With you being T3a G9, they may be intending to treat pelvic lymph nodes and seminal vesicles too, as there will be a higher chance of micro metastases there (where the cancer is most likely to go next), and for that, the 2Gy doses would be preferred, which means lots of fractions to make up the required total dose. You could ask if that's the case, and discuss the pros and cons of including pelvic lymph nodes and seminal vesicles too (often at a lower dose).
Another option for your diagnosis might be HDR Boost, where half the treatment is given by external beam radiotherapy (usually including pelvic lymph nodes and seminal vesicles for the same reason), and the dose in the prostate alone is boosted by a single session of HDR Brachytherapy (to a higher level than is possible via external beam). This is only likely to be offered if your hospital has the capability of HDR brachytherapy, but you can choose to be treated anywhere which will accept you, although possibly less convenient.