Sadly no. I'm not joking.
It has been a topic of discussion among average American citizens as far back as I can remember.
Could be an old wives tale but I've heard many sensible folks utter those words.
The thought that there is a cancer panacea out there being hidden because treatment is somehow more profitable is not so much old wive's tale as greater departure from logic than the anti vaxxer set.
Cancer is as much a unified concept as sneezing -- could be because you got dust up your nose, could be allergies (of which there are infinite), could be that you have that sunlight reflex thing, could be that you have a cold (again infinite numbers of things that cause this)... There are how many types of cells in the human body (
https://www.kenhub.com/en/library/anatomy/types-of-cells-in-the-human-body )? Each of those gets their own cancer, all with their own genetic lineages and approaches to the world.
Moreover it is not just the cancer that is the problem as much as the tumours growing, pressing on things that don't need to be pressed, siphoning off blood and resources. Take a pill that somehow gets absorbed by just cancer cells and kills them off (or maybe renders them inert) and you still have to do the surgery to remove it or face whole things dying inside you (see compartment syndrome, crush syndrome and other such things -- injecting yourself with dead and decaying tissue is seldom a great plan).
Maybe there will some day be some nano robots or something that can cure essentially anything. The baseline knowledge required to get there is so far above what we have now though that for "them" to be sitting on a cancer cure also means they are probably sitting on a cure for basically everything else out there -- the tissue and organ regeneration that would be required in this scenario alone would be worth trillions a month, the scanning and detection methods probably even more than that... you have basically cured ageing at this point. For something so prominent to be hidden from basic extrapolation from all biologists, chemists, pharmacologists, medics and everything in between*... just no.
*I am better with oxides than I am with oncology but if they have the ability to detect marginally different organic matter in large untreated samples massing in the kilograms, never mind whilst keeping the host alive, and alter it on such a level (also whilst keeping the host alive), then the materials that I could create with scanning tech that good with that would change the world... I can't say overnight as things would need to be shipped around the place so in about a fortnight. Or if you prefer I deal with inert things that I could sit in a vacuum and chill to just a fraction off absolute zero for months, pelt with gamma rays and positrons the entire time and anything else as extreme as that and still not get a clear enough picture to do what said detection methods would have to be able to do in a living patient.
We could look at numbers as well.
https://www.nature.com/articles/bjc201677
For 9 year treatments in the UK (very much an advanced medicine country) then the most expensive types (table 2) clock some £40000 if I round up.
In table 1 there are some 1206122 patients for all the cancers.
At said 40000 rate, assuming it is all profit and multiplying by 1206122 you have 48 billion over the course of 9 years. Or 5.3 billion a year in the most generous of circumstances. For reference the UK NHS budget is £125 billion for the financial year 2017-2018, and the social services bill to try to keep people that are in treatment from going bankrupt is not going to be low either. To that end sitting on a panacea would not only be one of the most unethical things done in history but if they are doing it for a profit then also one of the stupidest financial moves in history.