Recurrence?

Radiotherapy doesn't work that way.

Radiotherapy exploits a property of cancer cells that makes them much more vulnerable to radiotherapy damage than healthy cells are. The radiotherapy deliberately damages cells' DNA. Healthy cells can repair their DNA when it gets corrupted (which happens regularly in normal life), whereas a cancer cell became a cancer cell because it lost its ability to repair DNA corruptions, and picked up a set of DNA corruptions which made it malignant (dividing and multiplying uncontrollably). Radiotherapy generates enough additional DNA corruption to stop the cancer cells being able to divide and multiply, while the healthy cells can repair the DNA corruptions induced by radiotherapy. There is collateral damage to healthy cells too, and that is what generates side effects. The radiation dose is set at a level where the collateral damage doesn't risk killing the organ, because a necrotic organ the size and position of the prostate would be fatal. (I vaguely recall this is around 10% collateral damage, but I might be wrong here.) The organ must remain viable such that it can maintain itself, fight infections, etc. So after the radiotherapy, the cancer cells are no longer malignant and will simply die of old age over the next 18-36 months, plus a small percentage of healthy cells are similarly destroyed. The prostate is capable of regenerating tissue, and as the impacted cells die off, it may form new replacement tissue. A second effect of the repeated damage and healing cycles of radiotherapy is that fibrous tissue forms during the healing. The prostate is a muscular organ, and this stops it from being able to contract and expel its contents as it normally would.

On average, prostates are about 5% effective after radiotherapy, but this is heavily skewed to most not working at all, and a small number still up to 50% effective. This can depend on the extent of the cancer in the prostate at treatment time.

Radiotherapy doesn't make the organ vanish.