To minimize cognitive decline without increasing the intracranial tumor recurrence rate by optimizing whole brain dose fractionation pattern when WBRT is combined with stereotactic irradiation (STI) in patients with no more than 4 brain metastases. JROSG 13-1 was a single-arm multi-institutional phase II study of RD-WBRT. At multiple institutions in Japan, eligible patients; no more than 4 brain metastases and Karnofsky Performance Status ≥ 60; received RD-WBRT of 25Gy in 10 fractions and STI. Contrast-enhanced MRI was taken at baseline and 4, 6, 9, and 12 months and every 6 months thereafter. Neurocognitive function tests including Hopkins Verbal Learning Test-Revised, Controlled Oral Word Association and Trail Making Tests, and health-related quality of life questionnaires were performed at baseline and 4, 8, and 12 months and every 6 months thereafter. The primary endpoint was the free from incidence of new intracranial metastases at 6 months and that analyzed by Kaplan-Meier method. The free from incidence of new intracranial metastases at 6 months in the WBRT+SRS group of JROSG99-1 was 81% (95%CI: 67.4-93.6%), so this study allowed a 10% difference, up to 71%. Twenty analyzable patients were required to ensure 80% statistical power with α = 0.05. If the survival rate at six months is calculated as 60%, the minimum required number of cases was 33, but the final required number of cases was set to 40, assuming some cases of dropout. Secondary endpoints were the neurocognitive function, the intracranial local control rate, the adverse events and overall survival. Regarding neurocognitive function, early cognitive failure was defined as reliable change index decline on one or more tests at 4 months. We enrolled 40 patients with no more than 4 brain metastases at multiple institution in Japan from April 2013 to October 2018. Since the observation period ended in November 2019, the data has not yet been fixed. This presentation is preliminary results up to 6 months. The mean age was 68.3 years, 60% of primary cancers were non-small cell lung cancer, and 20 patients had solitary brain metastasis. The primary endpoint, the free-from incidence of new intracranial metastases at 6 months, was 73.8% (95%CI: 60.4-90.2%). The frequency of early cognitive failure was 65.7% (95%CI: 50.6-80.8%). RD-WBRT may not compromise free-from intracranial tumor control. However, the effect of minimizing early cognitive deterioration at 4 months was modest. The analysis of the status of neurocognitive function at 8 months or later is now underway.