boron neutron capture therapy is an experimental type of radiotherapy.
radiotherapy involves killing cells using ionizing radiation mainly through ionization
of cell dna or ionization of water in the cell near the dna forming free radicals which
then damage the dna. with the dna damaged the cell dies or loses the ability to reproduce
. there are three main types of radiotherap y. external beam radiotherapy uses a beam
of radiation such as high energy x-rays gamma rays electron beam or proton beam and focus
es it directly at the tumour. sealed source radiotherapy involves placing a radioactive
source directly into the body in or next to the tumour through surgery. and finally there
is the unsealed source radiotherapy which involves injecting or ingesting a soluble
radioactive substance designed to concentrate in tumours.
bnct is a distinct type of radiotherapy different from the three main types listed. it is a
combination of external beam neutron beam and unsealed source radiotherapy boron-10
amplifying the advantages while reducing the disadvantages of the two types.
bnct involves injecting a tumour seeking compound tagged with the isotope boron-10 for example
sodium tetra-borate borax or sodium boroc aptate. after allowing the substance to concentrate
at tumours a beam of epithermal neutrons is directed at this area. capturing a neutron
will induce fission in boron-10. the products of fission are an alpha particle and a lithium
-6 ion. these will ideally kill the tumour cell.
going through each of the component carefully first of all is the neutron beam. the epither
mal neutron beam is created from a nuclear reactor usually a small research reactor fuel
led from uranium and filtered from other radiation and slowed down into epithermal neutrons through
lead aluminium teflon and cadmium. 5 x 10 9 neutron/ cm2-sec then as the neutron beam
travels through the human tissue it loses its ke through elastic scattering thermal
ization the same way a moderator may slow a neutron down in a nuclear reactor. through
the loss of energy it becomes a thermal neutron ideal for the boron-10.
the neutron beam should ideally not affect the tissue and only be captured by boron-10
. this is unfortunately impossible and other than the scattering of the neutrons giving
away ke to the tissue there are several un wanted nuclear reactions which are usually
more destructive.
these are unlikely as the human body is composed of mainly oxygen carbon hydrogen and nitrogen
. out of these oxygen and carbon are very unlikely to capture a neutron as their neutron
cross sections are very small.
the neutron cross section of an isotope represents the probability of the isotope interacting
with the neutron. it is in the units barns b and one barn is 10 to the power of -28 meter
square. it is basically how large the cross section of the nuclei should be for the number
of neutrons interacted to be true. it has nothing to do with the actual size of the
nucleus.
the term barn came about when a physicist working on radioactivity found that according
to the size of the neutron and the number of neutrons that were captured or interact
ed by a uranium atom the cross section of the nucleus should be as big as a barn.
thus carbon and oxygen have extremely low neutron cross sections at least 5 to 6 orders
of magnitude lower than boron and have little to no chances of undergoing a nuclear reaction
. nitrogen and hydrogenon also have very small neutron cross sections 2 to 3 orders of magnitude
lower than boron and the nuclear reaction should be unlikely but due to the sheer ny
mver of nitrogen and hydrogen atoms they do produce some nuclear reactions.
one of this is the neutron proton reaction of nitrogen 14 producing an energetic proton
in exchange for capturing the neutron. pause another one is a reaction of hydrogen creating
deuterium and gamma rays through fusion. therefore it is important that the boron concentration
is very high in tumours so the neutron delivered can be held to a minimum to avoid these un
necessary background reactions.
now let's have a look at the boron. boron -10 is not radioactive it has a very high
neutron capture cross section and () when it captures a thermal neutron it changes to
boron-11 for a brief time then fissions. the binding energy produced by this reaction goes
into the energetic alpha particle the rec oiling lithium-6 ion and the gamma ray which
occurs 94% of the time.
the linear energy transfer of the alpha and lithium particles make them ideal. let. the
linear energy transfer represents the rate of energy transferred to particles per distance
. de/dx. therefore high let means lot of energy is given but since more energy is lost at
the same time it cannot go very far. the alpha particle and the lithium ion have very high
linear energy transfer and their path length how far they () travel before stopping is
smaller than the diameter of a tumour cell . this means assuming the boron has been deposit
ed in or next to the tumour cell it should only attack the cell and will do little damage
to normal cells.
overall bnct is a binary therapy involving neutron radiation and boron-10. unlike the
external beam therapy it is not limited to the surface nor does the radiation dose have
to increase increasing the background dose if the targeted tumour is not at near the
surface. unlike sealed source it is non-invasive and the radiation exposure can be controlled
. the biological compound injected is initially non-radioactive and safe unlike unsealed radi
otherapy.
due to these advantages there is one particular type of cancer glioblastoma multiforme gbm
that bnct is particularly effective against . gbm was in fact one the main reasons why
bnct was developed. as it is a brain tumour it is severely affected by the high background
radiation of external beam radiotherapy. sealed source radiotherapy cannot be used as surgery
is very risky.
ed. one of this is the lack of effective tum our seeking compounds. another issue is the
need of a nuclear reactor. this makes it difficult to treat the patients and a compact neutron
accelerator must be developed.
bnct being a binary therapy is an important factor. binary therapy signifies there are
two components that can be adjusted thus many more variables are involved compared to other
therapy. this is both an advantage and a weakness . the sheer number of variables means at the
beginning of development it takes much more time to develop and find the optimum values
. however at the end of the day they will have more flexibility and potential than other
forms raditherapy and will prove its worth .