Background/Aim: The anticipation of radiotherapy can cause distress and sleep disorders, which may be aggravated by the COVID-19 pandemic. This study investigated sleep disorders in a large cohort of patients with breast cancer before and during the pandemic. Patients and Methods: Twenty-three characteristics were retrospectively analyzed for associations with pre-radiotherapy sleep disorders in 338 patients. Moreover, 163 patients presenting before and 175 patients presenting during the COVID-19 pandemic were compared for sleep disorders. Results: Sleep disorders were significantly associated with age ≤60 years (p=0.006); high distress score (p<0.0001); more emotional (p<0.0001), physical (p<0.0001) or practical (p<0.0001) problems; psycho-oncological need (p<0.0001); invasive cancer (p=0.003); chemotherapy (p<0.001); and hormonal therapy (p=0.006). Sleep disorders were similarly common in both groups (prior to vs. during the pandemic: 40% vs. 45%, p=0.38). Conclusion: Although additional significant risk factors for sleep disorders were identified, the COVID-19 pandemic appeared to have no significant impact on sleep disorders in patients scheduled for irradiation of breast cancer.Breast cancer is one of the most common cancer types worldwide (1). In the vast majority of patients with primary breast cancer, the treatment regimen includes adjuvant radiotherapy (2). Being scheduled for radiotherapy can cause significant distress for patients, which may be associated with sleep disorders. According to previous studies, sleep disorders are quite common in patients with breast cancer (3,4).Controversy exists regarding the point in time during a patient's course when sleep orders are most prevalent. In a study that investigated sleeping habits in 33 patients with breast cancer and 23 with prostate cancer during a course of radiotherapy plus 6-month follow-up, sleep disorders occurred mainly before and at the beginning of the treatment (5). In a population-based epidemiological study including 465 patients with breast cancer and 263 with prostate cancer, sleep disorders increased during the period of treatment due to radiationrelated side-effects (6). To date, only a few studies have investigated the prevalence of pre-radiotherapy sleep disorders and corresponding risk factors (4, 7, 8). Therefore, a major goal of the current study was the identification of additional risk factors for pre-radiotherapy sleep disorders in a large cohort of patients with primary breast cancer.Distress and pre-radiotherapy sleep disorders may be aggravated by the Coronavirus Disease 2019 (COVID-19) pandemic. The majority of patients with cancer belong to the high-risk group because of immuno-suppression due to malignant disease or anticancer treatment (9-12). The studies reported so far have provided conflicting results regarding the impact of the COVID-19 pandemic on distress and sleep disorders in patients with cancer (13)(14)(15)(16)(17). No study has particularly focused on the impact of the COVID-19 pandemic on the prevalence of slee...