How does a diffusely positive PSMA in the prostate affect treatment planning in a patient with MRI and biopsy showing only one area of disease?  

There are three parts to my answer: First, mild to moderate PSMA uptake can be seen in benign conditions, including BPH and prostatitis (e.g., reviewed by Satapathy et al., PMID 32755196). Second, I don't see how the discrepancy between PSMA PET and MRI/biopsy would affect radiation treatment plan...