Influence of tumor volume change on intensity modulated radiotherapy for nasopharyngeal carcinoma analyzed by repeated CT simulation

Objective To analyze the influence of tumor volume change on intensity modulated radiotherapy(IMRT)for nasopharyngeal carcinoma by analyzing the repeated CT simulation.Methods Twenty nasopharyngeal carcinoma patients undergoing IMRT from July 2011 to November 2012 were selected in the study.The first CT simulation was conducted prior to radiotherapy,and the repeated CT simulation was finished after radiotherapy of 30 Gy.The first and the repeated CT images were fused in treatment planning system and GTV volume shrink rate was calculated.The original plan was used to recalculate the dose distribution on repeated CT.The dose volume histogram was used to calculate the dose difference of organs at risk including the brain stem and spinal cord.Results Compared with the first CT,GTV volume shrink rate of the repeated CT simulation was 28.7％,the maximum dose,1 cm3 volume and the average dose percentage of the brain stem and spinal cord were increased(t=0.83-3.17,P＜0.05).Conclusions GTV volume shrinked significantly after radiotherapy of 30 Gy in IMRT for nasopharyngeal carcinoma.The dose of the organs at risk increased accordingly. Key words: CT simulation; Intensity modulated radiotherapy; Nasopharyngeal carcinoma; Organ at risk