3-D Dose Distribution Difference Between Alloy-block And MLC In Nasopharyngeal Carcinoma Patients Undertaking Conventional Radiotherapy

Objective:The goal of this study was to evaluate the dose distribution of conventional radiotherapy for patients with nasopharyngeal carcinoma,using multiple leave collimator(MLC) or alloy-block as the individual shielding.The aim was to investigate the possibility of using MLC as an alternative of alloy-block in conventional radiotherapy for patients with nasopharyngeal carcinoma.Method:Ten patients with nasopharyngeal carcinoma were chosen in the study.Distribution according to T stage was as follows:T1-2,5 cases;T3-4,5 cases(UICC 2002 staging system).CT images were gotten and transferred to TPS,then gross tumor volume (GTV),clinical tumor volume(CTV) and critical risk organs such as brain stem,spine cord,optical nerves were contoured by radiation oncologist specialized in head and neck disease.Conventional facial-cervical field were adopted following with shrinking fields.The dose prescribed to GTV was 70Gy/35F,and 60Gy/30F to CTV.Individual block margin was drawn to avoid the critical risk organs while maximize the coverage of the tumor target.Thereafter,alloy-block and MLC(in the way of"mid-point"and "maximum") were used as individual shielding.Then the 3-D dose distribution of all regions of interest was calculated by the TPS(Pinnacle 3 V7.6) and the dose distributions between different shielding ways were compared.Results:The dose distributions of the tumor targets and critical risk organs between 3 different shielding ways had no statistic difference.Conclutions:MLC can be used as individual shielding as effectively as alloy-block in the conventional radiotherapy of patients with nasopharyngeal carcinoma.