The only word which fits into the first sentence is 'illness'. However, I am not sure what use radioactive material is for treating illness. "For cases of treating" does not make sense. You have cases of illness or disease (e.g. 'I had a bad case of flu'). 'For cases of treating' doesn't make sense. The second question, either word would work depending on the context.For example, "a medical technique for soldiers to treat severed limbs" OR "a medical technique for treating severed limbs" would also work. Does this help?