Boron-10 has been used experimentally in the treatment of inoperable
brain tumors. _Glioblastoma multiforme_, a particularly malignant form
of cancer, is an invariably fatal disease in which the patient has a
probable life expectancy of only 1 year. The tumor extends roots into
normal tissues to such an extent that it is virtually impossible for the
surgeon to remove all malignant tissue even if he removes enough normal
brain to affect the functioning of the patient seriously. With or
without operation the patient dies within months. This is therefore a
case in which any improvement at all is significantly helpful.
The blood-brain barrier that was mentioned earlier minimizes the
passages of many materials into normal brain tissues. But when some
organic or inorganic compounds, such as the boron compounds, are
injected into the blood stream, they will pass readily into brain tumors
and _not_ move into normal brain cells.
Boron-10 absorbs slow neutrons readily, and becomes boron-11, which
disintegrates almost immediately into alpha particles and a lithium
isotope. Alpha particles, remember, have very little penetrating power,
so all the energy of the alpha radioactivity is expended within the
individual tumor cells. This is an ideal situation, for it makes
possible destruction of tumor cells with virtually no harm to normal
cells, even when the two kinds are closely intermingled.
Slow neutrons pass through the human body with very little damage, so a
fairly strong dose of them can be safely applied to the head. Many of
them will be absorbed by the boron-10, and maximum destruction of the
cancer will occur, along with minimum hazard to the patient. This
treatment is accomplished by placing the head of the patient in a beam
of slow neutrons emerging from a nuclear reactor a few minutes after the
boron-10 compound has been injected into a vein.
[Illustration: SEQUENCE OF EVENTS IN NEUTRON CAPTURE THERAPY USING
BORON-10
_Neutron capture treatment of a brain tumor, using the Brookhaven
National Laboratory research reactor (center)._]
Control console
Treatment port (shutter shown open)
Observation window
Patient treatment room
Shutter elevator (hydraulic)
Control rod drives
Heavy concrete shield
Control rods
Experimental holes
Reactor core
Cooling air
Cooling water
[Illustration: _(1) A lead shutter shields the patient from reactor
neutrons._]
[Illustration: _(2) A compound containing the stable element boron is
injected into the bloodstream; the tumor absorbs most of the boron._]
[Illustration: _(3) After 8 minutes, when the tumor is saturated, the
shutter is removed and neutrons bombard the brain, splitting boron atoms
so that fragments destroy tumor tissue._]
[Illustration: _(4) Twenty minutes later the shutter is closed and the
treatment ends._]
The difficulty is that most boron compounds themselves are poisonous to
human tissues, and only small concentrations can be tolerated in the
blood. Efforts have been made, with some success, to synthesize new
boron compounds that have the greatest possible degree of selective
absorption by the tumors. Both organic and inorganic compounds have been
tried, and the degree of selectivity has been shown to be much greater
for some than for others. So far it is too early to say that any cures
have been brought about, but results have been very encouraging. The
ideal drug, one which will make possible complete destruction of the
cancer without harming the patient, is probably still to be devised.