Fig. 1

The delineation of PTV and NTV. a Axial T2-weighted image illustrating that the primary tumor and retropharyngeal lymph node involvement are located close together, making it difficult to distinguish between them. b Control CT image showing the PTV, including the primary tumor and retropharyngeal lymph node involvement, which was delineated according to the pretreatment MRI shown in Fig. 1a. c Axial T2-weighted image of neck lymph nodes with extracapsular spread, which was diagnosed on the basis of an irregular border and invasion into the adjacent fat and muscle. d Control CT image showing the NTV including metastatic cervical lymph nodes and nodal extracapsular spread, which was delineated according to the pretreatment MRI shown in Fig. 1c