Acceptable fetal dose using flattening filter-free volumetric arc therapy (FFF VMAT) in postoperative chemoradiotherapy of tongue cancer during pregnancy

•An optimal and safe irradiation method for pregnant woman remains controversial.•Flattening-filter free VMAT could reduce the fetal exposure significantly.•As adjuvant CCRT, 66 Gy to the involved nodes was delivered over 33 fractions.•Without any shielding device, the fetal dose of 0.03 Gy in total was estimated.•A vigorous liveborn girl was born vaginally and she had no complications until now.