During radiation treatments for cancer, high energy rays are used to destroy or damage the cells which are multiplying rapidly and causing illness. What would the most serious risk of this treatment be?

The high energy radiation causes damage to the cancer cell's genetic material by inducing double-stranded breaks in their DNA. This acts as a signal for the cells to begin the process of apoptosis - induced cell death. The cancer cells then are unable to reproduce and die.

However the radiation my reach neighboring healthy cells and also destroy their DNA. If these genes for apoptosis are also damaged in the process then the cells may begin dividing with damaged genetic material resulting in more cancer.