Most of the Male cancers (Testicular, Prostate) have to a degree a dependance on Testosterone.
Anti-Androgens (along with a low dose of Estrogen) is a well documented treatment of Male cancers.
I would suspect the risk is greatly reduced.
Post SRS, you still have the remains of the prostate gland, I know no cases of MTF TS, having Prostate Cancer (Although the risk of other cancers, breast etc) obviously increases.
Buffy