So, we've talked about, you know, what is a normal PSA; we've talked about PSA density;
can you talk about PSA velocity or doubling time?
Right, so in the early years when PSA was all we had—I'm talking about back in the
1990s—we didn't have these fancy scans; we didn't have OPCO; we didn't have SelectMDx,
and so there were attempts made to look at how fast the PSA is changing. It makes sense
because if there was a bad cancer you'd figure it's growing faster, the PSA would be rising
more quickly, and that might be a good indication as to whether there's a bad cancer behind
all this PSA activity. It turned out that it didn't work very well. It sounds great
on paper, but the problem is there are so many other reasons that PSA jumps around that
rapid rises in PSA were often times occurring from prostatitis. So, someone could have cancer
and prostatitis. So they could have cancer and a, you know, non-specific inflammation
or lab variations, and there just wasn't any added precision in the decision making process.
Now, PSA doubling time is useful for men that have had previous therapy where the prostate
has sort of been taken out of the picture. They've had previous radiation, previous surgery,
and then men that have cancer coming back after treatment, they look at the PSA doubling
time to try and gague how serious the cancer is and the PSA doubling time is very useful
in that situation (or the "PSA velocity"). PSA velocity and doubling time are essentially
the same thing, but there's a different equation used to provide the calculation. But the whole
idea is the rate of change over time and we all would understand intuitively, a rapid
change over time bespeaks of a more aggressive cancer. A very indolent change over time is
describing for us a low-grade process—sometimes it could be watched. So we have to make a
distinction: are we talking about PSA screening in someone that's never had treatment or are
we talking about evaluating the aggressiveness or a relapsed cancer. And so the doubling
time has application in relapsed cancer; it doesn't really have application in men that
are on a screening protocol. Seems like it should, but it has been carefully researched
and it really doesn't add much. It doesn't add any useful information. PSA doubling times
are also used in very advanced cancer. People that are hormone-resistant and as you would
imagine in that setting, also rapid rises are more serious and portend greater problems
than people that have a slow rise.
And for people who want to calculate the PSA doubling time, they're just counting how much
their PSA is doubling within a certain timeframe and that's the number that they would walk
into, correct?
Yeah, let's talk about the doubling. I don't use the velocity as much. That was used and,
as I pointed out, it wasn't practical–it's been more or less abandoned, but doubling
is pretty easy to calculate if you have just a little bit capacity with numbers. If you
take, say it's in June and your PSA is 6 and then in July it's 12, then it has doubled
in a month. So you just work backward from the most recent PSA. So, let's say you're
looking at a sequence of numbers. Let's say it's in June and it's 6 and you go back to
March and it's 3, then that's one doubling, three months. It's a three month doubling
time. So, it's not really difficult to estimate. Now, if you're looking at relapsed cancers,
what is a serious PSA doubling time? Just to give you all the context, the—well let's
talk about what isn't a serious PSA doubling time because it's nice to know that men that
have doubling times that require more than a year—men with relapsed disease that have
had previous surgery or radiation and the cancer is coming back and the doubling time
is greater than a year—those people have excellent long-term prognosis. Their survival
is similar to other people that don't even have prostate cancer. So, then, work down,
once you get under say 8 to 9 months it starts to become a significant problem, and then,
of course, people that have doubling times that are in the 1 to 3-month range have very
aggressive cancers.
Yeah, thank you so much for making those distinctions. I think that some patients have, in conversations
I've found, often think that if it's not in the screening situation and trying to figure
out what's going on there, they don't realize that doubling time has so much importance
when it comes to treatment.
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