There is minimal qualitative research on fear of cancer recurrence (FCR) in patients who are still undergoing treatment. This study explored how breast cancer patients’ illness beliefs changed during radiotherapy treatment, so as to provide their longitudinal perspective across sessions. These beliefs were mapped to Lee-Jones et al FCR model to assess its applicability to patients during this key treatment phase. A framework qualitative analysis was employed for verbatim interactions between patients (n = 8) and their radiographer (n = 2) over a minimum of three weekly review sessions (26 review consultations in total). Results proved suggested evolution and repetition of themes within and across sessions. Most themes were consistent with the early stages of the Lee-Jones et al model (antecedents and FCR) such as internal and external cues, cognitions and emotions. The crucial observation was that somatic stimuli were interpreted as side effects of radiotherapy treatment rather than cancer symptoms. Patients were still undergoing their last phase of major treatment, whereas the Lee-Jones et al model has been constructed to explain patients’ past treatment experience. New themes emerged, including current exercise, concurrent illnesses/problems, cancer treatment as a constant reminder (of diagnosis) and associated sleeping difficulties. Decatastrophising of symptoms and experiences relating to cancer and its treatment was also a prominent theme indicating a possible coping mechanism to reduce worries about treatment side effects and associated experiences. Finally, some evidence was found from failure of emotional/fear processing in patients due to early surface reassurance by health professionals – a possible explanation of how FCR might arise. Early detection of FCR and promoting support while patients are still undergoing treatment might prevent patients from developing FCR after treatment.