Radiotherapy is a critical component of treatment for the majority of women with breast cancer, particularly those who receive breast conserving surgery. Although medically beneficial, radiotherapy can take a physical and psychological toll on patients. However, little is known about the specific thoughts and feelings experienced by women undergoing breast cancer radiotherapy. Therefore, the study aim was to use qualitative research methods to develop an understanding of these thoughts and feelings based on 180 diary entries completed during radiotherapy by 15 women with stage 0 to stage III breast cancer. Thematic analysis identified four primary participant concerns: (a) a preoccupation with time, (b) fantasies (both optimistic and pessimistic) about life following radiotherapy, (c) the toll their side-effect experience takes on their self-esteem, and (d) feeling mystified by radiotherapy. These themes are consistent with previous literature on illness and identity. These findings have implications for the treatment and care of women undergoing breast cancer radiotherapy.