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Objective Radiation treatment enables a better local control and lower radiation-related toxicities.Also,various dose fraction and total treatment days (TTD) were widely used.This study is to investigate the optimal dose-fractionation of intensity-modulated radiotherapy in head and neck squamous cell carcinoma (HNSCC).Methods To keep the radiation dose of 70Gy or biological effect dose (BED) of 84Gy10 with the linear-quadric function,we calculated the tumor,early response (mucosal),late response BED and the tumor log10 cell kill with different fraction number and/or dose to find out the optimal fractionation.The tumor log10 cell kill was calculated by the function:Log10cell-kill=α×BEDtumor/2.303(α=0.35Loge/Gy),in which BEDtumor was calculated by the linear-quadric function with α/β ratio of 10Gy.Results with 33 fractions,the fraction dose and total radiation dose varied from 2.12-2.30Gy and 70.0-75.9Gy respectively and the tumor,mucosal and late effect BED were 69.6Gy-78.2Gy10,55.5-64.1Gy10,and 119.4-129.5Gy3 respectively and the tumor log10 cell kill was 10.6-11.9.While keeping the physical radiation dose of 70Gy or BED of 84Gy10,the fraction dose ranged from 2.20-2.80Gy with 25-35 fractions and TTD was 32-46,the tumor,early and late response BED were 67.5-82.3Gy10,53.1-69.8Gy10,and 113.5-119.8Gy3 respectively and the tumor log10 cell kill was 10.3-12.5.Taking the tumor,early,late response BED and tumor cell log10 kill into comprehensive consideration,the regimen with 30 daily IMRT fractions can balance the tumor control and radiation-related toxicity well.For every a single delivered day,the tumor BED decreased 1.4?% (0.8Gy10) and the tumor log10 cell kill did 0.1.Conclusion the optimal dose-fraction in IMRT of HNSCC is theatrically 30 daily fractions in six weeks.And the tumor BED will decrease as the prolonged overall treatment days.