I now will need to decide on SBRT or Brachytherapy

HDR brachytherapy only requires one session, Normally. Rather than five sessions of SBRT.

From AI

For Gleason 3+4 prostate cancer (intermediate-risk), both SBRT (Stereotactic Body Radiation Therapy) and HDR Brachytherapy are excellent, highly effective options, but neither is definitively "better" overall; SBRT offers non-invasiveness and fewer hospital visits, while HDR offers potentially higher cure rates with very durable long-term data, though it requires an invasive procedure. SBRT is simpler and more accessible, while HDR provides a very precise, high-dose internal treatment, with recent data suggesting SBRT may have similar or better long-term recurrence rates in some intermediate-risk groups, despite HDR's historical strength, making the choice dependent on patient factors like prostate size, lifestyle, and physician expertise.

HDR Brachytherapy
Pros: Delivers very high doses directly to the tumor, excellent long-term cancer control, often avoids need for hormone therapy, proven durable efficacy.

Here is an article that discusses the differences
https://pmc.ncbi.nlm.nih.gov/articles/PMC9105931/.

Will the Brachytherapy be a boost while the prostate bed is still being treated by SBRT?

Have you looked into the ASCENDE-RT (plus some other trials)? The comparison is between a Brachy boost and more traditional EBRT but the effect in survivial and recurrence risks is very large, for BCRFS something like 67% vs 85% for a mixture of intermediate and high risk cases.

@topf Both of the treatment options I am considering will be monotherapy. Not boost. Also, in UCLA, the Brachytherapy is for HDR only.

For me, given that the choices yielded similar outcomes, quality of life and side effects became the overriding issue along with effective targeting. I had the Mridian Linac SBRT machine with the built in MRI. The built in MRI was key for me. What they could see, they could treat and I had 5 sessions.
As a layman, and in addition to side effects and quality of life, my decision process included wanting to get rid of both the cancer and any micro cells that might be in and around the prostate, trying to avoid re-occurrence. I felt treating the entire prostate plus a small amount of healthy tissue around the prostate, 2 mm vs 3-5 mm for all other radiation machines, potentially achieved that goal.
I did not see enough information out there related to brachytherapy and accomplishing my goals in 2022 when I was first diagnosed.

It’s really a case of 6 of one , a half dozen of the other; both forms of treatment have success rates you can research and quantify.
The real question for me is in the Decipher score; while not glaringly high, it is still telling you that the G4 component is more aggressive. Remember, the ‘4’ does not indicate aggressiveness, but the number of anaplastic cells (ie more abnormal looking) relative to the less abnormal looking ones (G3). It is confusing and it IS the subject of debate from one pathologist to the next.
This is where the Decipher test really makes a difference - it doesn’t tell you how many abnormal cells you have, but how aggressive and possibly treatment resistant they might be.
Radiation is wonderful against PCa but does have some limitations in that it doesn’t always kill all the cancer cells. That’s where ADT comes in; hormones may reduce your recurrence rate from 2.7% to 1.3%?? Those numbers are small, but one represents TWICE the protection against recurrence.
A 6 month course of Orgovyx ‘won’t kill ya’ as my Urologist told me as I trembled at the thought of becoming a fat, eunuch with teats. Didn’t happen, and was just a minor inconvenience compared to surgery.
If I were in your shoes (and I’m not), knowing what I know now about tumor proximity to the capsule and the value of Decipher tests, I would opt for ADT (orgovyx - not lupron) as an added measure to weaken the ability of the cancer cells to repair the damage caused by radiation.
With a more aggressive PCa neither modality alone can kill the cells, but together their effect is more than additive - it is lethal. Best of Luck,
Phil

@heavyphil Your recommendation is valuable to me. You put the emphasis on hormone therapy after treatment, so the front end modality either SBRT or Brachy may not make much of a difference in reality, looks like.
Dr Kishan did mention the type of hormone medicine is new (unfortunately I did not write it down because I did not think I would need it) and has less side effect than Lupron, but there is side effect nonetheless. I wonder if I could elect to use it, and watch how I react to it, then if I can't endure, then ask to stop it after 3-4 months, for example.
I will have to ask UCLA about that.

"....I did a swab for Prostox at the clinic and now await for the result. If I am sensitive to high dose treatment, I will have to do low dose such as IMRT."
The Prostox tests may evaluate the delayed urinary tract effects of both SBRT or IMRT provided you request both. (both $900.00, one $500.00 if outside of the research network or covered by 'insurance') I suspect as it is derived from UCLA financial issues are not a consideration. The term brachytherapy should always be preceded by 'HDR' or 'LDR' . Low dose brachytherapy is a misnomer. If you add time/duration as a criterion LDR ('seeds') provides the optimal dose. Consider looking up the YOU-TUBE videos produced by the Prostate Cancer Research Institute [PCRI.org] Brachytherapy 101 is a good start.

@frank1956 I completed SBRT
for localized PCa in September; because my Gleason score was 9 additional ADT was not presented as optional. After a first 3 mos depot-Lupron injection, I was switched, at my request, to Orgovyx (mostly to avoid having to drive in to LA from my home in the Coachella Valley for the injection). The well-known side effects of ADT are unpleasant, but not unliveable, and there is an endpoint! My PSA at 3 months is down from 15.1 to 0.19, which is reassuring. My Rad Onc (in the same department with Dr Kishan, will consider limiting ADT to 12 months pending my continued therapeutic response and clinical course.

@frank1956 yes, Frank, he is probably talking about Orgovyx - it’s newer and it’s oral. You can stop it, but if you exercise as recommended your SE’s will be minimal.
Sexual side effects vary but that is to be expected. You usually begin ADT 8-10 weeks before radiation to weaken the cancer and halt growth and spread; then radiation, followed by another 8-12 weeks ADT.
Most RX’s call for 6 months but some on the forum have done 4…
Phil

@rbtsch1951 Thanks. Phil also mention Orgovyx. I will ask about that.
Did you have to drive to LA for all of your SBRT (total 6 including planning) treatments? Are you able to drive for each treatment or family member drove you to appointments?