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Comparison of 7 Epithermal Neutron Beams
 

Comparison of 7 Epithermal Neutron Beams

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    Comparison of 7 Epithermal Neutron Beams Comparison of 7 Epithermal Neutron Beams Document Transcript

    • RADIATION RESEARCH 164, 212–220 (2005) 0033-7587/05 $15.00 2005 by Radiation Research Society. All rights of reproduction in any form reserved. Epithermal Neutron Beams for Clinical Studies of Boron Neutron Capture Therapy: A Dosimetric Comparison of Seven Beams P. J. Binns,a,1 K. J. Rileya and O. K. Harlingb a Nuclear Reactor Laboratory and b Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, 138 Albany Street, Cambridge, Massachusetts 02139 cade using newly designed beams of epithermal neutrons Binns, P. J., Riley, K. J. and Harling, O. K. Epithermal for irradiating deep-seated lesions. The use of these higher- Neutron Beams for Clinical Studies of Boron Neutron Cap- ture Therapy: A Dosimetric Comparison of Seven Beams. Ra- energy epithermal neutrons attempts to ensure that satisfac- diat. Res. 164, 212–220 (2005). tory dose distributions of thermal neutrons are achieved at depth without requiring surgical intervention to reflect the A comparison of seven epithermal neutron beams used in clinical studies of boron neutron capture therapy (BNCT) in scalp or remove part of the skull. Thus far the studies have Sweden (Studsvik), Finland (Espoo), Czech Republic (ReZ), been restricted to dose-finding Phase I/II trials that seek first The Netherlands (Petten) and the U.S. (Brookhaven and Cam- to establish normal tissue tolerance with a view thereafter bridge) was performed to facilitate sharing of preclinical and to demonstrate some therapeutic effect (1–3). As the dif- clinical results. The physical performance of each beam was ferent programs conduct the extramural research necessary measured using a common dosimetry method under condi- to develop a successful paradigm and accrue individual tions pertinent to brain irradiations. Neutron fluence and ab- clinical experience, the need will arise to compare and pos- sorbed dose measurements were performed with activation sibly share results among the different participants. This foils and paired ionization chambers on the central axis both in air and in an ellipsoidal water phantom. The overall quality imperative will reduce duplication of effort while hopefully of each beam was assessed by figures of merit determined expediting optimization and determination of the possible from the total weighted dose profiles that assumed the pres- efficacy for the modality. ence of boron in tissue. The in-air specific beam contamination The description of the physical characteristics of an ep- from both fast neutrons and rays ranged between 8 and 65 ithermal neutron beam is a prerequisite for any exchange 10 11 cGy(w) cm2 for the different beams and the epither- of either experimental or clinical results between different mal neutron flux intensities available at the patient position facilities. This, however, is complicated by the differences differed by more than a factor of 20 from 0.2–4.3 109 n cm 2 s 1. Percentage depth dose profiles measured in-phantom in beam designs and an absence of a standardized meth- for the individual photon, thermal and fast-neutron dose com- odology of appraisal. Fission reactors are currently the ponents differed only subtly in shape between facilities. As- source of choice for clinical use that employ beam lines of suming uptake characteristics consistent with the currently various lengths and configurations to direct neutrons to the used boronated phenylalanine, all the epithermal beams ex- patient’s position (4). These beam lines contain different hibit a useful penetration of 8 cm or greater that is sufficient filter moderator assemblies that moderate and shape the fis- to irradiate a lesion seated at the brain midline. The perfor- sion energy spectrum of the source beam to the desired mance of the existing facilities will benefit from the introduc- epithermal range. Both photon shielding and collimation tion of advanced compounds through improved beam pene- trability. This could increase by as much as 2 cm for the pur- are used to remove unwanted background contaminants and est of beams, although the beam intensities generally need to to reduce beam spreading. No single, ideal neutron energy be increased to between 2–5 109 n cm 2 s 1 to maintain exists, and the facilities, because of practical considerations, manageable irradiation times. These data provide the first all produce a range of epithermal neutron energies. The consistent measurement of beam performance at the different neutron energy spectrum, field size and degree of colli- centers and will enable a preliminary normalization of the mation all affect beam performance, and these differ be- calculated patient dosimetry. 2005 by Radiation Research Society tween facilities. In addition, the beams are invariably con- taminated with photons and fast neutrons that are detri- INTRODUCTION mental to the depth–dose characteristics, unnecessarily in- creasing the dose to healthy tissue. The minimization of Clinical research into boron neutron capture therapy these contaminants is achieved with differing degrees of (BNCT) for brain cancer has advanced during the last de- success at the different facilities. 1 Address for correspondence: Nuclear Reactor Laboratory, 138 Albany An adequate description of the physical characteristics of Street, Cambridge, MA 02139; e-mail: binns@mit.edu. an epithermal beam for experimental as well as therapeutic 212
