鼻咽癌常规外照射放疗不同设野茎突后区剂量分析

  目的 探讨鼻咽癌(NPC)常规外照射不同设野时茎突后区的剂量学特点。方法 在1例NPC增强CT-SIM扫描后数字重建矢状位图像上，按目前常用的几种照射方法设野，I段采用面颈联合野，剂量．36Gy；Ⅱ段剂量34Gy，方法A、B、C均采用面颈分野，耳前野后缘分别位于外耳道后缘后0．5cm、1．0cm和1．5cm，方法D采用面颈联合缩野(后界避开脊髓)+颈后12MeV电子线野，方法E在方法A完成Ⅰ、Ⅱ段放疗后补充咽旁野16Gy。通过积分剂量体积直方图(DVH)及截面剂量分布图比较不同设野对茎突后区剂量的影响。结果 方法A、B部分茎突后区在Ⅱ段治疗中漏出照射野外，方法C有较多脊髓超过耐受量照射，方法D可满足临床剂量要求，方法E则靶区存在剂量热点。结论 NPC茎突后间隙肿瘤侵犯时应提倡使用方法D照射技术。

Dosimetric Impact of Different Portals on Post-styloid-process Region in Definite Radiotherapy for NPC

Objective To study the rationality of portal designation by evaluating dose distribution on post styloid process parapharyngeal space (PSPS) in different treatment planning for NPC. Methods Irradiated portals were designed base on DRR image from contrast 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 16 Gy 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 electrons through small lateral fields. AcQPlan 4.1.1 was applied for dose calculation, DVH and section dose distribution evaluation. Results In plan A and B, some part of the PSPS was outside 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 infiltrated, conedown facial cervical field plus electron posterior neck boost should be taking into consideration.