What is a spinal aneurysm?

My husband has been under treatments for an enlarged prostrate for over 10 years; he’s had three re-sections already. Yesterday, as part of his regular 3 month check-up, a spinal aneurysm was diagnosed. He is scheduled to go for a CAT scan in 3 weeks.
I was surprised the doctor did not seem so concerned and that the CAT scan wasn't scheduled sooner. I have been looking for information on the net but can't seem to find specific information about spinal aneurysms.  Remarkably, he says he hasn't had any pain or discomfort but we both theorized that perhaps we've been too busy thinking of the prostrate to have noticed.

Can anyone provide me with more information about this condition and what I should expect about treatments?