The standard was 18-24 months. The reason is that the doctors were not certain of catching all the cancer cells with existing scanning at the time. Therefore in order to be sure, they carry on longer. On the other hand, there is also the concern of over treatment. With the use of PSMA Pet Scan, the doctors are more confident of getting the cancer sooner. So there is the tendency to reduce to less than 18 months.
Hope that makes sense. Dont mind me. I am just another layman.