My PSA fell from 28 to 21 over a three week period during diagnosis. The reason I was diagnosed was because I had urinary retention and a trip to AandE. Remember prostate cancer has no symptoms.
The only explanation I can think for this is that, I suspect the retention was caused by infection not by cancer. MRI and biopsy showed G9 T3 cancer.
Without the urinary retention I assume I would not have been diagnosed early, but rather when it was too late. I guess there is a tiny possibility that there is some cell abnormality which looks like cancer but which isn't, but by normal modern standard of diagnosis I passed all tests for cancer, with the exception of my high PSA falling from very high to very high (yes I did mean to say very high twice).
Everyone's immune system is destroying new cancer cells everyday. Once a tumour gets established the immune cells obviously missed it and aren't likely to catch up. There are a few cases in the entire worlds population of people properly diagnosed with modern techniques where the tumour has disappeared, we are talking less than one in a million though. Let's hope it's your luck day.