Hello Jonadab,
With a Gleason 9 and T2c, there is a risk of micro mets outside the prostate (mets too small to see on scans). This would cause a prostatectomy to fail to cure as cancer would have been left behind. About 30% of prostatectomies require further treatment afterwards, but this is higher in the case of higher grade cancer (and lower for low grade cancer). Also, at age 77, you are above the cut-off point for NHS hospitals which offer prostatectomies, at least robot laproscopic ones, although if you are very fit for your age, it might be available privately or as an open prostatectomy.
You sound like you're having HDR Boost, which is a very good treatment for contained high risk cancers. It puts a high dose into the prostate by means of the external beam plus the brachytherapy boost, making it likely to be very effective against the G9, but the external beam can also treat the area around the prostate at a lower dose to mop up any micromets - often the pelvic lymph nodes and seminal vesicles are treated as that's where any micro mets are likely to be.
I had the same treatment as you at Mount Vernon 2½ years ago. I have almost no symptoms from the treatment now, with everything still working - I'm in a much better state now than I ever imagined at the outset. I now run the support group there (I'll send you a private message in case you're interested).