Extension of Local Disease in Nasopharyngeal Carcinoma Detected by Magnetic Resonance Imaging: Improvement of Clinical Target Volume Delineation

Purpose: To define by MRI the local extension patterns in patients presenting with nasopharyngeal carcinoma (NPC) and to improve clinical target volume delineation. Methods and Materials: Consecutive patients (N = 943) with newly diagnosed and untreated NPC were included in this study. All patients underwent MRI of the nasopharynx and neck, which was reviewed by two radiologists. Results: According to the incidence rates of tumor invasion, the anatomic sites surrounding the nasopharynx were initially classified into three risk grades: high risk (≥ 35%), medium risk (≥ 5-35%), and low risk (< 5%). Incidence rates of tumor invasion into anatomic sites at medium risk were increased, reaching 55.2%, when adjacent high-risk anatomic sites were involved. However, the rates were substantially lower, mostly < 10%, when adjacent high-risk sites were not involved. The incidence rates of concurrent tumor invasion into bilateral sites were < 10%, except in the case of prevertebral muscle involvement (13.1%). Among the 178 incidences of cavernous sinus invasion, there were often two or more simultaneous infiltration routes (60.6%); when only one route was involved, the foramen ovale was the most common (26.4%). Conclusions: In patients presenting with NPC, local disease spreads stepwise from proximal sites to more distal sites. Tumors extend quickly through privileged pathways such as neural foramina. The anatomic sites surrounding the nasopharynx are at low risk of concurrent bilateral tumor invasion. Selective radiotherapy of the local disease in NPC may be feasible.