Background: The use of a hypofractionated 15–16 fraction radiotherapy course replaced the standard fractionated whole breast irradiation (SF–WBI) more than 10 years ago, resulting in shorter waiting lists, lighter machine loads, and more patient compliance. Objectives: Assessment of Arm Lymphedema in breast cancer patients postmastectomy treated with adjuvant single-weekly hypofractionated radiotherapy to the whole breast. Aiming to improve the lifestyle and reduce the suffering of cancer patients. Patients and ethods: Adjuvant RTH to the chest wall was given to 30 post-mastectomy women who had infiltrating duct carcinoma of the breast that was histologically confirmed. The dose was 30 or 28.5 Gy in 5 fractions given once a week at a dose of 6.0 or 5.7 Gy. All patients were assessed for ipsilateral arm lymphedema by monitoring the arm circumference on both sides before radiation treatment and at 3, 12, and 24 months after radiation treatment. Results: The incidence of lymphedema grade 0 was (93.3%) in the study group before radiotherapy; at 12 and 24 months after the end of radiotherapy, it was (83.3%) and (80%), respectively (P = 0.044). Grade I was noted in 2 patients (6.66%) before RTH and in 4 patients (13.3%) at 24 months of follow-up (P = 0.682). One case (3.33%) showed grade 2 lymphedema at 3 months after radiation treatment, and 2 cases (6.66%) after 24 months of the end of RTH (P = 0.194). Conclusion: Breast cancer patients can achieve satisfactory results in terms of dosimetric parameters and lymphedema grades by receiving once-weekly whole breast irradiation.