    • A COMPARISON OF SEVEN EPITHERMAL BEAMS FOR BNCT 213 purposes requires an in-phantom assessment of the three that are based upon outputs of beam monitors such as fission counters to principal radiation constituents contributing to the dose, indicate the relative radiation fluence during the experiment. Measurements were performed in each beam on the central axis both namely the thermal neutron flux, which is used to determine in-air and in an ellipsoidal phantom of total volume of 2.50 liters that the dose from neutron capture in boron and other nuclei as was filled with distilled water (13). The geometry of the phantom is based well as the fast-neutron and the photon absorbed dose com- upon the Snyder brain model and has outer dimensions of 13.6 (X), 19.6 ponents. These physical characteristics vary depending (Y) and 16.6 cm (Z) for the axes of the three major ellipses. The phantom upon the geometry and material of the phantom in which was placed in the collimated fields with the X (13.6 cm) axis parallel to the beam and aligned with the central axis for each measurement using they are measured. Since there is no standard protocol for the patient positioning lasers. The phantom was positioned in the usual mixed-field dosimetry in epithermal therapy beams, differ- treatment position, that was, for most facilities, without an air gap be- ent methods using active and passive dosimeters are cur- tween the apex of the ellipsoid and the plane that forms the end of the rently used by the various research groups (5). The methods collimator. At the highly collimated HFR, however, it was placed 30 cm all have comparatively large uncertainties by general radio- downstream from the collimator exit. In-air measurements were per- formed with the detectors similarly positioned in the center of the field therapy standards because of the complexities of interpret- and either held in a retort stand or affixed across the collimator aperture ing dosimeter responses to the individual radiation com- with Mylar tape. ponents. These together with differences in the assumed Thermal and epithermal neutron fluxes were determined using the cad- tissue compositions for which absorbed dose is specified, mium difference technique with 0.05-mm-thick gold foils and 0.51-mm- the use of different elemental kerma coefficients, and def- thick cadmium covers (6). The foils were irradiated in runs between 0.5 and 16.1 h long for which the incident epithermal neutron fluence was initions of thermal neutron flux all preclude a simple eval- in the range of 3–19 1012 n cm 2. Foils were counted within 24 h of uation of the data routinely reported between the various the end of bombardment at the host institute using an HPGe detector with groups. an energy and efficiency calibration traceable to a European Standards To afford a direct comparison of the dosimetric proper- Laboratory. Irradiated foils from Studsvik, Espoo and Petten were also ties of the various BNCT research facilities currently avail- counted at MIT as a further test of consistency, using an HPGe detector calibrated with a mixed radionuclide source traceable to the National able in Europe and the U.S., the four different European Institute of Standards and Technology (NIST). Fast-neutron and photon facilities presently conducting clinical trials were appraised absorbed dose rates were measured using the dual-chamber technique in quick succession using a common experimental method with separate A-150 and graphite walled ionization chambers that have with a single set of uncertainties. Beams in Sweden (Studs- identical active volumes of 0.1 ml (8). Flow meters and gas mixtures vik), Finland (Espoo), Czech Republic (ReZ) and The were provided by each of the host laboratories, and chamber responses were corrected to standard environmental conditions using calibrated ther- Netherlands (Petten) were characterized under reference mometers and barometers. conditions pertinent to clinical irradiations using the dosim- The absorbed dose rates arising from thermal neutron capture in boron etry procedures developed and adopted by the Massachu- and nitrogen were calculated from measurements of the 2200 m s 1 neu- setts Institute of Technology (MIT). These studies comple- tron flux using bare and cadmium-covered gold activation foils (7, 14). ment those performed previously in the U.S. at MIT, Cam- The determined neutron flux was multiplied by the weight fraction of boron or nitrogen for the tissue or material of interest and kerma coef- bridge (M67) (6), Brookhaven (BMRR) (7) and more re- ficients of 8.66 10 6 and 7.88 10 10 cGy cm2 for boron and nitrogen, cently the new MIT fission converter beam (FCB) (8). The respectively. The energy-dependent responses of the two ionization cham- measured depth–dose data were used to determine various bers were converted to absorbed dose using values of the various physical figures of merit assessing the performance of each beam parameters described recently (8). Neutron energy spectra from the Eu- under uniform experimental conditions and provide, for the ropean beams were compared with those previously assumed for the M67, BMRR and FCB facilities to confirm the appropriateness of the different first time, a consistent set of characterizations of all facili- values with depth in phantom assigned to kt for the sensitivity of the A- ties accruing clinical results in the Western world. This 150 plastic walled chamber to neutrons. Fast-neutron absorbed doses are study is intended to illustrate the suitability of the beams reported for A-150 plastic that is tissue (muscle) equivalent (15). used to date for BNCT and