points represent individual mice; thus for counted for in this analysis. More data from a wider range neutrons there are pairs of points at the doses for the ‘‘inner’’ and ‘‘outer’’ of beam compositions are required to confirm whether this positions (see the Results). The error bars represent binomial 95% con- discrepancy is real and whether it can be accounted for by fidence intervals of the values for individual mice (six sections). For repair mechanisms or by the use of more complex survival clarity, the upper half of each error bar is shown for the photon data and the lower half for the 2.5-cm data, and no error bars are shown for the models, such as the dual killing model (16). 9.7-cm data. Note the axis break to show data with zero surviving crypts; The purpose of this experiment was to study the suit- some of these points represent two identical superimposed values. The ability of the regeneration of intestinal crypt cells in mice lines represent the fit to the LQ model (Eq. 1 and Table 2). as a possible transfer measurement to relate the biological properties of different NCT beams. In this respect, it is able, but there is some evidence of a lack of fit, and it can important to note that the stated RBEs do not represent the be shown statistically that if we allow the biological param- actual RBE values that should be applied in the clinical eter m to take different values for the three curves, a sig- prescription of absorbed dose, which, due to the intrinsic nificantly better fit is obtained (P 0.001). The formal lack variability of RBE, requires the use of clinically relevant of fit suggests that the shapes of the curves do differ some- biological systems and irradiation conditions (17). Conse- what for the different radiation sources, an observation that quently, the results should be interpreted as a ‘‘beam could be ascribed to differences in the mechanism of action weighting factor’’ or ‘‘RBEbeam’’ that is primarily useful in or to dose-rate effects. Crypt survival curves are not always well represented by the LQ model (16), and the discrepancy TABLE 3 is perhaps more likely to be accounted for by the difference Fitted Parameters of the LQ Model for the Beam in survival range sampled in the measurements of the two Components, with Separate / Ratios for beams, which differentially sample different components of Neutrons and Photons the survival curve. More data are required to investigate Parameter Estimate (SE) this further, but from Fig. 2 it is clear that a single RBE is ( ) (Gy) 0.050 (0.011) a good practical summary of the differences between ( / ) (Gy) 0.45 (0.11) beams. ( / )n (Gy) 0.11 (0.07) The NCT beams actually contain a mixture of rays and log(m) 1.43 (0.12) neutrons (the neutron components comprise only 23% and RBE ( )a at depth of 2.5 cm 9.7 (2.5) 10% of the dose at 2.5 and 9.7 cm, respectively; see Table RBE ( )a at depth of 9.7 cm 7.5 (2.0) RBE20b at depth of 2.5 cm 5.2 (0.6) 1). An alternative parameterization of the LQ model is to RBE20b at depth of 9.7 cm 4.0 (0.7) consider the RBE and the proportions of rays and neu- trons (Eq. 2) and to obtain estimates for the RBE of the Note. Since the RBE estimates for neutrons relative to the photon beam were significantly different for the two depths, separate estimates are neutron component of the beams. Since the / ratios provided. would be expected to differ for the two types of radiation, a The neutron RBE estimate extrapolated to zero dose. the RBE estimate will be dependent on dose. The param- b The neutron RBE at a survival level of 20 crypts per circumference.
