After psa reading of 36 and radiotherapy two years ago when reading went down to 1.5 recent mri scan found no cancer but psa raised to 5.2 then next psa test rose to 8.9 three months later why?

The PSA is not an absolute measure. You can have a high PSA reading for a number of reasons. Urinary Tract Infections (UTI) can increase the PSA reading very considerably, well above 30 on occasions. If that is suspected then a course of anti-biotics for about 10 days followed by another PSA test about 2 weeks to a month later would confirm if it was caused by an infection. A number of other things can cause the reading to increase. Ejaculation in the 7 days preceding the test can also raise the reading.