A diagnosed prostate adenocarcinoma attends the clinic. What is the SINGLE most appropriate follow up investigation?

A 79 year old man who is being treated with GnRH antagonist for a diagnosed prostate adenocarcinoma attends the clinic. What is the SINGLE most appropriate follow up investigation?

A. Serum AFP
B. Serum PSA
C. Serum acid phosphatase concentration
D. Serum ALP isoenzyme concentration
E. Prostate cancer antigen 3

Serum PSA is currently the best method of detecting localised prostatic cancer and monitoring response to treatment but it lacks specificity, as it is also increased in most patients with benign prostatic hyperplasia. The level of PSA over time would determine the next management of treatment.