emphasize the possible require- To compare the performance of each beam under more realistic con- ments of those that might be needed in future, more clini- ditions pertinent to the irradiation of brain tumors where boron would be present in tissue, the boron dose was added to provide the total effective cally based settings. It is also envisaged that these mea- dose profiles from the measured data. Two uniform sets of parameters surements could be extended to include other centers in the were applied that are representative of either boronated phenylalanine world as and when they commence clinical trials using ep- (BPA) or a more advanced boron delivery compound such as the por- ithermal neutrons. phyrins currently under development. Boron concentrations of 18 and 65 g g 1 were assumed in normal brain and tumor tissue, respectively, which are characteristic of the uptakes observed using BPA in clinical MATERIALS AND METHODS trials at MIT (16). Additional profiles determined for a more selective porphyrin compound assumed a boron concentration of 65 g g 1 in The epithermal neutron beams at the facilities of the R2–0 (Studsvik) tumor to match the loading achievable with BPA, but with only 0.65 g (9), FiR-1 (Espoo) (10), LVR-15 (ReZ) (11) and HFR (Petten) (12) re- g 1 in brain (17). Brain tissue was considered to be composed of 10.7% actors were characterized with the standard dosimetry apparatus and pro- hydrogen, 14.5% carbon, 2.2% nitrogen and 71.2% oxygen by mass (15). cedures used in clinical trials at MIT. This is a direct extension of pre- Skin doses were estimated by combining the activation foil measurements vious characterizations for the M67 (6), BMRR (7) and FCB facilities on the surface of the phantom with extrapolations of the photon and fast- (8). Irradiation conditions and beam-line configurations for the study were neutron absorbed dose rates measured in phantom. The composition of similar to those routinely used during therapy at the different facilities. skin was represented as soft muscle tissue containing 10.2% hydrogen, Beam intensity was assessed with the respective dose control systems 12.3% carbon, 3.5% nitrogen and 72.9% oxygen (15). Boron concentra-
    • 214 BINNS, RILEY AND HARLING TABLE 1 In-Air Beam Parameters Measured in Seven Different Epithermal Beams used for Clinical NCT MIT FCBa Studsvikb FiR-1 BMRRc ReZ HFR MIT M67c Reactor power (MW) 5.0 0.50 0.25 3.0 9.0 45 5.0 Aperture diameter (cm) 12 14 10 14 12 12 12 15 epi (10 n cm s 1) 4.29 1.43 1.18 1.11 0.60 0.33 0.20 9 2 Dfn (cGy min 1) 3.6 7.1 2.3 1.7 6.0 2.4 1.6 D (cGy min 1) 9.4 10.8 0.6 1.0 3.9 0.8 1.7 Dfn/ epi (10 11 cGy cm2) 1.4 8.3 3.3 2.6 16.9 12.1 13.0 D / epi (10 11 cGy cm2) 3.6 12.6 0.9 1.5 10.8 3.8 14.3 Dfn/ epi (10 11 cGy(w) cm2) 4.5 26.5 10.4 8.3 54.1 38.7 41.6 DTotal/ epi (10 11 cGy(w) cm2) 8.1 39.2 11.5 9.8 64.9 42.5 55.9 Notes. Overall uncertainties (1 ) of 15, 5 and 16% are associated respectively with the epithermal flux, photon and fast-neutron absorbed dose determinations. The data from the Brookhaven (BMRR) and two MIT (M67 and FCB) clinical facilities were obtained previously (6–8). The total specific doses were determined assuming an RBE of 3.2 for neutrons and 1.0 for photons. a Fission converter operating at 83 kW. b Beam line configured with 9-mm-thick 6Li filter installed. c Decommissioned. tions of 27 and 0.65 g g 1 were assumed in skin for BPA (18) and tron dose components. The individual and total specific porphyrin, respectively. Individual absorbed dose components were com- background doses were determined by weighting the pho- bined to yield estimates of the total weighted dose to normal brain and tumor tissue by applying RBE values of 1.0 and 3.2 for photons and ton and neutron contributions with RBEs of 1.0 and 3.2, neutrons (thermal and fast), respectively. Differences in the microdistri- respectively, and then summing the photon and fast-neutron bution of boron delivered by BPA in both tissue and tumor were also components. Epithermal neutron flux ranges from 0.2–4.3 accommodated using cRBE factors of 1.3 for normal brain, 3.8 for tumor 109 n cm 2 s 1 with the attendant beam purity varying (19), and 2.5 for skin (18). In the absence of boron microdistribution data significantly between the different facilities. A practical for an advanced compound, a conservatively high estimate of 3.8 was applied as the cRBE to the absorbed dose produced from thermal neutron limit of approximately 2.8 10 11 cGy(w) cm2 was pro- capture by boron in normal brain, skin and tumor. The measurements and posed (4) as a goal for negligible beam contamination. The calculations include no boron in the water-filled phantom that would oth- additional dose burden this represents is equal to only about erwise be accounted for in treatment planning. 