Avoiding over-treatment and minimizing treatment-related physical morbidity in lower risk BC is of growing importance but the effect of de-escalation on mental health, including FCR, must also be carefully considered. We sought to explore this association using a combination of validated psychometric assessment and qualitative interviews.

Our findings from this exploratory study provide preliminary but novel data that in a select but large sample of women with early BC, those who omit adjuvant RT after pre-operative MRI do not experience higher FCR than their counterparts who subsequently undergo RT, as well as women in usual care who do not have MRI but have RT. This finding was consistent whether treating FCR as a dimensional variable or using recommended cut-offs. A secondary analysis in which women with any positive nodes, a Grade 3 tumour or tumour size > 20 mm were excluded, yielded the same result. The results are made more compelling by the lack of significant differences between groups in time since diagnosis, age of participants, mental health treatment status, and neuroticism.

There are several possible explanations for our findings. It may be that RT is associated with higher FCR due to prolonging the treatment experience, with side effects serving as a reminder of cancer and/or being interpreted as signs of recurrence [20]. Certainly, interview data showed that many women did experience toxicities, and expressed a need for more education prior to RT. RT recipients consequently had more triggers for FCR and reported higher levels of FCR than women who omitted RT. Indeed, many women who omitted RT described very positive treatment experiences and BC having a minimal impact on their lives.

Another plausible explanation is that adjuvant treatment may be perceived as indicating more serious illness. Women who omitted RT in this study expressed relief that their cancer was ‘caught early,’ and others who received RT noted that adjuvant RT ‘compounded’ their illness experience. Those deemed ineligible for de-escalation may have interpreted ineligibility as signalling a significantly worse prognosis. The way in which news of ineligibility for de-escalation was communicated to patients is unknown, but this too may have contributed to FCR if conveyed as indicative of a poorer prognosis.

Another potential contributing factor explaining lower FCR in the cohort who omitted RT is the close monitoring and personalized care from dedicated trial staff in PROSPECT. This is supported by the qualitative data. Further, although women from all three groups reported high levels of trust in the recommendations of their treating team, the additional prognostic information afforded by MRI may have been protective for FCR by providing even more reassurance. Women who omitted RT did not perceive RT omission as under-treatment, rather as appropriate treatment.

Interpretation of these findings is limited by the cross-sectional, retrospective study design and recruitment from a single breast service. The groups studied were a priori clinically different in order to test the PROSPECT hypothesis. However, the absence of significant differences between the groups on age, time since diagnosis, neuroticism, and current or past mental health treatment is reassuring. It is possible that patients who declined participation in PROSPECT were more anxious and therefore disinterested in the option of treatment de-escalation. The exclusion of women not able to participate due to language barriers is recognized as a limitation.

There are several other psychological variables that could account for the differences reported here. Illness perceptions (personal ideas that patients have about their illness) may be important to consider. For instance, stronger beliefs about personal control over BC are related to fewer worries about whether cancer has been cured [21] and ideas about the chronicity, consequences, and emotional representation of BC have been associated with FCR [22]. Perceived risk of recurrence and appraisal of that risk are a key part of illness representations and are also associated with FCR [22, 23]. Future studies would benefit from including these variables. The way outcomes of investigations (e.g. pre-surgical MRI staging) and implications for treatment (e.g. eligibility for de-escalation) are communicated to patients and the meaning attributed to this information in terms of prognosis also warrants further scrutiny.

Nonetheless, the preliminary data presented here are novel and provide compelling grounds on which to include FCR in future studies of similar treatment de-escalation. If large-scale replication of PROSPECT confirms that omission of RT is associated with only a small rate of local recurrence in this select group, it may mean that QoL impacts, like FCR, become the deciding factor in determining whether or not to undergo RT.

Within the limits of this study, omitting RT in this setting does not appear to be associated with higher FCR, and this is reassuring for clinicians and patients attempting to limit treatment burden through de-escalation. Our findings may be of particular relevance to women ≥ 70 years with oestrogen receptor-positive, clinically node-negative T1 tumours for whom omission of RT is guideline-concordant. Further, providing clear communication, fostering trust in the patient-doctor and reassuring patients that their treatment plan is personalized may facilitate lower FCR.