I had nephrectomy in feb. After that I was diagnosed as metastatic renal cell carcinoma (lung cancer). But I don't have any symptoms as doc said. Why?

Congratulations. Renal cell carcinoma is notoriously capricious and unpredictable. It's common for a lung metastasis to vanish or stay stable for many years, especially after removal fo the primary tumor. I'm going to assume the diagnosis is accurate. Cherish being alive and be thankful for whatever time you have left -- fill it with love and good things.