Acute side-effects of radiation therapy (RT) are often underrecognized by the physicians of patients with breast cancer, according to the results of a patient-reported outcomes questionnaire completed by 9,941 patients with breast cancer who received RT following a lumpectomy at 29 practices from 2012-2020. Physician observations underrecognized patients’ frequent discomfort from swelling in 51.4% of cases, frequent discomfort from itching in 36.7%, moderate or severe pain in 30.9%, and severe fatigue in 18.8%. More than half (53.2%) of those who reported at least on substantial symptom among these categories had their symptom underrecognized by their physician. Demographic factors independently associated with symptom underrecognition included age younger than 50 versus age 60-69 (odds ratio [OR], 1.35), age 50-59 versus age 60-69 (OR, 1.21), black versus white race (OR, 1.92), and other versus white race (OR, 1.82). Also associated with differences in symptom reporting between patients and their physicians were conventional fractionation (OR, 1.15), supraclavicular field use (OR, 0.80), and treatment at an academic center (OR, 1.13).