How would you approach management of a large retroperitoneal mass that shows seminoma on biopsy?

Cure rate should be 90-95% with BEP X 3 ( EP X 4 if over age 50) . No need to consider a mixed tumor. Postchemo, he with be in P.R. and would then just observe residual mass with serial CT scans.

Also, do not forgot about Orchiectomy to take care of the primary tumor if not done yet (given the blood testis barrier with chemotherapy)