These are my thoughts on watchful waiting and so are from the perspective of the 30s-40s (and maybe 50s and 60s) age range.
First, watchful waiting is where you and your Doctor will detect an elevated PSA, probably do a biopsy which shows prostate cancer and then decide to wait and keep monitoring the level of PSA until it gets to a worrisome level. Obviously if the biopsy comes back with an aggressive report “watchful waiting” is usually out. Much depends on your age, DRE results, and biopsy results. If you are 95 years old, watchful waiting probably makes sense right now since the odds are that prostate cancer won’t be the thing to get you. If you are 35 or 45 things are different. Ditto if you are 55 or 65.
In my and most people’s understanding, at some point all cancers are contained. They may start as one cell, grow into 2, eventually into 100 and so on. Prostate cancers or other kinds, are at some point contained all in a small area. The problem comes with metastasis, that is, when they spread from the contained area to somewhere else in your body. At that point, cancer becomes much more difficult to treat because surgeons can’t just cut the cancer out and radiation oncologists can not just aim a beam of radiation at a particular area. The Doctors may treat you with surgery and/or radiation when there is metastasis, but will also likely treat you with chemotherapy to try and eradicate it everywhere.
The problem is that no one knows when the metastasis occurs. It could be any time that a cancerous cell slips out and survives elsewhere. What everyone does know is that at some point on one day the cancer is confined and a day later it is not. There is just no way to know when. Since you have no idea when that will occur, it is gambling that it will not be soon. Consequently if you are in good health otherwise and have at least a ten year life expectancy, I do not believe in “watchful waiting.” The day that the cancer “slips out” could be next week or next month. Or it could be five years. Whenever that happens, things become much more difficult for you and your Doctors. Since there is no way to know, it is playing the odds that it will be in years, not months. Admittedly, the odds of metastasis are lower the earlier the detection is, but do you really want to gamble with time?
Similarly, the larger the tumor, the larger the chance that a cell could slip out and while prostate cancer is usually slower growing, every day and every increase in size gives more time and more chances for a metastasis.
In short: I chose not to play the odds and do “watchful waiting”. Everyone has to make that decision for themselves, but I’m not a gambler when it comes to prostate cancer. You only get one chance at this life. Plenty of other people have made a different decision, but this is my perspective. Whatever decision you make, make it an informed one.