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.

however bnct is still in the development phase . there are several issues that must be address

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 .