10% of the unavoidable peak effective absorbed dose pro- The total weighted dose profiles obtained from the in-phantom mea- duced from neutron capture reactions with elemental hy- surements were then used to determine several figures of merit, namely the advantage parameters (20), to assess the different beams under uni- drogen and nitrogen during a brain irradiation with epither- form irradiation conditions. The advantage depth (AD) is defined as the mal neutrons alone. The three facilities, FCB, FiR-1 and depth at which the total weighted dose to tumor equals the maximum BMRR, possessing the lowest beam contamination of ap- weighted dose received by normal tissue during an irradiation and is a proximately 10 10 11 RBE cGy cm2 exceed the proposed measure of the maximum depth at which therapeutic benefit is obtained. diminimus level but are considerably less than the 39–65 The advantage ratio (AR) is the ratio of the integral tumor dose to the normal tissue dose averaged from the surface where the beam is incident 10 11 cGy(w) cm2 exhibited by the others. to the advantage depth. The advantage depth–dose rate (ADDR) is also Figure 1 shows the thermal neutron flux as a function of determined and specifies the therapeutic dose rate at the AD. The ADDR depth in phantom on the central axis for the different gives the total dose rate achievable to treat tumor at the maximum useful beams. The estimated uncertainties for all these measure- depth of the beam and is also the maximum absorbed dose rate to normal ments, which are omitted from the figure for clarity, range tissue. Last, the therapeutic ratio, defined as the quotient of the biologi- cally weighted dose to tumor and the maximum weighted dose to brain, between 4.4% at shallow depth to 6.0% at 10 cm depth. was determined from the measured dose profiles as a function of depth Curves are smoothed spline fits to the experimental data to in the water phantom. help guide the eye and are shown only for the four Euro- pean facilities. The data are normalized, for ease of com- parison, to each measured thermal-neutron flux maximum RESULTS given in Table 2. The shapes of the profiles are all generally Free in-air measurements of the epithermal neutron in- similar except for those of Studsvik and Petten, which peak tensity and purity provide a useful first-order indicator of at slightly greater depths. Although the experimental un- beam performance. The epithermal neutron flux and spe- certainties make separating the different curves difficult, it cific fast-neutron and photon absorbed doses measured in- is apparent that the HFR has the lowest relative thermal air for the specified aperture size at each of the seven beams flux on the central axis at the beam entrance point on the are shown in Table 1 scaled to the nominal reactor oper- phantom. The FiR-1 curve peaks at the shallowest depth ating power used during therapy. Fast-neutron absorbed and is then attenuated with depth at the same rate as the doses are those measured in A-150 plastic. The measure- FCB, M67 and BMRR beams. This is more rapid than for ments have estimated uncertainties (1 ) of 15% for epi- either of the Studsvik, Petten or ReZ beams. At Studsvik, thermal flux, 5% for the photon and 16% for the fast-neu- the deeper penetrating profile is attained with 6Li filtration,
    • A COMPARISON OF SEVEN EPITHERMAL BEAMS FOR BNCT 215 FIG. 1. Thermal neutron (2200 m s 1) flux obtained from the analysis FIG. 2. Photon absorbed dose measured with a graphite walled ioni- of gold foil activation at seven different clinical NCT facilities in Europe zation chamber are shown for seven different NCT facilities in Europe and the U.S. Measurements were performed in an ellipsoidal water phan- and the U.S. Measurements were performed in an ellipsoidal water phan- tom under conditions pertinent to clinical irradiations. The results for each tom under conditions pertinent to clinical irradiations. The results for each beam are normalized to the measured maximum given in Table 2 and beam are normalized to the measured maximum given in Table 2 and have an estimated uncertainty of 4.4% at shallow depths increasing to have an estimated uncertainty of 4.4%. 6.0% at 10 cm. attenuated at depths greater than 3 cm at a similar rate which preferentially decreases the number of lower-energy except for the HFR and M67 beams. The possibility of neutrons in the epithermal range, while at the HFR it is significant differences between the in-phantom photon en- likely related to high beam collimation. ergy spectra of these two beams compared to the others is The photon and fast-neutron dose components associated not considered a likely explanation. This is because the with the beams are shown in Figs. 2 and 3 as depth–dose majority of the photon dose is produced from the same curves normalized to the respective measured maxima stat- interactions of thermal neutrons with the medium irrespec- ed in Table 2. All photon dose maxima were observed at tive of the facility. Poorer shielding and the presence of depth in-phantom, but those for fast neutrons were gener- leakage radiation striking the phantom from the beam line ally obtained in-air. The estimated uncertainty for the pho- as well as a beam size approaching the phantom size (21) ton dose component is 4.4%. Beams with higher specific may explain the deeper penetration of the photons for the photon doses have proportionally greater photon doses in M67 beam. However, the different photon attenuation pro- the first 3 cm from the surface of the phantom where dose file for the HFR may be related to the high collimation of buildup is prominent. The photon curves peak at approxi- the epithermal neutron beam that ensures a more radially mately the same depth in all the different beams and are uniform irradiation of the phantom volume relative to less TABLE 2 Measured Maxima for Thermal Neutron Flux as well as Photon and Fast-Neutron Absorbed Dose Rates at the Seven Different NCT Facilities in Europe and the U.S. In-phantom maxima MIT FCBa Studsvikb FiR-1 BMRRc ReZ HFR MIT M67c 2200 (10 n cm s 1) 5.70 2.68 1.95 1.48 1.18 0.72 0.20 9 2 Photon dose rate (cGy min 1) 31.0 18.1 9.8 6.3 9.5 5.0 2.6 Fast-neutron dose rate (cGy min 1) 3.2 4.3 2.3 1.6 6.7 1.6 1.1 Note. These values are used to normalize the flux depth profile and depth–dose curves illustrated in Figs. 1–3. a Fission converter operating at 83 kW. b Beam line configured with 9-mm-thick 6Li filter installed. c Decommissioned.
