One advice is, look for the herb of known potent effects against specific cancer under investigation to act as a complementary and combination therapy with the chemo and/or radiotherapies. In addition, look for the synergistic action, it would be helpful not only in decreasing the bad side-effects of the toxic therapies but also improve the therapy as well.
You can test the already known effects of herbal compounds in the context of clinical trials. A well-designed placebo controlled trial would help. For example, mucositis is a disturbing common complaint of patients undergoing radiation therapy with or without chemotherapy for head and neck cancers. It is readily assessable by subjective and objective tools and rapidly yields results. Or for xerostomia (dry mouth) you may search the studies on the impact of Malva sylvestris. It is known as a moisturizer in Persian herbal medicine can relieve dry mouth, constipation and also insomnia without disturbing side effects.