A significant proportion of adjuvant-treated breast cancer patients experience cognitive decline, challenging the person’s ability to return to normal activities after treatment. However, not every patient experiences cognitive problems, and even in patients with impairments, determining clinically important cognitive decline remains challenging. Our objective was to explore differences in neuropsychological performance following adjuvant chemotherapy (CT) in patients with breast cancer.

We conducted a prospective observational study in an Oncology Breast Clinic and assessed neuropsychological performance before and after adjuvant CT and in non-CT-treated women with breast cancer and healthy controls (HCs). Standardised between-group differences and regression-based change scores were calculated.

CT-treated patients (n = 66) performed significantly different from non-CT-treated patients (n = 39) and HCs (n = 56). There was a significant effect on verbal fluency (p = .0013). CT performed significantly worse than non-CT and HC [effect size (ES) = .89, p < .001 and ES = .61, p ≤ .001, respectively] and from HCs with regard to proactive interference (ES = .62, p ≤ .001). Regression-based scores revealed more severe cognitive decline in the CT-treated group [24.24% (16/66)] than in the non-CT-treated group [15.20% (6/39)] and HC group [7.14% (4/56)]. Patients who underwent CT and showed cognitive decline were less educated and older, with significantly lower baseline scores.

CT-treated patients showed more vulnerability on cognitive control and monitoring than non-CT-treated breast cancer patients and HCs. Older patients with less education and lower baseline cognitive performance represent a group at risk for cognitive decline following CT. Identification of patients at risk for decline could improve targeted support and rehabilitation.