Objective To evaluate the dose distribution and clinical efficacy of hippocampal-sparing prophylactic cranial irradiation ( HS-PCI ) in patients with small cell lung cancer by using helical tomotherapy. Methods Clinical data of 49 patients with small cell lung cancer receiving HS-PCI using helical tomotherapy in Cancer Hospital between 2014 and 2017 were retrospectively analyzed. All patients received brain MRI to exclude the possibility of brain metastasis within 1 month after standard surgery or radio-and chemo-therapy. The prescription dose was 95% PTV,25 Gy in 10 fractions. The adverse reactions and cognitive functions of patients were observed before,6 months and 1 year after treatment,and the dose distribution in the hippocampal gyrus,survival rate and brain metastasis rate were analyzed. Results The median follow-up time was 16 months. The average dose in the hippocampal gyrus was 7. 23 Gy and 8. 46 Gy in the reduction region,which was reduced by 71. 88% and 66. 16% compared with the prescription dose. The maximum dose in the hippocampal gyrus was 10. 66 Gy and 15. 43 Gy in the reduction region. Among 49 patients,8 died,the 1-year survival rate was 85. 1% and the 2-year survival rate was 70. 3%.Nine patients (18. 3%) had brain metastases,and one of them with extensive multiple brain metastases (n＝13) presented with metastasis adjacent to the hippocampal gyrus. The main adverse reactions included mild headache, dizziness and brain edema,whereas no ≥ grade 2 adverse reactions occurred. At 6 months after treatment, the HVLT-R score was significantly decreased,and declined by 6. 78% at 12 months after treatment. The HVLT-R scores did not significantly differ in patients without brain metastasis before and 12 months after treatment ( P＞0. 05 ). Conclusion Application of HS-PCI using helical tomotherapy meets the dose requirement,effectively protects the cognitive function and yields slight adverse reactions.