Fig. 1

Illustration of target delineation in plan-I and plan-II. Magnetic resonance imaging (MRI)-I acquired before plan-I radiotherapy with cross-sectional T2-weighted images, T1-weighted images, and contrast-enhanced T1-weighted (T1 + C) images shows the primary tumor, which locates on the left side of the upper wall and extends into the nasal cavity, left medial pterygoid plate, and navicular fossa. In plan-I radiotherapy, the gross target volume of primary tumor (GTVnx-I) was outlined (red line). Clinical target volume 1-I (CTV1-I) (green line) is delineated with a 1.0-cm margin surrounding GTVnx-I. Clinical target volume 2-I (CTV2-I) (blue line) is delineated with a 0.5-cm margin surrounding CTV1-I. MRI-II acquired after 22 fractions of irradiation shows that the tumor greatly regressed. In plan-II radiotherapy, the residual tumor is delineated as GTVnx-II (red line). The regressing areas of the intracavitary area, the left medial pterygoid plate, and the navicular fossa lesion are included not in GTVnx-II but in CTV1-II (green line), which maintains the same as CTV1-I. CTV2-II is not delineated. GTVnx-I was copied to the CT-II for comparison (purple line)