The interviewer is interested in who you would shadow for a day and why to learn about your interests, goals, and motivations. If you were to choose a radiation therapist to follow, you should pick someone who is not only skilled and knowledgeable in their field but also compassionate and empathetic towards their patients. You can explain that watching and learning from someone who embodies these qualities would not only help you become a better radiation therapist but also inspire you to make a positive impact on the lives of your patients.
Your answer does not have to be someone famous, although that will not be a problem if it is. The radiation therapist you admire could be a professor, mentor, or a historical figure in radiology. Explain who you would shadow and how they would benefit you as a radiation therapist.