In advanced lung adenocarcinoma with metastases, the current standard of care does not in principle include aggressive cancer treatment with surgery and radiotherapy. Therefore, when chemotherapy cannot be continued, the patient is generally switched to palliative care. Our patient with stage IV lung adenocarcinoma in his 60s was receiving chemotherapy, which had to be discontinued due to severe side effects. As standard treatment was no longer indicated, he underwent radioimmunotherapy combined with WT1 dendritic cell vaccine therapy. As a result, the massive lung cancer shrank significantly and blood tests showed an improved immune profile. The growth of the lung cancer was suppressed, and the patient is completely symptom-free. After completing radioimmunotherapy, the patient continues to live a life similar to that of a healthy person. This case suggests that radioimmunotherapy can be useful as an active treatment in patients who are not eligible for standard treatment.