    • 216 BINNS, RILEY AND HARLING certainties depend upon the relative proportion of the fast- neutron component to the total dose. Only two curves are shown that represent the shallowest (Studsvik, FCB and HFR) and the deepest (M67) profiles for the normalized data sets, with points from the other facilities generally ly- ing between. The normalized fast-neutron dose profiles all appear to be generally similar in shape within the large stated uncertainties, and the doses are attenuated rapidly in the first few centimeters irrespective of the beam. The overall quality of the different beams is compared using advantage parameters (figures of merit) presented in Table 3. These were calculated from the weighted depth– dose profiles assuming a boron uptake characteristic of BPA and an advanced compound. Uncertainties associated with the different advantage parameters depend upon the relative error between the RBE weighted dose profiles for normal tissue and tumor in each beam. The effect on the weighted dose profiles of separately varying each absorbed dose component by its respective uncertainty was studied in a sensitivity analysis to give conservatively estimated uncertainties of 1, 6 and 3% for the AD, AR and ADDR, respectively. Skin doses for either type of compound did FIG. 3. Fast-neutron absorbed dose measured with an A-150 plastic not exceed the peak dose to brain for any of the beams and walled ionization chamber are presented for seven different NCT facilities in Europe and the U.S. Measurements were performed in an ellipsoidal consequently had no influence on the derived advantage water phantom under conditions pertinent to clinical irradiations. The parameters. Advantage depths (useful penetration) of 8.0 results for each beam are normalized to the measured maximum given in cm or greater are realized with all the beams studied if the Table 2 and have an estimated uncertainty ranging between 29 and 300% uptake characteristics of BPA are assumed. The AD must depending upon the depth in-phantom. Higher uncertainties are associated with greater depth in-phantom. The measured in-air fast-neutron dose be larger than the maximum of tumor depth for effective rates from Table 1 are plotted at a depth of 0 cm with data points for therapy, and values 8.0 cm or greater are needed to irradiate each beam normalized to the maximum measured either in-air (Studsvik, a target as deep as the midline of the average-sized head HFR, FCB, M67 and BMRR) or in-phantom (FiR-1 and ReZ). with bilateral field placements. The ADs of the facilities that are still operational vary by only 1.1 cm for the dif- ferent field sizes assessed. When considering BPA, a best forward-directed beams for field sizes approaching the di- value of 9.7 cm is realized by both Studsvik and the HFR, mensions of the phantom (21). Normalized fast-neutron which possess the most penetrating thermal flux depth pro- depth–dose data are depicted in Fig. 3 with the in-air values files. Use of an advanced compound increases the AD by also plotted at a depth of 0 cm. The uncertainties for these between 11 and 22% depending upon the beam. measurements are large, varying in the different beams be- The advantage ratio (AR) accounts for the detrimental tween 29% near the surface to 300% at depth. These un- effects of beam contamination in achieving the required TABLE 3 Advantage Parameters or Figures of Merit Derived from the Dosimetric Measurements for the Nominal Operating Conditions of the Different Epithermal Beams Using a Uniform Set of Boron Uptake Parameters Appropriate for BPA and an Advanced Delivery Compound in Parentheses MIT FCBa Studsvikb FiR-1 BMRRc ReZ HFR MIT M67c AD (cm) 9.3 (11.3) 9.7 (11.2) 9.0 (10.5) 9.3 (10.6) 8.6 (9.5) 9.7 (11.0) 8.0 (9.0) AR 6.0 (11.8) 5.6 (10.1) 5.8 (10.9) 6.0 (11.9) 4.2 (6.2) 5.4 (9.3) 3.5 (4.7) ADDR (cGy(w) min 1) 125 (60) 67 (41) 45 (24) 33 (16) 46 (35) 19 (12) 9 (7) Time to reach 12.5 Gy(w) (min) 10 (21) 19 (31) 28 (52) 38 (77) 27 (36) 66 (104) 139 (177) Notes. Boron concentrations of 18 and 65 g g 1 are assumed in normal brain and tumor tissue, respectively, for BPA and 0.65 and 65 g g 1 for an advanced compound. The applied RBEs are 1.0 for photons and 3.2 for neutrons. The cRBEs are 1.3 in brain and 3.8 in tumor for BPA and 3.8 in both tissue and tumor for an advanced compound. The time required to reach a normal tissue tolerance dose of 12.5 Gy(w) for these irradiation conditions is also given. a Fission converter operating at 83 kW. b Beam line configured with 9-mm-thick 6Li filter installed. c Decommissioned.
