why are some people more susceptible to a given carcinogen?

Genes.  It is fairly well accepted that susceptibility (let's say, a
higher susceptibilty) to cancers can/does occur in some families,
which suggests that it  is related to the genes.

I think Myo has hit the nail on the head.

In a similar vein, why don't heavy smokers all die from the same cause?

Did you have to mention that?!  I sure hope it is genes, and the good
ones from Dad, whose pipes I smoke.  He got pat 98 years.

Myo

Do you REALLY smoke a pipe?

My God ... NOBODY smokes a pipe these days, do they?

I think that it makes me look as intelligent as I really am.  You know
how hard it is to look intelligent.  The groom's father used to smoke
one, and it not only did that for him, but kept him from make the kind
of comments we all should avoid, so there could be an argument, that
one not only looks but also seems intelligent (well, not quite so
stupid  - speaking for myself only, of course).

Very seriously (the above was only "just seriously")  Helmut Schmidt,
erstwhile chancellor, when he gave up smoking cigarettes, sucked on an
empty pipe during TV interviews (maybe elsewhere, but I wasn't
present), reputedly to give himself something to do while he was
pondering an answer.  So, there you see, even just appearing to smoke
a pipe can result in your seeming to be more intelligent.
Of course, one could question a person his position securing oral
satisfaction right there on TV before millions.
(Remember the joke:  Do you smoke after ...?  Oh, I never noticed.)

More important to the question, however, is what I actually smoke.  A
lot of matches, but luckily the boxes say that they contain no
carcinogenic matter.


Keisha13-ga  -HI!- is probably wondering what's happened to the question.
By Gee-Ay standards, it is a very good one if it not only gets
answered for free by mere commenters but also provides entertainment.
Thanks, and regards,
"Put"

Myo

Did you say 'you had a groom'?

Wow there must be REAL money in your family ...

I shall now treat you with even greater respect and servility, Sir!

Yours,

A Most Humble One who verily begs forgiveness and yea even a few
crumbs from your table.

Probo, Bryan, 
I meant the father of the bridegroom in that other question.  IF you
knew how unkempt my wife says I am, you would know that we don't have
a groom.  And as for the crumbs from my table, they are all bits of
tobacco and ash from my pipes, so just return to attention, sir. 
Sorry to have kept you in that obeisant position so long, but I could
not respond sooner (g-a restriction).  I am presuming that you have
been stooping obiquitously, somewhat like "assuming the angle".  I
devoured "Stalky and Co." during my time in the US equivalent of a
public school.
But I am sure that that groom has at least one groom, to comb the
horses tails and manes, and maybe give the bride's mane a brush or
two.  What would she look like with a ponytail?  Don't make any
suggestions!
Cheerio, "Put"

Hi Keisha,

Hope this is helpful.

Carcinogenesis, or cancer creation, whether it be benign or malignant,
is a multi-step process.

Cells divide and grow all the time in the human body, and this is
normally a very tightly regulated and controlled process.  If a
divided cell does not meet certain specifications, your body will
destroy it.  Your genes are a program to run the cell.

If the copied genes in a new cell are too damaged, they will cause the
production of a protein that your body does not recognise, and it will
destroy that cell. Some cells are also programmed to die, e.g your
skin cell.

Cancer basically arises when one of these cells escapes the normal
checks and balances, and starts dividing itself like crazy.  It does
not respond to your bodys attempt to kill it or destroy it, or it
escapes detection by your body that it is out of control.

How it escapes the control system is carcinogenesis.

As your genes are the program, anything that messes with genes, mucks
up the program.

There are many things that damage your genes.  Sunlight, chemicals, radiation etc.

As you grow older, your genes "accumulate" little mutations and
changes.  Normally, these dont do much, as they are in a part of cell
that is not read much.

But say that a mutation occured in the part of the cell that told it
to die after a certain time.  If this went wrong, the cell wont kill
itself like programmed, and Bingo.  You have Skin Cancer.

This is a bit of an oversimplification, because it is NOT a SINGLE
mutation that causes cancer.  Cancer IS A MULTI-STEP PROCESS, that
requires many changes to key areas of the cell, for the cell to go
nuts.

Imagine a car.  A cancer is car that is careering down a road without
a driver.  For this to occur, there has to be petrol in the car, the
car has to be in gear, the accelerator has to be down etc.

Similarly in cell, the cell must have a mutations in 
- the part that stimulates it to divide
- the part that allows it to divide (Cell Cycle Regulator)
- part that allows your body to recognise it so that it escapes detection
- the part that releases chemicals that asks the body to provide it
with more blood so that it can grow.


This process was classically described in the creation of colorectal
cancer, where there were 5 genetic mutations identified, and all 5 had
to be present before you got invasive cancer.

Now, say you take 2 people, one that was born with 2 out the 5
mutations, and one that was born with no mutation.

Over time you accumulate genetic changes

Number 1 needs only 3 more mutations to get cancer
Number 2 needs 5!

If age was the only factor, 1 would get cancer way before 2, even if
they were the same for everything.

That is why your doctor asks you if anyone in your family ever had cancer.

But age is not the only thing that stimulates genes. There are many
carcinogens identified, which can stimulate or mutate one or two
genes.  These include
- Chemicals
- Radiation
- Smoking
-Smoking
-Smoking
- Did i mention smoking?

Cigarettes have MORE THAN 300 CHEMICALS in them, and they play havoc
on the cells DNA.  Thats why doctors HATE cigarettes.  They mess up
your system like nothing.

There are some familial conditions which greatly predispose you to
cancer, like Garder Syndrome, Turkot syndrome etc.

All these are basically conditions where people have inherited one or
more pre-existing mutations.

So these people are at higher risk of getting cancer.  If one of them
were to start smoking on top of that, they are effectively signing
their own death warrant.

Everyone knows an old uncle Bob who lived to 90 and smoked 2 packets
of cigarettes a day.

Uncle Bob was an exceptional person.  Maybe he had some detection
system in his immune system that escaped mutation (Maybe by chance) so
he never got cancer.  But for one Uncle Bob, there are a million
others who died needlessly.

So susceptibity as per your question, depends on
1.  Pre-existing genetics
2.  Existing genetics (how much you have accumulated since birth)
(Which in turn , depends on)
3.  Exposure to carcinogens.  More = Bad.

Nobody knows how many carcinogens there are out there.  New ones are
being found everyday.  In 50's everyone thought food was safe.  Now we
know a high processed food diet increases your risk of colon cancer.

Maybe anti-oxidants in green vegetables mop up chemicals that damage
your cells.  Who knows.

So the take home message is........................Eat Healthy + Quit Smoking.

Cheers
Dr. Sharma