The COVID-19 caused by the SARS-CoV-2 coronavirus is at the origin of a global pandemic. This pandemic has prompted the current health system to reorganize and rethink the care offered by health establishments. We report the early toxicity in patients infected with COVID-19 treated at the same time for early-stage breast cancer (BC). This is a monocentric prospective study of patients treated in our hospital between March 2020 and June 2020 and were diagnosed with COVID-19 infection. The inclusion criteria were to be irradiated for early-stage BC and to have a positive COVID-19 diagnosis on a PCR test and/or a lung computed tomography (CT) scan and/or suggestive clinical symptoms. Radiotherapy (RT) consisted of breast or chest wall irradiation with or without lymph node irradiation, with protocols adapted to pandemic situation. The treatment-related toxicity was graded according to the CTCAE (version 4.03). All 350 patients treated for early-stage BC were studied. Of them, 16 were presented with clinical symptoms of COVID-19 infection and of them, 12 had clinical, CT scan, and PCR confirmation. This entire cohort of 12 pts with median age of 56 (42–72) underwent their RT. During the radiotherapy, there were 9 pts presented radiation dermatitis, 8 (66%) were grade 1 and one was (8%) grade 2. Two patients with lymph nodes irradiation presented esophagitis grade 2. This prospective COVID-19 cohort, treated for early-stage BC demonstrated an acceptable toxicity profile with few low-grade adverse events. Longer follow-up is needed to confirm these findings.