16th International Congress on Neutron Capture Therapy
June 14-19, 2014 Helsinki, Finland
9Be(d,n)10B-based
A comprehensive study on
and deep tumor treatments.
neutron sources for skin
M.E. Capoulat1,2,3, D.M. Minsky1,2,3 and A.J. Kreiner1,2,3
1Department
of Accelerator Technology and Applications, CNEA, Argentina.
Overview
2School
of Science &Technology, San Martín University, Argentina. 3CONICET, Argentina.
Beam Shaping Assembly
The 9Be(d,n)10B was studied as a neutron source for BNCT,
with special emphasis on deep-tumor treatment application.
A preliminary study on the treatment of superficial tumors is
also presented.
A neutron beam produced by a deuterons of 1.45 MeV and a
8 micron–thick Be target was evaluated for deep–tumor
treatments in the context of a 30 mA deuteron current. This
target thickness–deuteron energy combination was chosen
so that all the (d,n) reactions occur at energies above 1.0
MeV. In this condition, most of the highest energy neutrons
(from 1.0 to 6.0 MeV) are eliminated, softening significantly
the primary spectrum, and hence, making it easier to
produce the desired epithermal beam.
An AlF3/Al beam shaping assembly (BSA) was optimized
with MCNP. The “best” treatment condition (i.e., highest
tumor-to-normal tissue dose and deepest treatable range) is
reached in a total treatment time of about 220 min., which
can be splitted into multiple sessions. Much more clinical
manageable times (1-2 hour) are also feasible, with fairly
good performances as well.
For superficial tumors (where a thermal spectrum is
required) the presence of the high energy neutrons is not so
critical due to the strong power of thermalization of heavy
water used as a moderator. Therefore, thick targets were
also considered as a possible option. The “best” condition
(highest tumor-to-normal tissue dose) was achieved with a
thick Be target and a 1.35 mA deuteron beam in a total
irradiation time of less than 1 h.
Thin vs. thick targets
The neutron spectrum from the 9Be(d,n)10B reaction shows a
strong contribution of neutrons of a few hundreds of keV that
belongs to a group of highly excited states (at 5.11, 5.16 and
5.18 MeV) in the residual nucleus 10B (see Fig. 1).
These states are energetically accessible for deuteron
energies of about 1 MeV, and are preferentially populated as
compared to the lowest-energy states in 10B, which, in
contrast, produce higher energy neutrons (i.e., the 1 to ~6
MeV “tail” of the spectrum).
Choosing “the right” target thickness – deuteron energy
combination:
The residual energy of the deuteron must be ≥ 1 MeV:
In this condition, all the reactions that occur below the 1 MeV
energy threshold (which would be present in a thick target)
are removed, and hence, the neutron production associated
with the “tail” of the spectrum is significantly reduced.
Selected configuration
Selectedenergy:
configuration
Deuteron
1.45 MeV
Deuteron
energy: 81.45
MeV
Target
thickness:
micron
Thin targets
produce softer
neutron spectra
Target thickness:
8 micron
Energy
loss in
the
target:
0.45 MeV
Energy
loss in
Residual
the target:energy
of the deuteron:
Residual energy
of the deuteron:
Design:
For deep-tumor treatments the moderating volume is filled
with Al and AlF3, as shown in Fig. 2. For skin–tumor
treatments, heavy water was used instead.
BSA Optimization:
Depth–dose profiles at the beam centerline in a Snyder’s
head phantom were simulated for 40 cm < L < 95 cm using
the MCNP code.
Fig. 2. Sketch of the BSA designed to produce an epithermal beam.
For thermal beams, AlF3 and Al replaced by heavy water.
Deep–seated tumors
Beam current:
Deuteron energy:
Target thickness:
30 mA
1.45 MeV
8 micron
Dose prescription:
Peak dose to normal brain = 11.0 Gy-Eq
BSA optimization:
Goal: To find the BSA configuration
that maximizes the dose to tumor
tissue and the treatable range.
Total dose to tumor and normal tissues:
Fig. 3. shows the peak tumor and skin doses as a function of the treatment
time.
Fig. 3. Peak doses in tumor and skin as a function
of the treatment time. .
Peak dose to normal brain is 11.0 Gy–Eq for all treatment times, according
the adopted prescription.
The maximum dose deliverable to the tumor is 59 Gy-Eq, in a total irradiation
time of ~ 180 min or more. Peak dose to skin is about 15 Gy–Eq for all cases.
Treatable ranges and depth of maximum tumor dose:
The maximum treatable depth (i.e., region where total tumor doses are ≥
40 Gy) is maximized for a 180-220 min. total irradiation time, resulting in
5.2 cm (Fig. 4).
Depth of maximum dose slightly decreases with the treatment time (i.e.,
with the moderator length) due to a higher thermal contribution in the
neutron spectra, as shown in Fig. 5.a.
Fig. 4. Maximum treatable depth and depth of
maximum tumor dose as a function of treatment time.
“Best” treatment condition:
180-220 min. irradiation time (i.e., moderator lengths from 70 to 74 cm)
Also note that a 2-hour irradiation allows working quite near the “best condition”, in a much more clinically
manageable irradiation time.
Fractionated BNCT:
The “best condition” involves too long irradiation times to perform a single application. Fractionated BNCT
is the solution. Compared to a single 1 h-irradiation, a fractionated scheme (e.g., 2-to-4 sessions of ~1 h
each) allows:
To increase total tumor doses without increasing dose in normal tissues, (i.e., best tumor/normal tissue
dose ratios) (Fig. 3 and 5.b.)
To maximize treatable depths (Fig. 4)
To significantly reduce fast dose to normal tissues (Fig.5.c.)
To increase specific dose (boron) in tumor (absolute and relative values).
0.45 MeV
1.0 MeV
1.0 MeV
Fig. 1. Neutron spectra from 1.45 MeV
deuterons on a 8 micron and a thick Be
target
Constraint on the thickness: Total neutron yield
Using a thin target unavoidably imply a loss in the total
neutron yield. Too thin targets (less than 5 micron) do not
produce a neutron beam intense enough to perform a
treatment in a clinically manageable time.
Fig. 5. Simulated neutron spectra (a) total dose profiles (b) and fast dose profiles (c) for different irradiation conditions.
Superficial tumors
Beam current:
30 mA
Deuteron energy (target thickness):
1.25 (thick), 1.35 MeV (thick) and
1.45 MeV (8 micron)
Dose prescription:
Peak dose to normal skin =
16.7 Gy-Eq
BSA Optimization:
Goal: To maximize the dose to skin tumor
For thermal neutron production, drawbacks from the "tail" of the primary spectrum are less critical due
to the enourmous power of thermalization of heavy water used as a moderator in this case. Therefore,
good dose performances (and even better) can be achievable with a thick target (Fig. 6).
1.35 MeV deuterons on a thick target produce the best dose performance, since higher doses in tumor
are achievable in shorter irradiation times (30-60 min.). This is mainly due to its relatively large neutron
yield (about 35-40% higher than the other target configurations)
Fig. 6. Skin tumor dose as a function of treatment time for three
target – bombarding energy configurations. Labels on data are the
lengths of the moderating volumes (in cm)