Setup Uncertainties With Intensity-modulated Radiation Therapy For Nasopharyngeal Carcinoma

Purpose: Intensity-modulated radiation therapy (IMRT)in the treatment of nasopharyngeal carcinoma provides the opportunity to diminish normal tissue toxicity profiles and thereby enhance patient quality of life. However, highly conformal treatment techniques commonly establish steep dose gradients between tumor and avoidance structures. Daily setup variations can therefore significantly compromise the ultimate precision of idealized IMRT delivery. This study was to investigate the setup accuracy of thermoplastic masks used for immobilization of nasopharyngeal carcinoma patients treated by simultaneous integrated boost IMRT and to determine adequate margins to account for those uncertainties.Methods and Materials: 19 patients with early stage nasopharyngeal carcinoma(T_1-2N₀M₀) treated by fractionated intensity-modulated radiation therapy underwent repeated CTs during their 6-week treatment course. We evaluated anatomic landmark coordinates in a total of 85 repeat CT data sets and respective x,y,and z shifts relative to their positions in the 19 treatment-planning reference CTs.The total magnitude vector and the 95% confidence interval (CI )of isocenter motion were calculated. Additionally, margins for setup uncertainties around clinical target volume(CTV) and organs at risk(ORs) were assessed according to the proposed recipes. Each patient was weighted once every week in period of treatment,the effect of weight loss on the accuracy of masks was assessed too.Results:The translation in x,y,and z-axes with their mean values derived from both positive and negative setup errors was -0.45mm...