    • A COMPARISON OF SEVEN EPITHERMAL BEAMS FOR BNCT 217 FIG. 5. Variations in therapeutic ratio as a function of depth deter- FIG. 4. Variations in therapeutic ratio as a function of depth deter- mined for the current clinical epithermal neutron facilities using param- mined for the different clinical epithermal neutron facilities using param- eters for an advanced compound as the boron delivery agent. Boron con- eters for BPA as the boron delivery agent. Boron concentrations of 18 centrations of 0.65 and 65 g g 1 are assumed in normal brain and tumor and 65 g g 1 are assumed in normal brain and tumor tissue, respectively. tissue, respectively. The applied RBEs are 1.0 for photons and 3.2 for The applied RBEs are 1.0 for photons and 3.2 for neutrons with a cRBE neutrons with a cRBE of 3.8 in tumor. The dashed horizontal line rep- of 1.3 in brain and 3.8 in tumor. The dashed horizontal line represents a resents a TR of one. TR of one. penetration and dose delivery to tumor. The advantage ratio 125 cGy(w) min 1 or between 7 and 60 cGy(w) min 1 with with BPA varies between 5.4 and 6.0 at most of the facil- the advanced compound. Irrespective of the class of com- ities, with beams that possess lower contamination at the pound considered, however, the range in ADDR between upper end of this range. Lower values of 4.2 and 3.5 are different beams is large, varying by more than a factor of associated with facilities having the highest beam contam- five for the existing facilities. The time to reach a tolerance ination. The FiR-1 beam, for instance, with the lowest com- dose to normal tissue of 12.5 Gy(w) at the specified ADDR bined in-air contamination of the four European facilities, is also given and ranges from 10–139 min and 21–177 min has a high AR of 5.8 even though the thermal-neutron flux with BPA and an advanced compound, respectively. profile appears to be the shallowest of those measured. This Estimates of expected clinical performance for the dif- is in contrast to the facility at ReZ, which offers a superior ferent beams are compared as a function of depth in phan- distribution of thermal neutrons at depth but which is ac- tom using the therapeutic ratio (TR). These curves are companied by much higher beam contamination and con- shown for brain irradiations in the different beams with the sequently a poorer AR of only 4.2. The FCB and BMRR two classes of compound in Figs. 4 and 5. The peak TR show an AR of 6.0, which is the best in this comparison. values are observed at depths of 2–3 cm and are large, These two examples illustrate the benefit of providing ad- equate depth profiles of thermal flux in combination with ranging between 3.3 and 6.3 with BPA and favoring most a low specific fast-neutron contamination. The AR increas- those facilities with lower specific dose contamination. Us- es markedly if an advanced compound is considered and ing an advanced compound significantly improves the peak almost doubles for beams with the lowest specific dose TR values for all the existing facilities and extends the use- rates for the RBEs considered. If a cRBE lower than 3.8 ful therapeutic penetration of these beams by up to 2 cm. was found appropriate for normal tissues (as with BPA), At a depth of 8 cm (the brain midline), the TR for the FCB the dose to normal tissue would decrease and the AD and is highest for the advanced compound, with a value of 3.1, AR values would increase still further. which is higher than for Studsvik (2.9), HFR (2.8), FiR1 The advantage depth–dose rate (ADDR), which is the (2.8) and ReZ (1.5). Again these results for overall beam therapeutic dose rate at the AD, is also given in Table 3. If performance exemplify the advantage of high purity linked parameters for BPA are used, the ADDRs range from 9 to to adequate thermal flux penetration.
