Background: Stereotactic radiotherapy (SRT) is a less invasive, widely accepted treatment for brainmetastases, particularly in patients with poor general condition. However, large cystic metastases can complicate radiotherapy because of their morphology. This study evaluated the efficacy and safety of continuous cyst drainage under natural pressure combined with fractionated SRT (f-SRT) for these lesions. Methods: We retrospectively reviewed patients treated at Yokohama Rosai Hospital betweenAugust 2022 and February 2024. Eligible patients had cystic brain metastases ≥10 cm³ and underwent cyst drainage under local or general anesthesia, followed by f-SRT (35 Gy in 5 fractions). Outcomes included Karnofsky Performance Status (KPS), tumor and cyst volumes, local control, and neurological symptoms. Results: Four patients (one male, three females; median age, 52 years) were analyzed. Primarycancers were lung (n=1), breast (n=1), breast and lung (n=1), and esophageal (n=1). After drainage, median KPS increased from 65 to 70, and neurological symptoms improved in three cases. Tumor volume decreased from 35.6 cm 3 to 12.4 cm 3 (65.5% reduction). No treatment-related complications or radiation-induced adverse events were observed. Conclusion Continuous cyst drainage under natural pressure combined with f-SRT appears safeand effective for large cystic metastases, enabling continuation of f-SRT in patients who are not suitable candidates for resection and providing a minimally invasive treatment option.