Effects Of The Setup Error On The Dose Distribution Of Brainstem For Nasopharyngeal Carcinoma

Objective:To investigate the impact of patient setup accuracy to brain stem overdosage in advanced nasopharyngeal carcinoma irradiation by using cone-beam computed tomography(CBCT) online setup verification. And the relationship between weight changes and setup error has also been investigated.Methods:4patients had advanced nasopharyngeal carcinoma treated by intensity modulated radiotherapy(IMRT) were enrolled in this investigation as volunteers. Nasopharyngeal carcinoma irradiation could be improved by online setup verification and rigid image registration. Setup error of CBCT were acquired immediately before irradiation treatment then were analyzed as described above. CBCT scans were performed once before taking all fractions during the entire course of radiotherapy and recorded CBCT online-guided correction data. Then match every setup corrections which were acquired from each fraction to Radiation Treatment Planning System without changing radiotherapy dosage and recalculate Treatment Planning. In every radiotherapy plan, record the area where was more than163.64cGy dosage of the brain stem. Then count and superimposed the overdosage area of brain stem on the same level in every patient. The weight changes in patients during the treatment process were recorded, then analyzed relationships between the weight changes and setup error.Results:116sets of KVCBCT image were analyzed in4cases, and the Z/Y/X and Rtn data of the average daily CBCT errors were0.983±1.047mm,0.983±0.646mm,0.897±0.848mm and0.252°±0.487°without correction. The percentage of weight loss in patients was from11.67%to8.2%during the treatment process. And setup errors between CBCT and the weight loss did not show obvious statistical significance(P>0.05).Conclusions:Obtained setup error of CBCT proved that the errors of advanced nasopharyngeal carcinoma could lead to significant impact on brainstem dosage during the process of radiotherapy treatment. Patients who need radiotherapy treatment might be benefit from CBCT once a day.