Cancer antigens such as PSA and CA-125 can also increase due to non-cancerous causes. Pretty much all of them increase in any kind of inflammation whether it is due to bacteria or something else and more. So their main use is getting a baseline once the cancer is diagnosed and monitoring it later on.
Funny story, a professor once had a patient with prostate cancer, treated and cancer free afterwards. Every 6 months the patient would visit for screening and his PSA was through the roof( think >1k while normal levels are at most around 3ng/mL). After performing CT scans etc the patient was found cancer free. Another PSA test would return normal after a couple days. This story repeated several times with the professor slowly sliding into madness from it.
Turns out the patient was getting anxious about the screening the day before so he smoked some weed which increases PSA secretion. And of course weed is illegal here so he didn't want to admit it.
That's why getting an early PSA baseline is important, so you know what value is "good". Also you might not want to take drastic action based on a single "bad" PSA.
Cancer antigens are NEVER to be used for diagnosis. Chemotherapy is too destructive to be used without 100% certainty. We've had a woman with high CA125 and it turns out it was tuberculosis in the reproductive system... Everyone wanted to start chemo, the symptoms matched etc...
But we might use the word baseline for different things.
As PSA is influenced by the patients physiology and activity [1], it can be helpful to understand what the non-cancer baseline/standard PSA value for a specific patient is, this would need to be established by multiple tests over a couple of years. This can help in distinguishing noise from signal later.
After surgery the PSA baseline is zero (by definition, the generating organ has been removed). Any value above zero indicates either metastasis in other body parts or a really bad surgeon.
After non-surgery primary treatment (radiation, ...) the PSA does not (necessarily) fall to zero, so here one takes the lowest value after treatment (with a grace period) as the baseline.
[1] https://www.cancer.org/cancer/prostate-cancer/detection-diag...
Importantly, it doesn't seem to affect mortality rate of prostate cancer, nor all cause mortality.
People focus a lot on 5 year survival rate, but sometimes they don't really understand what this means. If I die at 75 does it matter if my cancer is detected when I'm 68 vs 73?
Yes, it matters in both directions.
One way is that the cancer might cause you to suffer in other ways, e.g. by metabolizing all of your food, causing you to lose large amounts of weight. If this is addressable (not at all a sure thing!), then knowing earlier is probably better.
The other way is that you might not be suffering. In that case, given that your death is fixed, knowing earlier is obviously worse.
I've never seen someone die of cancer without being treated beforehand. I understand it's a horrible, painful death. That said, the treatments we apply to someone dying of cancer also make for a horrible, painful death; there definitely is a point at which the treatment is worse than the cancer.
Another wrinkle is that treatment may give you a few horrible months, then a few not-so-bad-years before your cancer comes back, and then the horribleness resumes. Those years in the middle are worth something, even if the time at which you die is fixed.
If we ignore brief-to-medium remissions, and the hypothetical patient's time of death is fixed, then the obvious recommendation is to avoid all treatment and commit suicide when your quality of life drops too low. But the uncertainty we have in reality makes that difficult.
But in the lower age groups 40-60 early detection should IMHO be the aim.
I’ve spoken with the chief surgeons of two major prostate cancer centers. They agree this was not a good recommendation by the US task force.
Also I should add that the risk of impotence / dysfunction is much higher with late-detected cancers.
https://jamanetwork.com/journals/jamaoncology/fullarticle/25...
And yes, there is an (I would say) ongoing debate about the value of using PSA to detect early cancers. Obviously detecting cancer early is a win. But the downside of this early detection is over-treatment of low-risk cancers.
Standard therapy for prostate cancer is the removal of the prostate, but this surgery has a somewhat high (up to ~10-15%) risk of impotence and incontinence (depending on the quality of the surgeon), which can obviously impact life quality quite massively.
Current strategy (in the US) seems to basically stop broad usage of PSA testing, which gets rid of the problem of detecting low risk cancers, but at the cost of delaying the identification of high risk cancers. This is visible in the falling rate of newly diagnosed prostate cancer patients in the US.
This feels like not solving the actual problem (distinguishing low risk from high risk tumors) but glossing over the problem by not actively working on detecting it anymore.