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How accurate is the Strum model of PSA leakage?

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Posted 2/16/2011 9:30 AM (GMT 0)
Hi -

Does anyone have a sense of how accurate the Strum model for PSA leakage is, and how to interpret discrepancies?

My situation (after RP):

A 40 cc prostate
Gleason 3+4 (25% was G4, 75% was G3)
Tumor volume 3 cc (7.5% of prostate volume)
PSA pre-op was 5.6 tested just a few days before the surgery.

According to the model, PSA should be as follows:
37 cc of health prostate tissue should generate abut 2.5 nl of PSA
2.25 cc of gleason 3 should generate about 9.6 nl of PSA
.75 cc of gleason 4 should generate about 1.5 nl of PSA
so total PSA should be about 13.6, but it was actually 5.6 (measured just a few days before the operation)

So this means either the tumor was much smaller than 3 cc (about 1 cc would make the math work) or else the tumor was much higher grade that 3+4. I doubt the former since the size of 3 cc is pretty consistent with the percentage of biopsy core length that was positive. And I doubt the latter since the biopsy was graded at two different labs (3+3) the first time and (3+4) the second time, and then the path report showed 3+4.) So that's all pretty consistent. Or it's possible that the model simply has a lot of variation from one to the next and the discrepancy doesn't really mean much.

If you want to do your own math, see
www.prostate-cancer.org/education/riskases/Strum_StrategyOfSuccess2.html and look under the heading "prostate gland volume"

Any thoughts? Thanks in advance.
Posted 2/16/2011 12:22 PM (GMT 0)
Hi proscapt,

I dont think your calculations are right.
3 ccs of 3+4 tumour shoud give you 2.92x3cc=8.8nl of psa + 2.5 nl from healthy tissue. This adds up to about 11.3nl. However this is still much higher than your actual reading.
I had the same issue with my husbands reading of 2.69 before the op. 2.5 was from healthy prostate tissue. Pathologist 1 said he had 0.2cc of cancerous tissue 3+4. This lines up with a pre op score of 2.5+0.2x2.92=3.1 which is close. Pathologist 2, a pathologist favoured by dr strum thought that the tumour was more like 0.7cc which would indicate a preop reading of close to 4.6.

I think that the reason these numbers don't tie up is thst pathologists over or under estimate tumour and prostate volumes and dr strums model is not perfect. Most people with healthy prostates have a reading below 1 for their psa my dad's is a steady 0.7 and he is aged 75. According to dr strums model his prostate should be tiny, but actually he had surgery a few years ago for bph and i remember it was not.

Hope this helps.
Posted 2/16/2011 6:06 PM (GMT 0)
Some doctors have been using .1 instead of .666 to measure benign prostate psa. I would imagine that just with anything else there are variations between individuals. Also transition zone tumors created a lot more psa for their volume than tumors in other locations.
It's just one peice of information among many that give you a better picture of your status.
JT
Posted 2/16/2011 9:26 PM (GMT 0)
the math doesn't seem right - is it possible that gleason 3 leaks PSA at a greater rate than gleason 4 ? you have 3 time time the gleason 3 as gleason 4, but the gleason 3 seems to contribute 6 times as much PSA roughly. (9.6 vs 1.5)
Posted 2/17/2011 2:07 AM (GMT 0)
sfigato,

Thanks for your feedback, but I think the math is right. According to the Strum model, higher grades of PC leak much less PSA because so much genetic change has occurred that they no longer generate much PC.

G3 generates about 4.25 ng per cc of tumor volume.
G4 generates about 2 ng per cc of tumor.

2.25 cc of G3 + .75 cc of G4 should generate 9.6 and 1.5 ng of PSA respectively.
Posted 2/17/2011 2:26 AM (GMT 0)
Who is strum? Thanks
Posted 2/17/2011 2:34 AM (GMT 0)
Trepidation - He's Stephen B. Strum, a very highly regarded oncologist specializing in PC, located in Ashland Oregon.

See the URL link in the first post in this thread for more.

Post Edited (proscapt) : 2/16/2011 8:11:02 PM (GMT-7)

Posted 2/17/2011 6:22 PM (GMT 0)
proscapt....

Just got back from the oncologist who is doing the clinical trial I am in. He said that the psa leak was 3.32 per cc for G6. I have no idea where he got his figures from, but thought I would pass that along. Punch in his numbers and see what you come up with. (BTW, my tumor volumn didn't calculate using the 3.32 either.) I had a 6 cc tumor and 50 cc gland but psa was 4.4.
Posted 2/17/2011 6:35 PM (GMT 0)
Thanks, LV-TX... that gets closer to working out arithmetically.

My guess is there's probably pretty wide confidence interval (standard deviation) at work here. Maybe the point is that the leakage math is valuable in the pre-treatment context when it helps estimate the likelihood of EPE and SVI, or when it helps to raise suspicion about an apparently clean biopsy -- but maybe not so useful in a post-treatment setting, when the path report supersedes it.
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