I wish I could offer an answer for predicting life span with lung cancer. Although I certainly worried enough about it to be a professional predictor, my prediction experience hasn't been very good. I went from death's door at age 53 to celebrating my 69th birthday tomorrow. The survival statistics your doctor may be citing are based on a 5-year history and small cell treatment breakthroughs have just started to occur within the last year. Read Leo's experience with small cell lung cancer here!
My fretting over lifespan during my 3 years of treatment resulted in two things I hope you avoid: severe depression and foregoing the opportunity to experience the joy of live during my treatment and between treatments.
I don't know the details of your clinical trial but table below shows some of the immunotherapy drugs currently being studied. Keytruda is a FDA approved small cell treatment drug that has demonstrated tumor killing power and is good stuff. So if you are in an immunotherapy trial, I think you are in a good treatment space. This could be a lottery where you win either way!
Stay the course.