> How do you think your blood test can tell the difference between slow-growing and fast-growing prostate cancers?

Would it need to? If your test ever came back postive, you'd go into the doctor, presumably retest to confirm, and then start appropriate treatment, regardless of how fast-growing the discovered cancer was.

To be clear: I'm not arguing, just genuinely curious as this isn't a subject I know much about.

The appropriate treatment depends on the risk of death. Chemotherapy is among the worst - probably 20% of people who get chemo die because of the chemo. If there's a 20% chance that the cancer will kill you, there's zero utility to a treatment that has a 20% chance of killing you.

Since none of the treatments we have are 100% effective, they can be worse than useless, actually increasing the overall death rate, and ensuring that the end of life is long, drawn-out, and horribly painful.

Understood. But my original question was inspired by greg's claim that early diagnosis of fast-growing cancers typically won't help, because they're so fast. I was asking if it would feasible to do monthly at home blood tests as a way to detect even rapid growth cancers (and assuming that would be enough time).

The point you're making seems orthogonal to me. Unless you're claiming that treatment is never worthwhile for such cancers, no matter how early they're detected, but Greg's original statement seemed to imply otherwise.

I read Greg's claim (and it's similar to mine) as meaning that there's no test to distinguish between fatal cancers and benign cancers, and this test doesn't claim to be such a test. Even if you have a test that's 100% accurate at detecting cancer, it's only useful if it only detects cancers that are likely to be fatal, otherwise unnecessary treatment will be expensive, both in dollars and in deaths.

>Chemotherapy is among the worst - probably 20% of people who get chemo die because of the chemo.

That's likely not right. I would believe something more boxed in...like 20% of patients who die within 30 days of starting chemotherapy died from it versus the cancer.

That's a recipe for over-treatment, which is already commonly happening for prostate cancer. As I mentioned earlier, it's very important to study outcomes! And not just do what seems reasonable.

Meaning you'd be treating false positives? I assumed the blood tests were coarse-grained, but surely there are more reliable tests that you could do if you tested positive on a blood test. Or are there not?