Background This study aimed to determine radiotherapy (RT)-related changes in cancer-related fatigue (CRF), using Multidimensional Fatigue Inventory-20 (MFI-20), and fatigue-intensity rating (FIR) instruments at three different timepoints and to identify the optimal thresholds of MFI-20 scores which would correlate with moderate-to-severe fatigue warranting an intervention in breast cancer patients treated with RT. Methods Eighty-eight breast cancer patients treated with surgery followed by RT were included in the study. CRF was assessed with both FIR and MFI-20 tools at three different timepoints: within the week prior to RT (pre-RT), last week of RT, and 6 weeks after RT completion (post-RT). Changes in measurements, correlations between measurements and optimal cutpoints of MFI-20 scores were analyzed. Results While FIR scores significantly changed over time (eta(2): 0.179), changes in MFI-20 scores were relatively small (eta(2): 0.076). Comparisons of the last week of RT versus post-RT scores showed small-to-moderate decrease for MFI-20 and FIR. FIR and MFI-20 scores were correlated at all timepoints and most correlated during and after RT (r = 0.525 95%CI 0.346-0.667, r = 0.791 95%CI 0.692-0.860 and r = 0.716 95%CI 0.589-0.808, respectively). Furthermore, the most correlated MFI-20 subscale with FIR was general fatigue (r = 0.603 95%CI 0.442-0.725, r = 0.821 95%CI 0.734-0.881 and r = 0.754 95%CI 0.641-0.835, respectively). Optimal cutpoints of the MFI-20 total scores corresponding to FIR scores >= 4 was 43.5 for all timepoints and the MFI total scores corresponding to FIR score >= 7 were 53.5, 52.5 and 60.5, respectively. Conclusions MFI-20 and FIR scores are highly correlated measures of CRF among breast cancer patients treated with RT. An MFI-20 score of >= 43.5 is suggested as a clinically significant score indicating moderate-to-severe fatigue, while an MFI score of >= 52.5 is indicative of severe fatigue.