Dosimetric Impact of Different Portals on Post-styloid-process Region in Def inite Radio-therapy for NPC

Objective 　To study the rationality of portal designation by evaluating dose dist ribution on post-styloid-process parapharyngeal space ( PSPS) in different t reatment planning for NPC. Methods 　Irradiated portals were designed base on DRR image from cont rast CT scan of an adult male NPC patient . All plans included section I and section II in which 36 Gy and 34 Gy would be delivered to the target respectively. Section I of each plan was the same with opposite lateral facial-cervical fields. For section II, posterior border was set beyond external auditory canal at 0. 5 cm for plan A, 1. 0 cm for plan B, 1. 5 cm for plan C, 0. 5 cm plus 16Gy of parapharyngeal space boost for plan E. In plan D, the posterior border of the lateral field was displaced anteriorly to shield the spinal cord, making a conedown facial-cervical field, while the posterior neck was supplemented with 12-MeV elect rons through small lateral fields. AcQPlan 4. 1. 1 was applied for dose calculation, DVH and section dose dist ribution evaluation. Results 　In plan A and B, some part of the PSPS was out side the irradiated fields. PSPS was included in plan C but the spinal cord received dose beyond limitation. Only plan D was safe and satisfied the clinical requirement . Parapharyngeal space boost as designed in plan E produced hot spot . Conclusion 　When PSPS was infilt rated, conedown facial-cervical field plus electron posterior neck boost should be taking into consideration.