The prostate is surrounded by venous plexi and the perineum has a rich blood supply.
2005, Brian J. Moran, “27: Using the perineal pressure applicator device”, in Adam P. Dicker, Gregory S. Merrick, Frank M. Waterman, Richard K. Valicenti, Leonar G. Gomella, editors, Basic and Advanced Techniques in Prostate Brachytherapy, page 251