A Dosimetric Research Between Peripheral And Central Distribution Of ¹²⁵I Seeds In Tumor Therapy

125I seeds interstitial implantation with the characteristics of accurate position, highly comformal dosimetric distribution and dose in tumor target, low dose level of normal tissues, few complications, and easy to protect,was widely used in prostate tumor,lung tumor,pancreatic tumor, liver tumor and other malignant tumors which have achieved obviously clinical effects[1-5].The position of 125 I seeds is one of the most direct and important factors that affects the dose distribution.The dose distribution makes enormous impact on clinical curative effects and complications. At a former time,due to lack of effective methods to control accurate position of 125 I seeds,the position of 125 I seeds were mostly depends on the experience which was implanted in tumors by the spacing of 0.7cm~1.5cm.This practice easy to cause a big error, unable to calculate the whole target of radiation dose,and hardly can konw the dose distribution.Because the more complications,the clinical applications is limited.The rapid development of TPS and imageology are promote the development of brachytherapy. At preset,there are mainly two types to decide the 125 I seeds location. One method is based on the principles of the Paris system,emphasizes on uniform distribution, the Paris system is not designed for 125 I implantation and inevitably caused a does error.Another way makes by the simulation of TPS.Base on TPS,The prescription dose can be acquire by different 125 I seeds position when the activity of 125 I seeds and tumor volume were equal. Because of no uniform standard for the three-dimensional distribution of 125 I seeds in the different parts、size、pathological types、sharp of the tumors, different doctors choose the position of 125 I seeds in different methods,which lead to different curative effects and complications.It is a primary problem need to be solved for uniform standard in the malignant tumor therapy of 125 I seeds.Meanwhile for some special anatomical structures(e.g.retro-peritoneal lymph node metastatic carcinomal which surrounded by endanger organs such as intestinal canal) make the postoperative 125 I seeds position can not match with the preoperative plan that lead to the differences between postoperative verification dose with the preoperative plan.For these patients which unable to completely conform to the TPS plan,which positions of 125 I seed is more advantage to the dose distribution in the tumor target is a problem to be solved.Objective:This research aim to study the dose comparison and assess the quality of the plan between central and peripheral distribution of 125 I seeds by the Computer treatment planning system(TPS),when the prescription dose, tumor volume and the activity of the 125 I seed are the same. In order to provide the theoretical basis for the clinical choice of 125 I seeds distribution.Methods:To scan a white paper which marked with 5cm scale by the laser scanner, save it on the computer by JPEG format.Established images accord to the stimulative tumor, the images distance was 5mm by using image conversion program, then transmitted it to the TPS, adjust the image origin point o of coordinates was(0.2,0.2), origin point of template was(-0.25,-0.25) and matrix density was 128×128. The TPS was used to create 4 cubes and 4 cylinders which represented for the tumor. The side lengths of the 4 cubes were 2cm, 3cm, 4cm and 5cm respectively.The diameter and height of the 4 cylinders were 2cm, 3cm, 4cm and 5cm respectively. Making these simulation sharp represent to the corresponding size of the tumor. The prescription dose(PD) was 145 Gy.The activity of 125 I seed was 0.6mci, the 125 I seed layer distance was 10 mm. The needles direction were perpendicular to the 125 I seeds implanted plane and template. To make the D90(the dose received by 90% of the target) close to PD.According to the different distribution of 125 I seeds,the peripheral distribution was defined as A group(more than ninety of percent of 125 I seeds were implanted outside in a third volume of the tumor) and the central distribution was defined as B group( more than ninety of percent of 125 I seeds were implanted inside in two-thirds volume of the tumor). T o record the D90, V90(the percentage of the GTV volume receiving 90% the prescription dose), Max Dose, Mean Dose and the number of seeds showed on Dose volume histogram(DVH) and calculate the HI(homogeneity index)、EI(external volume index)and CI( conformal index).Results:The mean D90 of A and B groups were 146.41±1.73 Gy, 145.07±2.06 Gy respectively, There was no significant differences between the two groups(t=-1.733,P=0.127); The mean V90 of A and B groups were 95.89±1.01%, 94.39±1.29% respectively, There was significant difference between the two groups(t=-3.578,P= 0.009); The mean max dose of A and B groups were 1431.74±73.99 Gy, 1808.45±196.89 Gy respectively, The difference between the two groups was statistically significant(t= 7.758,P= 0.000); The mean mean dose of A and B groups were 190.93±4.04 Gy, 198.81±1.69 Gy respectively, The difference between the two groups were statistically significant(t= 5.633,P= 0.001); The mean number of seeds of A and B groups were 61.00±40.89, 83.88±60.78 respectively; The difference between the two groups were statistically significant(t= 3.214,P= 0.015);The mean HI of A and B groups were 52.45 ± 13.47%, 27.49 ± 3.78% respectively; The difference between the two groups were statistically significant(t= 4.778,P= 0.002); The mean EI of A and B groups were 2.32±2.23%, 6.60±4.50% respectively; The difference between the two groups were statistically significant(t= 3.412,P= 0.011); The mean CI of A and B groups were 0.89±0.02, 0.84±0.04 respectively; The difference between the two groups were statistically significant(t=-3.529,P= 0.010).Conclusions:1 The dosimetric distribution is influenced significantly by the locations of 125 I seeds. The prescription dose can be achieve by peripheral and central distribution of 125 I seeds when the PD, tumor volume and the activity of 125 I seeds were equal. but the central distribution easily formed a large area of high dose, the peripheral distribution’s dosimetric parameter level was more appropriate.2 When the PD, tumor volume and the activity of 125 I seeds were same, the distribution of 125 I seeds was peripheral, the dosimetry distribution in tumor target was more uniform and suitable, the volume of PD outside the tumor target was more less.3 When the PD, tumor volume and the activity of 125 I seeds were same. The number of 125 I seeds in the peripheral group was far less than central groups. The more greater tumor volume, the more obvious difference.