2. Improved dose targeting for BNCT using 6Li filtration ● P. J. BINNS et al. 1485
absorbed boron present in tissue produce an adventitious
dose to the patient during irradiation. In the first approx-
imation, beam design aims to minimize the background
contamination while providing sufficient intensity and
penetration to irradiate deep seated targets. Although this
has been satisfactorily achieved with the fission converter
beam (3, 4), possible improvements that are treatment or
patient specific may further optimize delivery of thermal
neutrons to the target volume with a particular field
placement. During the phase I/II brain cancer trials con-
ducted at Harvard-MIT that have studied toxicity of
NCT, the prescription dose was first specified as the
maximum dose to normal tissue (5) and subsequently as
a whole brain average including the contoured tumor
volume with treatment plans optimized to maximize dose
to the tumor while respecting dose limits to other organs
at risk such as skin, eyes, optic nerves, and optic chiasm Fig. 1. Schematic of the horizontal beam line for the fission
converter– based epithermal neutron beam at the Massachusetts
(6). In practice this requires multi-field plans to achieve Institute of Technology (MIT) research reactor (MITR II). The
adequate coverage of the whole brain. Any option that beam line consists of an aluminum and polytetrafluoroethylene
could improve neutron penetration to boost tumor dose at (PTFE; Teflon®) filter moderator as well as cadmium and lead
or beyond the head centerline would be beneficial when filters. The housing for the lithium filter extends the existing beam
tumor control and efficacy are studied more systemati- line by 18 mm and fits between the lead annulus housing the four
beam monitors and the steel plate on to which the patient colli-
cally. mator is mounted.
Adding a 6Li filter to the neutron beam is a relatively
simple and inexpensive modification that can increase the
average energy of the epithermal neutrons in the beam and METHODS AND MATERIALS
should improve neutron penetration at the expense of re- Epithermal neutron beam
duced neutron intensity. Lithium-6 has a nuclear cross sec- The FCB provides a high intensity beam of epithermal neutrons
tion that is inversely proportional to the speed of the inci- from a source of fission neutrons generated in a subcritical array of
dent neutron and preferentially absorbs neutrons of lower uranium fuel that is housed in a separate vessel outside the reflec-
energies to enhance the relative intensity of neutrons in the tor region of the reactor. The converter is driven by thermal
higher part of the epithermal range without producing any neutrons from the reflector region surrounding the core of the MIT
Research Reactor (MITR II), which currently operates at a maxi-
significant undesired secondary radiation. A fixed 6Li filter
mum power of 5 MW. A shielded horizontal beam line 2.5 m long
has been incorporated previously at the Studsvik epithermal directs neutrons from the converter to the treatment room. The
neutron facility in Sweden (7) that has proven to provide present configuration of the FCB is shown in Fig. 1. The fission
improved penetration of the thermal neutron dose compo- neutrons emanating from the converter are moderated and filtered
nent (8). Accommodating modifications such as additional by the D2O coolant, aluminum, fluorine, cadmium, and lead,
beam filtration had been considered in the final design and resulting in a beam possessing a broad energy distribution of
construction of the FCB although at that time inclusion of a epithermal neutrons with an average energy of approximately 2
6
Li filter was postponed (3). Following the initial clinical keV. The beam has minimal unwanted contamination from pho-
trials using the FCB, a study was initiated to examine and tons (specific photon absorbed dose 3.5 0.5 10 13 Gy cm2)
as well as fast (specific fast neutron absorbed dose 1.4 0.2
quantify the dosimetric advantages that using optional 6Li
10 13 Gy cm2) and thermal neutrons. This epithermal beam en-
filtration could provide during therapy without significantly ergy distribution results in a thermal neutron maximum at a depth
degrading the inherent beam characteristics and without in tissue of between 2 and 3 cm. Previous computational studies
unduly limiting neutron intensity. using ideal, mono-energetic neutron beams showed that neutrons
The feasibility of a 6Li filter was evaluated theoretically with energies between approximately 5 eV and 20 keV possess the
using established Monte Carlo calculations of the beam line best depth dose characteristics for BNCT (10, 11). Neutrons out-
and a mechanical design study was completed (9). A re- side this energy range are generally less useful since they contrib-
