Study On The Correlation Of Intracranial Tumor Volume With Dose And Activity Of Iodine-125 Seed

Objectives A self-developed intracranial tumor model was used to explore the correlation between the volume of intracranial tumor and the activity and number of iodine-125 seed implanted under a specific prescription dose and distribution pattern.Preliminary imaging techniques were used for step verification,dose distribution of target area and the dose of the organ at risk were observed.Methods The intracranial tumors were simulated by placing rubber balls with diameters of 2.0cm,2.5cm,3.0cm,3.5cm,4.0cm,4.5cm and 5.0cm in human brain models developed by ourselves,and then Computed tomography（CT）was performed.Save CT scan images as Digital imaging and communications in medicine（DICOM）,The DICOM format was then fed into 3D-Treatment planning system（3D-TPS）for preoperative Planning.Two senior physicists performed the preoperative simulation plan,and radiotherapists with more than 5 years of clinical experience selected the better plan.The range of Body and Gross target volume（GTV）of intracranial tumor were delineated in3D-TPS.The prescribed dose of tumor was set at 120Gy,and the single-hole fan-shaped needle was adopted.The target areas of each tumor size were loaded with iodine-125 seeds with 0.5m Ci,0.6m Ci,0.7m Ci and 0.8m Ci activity respectively.The distribution sources were adjusted according to Dose volume histogram（DVH）and isodose lines.When Dose90(D₉₀)in DVH diagram reached the prescribed dose,the source was stopped,the tumor volume was calculated,and the number of implanted seeds was recorded at this time.The scattergrams of the two groups of variable values of seed number and tumor volume were drawn,and the data analysis and curve fitting were performed with SPSS 25.0 statistical software.The fitting curves of the relationship between seed number and tumor volume under each seed activity were drawn to explore the relationship between seed number and tumor volume.The data of 20 patients with single intracranial tumor admitted to Hebei General Hospital from September 2020 to September 2021 were collected,and the head CT images were saved into DICOM format and imported into 3D-TPS.The Body,GTV and Brainstem ranges were delineated in 3D-TPS by two experienced physicists according to the prescribed dose,seed activity and needle placement method as described above.The better plan shall be selected by a radiotherapist with more than 5 years of clinical experience.The tumor volume of each patient was calculated.The number of seeds required,Conformity index（CI）,External index（EI）and Homogeneity index（HI）were recorded under different seed activity.The shortest linear distance,Dose max(D_max)and D₉₀ of the brainstem to the edge of the target area were recorded when the seed activity was 0.8m Ci.The scattergrams of the two groups of variable values of seed number and tumor volume were drawn.The data analysis and curve fitting were performed with SPSS25.0 statistical software.The curve fitting was carried out for the two variables of seed number and tumor volume,and the relationship curve and corresponding curve formula of the two variables were obtained.Further study was conducted to further explore that when the prescription dose was 120Gy,four kinds of activity seeds（0.5m Ci,0.6m Ci,0.7m Ci and 0.8m Ci）were used as a function of the number of seeds and tumor volume.The dose distribution of tumor target was evaluated according to the above dose parameters.Through bioequivalent dose conversion,the maximum tolerated dose of brainstem in external radiotherapy was converted into the maximum tolerated dose of brainstem implanted with seeds,to evaluate whether the D_max value of brainstem exceeded its maximum tolerated dose,and to explore the safety of the preoperative plan simulated by seed implantation.Results 1 When the prescribed dose was 120Gy and the seed activity was 0.5～0.8m Ci,the R² values of the quadratic function and the cubic function in the curve fitting were equal and highest,the R² value of cubic function was the highest when the activity was0.7m Ci.When the seed activity of 0.5～0.8m Ci was selected,the tumor prescribed dose was 120Gy,the relationship between the number of intracranial tumor seeds and the volume of tumor in iodine-125 treatment showed a cubic function trend.2 When the prescribed dose was 120Gy and seeds of 0.5～0.8m Ci were used,the curve fitting results of seed number and tumor volume in 20 patients with intracranial tumors showed that R²value of cubic function was the largest,the fitting degree is the highest,which is in good agreement with the relationship curve obtained by the model.It is preliminarily believed that when the prescribed dose is 120Gy and the seed activity is 0.5～0.8m Ci,the accuracy of iodine-125 implantation for intracranial tumor seed number Y and tumor volume V is a cubic function.The relation between them is as follows:1)When the activity is 0.5m Ci,Y=0.000245 V³-0.028 V²+1.915 V+3.718,2)When the activity is 0.6m Ci,Y=0.000256V³-0.026 V²+1.615 V+3.059,3)When the activity is 0.7m Ci,Y=0.000109 V³-0.013V²+1.226 V+3.088,4)When the activity is 0.8m Ci,Y=0.000187 V³-0.0199 V²+1.283V+1.760.3 After bioequivalent dose conversion,the maximum tolerated dose of brainstem in brachytherapy was 78.90Gy.In all patients implanted with 0.8m Ci seeds,the brainstem D_max value was less than 78.90Gy.The maximum D_max value was 72.00Gy when the brainstem was 0.50cm away from the edge of the tumor target,the target D₉₀ was124.00Gy,and the tumor volume was 48.89cm³.When the brainstem was 2.50cm away from the tumor target,the target D₉₀ was 120.00Gy,and the tumor volume was 38.30cm³,the brainstem D_max was the lowest,15.80Gy.4 CI and HI were normally distributed（P>0.05）,CI was（0.76±0.06）,HI was（29.75±3.79）%.EI obeys skewness distribution（P<0.05）,represented by 0.17（0.13,0.21）.Conclusions 1 With human intracranial tumor model and clinical data,the cubic function relationship between tumor volume and the number of iodine-125 implanted seeds was determined under a given radioactivity by fitting dosimetric parameters and planning system.This can be used as a clinical reference for preoperative planning of iodine-125implantation.2 Intracranial tumor seeds can be implanted by adjusting the distribution mode and optimizing the dosimetric parameters to accurately avoid organ at risk dose tolerance,so as to ensure the safety.3 Fan-shaped needle pattern can reduce the damage to the vital tissues in the skull and brain,which,however,will have an effect on the dose uniformity in the the target area.Figure 21;Table 5;Reference 132...