Why is volume of the cancer not used?

That's a good question. While I certainly pay attention to the overall volume too, it hasn't been found to predict the progression of the cancer for prostate cancer. The aggressiveness of the cells (how different they look from non-cancerous cells) is apparently a better predictor. They do however now also quote the % of that grade of cell in the biopsy. So 80% 4 and 20% 3 might be a bit more concern than 60% 4 and 20% 3. You have a 4+4, which means that the aggressiveness of the cancer cells in that sample was graded by your reading pathologist at 4 on a scale of 3 (somewhat abnormal) to 5 (as very abnormal,) and that was more than twice as common as 1,2,3, or 5 graded cells in that sample. This is definitely an intermediate unfavorable result as they say, not based on how far your cancer has progressed, but rather how rapidly it seems likely to progress going forward.
You're right, though, that a good mpMRI provides important input as to how much cancer is already present. 12-20 needle biopsies not so much! A resected prostate after removal provides great guidance as to volume, a little late to provide the help you're looking for.
mpMRI's still vary widely as to both clarity and interpretation, hence the lack of reliance on that alone. I ended up having 2 before my surgery, and the one with the more advanced cancer was completely missed by the first MRI, on less modern equipment with less informed interpretation.

Thanks for the somewhat uncertain followup, not sure that explains to me how 5 times the amount of pathology is less of an issue. Nice theory perhaps, some odds or probabilities presented is interesting but perhaps not clinically relevant- the math seems to not add up. Whats worse 1 break in a single bone or 5 breaks in a single bone? I would say 5 is more serious. Perhaps they have used Gleason pathology numbers so long (or too long now with modern imaging giving additional information) just nobody considered volume.

It's hard to imagine a serious practitioner or researcher not considering all the available data, but I'm sure it's also true that there are biases on the part of everyone. I'm sorry you didn't find my explanations more helpful, but I guess I shouldn't be too surprised. I'm sure you didn't make your own comments in a vacuum!
For what it's worth, I would just say that since prostate cancer can progress at quite varying rates, the risk of it spreading beyond the prostate is better calculated by the aggressiveness of the cells than the current progress within the prostate. Since removing the prostate remains a standard of care, even in the presence of alternatives, the size of the nodule is perhaps not as important as how many parts of the prostate have nodules, how aggressive the cells producing the nodule(s) are, and so forth. I am of course not saying that volume of cancer doesn't matter, but just that it is perhaps not as good a guide, especially alone, for the wide range of prostate cancers that are lumped together as "prostate cancer." Also, if the volumes are actually quite low, like 1-5% as opposed to 10-50%, absolute size, again, may be not the best factor in determining time to metastasis (when the cancer leaves the prostate and begins to spread through the body and into the bones.)

Yes but 4 is 4 with both of my examples. I didn't say compare 4 to 4+, or 4 to 3, or 3 to 4, or some other difference. I am comparing 4 to 4. With 4 in one having the same aggressiveness as 4 in the other. Maybe there are other factors in people making differences but pathology 4 is pathology 4 in both my examples. Yes 4 is bad but both samples are 4. I think your point is relied on comparing 4 to some other number with differing aggressive characteristics. That is not what I am discussing, at least I think 4 = 4 faulty as that may be based on pathologist differences and so on that can happen but even so let us say we use the same pathologist and a 4 is a 4. It is exactly why the model is not seemingly up to date and seems based on how they did pathology and imaging from years ago or something, at least seems like to me.

So in case it is not clear:

*Case 1 has a 3+4 lesion at 2.1 cc, the 4 at 49% and the 3 at 51%. So that means the part that composes 4 is about 1 cc total. Total count of 4 = 1 cc.
*Case 2 has a 4+4 lesion but only about 0.2 cc. Total count of 4 = 0.2 cc

Case 2 has only 20% of the amount of 4 of Case 1
Case 2 will have the most serious recommendations possible, RP or Radiation with ADT.

The aggressive nature of the beast to either lie there and rest peacefully growing slowly within its little bubble or jump on a train rail to travel and see the countryside would absolutely lead the decision making..
Just my simple minded thoughts

Again you are saying aggressiveness, and that means 4. Which case has more 4? Doesn't matter if one is simple or not, which case has more aggressive tissue?

*Case 1 has a 3+4 lesion at 2.1 cc, the 4 at 49% and the 3 at 51%. So that means the part that composes 4 is about 1 cc total. Total count of 4 = 1 cc.
*Case 2 has a 4+4 lesion but only about 0.2 cc. Total count of 4 = 0.2 cc

Looks to me like Case 1 has way way way more aggressiveness in it.

You can not know at what time frame the grade 4 started .. you would need time study of several biopsies, without treatment. these studies have most likely been recreated in the labs with micros, and this is the science they base protocol on..these protocols most likely get reevaluated with every new proven theory..this being why the major clinics have the latest and usually best available treatments

Why would you need ANY of that. You know the highest level of aggressiveness in the lesion/prostate (that from pathology reports saying there is 4 in there in these examples) and you know pretty close to exactly how much. That provides everything needed. You don't need what you are saying at that point, yes for history maybe is nice information, but a history should not dictate anything it is just "history".

4 mutant radically
A petrie dish with 0.2cc 4+4 Gleason can grow at a much higher doubling time
Than larger amount of 1.0cc 3+4