Over an informal dinner in a Chevy Chase home, 25 doctors peppered one another with questions about prostate cancer treatments and clinical trials. The District has the highest incidence and highest death rate of prostate cancer in the country, according to federal health statistics. But doctors say there is no easy way for doctors to find out what trials may be available for their patients. Difficulty in finding such information was the inspiration for the informal dinner and gathering of doctors. What’s unusual about them is that the doctors hail from a diverse group of hospital systems, many of them competitors. Such collaboration is rare, according to doctors and other experts.