movable filter in a sliding drawer assembly was constructed ute to the normal tissue dose without appreciably improving the
and installed into the existing beam line, which required build-up of thermal flux at depth. The filtered beam enters a 1.1-m
tapered portion of the beam line with lead walls that reflect some
only minor modification. Dosimetric measurements were
of the source neutrons (that might otherwise leave the beam)
then performed to confirm the performance of the filter and toward the patient and forms a beam collimator. A circular cone
to provide calibration data for treatment planning. Lastly shaped, 40 cm long final patient collimator constructed of lithiated
some treatment plans from previously treated clinical cases and boronated epoxy mixed with Pb shot extends beyond the
were evaluated with the new filter to show the tumor dose shielded beam line into the medical room to provide a beam that is
enhancements that can be achieved. spatially and directionally well defined. Different circular field
3. 1486 I. J. Radiation Oncology ● Biology ● Physics Volume 67, Number 5, 2007
sizes are obtained by augmenting the collimator with additional
end pieces that provide an epithermal flux at the patient position of
approximately 5 109 n cm 2 s 1 with the converter operating at
83 kW (4).
Four fission counters positioned in the periphery of the beam
near the base of the patient collimator serve as integral monitors of
the neutron fluence as it is delivered to the patient. Signals are fed
to nuclear instrumentation modules (NIM) electronics and irradi-
ations are administered with a programmable logic controlled
(PLC) system that automatically terminates the irradiation when
the integrated counts from the beam monitors reach the prescribed
targets.
Computational methods
The beam line of the MIT FCB has been modeled in detail (12,
13) using the general purpose Monte Carlo transport code MCNP
4C (14) and was properly benchmarked by measurements for
treatment planning purposes (1, 4, 15). This model was used for
assessing the influence of the filter on beam performance for
preliminary as well as final filter designs. Energy dependent weight
windows were used for variance reduction and calculations were
performed in several stages using surface source files written at Fig. 2. Schematic drawing of the lithium filter housing assembly
various positions along the length of the beam line (12) to help that was designed to fit into the existing beam line. The numbered
determine the best combination of filter thickness and location in components are as follows: (1) fixed steel holder, (2) sliding steel
the beam line. Neutron and photon surface source files were holder, (3) boronated polyethylene shield ring, (4) aluminum filter
written at the plane of the beam aperture. Fast neutron and photon housing, (5) roller bearing track, and (6) location of push rod
absorbed dose rates (Dfn and D , respectively), epithermal (1 eV to switch.
10 keV) neutron flux ( epi), and the epithermal neutron current to
flux ratio (J/ epi) were calculated in-air at the patient position. Flux
tallies were integrated against photon and neutron kerma coeffi- aluminum ring with an outer diameter of 414 mm that is sealed
cients (16) to determine the absorbed dose to brain tissue (17). The front and back between two thin (0.254-mm-thick aluminum) air
beam was subsequently transported into an analytical model of the tight covers sealed by two graphite gaskets. This protects the
water-filled ellipsoidal phantom (18), in which beam calibration lithium from exposure to air and oxidation. The filter holder was
measurements were performed. Neutron fluxes and absorbed dose designed for convenient installation and removal of the filter and
rates were calculated in 5 mm long cylindrical cells 12 mm in consists of a frame with surrounding shielding made from borona-
diameter along the central axis of the beam. All calculations were ted polyethylene and steel that is fixed in the beam line. The beam
normalized to a fission converter power of 83 kW. line was lengthened by 18 mm to accommodate the assembly. The
holder, which operates like a vertical drawer, is split in two,
allowing one side to slide out on two roller bearing tracks for
Design and construction of lithium filter and holder
insertion or removal of the filter, which is fastened to the drawer
The initial design criteria were established from preliminary
by screws in the three tabs visible in Figures 2 and 3. Once closed,
calculations that showed a lithium filter 8 mm thick (95% enriched
6 the filter drawer is secured in place with a locking pin and, when
Li) offered a reasonable compromise between improved penetra-
the filter is installed, a push-rod switch is activated that interlocks
tion and loss of beam intensity (9). Determining the best location
with the automated dose monitoring and control system.
