Hi myav. Welcome to the forum.

I'm not a doctor, but I had surgery for prostate cancer 10 years ago (almost to the day) when I was 56. The fact I'm still here should maybe give you some hope and reassurance.  In those 10 years I've been on the committee of my local prostate cancer support group and we've seen many men in a similar situation to your husband.

With a PSA of 18, it is usual and good practice to have a biopsy.  The MRI will guide the biopsy; I can't deny that cancer is a possibility, but it's not a certainty.  If it is cancer then there is every reason to hope that it won't have spread, and that a cure could be achieved through surgery or other treatment.  However, you won't know until all the results are in, and waiting is really difficult.

There are two excellent prostate cancer charities where you can find more information: Prostate Cancer UK, and Tackle Prostate Cancer. If you can face it, it might be worth finding out the next step, which is to understand the prostate cancer T gradings (T1, T2, T3 and T4) and what the implications are for treatment options. 

I know it's really difficult for you right now, and cancer is a scary word, but the situation isn't hopeless.