  5. 5. RBE OF THE MIT EPITHERMAL NEUTRON BEAM 809 comparing the relative biological potency of a beam under 7. P. J. Binns, K. J. Riley and O. K. Harling, Dosimetric comparison of six epithermal neutron beams using an ellipsoidal water phantom. In well-defined irradiation conditions. This will enable varia- Research and Development in Neutron Capture Therapy (W. Sauer- tions in irradiation conditions within a single beam (e.g. as wein, R. Moss and A. Wittig, Eds.), pp. 405–410. Monduzzi, Bolo- a function of depth) or between different beams at the same gna, 2002. depth to be quantified and if necessary incorporated in the 8. K. J. Riley, P. J. Binns, D. D. Greenberg and O. K. Harling, A phys- ical dosimetry intercomparison for BNCT. Med. Phys. 29, 898–904 specification of the clinical dose. (2002). 9. J. Gueulette, M. Beauduin, V. Gregoire, S. Vynckier, B. M. De Cos- ´ ter, M. Octave-Prignot, A. Wambersie, K. Strijkmans, A. De Schrijver ACKNOWLEDGMENTS and P. Chauvel, RBE variation between fast neutron beams as a func- tion of energy. Intercomparison involving 7 neutrontherapy facilities. The authors thank Profs. Jeff Coderre and Otto Harling for their com- Bull. Cancer/Radiother. 83 (Suppl. 1), 55s–63s (1996). ments during the preparation of this manuscript. This work was supported 10. O. K. Harling, K. J. Riley, T. H. Newton, B. A. Wilson, J. A. Bernard, by IAEA (contract no. 11063/RO) and by the U.S. DOE (contracts DE- L. W. Hu, E. J. Fonteneau, P. T. Menadier, S. J. Ali and P. M. Busse, FG02-97ER62489 and DE-FG02-96ER62193). The fission converter-based epithermal neutron irradiation facility at the Massachusetts Institute of Technology reactor. Nucl. Sci. Eng. Received: October 15, 2003; accepted: August 1, 2005 140, 223–240 (2002). 11. H. R. Withers and M. M. Elkind, Microcolony survival assay for cells of mouse intestinal mucosa exposed to radiation. Int. J. Radiat. REFERENCES Biol. 17, 261–267 (1970). 12. C. S. Potten, M. Rezvani, J. H. Hendry, J. V. Moore and D. Major, 1. G. L. Locher, Biological effects and therapeutic possibilities of neu- The correction of intestinal microcolony counts for variation in size. trons. Am. J. Roentgenol. Radium Ther. 36, 1–36 (1936). Int. J. Radiat. Biol. 40, 321–326 (1981). 2. J. A. Coderre, M. S. Makar, P. L. Micca, M. M. Nawrocky, H. B. 13. K. J. Riley, P. J. Binns and O. K. Harling, Performance characteristics Liu, D. D. Joel, D. N. Slatkin and H. I. Amols, Derivations of relative of the MIT fission converter based epithermal neutron beam. Phys. biological effectiveness for the high-LET radiations produced during Med. Biol. 48, 943–958 (2003). boron neutron capture irradiations of the 9L rat gliosarcoma in vitro and in vivo. Int. J. Radiat. Oncol. Biol. Phys. 27, 1121–1129 (1993). 14. R. D. Rogus, O. K. Harling and J. C. Yanch, Mixed field dosimetry of neutron beams for boron neutron capture therapy at the MITR-II 3. P. R. Gavin, S. L. Kraft, R. Huiskamp and J. A. Coderre, A review: research reactor. Med. Phys. 21, 611–1625 (1994). CNS effects and normal tissue tolerance in dogs. J. Neuro-Oncol. 33, 71–80 (1997). 15. ICRU, Determination of Absorbed Dose in a Patient Irradiated by Beams of X or Gamma Rays in Radiotherapy Procedures. Report 24, 4. J. A. Coderre and G. M. Morris, The radiation biology of boron International Commission on Radiation Units and Measurements, Be- neutron capture therapy. Radiat. Res. 151, 1–18 (1999). thesda, MD, 1976. 5. J. Benczik, T. Seppala, M. Snellman, H. Joensuu, G. M. Morris and 16. S. A. Roberts, J. H. Hendry and C. S. Potten, Intestinal crypt clon- J. W. Hopewell, Evaluation of the relative biological effectiveness of ogens: a new interpretation of radiation survival curve shape and a clinical epithermal neutron beam using dog brain. Radiat. Res. 159, clonogenic cell number. Cell Prolif. 36, 215–231 (2003). 199–209 (2003). 17. J. Gueulette, H. G. Menzel, P. Pihet and A. Wambersie, Specification 6. O. K. Harling, International dosimetry exchange: Status report. In of radiation quality in fast neutron therapy: Microdosimetric and ra- Research and Development in Neutron Capture Therapy (W. Sauer- diobiological approach. In Recent Results in Cancer Research (R. wein, R. Moss and A. Wittig, Eds.), pp. 333–340. Monduzzi, Bolo- Engenhart-Cabillic and A. Wambersie, Eds.), pp. 31–53. Springer- gna, 2002. Verlag, Berlin, Heidelberg, New York, 1998.