Characteristics of local extension based on tumor distribution in nasopharyngeal carcinoma and proposed clinical target volume delineation.

Objectives: To summarize the characteristics of local extension of eccentric and central nasopharyngeal carcinoma (NPC) by magnetic resonance imaging (MRI) and to improve clinical target volume (CTV) delineation.

Methods: MRI of 870 newly diagnosed NPC patients were reviewed. According to tumor distribution features, the NPCs were divided into eccentric and central lesions.

Results: All local invasions presented as continuous invasion from gross lesions and structures adjacent to the nasopharynx were more likely to be invaded. There were 240 (27.6%) and 630 (72.4%) cases with central and eccentric lesions, respectively. The spread of eccentric lesions was centered on the ipsilateral Rosenmüller's fossa; and most anatomic sites had significantly higher invasion rates in the ipsilateral side than the contralateral side (P < 0.05). However, they were at low risk of concurrent bilateral tumor invasion (<10%), except the prevertebral muscle (15.4%) and nasal cavity (13.8%). The extension of central NPCs was centered on the nasopharyngeal superior-posterior wall and was more common in the superior-posterior direction. Furthermore, bilateral tumor invasion into the anatomical sites was common.

Conclusion: Local invasion of NPC was characterized by continuous invasion from proximal to distal sites. The eccentric and central lesions showed different invasion features. Individual CTV delineation should be based on the distribution characteristics of tumors. The eccentric lesions had a very low probability of invasion into the contralateral tissue; thus routine prophylactic radiation of contralateral parapharyngeal space and skull base foramina may not be necessary.