Objective: Intensity-modulated radiotherapy (IMRT) in nasopharyngeal carcinoma (NPC) demonstrated its advantages in dose-conformity,local control and normal tissue sparing.Geometrical accuracy,as one of prerequisites for these advantages,is highly required throughout the course of IMRT.Yet considerable geometrical variations might occur during 6-7 week-long IMRT,which resulted from many factors such as the volume,position changes of tumor and/or organ at risk (OAR),weight loss of patients,etc.The purpose of this study was to quantify the volumetric and positional variations of the target volume during IMRT.Material and methods: 20 patients (pts) with locally advanced NPC who received concurrent (13pts) or sequential (7pts) chemoradiotherapy were recruited prospectively.All patients underwent a planning CT and six repeat CTs (every five fractions) and each repeat CT was registered rigidly to planning CT respectively.Gross tumor volume (GTV) and elective clinical target volume (CTV) were delineated manually on each axial CT images.CTVs of primary tumor and lymph node were expanded with 5mm margin from corresponding GTV with necessary modification.The volume loss,system and random error,the mean and 3-dimensional vector displacement were calculated and compared statistically.Results: the volume of primary tumor,small (>1cm,≤3cm) and large (>3cm) positive neck lymph nodes decreased with a rate of 2.6%,3.7%,3.9% per treatment day.The CTV of primary tumor,lymph node,and elective region decreased 1.5%,2.3% and 0.3% per treatment day respectively.The average displacements of GTVs and CTVs in all directions were <1.3mm.The GTV,CTV of the large and small lymph nodes averagely shifted 0.8-1.3mm and 0.6-1.2mm to the medial direction respectively.The average 3-dimensional displacements of GTVs and CTVs were 3.4-4.3mm and 2.5-3.7mm respectively.The volume loss and displacements in most of directions of patients with concurrent chemoradiotherapy was significantly larger than those with sequential therapy.The volume loss and displacements of GTV of large node and elective CTV in males were significantly larger than in females.Conclusion: During chemo-IMRT for patients with NPC,both the volume and position of TV experience the significant changes throughout the treatment course and these variations show the obvious time trend.Volume reduction becomes larger and larger as the increase of radiation fraction delivered.The average 3D displacements of TV are of 2.5-4.3mm and the neck lymph nodes shift medially with a mean value of 0.8-1.3mm.It seems to be more reasonable to re-plan individually in the first three weeks and take various factors such as treatment modality,gender,the size and anatomical position,weight loss etc.,into account comprehensively.Treatment adaptation for anatomical changes is less needed in the presence of neo-adjuvant chemotherapy.And the impact on dose distribution resulting from anatomical variations and the optimal adaptive strategies remain the topics of further investigation.