    • 218 BINNS, RILEY AND HARLING TABLE 4 An Illustration of the Influence of Aperture Size and 6Li Filtration on the Advantage Parameters for the FCB Using Boron Uptake Parameters Appropriate for BPA and an Advanced Compound in Parentheses ADDRa Time to reach Aperture diameter (cm) AD (cm) AR (cGy(w) min 1) 12.5 Gy(w) (min) 16 9.7 (12.0) 5.7 (10.8) 173 (88) 7.4 (14.2) 12 9.3 (11.3) 6.0 (11.8) 125 (60) 9.6 (20.8) 12 (with 8-mm 6Li filter) 9.8 (11.7) 5.7 (10.7) 66 (32) 18.9 (39.1) Notes. Boron concentrations of 18 and 65 g g 1 are assumed in normal brain and tumor tissue, respectively, for BPA and 0.65 and 65 g g 1 for an advanced compound. The applied RBEs are 1.0 for photons and 3.2 for neutrons. The cRBEs are 1.3 in brain and 3.8 in tumor for BPA and 3.8 in both tissue and tumor for an advanced compound. The time required to reach a normal tissue tolerance dose of 12.5 Gy(w) for these irradiation conditions is also given. Results for the 12- and 16-cm fields are from measurements (8) except those with the 6Li filter, which were calculated using the experimentally validated beam model of the FCB. a Fission converter operating at 83 kW. DISCUSSION figures of merit for an epithermal neutron beam can be quite pronounced, as illustrated for the FCB in Table 4. The measured epithermal neutron intensities vary con- Increasing circular field size from 12 to 16 cm in diameter siderably among the different facilities, spanning over an increases the AD by 0.4 cm to 9.7 cm, decreases the AR order of magnitude between the least and most intense. Re- from 6.0 to 5.7, and significantly improves the ADDR from sults are for the typical operating powers of the reactors 1.25 to 1.73 Gy(w) min 1 if BPA is considered as the boron that produce the neutron sources. Higher power operation delivery compound. These changes in AD and AR are sim- is possible at some of the facilities, such as Studsvik, where ilar in magnitude to those that would be expected by in- the reactor can run at 1 MW rather than the current 0.5 MW, and at MIT, where increases of up to 250% could be corporating a final 8-mm-thick 6Li filter in the beam line realized (22). Beam purity is most easily assessed by the for the 12-cm-diameter field. These results were obtained contamination evident from in-air measurements and using the experimentally validated beam model of the FCB should be minimized or reduced to levels that have an in- (8). The inclusion of the 6Li filter, however, reduces beam significant effect on in-phantom beam performance. Total intensity considerably, by almost half, whereas increasing specific dose rate can be a good predictor of overall in- field size has the opposite effect. Field size may also ex- phantom beam performance since all the beams with low plain in part the better penetration observed at the HFR. specific dose rates of approximately 10 10 11 cGy(w) Although a field with a 12-cm-diameter aperture was stud- cm (i.e. FCB, FiR-1 and BMRR) exhibit good figures of 2 ied, the phantom was positioned 30 cm from the collimator merit. Having a higher than optimum specific dose rate aperture at the HFR where anything less than ideal colli- does not, however, preclude good in-phantom performance, mation will produce a divergent beam. Previously a full as illustrated by the Studsvik and HFR beams. The gener- width at half maximum of 13.2 cm has been reported for ally good overall performance of these two beams can be the beam (24) at this location. This effectively larger field understood qualitatively in terms of the known dosimetric impinging upon the phantom should contribute to pushing behavior of epithermal neutron beams (21, 23). The Studs- the thermal flux deeper, as observed. vik beam uses a similar filter-moderator arrangement of Tef- The measured flux and dose–depth profiles are generally lon and aluminum as the FCB to produce a useful spectrum similar for all the beams, although small discernible differ- of epithermal neutrons but incorporates an additional 9- ences are evident that influence the figures of merit derived mm-thick 6Li filter that removes lower-energy neutrons for each beam. The thermal flux profile on central axis is through absorption (9). This filter hardens the final beam the most important of the quantities measured since this is incident upon the patient, enhancing penetration of the ep- used to directly determine the 10B dose component in tissue. ithermal neutron flux and the resultant thermal neutrons This dose contribution is significant with a compound like (23). Combined with this is the influence of field size, BPA. However, thermal neutron flux should not be consid- which for increasing dimensions up to those of the target ered as the sole criterion for judging beam performance. (ellipsoidal water phantom) improves the depth profile for The Studsvik and HFR beams have better penetrating ther- thermal neutron flux and the advantage depth. At Studsvik, mal neutron profiles (and ADs) than the FCB, FiR-1 or the field aperture has an area of 140 cm2 compared to 113 BMRR but inferior advantage ratios. The more penetrating cm2 for that studied at the FCB. Predictions for a colli- thermal neutron component of the ReZ beam does not pro- mated, forward-directed beam show that the AD and duce superior figures of merit compared to the FCB, FiR- ADDR increase as the AR decreases with increasing field 1 and BMRR beams, because the better penetration at ReZ size (21). The effect of changing field size on in-phantom is insufficient to offset the deleterious effects of the high
