While normal radiation, can cause problems after many years proton radiation has been used in children and younger people because it has a very low incidence of side effects.

If you watch the 2023 PCRI Video and see Dr. Carl Rossi talking about radiation you find some interesting things. For one he’s been doing Proton radiation treatments since 1994 when they built a building for the machine in San Diego. The first patient was the person that invented it, He lived to over 100. He has done Proton radiation on over 13,000 prostate cancer patients.
That video link is

And then there is the side effect question, Something Stanford looked into. I know that for me after having salvage radiation 11 years ago, the only problem I’ve had is some incontinence, And that could be due to the surgery as much as the radiation.

Stanford Edu

In a study of about 145,000 men with prostate cancer, the team found that the rate of developing a later cancer is 0.5% higher for those who received radiation treatment than for those who did not. Among men who received radiation, 3% developed another cancer, while among those who were treated without radiation, 2.5% developed another cancer.
Here is a link to the full study from Stanford
Prostate radiation only slightly increases the risk of developing another cancer, Stanford researchers find
https://med.stanford.edu/news/all-news/2022/070/prostate-radiation-slightly-increases-the-risk-of-developing-ano.html