Abstract: Objective To explore reasonable fractioned doses， optimized field and application time of split-filling and simultaneous boost irradiation（SF-SBI）in patients with locally advanced nasopharyngeal carcinoma（NPC）involving extensive，C-shaped invasion of the skull base，posterior cranial fossa and post-styloid parapharyngeal space. Methods Six patients with locally advanced NPC treated at the Cancer Center of Sun Yat-sen University between January 2004 and June 2008 were enrolled in this study. Different SF-SBI schemes were used for the patients，and dose volume histograms（DVHs）were used to analyze dosimetric characteristics of boost tumor volume（BTV），primary tumor planning target volume（PTVnx）and organs at risk （OARs）.Clinical efficacy and complications were e-valuated for each patient. Results The mean dose（Dmean）of BTV was 73.8~76.2 Gy and the target volume percentage contained by the 95% prescribed dose curve（V_95%）of BTV was 95.8%~99.9%.The Dmean and V_95% of PTVnx was 70.3~75.9Gy and 96.0%~99.9%，re-spectively.No volume received more than 80Gy for PTVnx and BTV.The V_95% of PTV1 and PTV2 was 98.8%~99.9% and 98.9%~99.9%，respectively.The volume percentage at 50 and 60Gy（V₅₀，V₆₀）were 13.2%~46.3% and 2.6%~12.8%，respectively；the maximum dose for 33% volume（D₃₃）of the brain stem was 23.8~53.9Gy.The Dmean，D₃₃ and V₆₀ of the ipsilateral temporal lobe were 23.8~32.2Gy，26.4~39.2Gy and 19.5%~25.7%，respectively.The temporal lobe percentage volume of patients A，B and C was 9.5%， 32.2% and 28.7%， respectively. The Dmean and D₅₀ of the ipsilateral temporo-mandibular joint were 69.1~76.2Gy and 62.2~69.8Gy，respectively. Follow-up time was 3.5 to 7 years. Except patient A，who showed partial remission（PR），the remaining patients showed complete remission（CR）during or after radiotherapy.None of the patients showed local tumor progression or tumor recurrence，and none showed signs of brain stem injury.Patients treated with a large fractioned dose（2.4~2.7Gy）suffered radiation encephalopathy，cranial nerve in-jury or severe trismus. Conclusion This study describes an improved SF-SBI conformal radiotherapy technique for personalizing treatment of patients with locally advanced NPC.The fraction dose in this technique is 2.3Gy for BTV and most of GTV from the beginning of radiotherapy.In this approach，the radiotherapy field can be reduced based on observed regression of individual tumors.

Key words: Nasopharyngeal neoplasms/radiotherapy, Simultaneous boost irradiation, Dosimetry