for the filter in the beam line was also considered and an assembly
that mounted between the lead annulus housing the 4 beam mon-
itors and the steel plate attached to the patient collimator appeared
the most expedient (Fig. 1). Consideration was given to placing the Measurement methods
filter as far upstream as possible to reduce beam line activation Measurements of neutron flux were performed both in-air and in
(and hence dose from photons incident upon the patient) from the an ellipsoidal water phantom on central axis for the 12- and 16-cm
low energy neutrons it would absorb and to providing easy acces- diameter field sizes using bare and Cd covered gold foils (4). A
sibility for routine installation and removal. A stringent series of lateral irradiation of the brain was simulated by the ellipsoidal
safety analyses was also performed to ensure that there would be water phantom positioned with the smallest axis on the beam
no undue concerns from tritium production and gas pressure on the center line (18). Absorbed dose rates were also determined using
aluminum covers, nuclear heating from neutron absorption in the separate graphite and A-181 brain equivalent plastic walled ion-
lithium as well as possible degradation of neutronic performance, ization chambers (IC-18s, Far West Technology, Goleta, CA),
combustion or rapid oxidation. each with a sensitive volume of 0.1 cm3 to quantify respectively
Based on these findings, an assembly was constructed and the photon and neutron absorbed dose components in the mixed
installed, a schematic of which is presented in Fig. 2. The lithium radiation field (19). All measurements were performed with the
filter consists of a disk 8 mm thick and 345 mm in diameter that is converter operating at between 58 and 80 kW (MIT research
slightly greater in area than the neutron beam at its mounted reactor operating at 3.5 to 4.8 MW) and have been scaled to a
position in the beam line. The lithium disc is pressed into an converter power of 83 kW.
4. Improved dose targeting for BNCT using 6Li filtration ● P. J. BINNS et al. 1487
biologically weighted dose to tumor and the maximum weighted
dose to brain, was determined from the measured dose profiles as
a function of depth in the water phantom.
Treatment plan calculations
A theoretical study was performed to assess the expected clin-
ical performance of the filter for boronophenylalanine-fructose–
mediated therapy by analysis of archived treatment plans for GBM
patients treated using the epithermal neutron beam with the 12-
cm-diameter field (6). Plans were prepared using the new MCNP
model of the beam with and without the filter added. The beam
models were benchmarked by comparing calculated and measured
dose profiles in the ellipsoidal water phantom. Although the orig-
inal treatments were planned using NCTPlan (15), these retrospec-
tive treatments were planned using the radiotherapy planning
system called MiMMC (Multi-Modal Monte Carlo) that is cur-
rently being developed at the Beth Israel Deaconess Medical
Fig. 3. The newly built removable 6Li filter assembly sits in a Center, with individual models constructed for each subject from
drawer that easily slides into or out of the beam line upstream of CT images. The MCNP5 program version 1.40 was used for
the patient collimator. The filter drawer (shown half withdrawn) is radiation transport and dose calculations (26). This version of
secured in place with a locking pin and when installed activates a MCNP5 incorporates, as a standard feature, special modifications
push-rod switch that interlocks with the automated dose monitor-
for rapid calculations in lattice geometries that significantly accel-
ing and control system. The beam line was lengthened 18 mm to
incorporate the 6Li filter assembly. erate these treatment planning calculations (27, 28). In this anal-
ysis the actual average blood concentration at the time of irradia-
tion for each field was used. The same RBE and cRBE factors
To compare the performance of the filter in the beam under described earlier were also used as biologic weighting factors in
more realistic conditions pertinent to clinical irradiations of brain these treatment planning calculations. Tissue compositions for
tumors, total biologically weighted dose profiles were determined brain and cranium used in transport calculations were from the
for tissue and tumor from the measured data. The absorbed dose International Commission on Radiation Units and Measurements
rates arising from thermal neutrons captured by boron were deter- (ICRU) Report 46 (29), with energy-dependent kerma coefficients
mined from the product of the 2200 m s 1 neutron flux obtained adapted from ICRU and the National Institute of Standards and
from the foil activation measurements (20, 21) and a kerma coef- Technology (NIST) by Goorley et al. (17).