    • A COMPARISON OF SEVEN EPITHERMAL BEAMS FOR BNCT 219 specific beam contamination (65 10 11 cGy(w) cm2). The apparent if full advantage is to be taken of future improve- majority of the specific beam contamination at ReZ is from ments in compounds. This is illustrated by plotting the ther- fast neutrons that have not been filtered from the beam and apeutic ratio as a function of depth for the different beams that presumably contribute to a harder neutron energy spec- with the two classes of compound. The more the normal trum and a better depth penetration of thermal neutrons. tissue dose is reduced, whether from beam contamination The ReZ beam does offer superior performance compared or the delivery compound selectivity, the higher the TR to the M67 facility that had a comparable specific dose of peak and the greater the improvement in performance of a 56 10 11 cGy(w) cm2 but a less penetrating thermal com- particular beam for BNCT at each depth. The comparative- ponent. ly high uptake of boron in normal tissue from the BPA Of practical significance to BNCT is the ADDR, which compound currently employed produces a large normal tis- determines how quickly a clinical irradiation field can be sue dose that obscures the importance of beam purity, al- completed. The ADDRs for the conditions studied depend though the detrimental effect of high specific doses (56–65 mostly on the intensity of the epithermal neutron source 10 11 cGy(w) cm2) is evident in the limited TR and AD available. A large variation between 9 and 125 cGy(w) attained in the ReZ and the M67 beams. Should future com- min 1 is apparent amongst the various facilities if BPA is pounds with better selectivity result in lower boron con- considered. The FCB, Studsvik, FiR-1 and ReZ facilities centration in normal tissue, beam contamination will be can deliver a tolerance dose to normal tissue of 12.5 Gy(w) more important in limiting the physical performance of a in less than the 30 min suggested as reasonable for clinical beam for BNCT. This is illustrated in Fig. 5 by the mark- studies (4). The use of an advanced compound with a high- edly improved TRs with an advanced compound. The rel- er tumor to normal tissue selectivity, although increasing ative improvement in the TR for the FCB and FiR-1 beams, the tumor dose relative to normal tissue, significantly which have the lowest measured specific dose, is large and lengthens irradiation times compared to those for BPA with exceeds that for any of the other beams at all depths. the same boron concentration in tumor. This is because the Apart from providing a consistent measurement of beam boron in normal tissue would be much less than with BPA, performance, it is also envisaged that these data will be but the clinician would probably still want to irradiate until used in a preliminary comparison of the calculated patient normal tissue tolerance is approached. Nonetheless, the dosimetry from the participating clinical centers. The re- FCB, Studsvik and ReZ beams would be able to complete sults will be used in the near future to normalize comple- irradiations in about 30 min or less. In practice, however, mentary treatment planning calculations from each center it might also be feasible to increase the ADDR when using that are currently being prepared for the identical irradiation an advanced compound by increasing the overall boron conditions. These will use CT images of the ellipsoidal wa- loadings in both tumor and normal tissue above those spec- ter phantom2 together with the RBE as well as cRBE values ified, although this will eventually be limited by drug tox- and actual boron concentrations appropriate for each infu- icity. Routine therapy would require high patient through- sion schedule. This separate study will be an integral test put as well as multiple beams possibly for each irradiation relating the absorbed doses specified by treatment planning fraction, and ADDRs of 1 Gy(w) min 1 or greater would calculations with those actually measured at all the different be desirable. Some of the current facilities already have the centers using a common dosimetry method to facilitate a intensities required or the capacity to attain them. direct comparison of clinical dosimetry protocols. The improvements that a more selective capture agent This study illustrates that the dosimetric performance of could bring to the figures of merit depend upon the beam. an epithermal neutron beam for BNCT can be properly as- AD and AR increase for all the facilities currently pursuing sessed only by using figures of merit that account for all clinical trials. The advantage depth improves by between radiation components in the mixed field rather than by com- 11 and 22% for the different facilities, favoring those beams parison of any single dose component. The adequacies of with lower specific doses. A significant improvement is ob- the different epithermal neutron beams investigated are served for the AR, increasing between 80–97% for the demonstrated by advantage depths of 8.0 cm or greater cleaner beams. An advanced compound would offer better when uptake parameters consistent with BPA as a boron dose distributions for tumor relative to normal tissue and delivery agent are assumed. The advent of more advanced could provide more flexibility in treatment planning. In ad- compounds with a better tumor to normal tissue selectivity dition, the useful penetration of a beam would be extended will, however, place further demands on the existing and by up to 2 cm in some cases. This could be important if future facilities if the full potential of NCT is to be realized. sites other than the brain are to be considered as future The challenge in epithermal neutron beam design is to candidates for NCT. The greater tissue selectivity promised achieve a TR greater than unity that extends to the greatest by improved compounds will allow a higher tumor dose possible depth while maintaining adequate intensity for during NCT and will effectively shift the AD deeper, de- clinical studies and eventually routine therapy. It is clear crease the ADDR, and increase the irradiation time. The that by hardening the epithermal energy spectrum from fis- need for an epithermal neutron source that has the com- bined attributes of high purity and high intensity becomes 2 W. S. Kiger III, private communication, 2004.
    • 220 BINNS, RILEY AND HARLING sion sources through 6Li filtration (Studsvik), high beam Metamorphosis of a 35 year-old TRIGA reactor into a modern BNCT facility. In Frontiers in Neutron Capture Therapy, Vol. I (M. F. Haw- purity (FCB, FiR-1, BMRR), and high collimation (HFR), thorne, K. Shelly and R. J. Wiersema, Eds.), pp. 267–275. Kluwer a favorable effect on the maximum useful beam penetration Academic Publishers/Plenum Press, New York, 2001. is achieved. However, all these approaches to enhanced 11. M. Marek, L. Viererbl, S. Flibor, J. Burian and J. Rejchrt, Validation beam performance are costly in intensity and emphasize the of the epithermal neutron beam at LVR-15. In Proceedings of the Ninth International Symposium on Neutron Capture Therapy, pp. 41– importance of producing neutron sources of the highest 42. Osaka, Japan, 2000. [Abstract] possible strength. 12. R. L. Moss, F. Stecher-Rasmussen, K. Ravensberg, G. Constantine and P. 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