Just to add that the unit that measures the amount of radiation absorbed into a volume of tissue is the Gray (Gy). However, this doesn't by itself give you the biological effect (BE), because that also depends on the rate at which the radiation is delivered. Radiation delivered at a high power over a short duration has much more BE than the same dose delivered slowly over a long time.
LDR brachytherapy using Iodine125 is 150-170Gy over ~28 weeks.
The classic external beam radiotherapy dose is 37 x 2Gy = 74Gy over 7½weeks.
Hypofractionated external beam radiotherapy dose is 20 x 3Gy = 60Gy over 4 weeks.
Ultrahypofractionated external beam radiotherapy dose is 5 x 7½Gy = 37½Gy over <2weeks.
HDR Brachy monotherapy (rare in the UK) is 2 x 15Gy = 30Gy over 2 weeks.
So here we see a variation between 170Gy and 30Gy over different durations, but the BE is roughly the same in all cases. I don't know of any units for BE, but basically, you get given the max lifetime BE, that being most likely to cure the cancer without doing you excessive collateral damage. That's the reason you can't have it more than once to the same tissue.
When radiotherapy fails, it most often fails outside the target area because the cancer had unknowingly already escaped. These areas can often be retreated providing the spill of BE they received during the first radiotherapy treatment is low enough that they still have sufficient reserve from their max lifetime treatment to do a useful retreatment, and treatment plans can be constructed which don't take any other tissues over their max lifetime BE. This is more likely to be possible the further away the recurrence is from the original treatment site. Having said that, radiotherapy can occasionally fail in the original treatment volume, in which case curative options might be salvage prostatectomy or one of the focal treatments.