It wasn't what you (or I) were hoping for, but you have several options (not just two). Take a deep breath. First thing to do is get a second reading of your biopsy slides. It's amazing the differences I've seen from one lab to another. Epstein (at Johns Hopkins) and Bostwick are known for their expertise. Just ask your urologist to get a second opinion from one of them. It's pretty routine, but they won't do it unless you ask. Ask.
Your question was:
What does bilateral perineural invasion mean??
This means that cancer was found near the neurovascular bundles on both sides of your prostate that innervate and supply blood to your penis. It means that nerve-sparing surgery would be difficult, if not impossible. It also is associated with more incontinence from surgery. Also, there is increased risk that it is not fully contained in your prostate. All of these mean that surgery is probably not a good option for you. I'm surprised your Uro even recommended it. But it's not all that surprising -- that's what Uros do. I look at his argument for surgery the other way around -- because you are young, you have more years to live with the side effects of treatment, so why not choose the one with the least side effects, cure rates being about the same?
I usually think an MRI at this point is a waste of time and money. However, with your small prostate and high cancer volume it might be prudent. Sometimes the cancer bulges out of the prostate and grows into the prostate bed (called "extracapsular extension"). If this has happened, digging the prostate out and getting it all surgically is almost impossible. They can see that on an MRI. On the other hand, if you're convinced that some kind of radiation is the way to go, they will probably do an MRI or CT anyway (for me, they did both and fused the images).
You would think that you could discuss all your options with one doctor. I wish! You will actually have to discuss each kind of treatment with a doctor who specializes in only that treatment. Pain in the *ss, right?(see below)
Your type of PC, if confirmed by a specialty lab, is known as "intermediate risk." NCCN cancer centers recommend IGRT/IMRT with or without
brachytherapy (called a "boost")with or without
hormone therapy started a couple of months before the radiation (called "neoadjuvant") and continued through the radiation and possibly 4-6 months after (called "adjuvant"). There are a lot of permutations in that last sentence. No one really knows which combination is the best. The more treatments you pile on, the better the chance of a permanent cure but the higher the incidence of side effects, especially ED. So you can "overkill" the cancer and end up impotent.
Another option is CyberKnife. In men with Intermediate Risk PC, it has a 91% cure rate after 5 years, and potency preservation is about 80%. Urinary and rectal symptoms are just irritative and transient. Concomitant or pretreatment hormone therapy does not improve the outcome, but only worsens the side effects.
Another option is HDR brachytherapy. It has about the same cure rate and side effect profile as CyberKnife after 8 years. It is often given along with IGRT/IMRT but the outcome is just as good and fewer side effects if given as a monotherapy. Like CyberKnife, it works just as well without hormone therapy. Depending where you are, it may be hard to find someone who offers it as a monotherapy.
LDR brachytherapy (seeds) may sometimes be given as a monotherapy for intermediate risk disease. More often, it's given as a boost to the IGRT/IMRT. They might use higher dose rate radioisotopes like Cesium-131 rather than I125 or Pd103. Results vary widely according to who does it, with UWSeattle, MDAnderson and MSW having the best results. Potency preservation not as good as HDR generally.
Protons may be given as a boost to IMRT for intermediate risk PC. Published results are about the same as IMRT with possibly worse sexual side effects.
With all forms of radiation, it is important to deliver enough to the prostate and a small margin outside of it, but to leave the bladder, rectum, and penile bulb unaffected. The effectiveness of IGRT/IMRT and protons are improved, but sexual side-effects are increased by hormone pre-treatment. The effectiveness of CyberKnife and HDR brachy are not improved by hormone pre-treatment. IMRT and protons have the longest track record. Brachy and protons seem to have the steepest learning curves.
That should get you started. I wish I could recommend a single Radiation Oncologist who can give you an unbiased view of all the options. Lacking that, what I did was talk to specialists in each of those. For some reason, they all wanted to give me the DRE finger