The Study On Three-dimensional Treatment Planning System Simulate The Dose Distribution In And Outside The Tumor Target With Different¹²⁵I Seeds Activity

Objective:Radioactive¹²⁵I seeds interstitial implantation is a method ofbrachytherapy treatment.It can multiplied improve tumor target dose,but makethe surrounding have dropped steeply.Thus the tumor target has high dose ofanti-tumor effect but on the surrounding normal tissue the pathologicaldamage is small. Due to the advantage of safe,micro-wound,good therapeuticeffect,less complication,low pollution,easy to protect,seed implantationbecome more and more important in therapy of tumor.The technique of seedimplantation as the standard therapy in prostate cancer has reached a mature inforeign,and used in other solid tumor widely in China, such as head and necktumor,thoracic tumor,abdomen tumor,pelvic tumor and soft tissue sarcoma,andhave received obviously effect.Effect of radioactive¹²⁵I seed implantation isinfluenced by many kinds of factors,such as the matched peripheral dose, theparticle source activity,the source space and so on. Any radiation are basedon dosimetry. The deaths rate and local recurrence rate will reduce3%,whenthe dose increase every1gy,but high dose will lead to damage to the adjacenttissues and organs, and increase the complications. So we think that tumorradiation dose depends on the balance of surrounding normal tissues tolerancedose and tumor killed doses. Dose distribution is one of the most direct andimportant factor that affect curative effect. The dose rate of radioactive¹²⁵Iseed is related to the activity,with activity decreased,dose rate decreasesexponentially.Activity is one of the key factors that affect dose distribution.The activity of the clinical commonly used is from1.11×10⁷to3.33×10⁷,but how to choose the seed activity is no uniform standard, and is theclinical application of problems to be solved.In this study,the¹²⁵I seedsimplantation in differert activity were simulated by using the three-dimensional compputer treatment planning system（3D-TPS）,when theperipheral dose and volume are the same. According to investigate thedosemetric differences in and outside the tumor target provide theoretical basisfor selecting¹²⁵I seed activity in clinical implantation.Methods:There groups were established in this experiment by seedactivity.The activity in group A was1.11×10⁷（0.3mCi），the activity ingroup B was1.85×10⁷（0.5mCi）and the activity in group C was2.96×10⁷（0.8mCi）.With laser scanner scanning a marked with5cm scale whitepaper, save it on the computer by JPEG format.Established9images that layerdistance was5mm using image conversion program, then transmitted to theTPS, adjust the image origin point o of coordinates as （0.2，0.2）, origin pointof template as （-0.25，-0.25）, matrix density as128x128. The tumour volumedefined GTV was assumed three spheres which radius were2cm and with thepoint O as the center outlining by TPS.The prescription dose （PD） was145Gy.The¹²⁵I seeds were implanted according to peripheral distributionusing1.11×10⁷,1.85×10⁷,2.96×10⁷respectively,make the D90（thedose received by90%,80%of the target） close to PD. The seed layer distancewas5mm.The D90,D80,V100、V150、V200（the percentage of the GTVvolume receiving100%,150%,200%the prescription dose）,Min Dose,MaxDose,Mean Dose and the number of seeds showed on Dose volume histogram（DVH）were recorded.Calculate the EI （external volume index） and record themached peripheral dose（MPD） of different activity when GTV outside enlarge0.5cm,1cm,1.5cm,2cm,2.5cm,3cm,3.5cm and4cm. Calculate the medical costof the three groups respectively. Investigate the dosemetric differences in andoutside the tumor target of different activity of¹²⁵I seeds.Results：The D90which showed on the DVH in the groups of A,B and Cwere145.54Gy,145.78Gy and145.46Gy respectively,D80were155.36Gy,155.3Gy,157.41Gy respectively;V100were90.4%,90.2%,90.4%respectively;V150were33.5%,32.9%,36.6%respectively；V200were16.3Gy,16.1Gy,18.2Gy respectively;MinDose were95.49Gy,91.31Gy,91.74Gy respectively;MaxDose were795.5Gy,1210.71Gy,1850.42Gy respectively;MeanDose were 185.06Gy,185.50Gy,187.92Gy respectively. The number of seeds were76ingroup A which had the most seeds,and36in the group C which had the leastseeds. The number of seeds were52in group B. The group A had the least EIwhich was3.0%.The group C had the biggest EI which was21.4%, The EI ofgroup B was13.7%.When the GTV outside enlarge0.5cm,the group A hadthe least peripheral dose which was79.41Gy, the group C had the biggestwhich was100.82Gy, and the group B was93.88Gy. When the GTV outsideenlarge1.0cm,the group A had the least peripheral dose which was47.96Gy,the group C had the biggest which was62.01Gy, and the group B was55.94Gy.When the GTV outside enlarge1.5cm,the group A had the least peripheraldose which was29.25Gy, the group C had the biggest which was39.45Gy,and the group B was34.44Gy.When the GTV outside enlarge1.5cm,thegroup A had the least peripheral dose which was19.54, the group C had thebiggest which was25.25Gy, and the group B was22.36Gy. The group A hadthe most mecical cost which was1289.29yuan/cm³,the group C had the leastwhich was610.71yuan/cm³, and the group B was882.14yuan/cm³.Conclusions:1The prescription dose can be achieved by different¹²⁵I seeds activitywhen the peripheral dose and volume were equal,but the activity more higher,the more easy to form dose hot particles.2More higher is the activity of¹²⁵I seeds,more greater is the dose ofoutside the tumor target,and more bigger is volume outside the tumor targetreceived prescription dose,even the peripheral dose and volume were equal.3When the peripheral dose and volume were equal,the higher is theactivity of¹²⁵I seeds,the lower the cost of medicine will be.4The dose distribution in and outside the tumor target with different¹²⁵Iseeds activity could be caculated accurately and predicted by TPS which provides theoretical basis for selecting¹²⁵I seed activity in clinicalimplantation.