Cell and DNA Damage and Repair from Different Kinds of Radiation

A very interesting fact from the book Light: The Medicine of the Future (1990) by Jacob Liberman is that far-ultraviolet radiation (UV-C), 100-280 nm, causes DNA damage and shortens cells' life spans, according to a study by Dr. Smith-Sonneborn.

However, she found that near-ultraviolet radiation (UV-A), 320-400 nm, repaired cells and reverses aging. Even more, it repairs DNA while damaged DNA is known to cause cancer. This suggests that sunlight can repair cells and reverse aging since sunlight that reaches the earth is mostly above 270 nm, according to Wikipedia:

Interestingly, this UV-A radiation is below 400 nm which means it does not penetrate glass (glass only allows 380-780 nm) nor is it produced by default by LEDs.

So based on Dr. Smith-Sonneborn's studies, cancerous cells could be repaired by using UV-A radiation at 320-400 nm; available from the sun.

This is not at all what radiation therapy uses for cancer since these therapies use X-rays and gamma rays at wavelengths below 10 nm. Given that UV-C radiation could repair cells and reverse aging, wouldn't that be a better approach to use than to bomb the place via highly damaging sub 10 nm radiation?