Study On Dose Evaluation In Intensity-modulated Radiotherapy Guided By Multimodal Images For Nasopharyngeal Carcinoma

During intensity-modulated radiotherapy（IMRT）for nasopharyngeal carcinoma（NPC）,the volume of the target volume and the organs at risk（OARs）will change constantly,which may lead to the inconsistency between the actual dose received and the dose evaluated in the initial plan.Accurate dose assessment is critical to ensure that the tumor target volume is dosed adequately and to reduce the dose of organs at risk.Magnetic resonance imaging（MRI）has obvious advantages in identifying soft tissues of NPC,it has been applied to the simulation positioning of radiotherapy and the online dose assessment,but MRI is lack of electron density information to calculate the dose.MRI and computed tomography（CT）registration are used to contour the target volume in clinical applications,but the errors generated during the registration process can seriously affect the accuracy of radiotherapy for NPC.Using MRI as the only imaging method to guide radiotherapy for NPC can eliminate this error,simplify workflow and reduce radiation exposure,so it will be a trend to make IMRT plans based on MRI.Therefore,the main content of this study is divided into two parts:The first part is the study of dose assessment of NPC during IMRT based on deformable image registration（DIR）.In this study,18 patients with NPC were selected.All patients underwent simulation CT and replanning CT scan after 20-25 fractions of radiation therapy.The treatment plans designed on simulation CT and replanning CT were called Plan_1st and Plan_2nd,respectively.The Planreg and Plandef were obtained from Rigid image registration and DIR of Plan_2nd and Plan_1st are carried out by Velocity software.The dosimetric differences between different plans were compared.The results showed that compared with simulation CT,the volume of the tumor target volume and the left and right parotid glands in replanning CT were reduced by 20.8%,36.8%,and 37.5%,respectively.Compared with Plan_1st,the dose and volume indexes of OARs in Plan_2nd decreased significantly（decrease 17.0%-60.1%on average）.the dose and volume indexes of tumor target volume and OARs in Plandef decreased 0.3%-4.9%.Studies have shown that the volume of tumor target volume and parotid gland shrinks during IMRT for NPC,and secondary planning can reduce the radiation dose of OARs.Dose accumulation using DIR technology can more accurately assess the dose volume index of e target volume and OARs during IMRT for NPC.The second part is the study on designing the plan of IMRT for NPC based MRI-only using the CT value assignment method.In this study,32patients with NPC were selected.Populate CT values were obtained by contouring and analyzing tissue of patients including bone,air and soft tissue on their simulation CT.Pseudo CT were generated by different assignment methods based on simulation CT:（1）CT1:CT value of all tissues was set to0HU;CT2:CT value of air cavity was set to Populate CT value based on CT1;CT3:CT value of Bone was set to Populate CT value based on CT2;CT4:CT value of each soft tissue were set to Populate CT value based on CT3.The plan of IMRT as Plan0 was designed base on simulation CT.Then Plan0 was transplanted to four pseudo CT to recalculate the dose,and Plan1,Plan2,Plan3 and Plan4 were obtained and difference of dosimetric parameters were compared with Plan0.Plan of IMRT for NPC was designed base on MRI-only by using the assignment method with CT value of each tissue were set to Populate CT value.The results show that the difference of dosimetric indexes of Plan1,Plan2,Plan3,Plan4 decreased in turn compare to Plan0.The areas with great difference in point-to-point dose were mainly distributed at the junction of soft tissue with air cavity and bone,as well as the skin area through which the field passes.Studies have shown that the assignment method of each tissue and organs were set to Populate CT value compared with other methods has the least influence on the dose calculation of plan IMRT for NPC（The dose deviation is controlled within 1.3%）,which could meet the clinical requirements.So it can be used as the first method of assignment when designing plan of IMRT for NPC based on MRI.