From what little I can find on the UK law at least, it is only illegal if you do not have "sufficient" scientific proof to back up your claim which to me at least seems perfectly reasonable. This law was put in place to stop people from doing things like selling magnets and colloidal silver claiming that it can cure or decrease risk of cancer.

If you can think of a worse scenario than another cancer forming I'd like to know what it is. You also talk as if people who've looked into this more than either of us weren't already using beta radiation to treat cancers. The only difference between this and other radiation therapies is that the radiation is more precisely targeted. If you can make a convincing case that this is somehow more dangerous than current treatments I will gladly concede the point.

5mm may be fairly deep, but remember that the amount of radiation drops of exponentially AND radiation is much better at killing cancerous cells than it is at converting healthy cells into cancerous ones. Worst case scenario is you get a new cancer where the old one already was, which you are probably watching very closely so it'll be picked up early.