ficient of 8.66 10 8 Gy cm2 (17). Depth dose profiles were
determined assuming boron concentrations of 18 and 65 g · g 1
in normal brain and diffuse tumor tissue, respectively, that approx- RESULTS
imately represent the uptakes observed using boronophenylala-
nine-fructose (BPA-F) (22). Biologically weighted depth dose Measurements and calculations
profiles were then obtained by applying relative biological effec- Biologically weighted dose profiles as a function of depth
tiveness (RBE) values of 1.0 for photons and 3.2 for thermal and for both normal tissue and tumor based on the measured and
fast neutrons. Differences in the effective microdistribution of the calculated results in the ellipsoidal water phantom with the
boron delivered by BPA in both tissue and tumor were also 6
Li filter installed for the 12-cm-diameter field aperture are
accommodated using compound relative biological effectiveness shown in Fig. 4. These results are scaled to the converter
(cRBE) factors of 1.3 for normal brain and 3.8 for tumor (23, 24).
operating at 83 kW. The Monte Carlo simulations, based on
The total biologically weighted dose is determined as the sum of
the previously validated beam model, are depicted as curves
the individual dose components (i.e., photon, fast neutron, thermal
neutron, and boron) after weighting each with their respective RBE and have a statistical uncertainty of approximately 1% (1 ).
or cRBE factors. Measurements are illustrated by data points with uncertain-
The total weighted dose profiles obtained from the in-phantom ties of 4% for photons and between 13% and 22% for the
measurements were then used to determine several figures of total neutron dose component, depending on the depth in
merit, namely, the advantage parameters (25), to help quantify the tissue (19). Uncertainties in the boron dose are only attrib-
changes in beam performance apparent with the filter. The advan- uted to errors associated with the thermal neutron flux
tage depth (AD) is the depth at which the total weighted dose to determination, which are between 4% and 7%, depending
tumor equals the maximum weighted dose received by normal on depth. No uncertainties are assessed for the applied
tissue during an irradiation and is a measure of the maximum depth weighting factors. These depth profiles illustrate the con-
at which therapeutic benefit is obtained. The advantage ratio (AR)
ceptual advantage of BNCT with marked skin sparing, a
is the ratio of the integral tumor dose to that of normal tissue
averaged from the surface where the beam is incident to the
dose build up to a maximum at a depth of approximately 3
advantage depth. The advantage depth dose rate (ADDR) specifies cm in normal tissue, and tumor doses that exceed the
the therapeutic dose rate at the AD to give the total dose rate maximum normal tissue dose at all depths up to 9.9 cm (the
achievable to treat tumor at the maximum useful depth of the beam advantage depth). The calculations, which include in the
and is also the maximum absorbed dose rate to normal tissue. model of the beam line the filter assembly as it was actually
Finally, the therapeutic ratio (TR), defined as the quotient of the built, are in good agreement with the measurements. Figure
5. 1488 I. J. Radiation Oncology ● Biology ● Physics Volume 67, Number 5, 2007
Fig. 4. Measured (points) and calculated (curves) biologically Fig. 5. Therapeutic ratio vs. depth determined from measurements
weighted dose profiles produced in an ellipsoidal water phantom in the ellipsoidal water phantom with and without the 6Li filter in
by the fission converter based epithermal neutron beam with an the fixed horizontal beam line of the fission converter beam (FCB)
8-mm-thick 6Li filter installed. The maximum dose to normal for the 12-cm-diameter aperture. The depth at which the therapeu-
tissue is indicated. The advantage depth (AD) is where the tumor tic ratio curve crosses the horizontal line at unity is the maximum
dose falls to the maximum normal tissue dose and the advantage useful penetration for therapy with the boron compound borono-
depth dose rate (ADDR) is the dose rate to tumor at this depth. The phenylalanine-fructose in the FCB.
