Fig. 1

Recommended clinical target volume according to the percentage of local recurrence in cases of early and locally advanced stage of nasopharyngeal carcinoma. A–H Demonstrated the risk of local recurrence of each anatomic structure and recommended delineation of clinical target volume (CTV) in T1–T2 case with nasopharyngeal carcinoma, at the level of cranial border of C2, midpoint of C1, hard palate, midpoint of maxillary sinus, foramen lacerum, great wing of sphenoid bone, inferior orbital fissure and midpoint of orbit. I–P Demonstrated the risk of local recurrence of each anatomic structure and recommended delineation of clinical target volume (CTV) in T3–T4 case with nasopharyngeal carcinoma, at the level of cranial border of C2, midpoint of C1, hard palate, midpoint of maxillary sinus, foramen lacerum, great wing of sphenoid bone, inferior orbital fissure and midpoint of orbit. The color bar showed the risk of local recurrence. The pink and red line showed the iso-risk line of 15% and 30% of local recurrence. In our study, the iso-risk line of 15% was selected as the recommended contouring of CTV. It was noteworthy that lateral pterygoid muscle, posterior ethmoid sinus, inferior orbital fissure and cavernous sinus were not included in the delineation of CTV in early-stage disease, but were encompassed by CTV in locally advanced disease