The hypothesis here is an intriguing one and seems to suggest that there is some way of measuring two competing processes: tumor regression due to treatment and tumor growth despite treatment. This would require some way of quantifying each of these processes. The tool for doing this is described as an R package called tumgr9 and seems to include sequential PSA measurement, though what the other elements are is not clear. The value of this study would be in demonstrating what additional value the tumgr9 score adds to the PSA measurement and how this can be used to distinguish between the competing mechanisms of tumor growth and regression as a tool for research and clinical practice. I think we need greater clarity about this.