The number of patients with head and neck squamous cell carcinoma (HNC) with mediastinal involvement is small, and appropriate treatment techniques have not been widely discussed. This study aimed to compare the efficacy of radiotherapy planning techniques in reducing lung irradiation while retaining target coverage. Among all HNC patients with mediastinal involvement who underwent radiotherapy between 2007 and 2014 at our hospital, seven consecutive patients were included in this study. Four different treatment plans were generated for each patient as follows: seven-field intensity-modulated radiation therapy (IMRT), modified IMRT in which the lateral beams avoided the lungs, three-full-arc volumetric-modulated arc therapy (VMAT), and VMAT with lung avoidance. We compared the outcomes of IMRT and VMAT plans using the paired t-test. After modifications were made to avoid lung irradiation, IMRT values for V5Gy and V20Gy decreased from 713.2 to 503.6 cm3 (p = 0.011) and from 338.8 cm3 to 267.0 cm3 (p = 0.058), respectively. In the case of VMAT, lung V5Gy and V20Gy values decreased from 754.8 to 601.0 cm3 (p = 0.004) and from 328.5 to 255.7 cm3 (p = 0.020), respectively. Other factors did not significantly differ between the plans. In both IMRT and VMAT planning, lung doses were significantly reduced following the modification of the beams that cross the lungs with target coverage maintenance.