One of my biggest problems with cancer is that I can never quite get my head round what it is, a disease attacking the dna, or the body attackling itself, or what? When you ask semi-medical people (nurses

) they say it's "just cancer"
Cancer is where your own DNA becomes corrupted (best way to explain it) it happens all the time in the body given the millions of cells we are made from and so there are systems in place to prevent any issues (apoptosis) the cells are supposed to kill themselves off if there is something seriously wrong, or in less severe cases there are ways the DNA can mend itself, like a form of error correction.
But when these systems fail we have cancer, the mutated cell duplicates itself with the error, which also duplicates itself and so on and on till we end up with a mass of faulty cells, which are still part of the body so the immune system doesn’t attack them, and actually builds blood vessels into the tumour to keep it alive, in some cancers they can be localised to one place but in other cases the cells and shoot off to other parts of the body and start growing the tumours elsewhere.
That’s a pretty simplistic way of explaining it, and it does get a bit more complex with some cancers but it’s not a disease like catching a virus, it’s something going fundamentally wrong in the cells then the normal mechanisms for dealing with those things fail.
Hopefully that makes sense.
Chemotherapy is designed to weaken the whole body and attack all cells in the hope it kills of the relatively fragile and faulty cancer cells, the issue with that is all cells get attacked.
Radiotherapy is more targeting, it attaches the tumour more directly killing the cells with radiation unfortunately it tends to damage any cells the radiation beam passes through.
There is proton beam therapy, a lot like chemotherapy but you have more control over the depth and scattering of the radiation.
There are new therapies which program the body’s immune system to see the cancer cells as and invader and so the immune system starts to attack the cancer,
These actually work really well on advanced cancers but they can’t always be used.
There are a number of other experimental treatments as well generally though if you are making a medication specifically to target only one person’s cancer then these treatments are stupidly expensive sometimes in the £1000’s per dose.
If the cancer has spread (metastasised) then it is much harder to treat because it is all over the body, a bit like a car being “riddled” with rust, unfortunately with cancer it tends to attack important organs like the liver lungs brain and bones which you can’t just cut out or zap with radiation, that’s when it often becomes to advanced to treat and people eventually die as the cancer consumes the body’s resources for itself or damages the organs to the extent they stop working