Hello, 
At your age, the PSA of 17 is really high although you feel healthy great and doing your hobby activities. 
However, your 
urologist might have run other tests like ct-scan of abdomen, 
sonography, and others that might have some clues to support the 
radiation therapy. 
Apart the good effect in neutralizing cancer cells with the similar cure rates as prostaectomy, radiation will cause less side effects when it comes to 
urinary incontinence and long-term sexual side effects.
So as your doctor has all your personal history, your disease history, and other comorbidities, and examine your actual health status, please follow your urologist's advice.
Hope I have answered your query. Let me know if I can assist you further. 
Regards,
Dr. Albana Sejdini, General & Family Physician