Study On Dose Accumulation And Evaluation Of Radiotherapy In Locally Advanced NSCLC Patients With Brain Metastasis

Lung cancer is the malignant tumor with the highest morbidity and mortality in China.80%of lung cancer patients are non-small cell lung cancer?NSCLC?.The initial development of NSCLC is slow and difficult to detect.Most of the patients have already metastasized when receiving treatment.Brain metastasis is one of the common metastases.As a secondary malignant tumor,brain metastases have the characteristics of rapid onset,short course of disease and poor curative effect.Radiotherapy is one of the main treatment methods for NSCLC and its brain metastases.The key to ensure the accuracy and efficiency of radiotherapy lies in the accurate delineation of tumor targets and organs at risk?OARs?and accurate dose prediction.In the course of radiotherapy in patients with NSCLC,the tumor target is constantly retreating,especially in large volume locally advanced NSCLC patients.It is usually necessary to relocate the tumor after a period of radiotherapy and modify the radiotherapy plan accordingly.Because the one-course radiotherapy plan and the modified two-course radiotherapy plan are based on different CT images,the dose accumulation can not be carried out directly,which brings a difficult problem to accurately predict the dose of tumor target and OARs.For this reason,we studied the dose accumulation of locally advanced NSCLC two-course radiotherapy planning based on rigid registration and deformation registration.Thirty patients treated with IMRT for locally advanced NSCLC with twice 4DCT scans acquired before radiotherapy and after 20 fractions of radiotherapy.The initial treatment plan?Plan₁?based on the average density projection CT(CT_1-avg)of the first 4DCT images and the second treatment plan?Plan₂?based on the average density projection CT(CT_2-avg)of the second 4DCT images were calculated.Then,the dose distribution of Plan₂and the dose distribution of Plan₁were accumulated by rigid registration accumulation and deformation registration accumulation to obtain Plan_rigand Plan_def,respectively.Finally,the volume changes of GTV between CT_1-50and CT_2-50and organs at risk?OARs?between CT_1-avgand CT_2-avgwere compared,as well as the dose-volume parameters among Plan₁,Plan₂,Plan_rigand Plan_def.Compared with CT₁,the mean volume of GTV and heart on CT₂decreased by 44.2%and 5.5%,respectively,while the mean volume of ipsilateral lung,contralateral lung and total lung increased by5.2%,6.2%and 5.8%,respectively.The differences of above volume parameters between CT₁and CT₂were statistically significant.Compared with Plan₁,the D₉₅,D₉₈and V_100%of target volume IGTV and PTV in the Plan₂didn't change significantly,and that in Plan_rigand Plan_defdecreased slightly.The dose-volume parameters of spinal-cord,heart,ipsilateral lung and total lung in the Plan₂,Plan_rigand Plan_defwere significantly lower than Plan₁.Among them,the V₃₀and D_meanof heart decreased by27.3%,16.5%,15.3%and 15.2%,6.6%,5.6%,respectively;The V₂₀and D_meanof total lung decreased by 15.6%,4.5%,3.7%and 15.7%,6.2%,5.1%,respectively.Some dose-volume parameters(including D₉₅and D₉₈of target volume,V₄₀of heart,V₂₀and D_meanof the ipsilateral lung and the total lung)of Plan_defwere slightly higher than the Plan_rig.The Dice similarity coefficients?DSCs?of OARs after deformation registration were significantly higher than that after rigid registration?P<0.05?.The results show that in the IMRT of patients with locally advanced NSCLC,the dose-volume parameters of the one-course plan can well reflect the target dose,but will overestimate the dose of OARs.The dose-volume parameters of accumulation dose obtained by rigid registration or deformation registration of two plans can better predict OARs dose,and the dose-volume parameters obtained by deformation registration are better.Patients with NSCLC brain metastasis often need radiotherapy,but conventional CT simulation localization has the problem of unclear boundary of brain metastases,which is a difficult problem of accurate radiotherapy for brain metastases.MRI images have better soft tissue resolution and can be used to accurately outline the boundaries of brain metastases and soft tissue organs,but MRI images lack of electronic density information and can not be directly used in radiotherapy planning.In order to solve this problem,we propose three methods to assign CT values to MRI images,and verify the accuracy of the three CT value assignment methods in dose calculation.In this study,on the basis of 21 patients with NSCLC brain metastases who received brain radiotherapy in the previous part,14 patients with NSCLC brain metastases were further expanded into the group,a total of 35 patients with brain metastases treated with radiotherapy.Each patient underwent CT and MRI simulated localization on the same day before radiotherapy.Based on the CT simulation positioning images,three dimensional conformal radiation therapy?3D-CRT?or intensity modulated radiation therapy?IMRT?plans were calculated as the original plan?Plan1?.The CT and MR images were rigidly registered and then the main tissues and organs were delineated on CT and MR images.The average CT values of each tissue and organ was calculated.Three groups of pseudo CT were generated by three CT values assignment methods based on the CT images:whole tissue was assigned 140HU;cavity,bone and other tissues were assigned-700HU,700HU and 20HU,respectively;different tissues and organs were assigned corresponding CT values.The dose distribution of Plan1 was recalculated on three groups of pseudo-CT to obtain Plan2,Plan3 and Plan4,respectively.Finally,the dosimetric difference between Plan1 and other plans?including Plan2,Plan3 and Plan4?were compared.The average CT values of bone and cavity were?735.3±68.0?HU and?-723.9±27.0?HU,respectively.The average CT values of soft tissues was mostly distributed in-70 to 70 HU.The dosimetric differences between Plan2,Plan3,Plan4,and Plan1 decreased in turn.The differences of maximum dose of lens were the biggest,which can reach more then5.0%,1.5-2.0%and 1.0-1.5%,respectively,and the differences of other dose parameters were basically less than 2.0%,1.2%and 0.8%,respectively.In the pixel dosimetric comparison,the areas with a difference of more than 1%in the local target cases were mainly distributed in the skin near the field.On the other hand,those in the whole brain target cases were mainly distributed at the bone,cavity,bone and soft tissues junction,and the skin near the field.In addition,the dose calculation error of CT value assignment methods in 3D-CRT plan was slightly larger than that in IMRT plan,and that in whole brain target cases were significantly larger than that in local target cases.Different CT value assignment methods have a significant effect on the planned dose calculation of radiotherapy for brain metastases.When appropriate CT values are given to bone,air cavity and soft tissue,respectively,the deviation of dose calculation can be basically controlled within 1.2%.And by assigning mass CT values to various tissues and organs,the deviation can be further controlled within 0.8%,which can meet the clinical requirements.Conclusion:In the two courses radiotherapy of NSCLC patients,the dose index of one-course planning can be used to predict the target dose,while for OARs dose,the dose index after deformation matching dose accumulation is more accurate.In the radiotherapy of brain metastases,radiotherapy planning based on MRI images can not only more accurately outline the target area and soft tissue organs,simplify the radiotherapy process,but also have sufficient dose calculation accuracy.With the development of MRI simulation positioning and MRI image-guided radiotherapy technology in more and more units,radiotherapy planning based on MRI images will have more and more broad prospects.