Question: Why is cancer so hard to beat? Can't scientists invent a super cell that kills what it's told to?

Susan Cartwright answered on 19 Jun 2015:

The main problem is that cancer is not one disease: it’s an entire family of diseases, with different causes. Cancer is really a symptom – cancerous cells grow out of control, rather than dividing only when the body tells them to – rather than a single disease, so “curing cancer” is a bit like “curing a fever” – you may use the same medications to treat the symptoms, but to effect a cure you need to know which fever-causing illness you are dealing with.

Another problem is that cancers metastasize: that is, they release cancer cells into the bloodstream, which can then travel through the body and lodge somewhere else. Therefore you can “cure” the original cancer, say a tumour in the breast, only to have the same cancer pop up again somewhere completely different, perhaps years later. Oncologists try to catch cancers early, before they have chance to do this, but with some cancers that do not present any obvious symptoms (no pain, no visible lump or growth) it can be extremely difficult to do this. (This is one reason why pancreatic cancer is almost always fatal: it does not cause symptoms until it is too advanced to treat effectively.)

The body’s immune system is designed to kill foreign cells, and it is quite possible that many potential cancers are detected and destroyed by the immune system before they become dangerous. (One of the symptoms of AIDS, which destroys the immune system, is that the patient develops rare types of cancer – presumably a functional immune system attacks these effectively.) There is a field of research – cancer immunology – which is attempting to sensitise the immune system to more forms of cancer, so that the body’s own defences attack the tumour. The great advantage of this technique, if it can be made to work, is that it would be specific to the cancer cells, rather than indiscriminately killing all rapidly dividing cells like chemotherapy, and it would also attack the metastatic cancer cells hiding elsewhere. However, i suspect that even if this works it will not be a one-size-fits-all cure: different cancers will require different immune-system primers, and some will certainly be easier than others.

We already know that different cancers are associated with mutations in different genes – for example, the protein p53 works to repair damaged DNA and helps to prevent cancer (it is a tumour suppressor). Mutations in the gene for p53 are associated with higher risk of many types of cancer – so a therapy that inserted a functional p53 gene into cancers might well be very effective – but it isn’t implicated in all cancers, so even this would not be a “magic bullet”.

The lack of a cure for cancer is not caused by lack of work on the problem. It’s genuinely hard!