advantage ratio (AR) is the ratio of the integral tumor dose to that
in normal tissue averaged from the surface where the beam is
incident to the AD. intensity compared with the open field, and this is the
trade-off for improved penetration. Although beam intensity
is attenuated by up to 52% for the 12-cm field, for example,
4 also indicates the various advantage parameters that re- irradiation times with the FCB are still clinically acceptable,
flect the expected therapeutic effects in a realistic configu- as a weighted maximum normal tissue dose of 12.5 Gy can
ration. A comparison between measured and calculated be delivered in a single field of approximately 22 min with
figures of merit with the 6Li filter in the beam line is given the converter operating at 83 kW. This loss in beam inten-
in Table 1. The estimated uncertainties for the predicted sity when using the filter will be largely recovered through
advantage parameters are again small, being between 1% a combination of refueling the converter with fresh fuel,
and 2%; those estimated for the measurements are 1% and which has already been completed and enables operation at
6% respectively for the AD and AR (8), and the measured 102 kW with the reactor at 5 MW, and a pending application
ADDR has an uncertainty of 5%. Agreement between the with the US Nuclear Regulatory Commission to increase the
predicted and measured values is good, as should be ex- MITR II operating power from 5 to 6 MW.
pected, given the good agreement reported previously for The benefit of the 6Li filter to beam performance in
the earlier configuration of the beam line without the filter improving penetration of the thermal neutrons is exempli-
(4). Increased ADs of 10.0 and 9.9 cm are realized with the fied by the increase in therapeutic ratio at depth as shown in
filter for the 16- and 12-cm fields respectively. The increase Fig. 5. The presence of the filter slightly reduces the max-
of 6 mm for the 12-cm field is larger than the 3 mm for the imum TR attained at shallow depths where the value is
16-cm field and is qualitatively consistent with earlier pre- already high (TR 6.3), and shifts the peak in this curve 5
dictions when increasing neutron energy up to approxi- to 10 mm deeper to approximately 3 cm. Thereafter the TR
mately a keV in a forward-directed beam (10). However, the for the 6Li filtered beam rises above that for the open beam,
inclusion of the 8-mm-thick filter markedly reduces beam providing an AD of 9.9 cm that is 6 mm deeper than without
Table 1. Comparison between measured and calculated (in parentheses) figures of merit with the
6
Li filter in the beam line
Aperture diameter ADDR Time to reach 12.5 Gy
(cm) AD (cm) AR (cGy min 1) (min)
16 10.0 (9.8) 5.5 (5.3) 64 (64) 19.5 (19.5)
16 (No filter) 9.7 5.9 159 7.9
12 9.9 (9.8) 5.7 (5.7) 55 (56) 22.7 (22.3)
12 (No filter) 9.3 6.0 114 11.0
Abbreviations: AD advantage depth; AR advantage ratio; ADDR advantage depth dose
rate.
6. Improved dose targeting for BNCT using 6Li filtration ● P. J. BINNS et al. 1489
Fig. 6. Three field treatment plan for tumor dose calculated for both open (solid lines) and 6Li filtered (dotted lines)
epithermal neutron beams shown in transverse, coronal and sagittal cross sections. The tumor is indicated by the dark
blue contour. Both plans provide the same maximum biologically weighted dose to brain (12.1 Gy) that was given for
the open field but the tumor coverage is improved at depth by using 6Li filtration. The doses assume a uniform
distribution of boron and are biologically weighted (denoted by Gyw in the figure for emphasis).
