Study on the effects of different CT values assignment methods on dose calculation of brain metastases radiotherapy / 中华放射医学与防护杂志

ABSTRACT

Objective To study the effects of different CT values assignment methods on the dose calculation of radiotherapy plan for brain metastases,which will provide a reference for radiotherapy treatment planning based on MR images.Methods A total of 35 patients treated with radiotherapy for brain metastases were selected,with pre-treatment CT and MR simulated positioning performed at the same day.Based on the simulation CT 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 were calculated.Three groups of pseudo CT were generated by three CT values assignment methods based on the CT imageswhole tissue was assigned 140 HU;cavity,bone and other tissues were assigned-700 HU,700 HU and 20 HU,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.Results 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 from-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 than 5.0％,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 pixelwise dosimetric comparison,the areas with more than 1％ difference 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.Conclusions Different CT value assignment methods have a significant effect on the 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.