How will I know EBRT has worked?

I'm writing to share with you my journey with Prostate Cancer. I am 73 years old, from Calcutta, India and getting treated at Tata Medical Centre. Treatment protocols could be different in India compared to the US.

I was detected with Prostate Cancer (Acinar Adenocarcinoma) Stage 3B/4A (?), with a PIRADS MRI score of 5 and Biopsy Gleason Grade 4+4.

Around seven years ago, I came down with urinary issues (including slight difficulty peeing) and I reported to my doctor who prescribed me antibiotics and Tamsulosin 0.4 mg which he said he was prescribing for prostatitis. In a month's time I was fit and these medications were not needed any more. However, what needs to be noted here is that he did not refer me to a Urologist. My world was rosy again and I continued living life to the brim, as I've always done. As the years went on and I stepped into my seventies, some common old-age issues started showing up, like more frequent urge to urinate, slight dribbling after peeing (including feeling of "I could have peed a little more"). But then, what the hell, I lulled myself with the thought that these were all part of old age and so carried on living life as before. It’s said Prostate cancer is usually a very slow-growing one and with almost no symptoms at all. Even in my case I would never have learnt that I have got Cancer, but for the fact that in mid-July last year I came down with a viral infection with fever of 102+. I went to my doctor (not the one I had been to seven years ago) and when my RT-PCR test for COVID returned negative, he put me on antibiotics for five days. The high fever went down but a low-grade fever (99.5-99.8) continued. He gave me another course of antibiotics and even then when the low-grade fever did not come down, he started investigations. Amongst several investigations, one was a simple USG whole-abdomen which revealed that my prostate was enlarged. I got a PSA test which was 2.6 (very misleading, as this result is normally considered very good at our age). But he was not happy, he insisted that I see a Urologist. I went to a Urologist who did a DRE (Digital Rectal Examination) and told me to immediately get two tests done. Firstly, a Multi Parametric MRI of the prostate (which gave a PIRADS score of 5) followed by a TRUS-guided biopsy (which gave a Gleason score of 8). Thereafter, I underwent a GA-68 PSMA PET Scan that revealed that the cancer in the prostate had metastasized to nearby lymph nodes, bladder, seminal vesicles, etc. and the doctors concluded that my case was that of an aggressive locally-advanced prostate cancer.

Surgery was ruled out and it was decided that I would undergo Radiation therapy with ADT.

I was put on Bicalutamide for 3 weeks before starting off on 3-monthly Goserelin 10.8 implants. At the same time I was started off on Abiraterone 1000 mg daily, and 14 weeks after that I was put through Radiotherapy (Tomotherapy) 60 units (3 units x 20 fractions, 5 days a week). This is where I wish to highlight the side effects of the EBRT. The first week went without any issues, but by the time I was through to the end of the 2nd week, I started having urinary issues, like urgency, incontinence, burning/paining and incomplete voiding. Two days after I finished my Tomo sessions, the side effects aggravated tremendously and as of today, 10 days after I completed Tomo sessions, I have started feeling some relief. There were some GI issues like a little mucus in stool and increase frequency, but nothing which really bothered me. What was painful was the urination part.

I will continue to remain on Goserelin and Abiraterone for 2 years.

When I asked my Radio Oncologist how do I know whether the RT had worked or not, and whether I would undergo a PET scan in the near future, he said that there was no need for any further PET Scan and that it would suffice to only monitor my PSA level 3-monthly.

My apprehension is that even before treatment my PSA was only 2.6 (now its 0.15 after 4 months of Goserelin and Abiraterone), and whether monitoring only the PSA would suffice to know the progression of cancer, and whether it has or not metastasised further. Will be grateful for inputs.