From UroToday:

We examined the association between percent tumor involvement in the radical prostatectomy specimen and the outcome measures of pathological stage and biochemical progression using multivariate logistic regression and Cox proportional hazards analysis, respectively, in 2,220 patients from the Duke Prostate Center radical prostatectomy database.

This was a study that showed that if you had more cancer in the prostarte, you had a higher risk of teh cancer coming back after surgery. Although this is obvious, it is important to show these type of things with studies.

Would I would like to see is if patients with the same gleason score and stage (meaning the extent of cancer spread) have different rates of recurrences.

For example, 2 men with organ confined prostate cancer that both have gleason 6 cancers. If 1 man has 1% of tumor volume in his prostate and the other 30%, I would imagine the man with 30% would be more likely to have a recurrence. I am not aware of any studies that have looked at this.