the filter and extends well beyond the midline of the average as compared with the plan with open beams. This is because
sized head. the mean normal brain dose is also higher relative to the
maximum dose at depth with the filtered beam. As shown in
Treatment plans Fig. 7a, in all three cases, dose–volume histograms (DVHs)
The advantage of incorporating the removable 6Li filter for the normal brain were very similar (5.0% reduction in
for a planned treatment of a GBM is shown in Fig. 6. This maximum brain dose when the mean brain doses are
example is a real case history from the last dose escalation matched), but the minimum tumor dose increased for the 6Li
study performed at MIT, in which the patient presented with filtered beams.
a right parietal GBM that was approaching the maximum Similar comparisons were performed for other cases, in
allowable volume of 60 cm3 for inclusion in the trial. The which it became evident that inclusion of the 6Li filter did
mean biologically weighted dose to normal tissue in the not always offer an advantage. As an illustration, Fig. 7b
whole brain volume that includes the tumor was the pre- also shows DVHs for a patient with multifocal lesions who
scription dose (7.7 Gy) with a maximum weighted dose to underwent irradiation in the same trial, but in the lower 7.0
normal brain of 12.5 Gy. The three-field treatment plan Gy mean brain dose cohort. In this case, the treatment plan
delivered used the 12-cm diameter aperture with posterior- was recomputed for the open and filtered beams with the
vertex, left-lateral, and right-lateral fields that were opti- same weights as originally used. Poorer tumor coverage
mized with equal beam weightings. Biologically weighted (16% lower minimum dose) resulted for the shallow tumor
isodose contours for tumor dose are shown in transverse, because, in this instance, the unfiltered beam had sufficient
coronal, and sagittal views through the contoured tumor penetration to reach the relatively shallow target volume,
volume. The boron concentrations assumed in normal brain whereas for the deeper tumor dose coverage was improved
(19.3 g · g 1) and tumor (67.4 g · g 1) were based on marginally by 1.8%. Further optimization of the beam
blood samples analyzed during therapy. Differences in the weights could not improve the tumor dose coverage with the
6
tumor isodose contours for the open and 6Li filtered beams Li filtered beams.
are particularly apparent in transverse view, where, for
example, the tumor volume is completely covered by the
DISCUSSION
40-Gy isodose line with the filter installed. Better beam
penetration is achieved at the brain centerline and the spar- As the dose escalation trials associated with BNCT re-
ing effect of dose build-up is more pronounced with the 6Li search progress, more sophisticated treatments plans have
filtered beam configuration. Quantifying the relative effec- been developed, changing from either single-field or paral-
tiveness of the lithium filter depends on the mode of the lel opposed irradiations to multiple noncoplanar fields that
dose prescription. When the maximum normal brain doses are arranged to maximize the dose delivered to the target
are matched for the different beam configurations, the 6Li volume. A new 6Li filter has been designed, constructed,
filtered beam provides a minimum tumor dose (biologically and installed in the MIT fission converter-based epithermal
weighted) at the midline on central axis that is 9.0% higher neutron beam. This improvement in the clinical beam
than with the unfiltered beam. However, when the prescrip- should prove beneficial when tumor control and efficacy are
tion is specified as the mean normal brain dose as was the studied more systematically using delivery agents that can
case during the last phase I trial (6), the 6Li-filtered beam selectively accumulate boron more uniformly in all tumor
appears comparatively less advantageous with the minimum cells.
tumor dose (biologically weighted) increasing by only 3.7% Treatment planning calculations of clinical cases that
7. 1490 I. J. Radiation Oncology ● Biology ● Physics Volume 67, Number 5, 2007
100 100
a b
Shallow
80 80 Tumor
Deep
Volume (%)
Volume (%)
Brain Tumor Brain Tumor
60 60
40 40
20 20
0 0
0 20 40 60 0 20 40 60
Dose (Gyw) Dose (Gyw)
Open Beam
Li Filter Rx: Brain Mean
Li Filter Rx: Brain Max
Fig. 7. Dose–volume histograms for the normal brain and tumor for plans using open or 6Li filtered epithermal neutron
beams for two different treatments: (a) patient with a deep right temporo-occipital tumor (mean biologically weighted
brain dose 7.6 Gy) whose plan is shown in Figure 6; and (b) patient with multifocal disease having both shallow occipital
and deep thalamic lesions (mean biologically weighted brain dose 7.0 Gy). Plans were computed for the 6Li filtered
beams with two different dose prescriptions (i.e., the same mean brain dose and the same maximum normal brain dose
as the open beam plan) for each patient. The doses assume a uniform distribution of boron and are biologically weighted
(denoted by Gyw in the figure for emphasis). The 6Li filtration improves dose coverage of deep tumors but can
significantly reduce doses for shallow sites.
