The patient was a 73-year-old woman. She had been treated for squamous cell carcinoma of the lung (cT3N3M0, Stage IIIC) at our department. The patient had low back pain due to retroperitoneal lymph node metastasis; in June 2022, this was exacerbated as lung cancer progressed. She had difficulty in body movements due to edema in both lower limbs, in addition to the pain. Consequently, she was urgently admitted on July 8 and received radiotherapy (30 Gy/10 fractions) for retroperitoneal lymph node metastasis. She was being given tapentadol at a dose of 200 mg/day for relief of her pain. However, she was switched to fentanyl patch at a dose of 1200 µg/day during her hospitalization, which resulted in relief of low back pain. The underlying disease causing the edema was investigated. Based on physical and laboratory findings and medical history, lymphedema associated with retroperitoneal lymph node metastases was diagnosed. On day 31 of hospitalization, the patient was allowed to be temporarily discharged from the hospital because the edema had improved and the activity of daily living around the bed had increased. Treatment methods for lymphedema associated with lymph node metastasis have not been established, but the efficacy of radiotherapy has been reported. We have herein reported a case of lymphedema that was improved by radiotherapy after it was differentiated from other diagnoses.