assume homogeneous uptake of boron throughout tumor 3 cm as determined from the point of intersection for the
and normal tissue but at different concentrations show the filtered and unfiltered depth dose profiles in Fig. 5) provides
potential benefit of including the 6Li filter in the epithermal no obvious benefit because of the reduced beam intensity,
neutron beam when irradiating deep-seated tumors. The the decreased therapeutic ratio, and the poorer DVHs ob-
minimum dose to tumor can be increased by 4% to 9% for tained for tumor. This highlights the need for a 6Li filter that
the same mean biologically weighted dose to normal brain is readily removable from the beam line for optimum dose
depending on how the dose is prescribed, i.e., using the delivery.
mean or maximum brain dose. Irrespective of the mode of The 8-mm filter thickness was optimized to provide en-
prescription, however, the increased tumor dose achieved hanced penetration of the thermal neutron distribution in-
when using the filter while maintaining the same normal side the head. The design premise was to produce a beam
tissue dose must be considered in the context of the dose– with the highest possible AD while also maximizing the
response curves for tumor control probability (TCP) and ADDR and AR. This was achieved through tailoring the
normal tissue complications that are usually steep and sim- incident energy spectrum by preferentially removing neu-
ilar in shape. In particular the increase in beam penetration trons of the lowest energies in the epithermal range to
provided by the additional filtration should initially help to improve the therapeutic characteristics of the beam. Mea-
broaden the therapeutic window that BNCT trials are seek- surements inside an ellipsoidal water phantom confirmed
ing to determine and thereafter provide added flexibility the accuracy of the Monte Carlo calculations of the beam
when trying to optimize dose delivery to the target volume line and provided beam data essential for performing treat-
that may well extend to the midline of the head. As an ment planning. Predicted gains in beam performance were
example, Laramore et al. (30) have predicted a TCP for realized by an increase in the AD to 9.9 cm for the 12-cm-
high-grade glioma using BNCT and epithermal neutron diameter field aperture with a concomitant loss in beam
irradiations based on clinical responses to fast neutrons that intensity of 52%. Although the gains in penetration
shows a narrow dose (biologically weighted) interval of achieved with the 6Li filter are comparable to those that can
only 5 Gy between 30 and 35 Gy for little and complete be realized by increasing field size or beam collimation (13),
tumor control probability respectively. An increase in tumor using the filter will provide a more homogeneous distribu-
dose of 9% in this interval, as can be achieved using the 6Li tion of thermal neutrons and thus dose at depth in both
filter, could produce a large increase in the predicted TCP tumor and normal brain. Although spectral shaping is costly
(up to approximately 70%) without increasing the incidence in beam intensity, this is feasible for high intensity reactor
of side effects. This would be highly desirable. Using the sources such as the FCB where treatment times for a single
filter when irradiating tumor sites at shallower depths (i.e., irradiation to reach a maximum normal tissue dose of 12.5
8. Improved dose targeting for BNCT using 6Li filtration ● P. J. BINNS et al. 1491
Gy can still be achieved in less than 23 min. Furthermore depending on clinical need. The filter is interlocked to the
loss of beam intensity will be made up at the MITR-II by automated dose control system that requires confirmation
reloading the converter with fresh fuel and planned power of installation when setting the prescribed monitor units
increases of the reactor. for treatment. This has been fully tested and, together
The newly designed filter drawer required only minor with the completion of treatment planning calculations, is
modifications to the existing beam line that allows easy now commissioned for routine use in the next series of
installation of the filter disk, and inclusion is optional clinical trials.
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