The Effect Of Post-implant Tumor Target Volume Reduction To Dosimetry In Lung Malignant Tumor ¹²⁵I Seed Implant

Radioactive 125 I seeds interstitial implantation is a kind of brachytherapy treatment. It can make the tumor target region acquire high dose irradiation, and the surrounding normal tissue dose drop steeply. This treatment method has advantages of safety, minimally invasive, curative effect, less complications, low radiation pollution, easy protection, and so on. At present, radioactive 125 I particles implantation therapy has become a standard treatment for early stage prostate cancer, and is also widely used in many solid tumors in our country.The biological effect of radiation therapy is directly related to the dose which the tissue adsorbs. The actual absorbed dose of tumor and organs at risk is most direct and important factor that influences tumor therapeutic effect and complications. The post-implant dosimetry using three-dimensional treatment planning system(TPS) has become the gold standard for assessing implant quality and the actual dose delivered to the prostate and normal surrounding structures. Other malignant tumors also use the post-implant verified peripheral dose as the actual absorbed dose in clinical. The peripheral dose calculated using TPS is the absorbed dose of tumor target until the 125 I seed decaying completely as the premise of tumor target and position of the particle not changing. The target range of prostate and particle spatial distribution do not change almost after implantation, and the post-implant verification dose is its actual absorbed dose. While the other malignant tumors are not different from prostate cancer, the tumor target volume become smaller gradually because of the radiation damage effect and the seed position of the particles also change. Therefore the actual absorbed dose of tumor and organs at risk also change.This study simulate the malignant tumor’s verification plan of different time after implanted using TPS to explore the dosimetry change in different time after the operation. and selected lung malignant tumor patients that received 125 I seed implantation to investigate the effect of target change to dose after 125 I seed implantation. We look forward to provide the reference for clinical application. Part 1: A simulation study on the effect of post-implant tumor target volume reduction to dosimetry using three-dimensional treatment planning systemObjective: This study aimed to discuss the effect of tumor target volume reduction with different speed to dosimetry after 125 I seed implant.Methods: Assuming that tumor shape is sphere, then a sphere whose diameter is 4 centimeter was outlined by treatment planning system(TPS) as the gross tumor volume(GTV).The Prescribed dose(PD) is set to 100 Gy and 125I seed activity is 1.85×107Bq(0.5m Ci). The seed was loaded with peripheral distribution per layer and its numbers were 36. Finally the D90 was worked out by TPS, and its value was 107 Gy. If the tumor volume reduced with different speed every month, we set several groups according to reduction speed: A group’s volume was not change after 125 I seed implant, B group reduced 5 percent of initial tumor every month, C group 10 percent, D group 15 percent, E group 20 percent, F group 25 percent, G group 30 percent, H group 35 percent, I group 40 percent, J group 45 percent, and K group 50 percent every month. Then, we used TPS to simulate the verification plan of every group in 1 to 6 month after implant and worked out all D90, and observed every group’s change trend of post-implant peripheral dose.Results: The D90 of A to C groups in 1 to 6 month after implant showed a trend of gradual decline, however, D to K groups tended to fall at first and then rise. The post-implant D90 and tumor target volume of D and E groups can be fitted three function curve respectively. Equation of D was Y=5670.4-119.4X+0.35X2+0.21X3, R2=0.998, Equation of E was Y=7004.6-8.78X-6.53X2+0.29X3, R2=1. The D90 of D and E groups became steady in 5 to 6 month after implant, when the tumor was going to disappear.Conclusions: The reduction of tumor target volume has significant effect to the peripheral dose. It may be more appropriate that the tumor volume reduces 15 to 20 percent of initial tumor every month. It prompts that the prescribed dose may be insufficient if tumor volume reduced less than 15 percent of initial tumor every month and we can consider reimplant. And, it prompts that the prescribed dose may be too high if tumor volume reduced more than 20 percent of initial tumor every month. Part 2: The effect of post-implant tumor target volume reduction to dosimetry in lung malignant tumor 125 I seed implantObjective: To study the effect of the lung malignant tumor target volume change after one month implanted to dosimetry.Methods: There were fourteen lesions received 125 I seed implantation in eleven patients which have lung malignant tumor. The age ranged from 29 years to 89 years, median age was 71 year. There were 10 males and 1 female. The tumor size ranged from 1.5 centimeter to 7.2 centimeter. And the primary lung cancer had 7 cases, the metastatic lung cancer had 4 cases. All lesions were percutaneous implantation, and the radioactive activity was 1.11×107Bq~2.22×107Bq, the 125 I seed numbers were 20~82.The post-implant verification plan were made immediately after implantation, and the peripheral dose was 36~102Gy. The dose of normal lung tissue and spinal cord were below the tolerance dose. Some relevant complications were not found in our follow-up. We made verification plan again in 1 month after implant, and set as two groups according to whether the tumor target change or not. The group of tumor target no changing(A group) was worked out by the CT image of day 0 suppose the target tumor were not change; The group of tumor target changing(B group) was worked out by the CT image of day 30; Reaching two groups’ V(tumor target volume), D90, V90, V100, V150 respectively, and compared whether the parameters between the two groups had statistical significance or not. The percentage difference of V and D90 between A group and B group were calculated by the formula Vd%=(VA-VB)/VA×100% and D90d%=(VB-VA)/VA×100%, and observed the relationship between Vd% and D90d%.Results: The mean V, D90, V90, V100, V150 of group A were 32.80±28.89cc、69.17±31.94Gy、89.48±7.54%、85.67±8.88%、66.71±14.42%;The mean V, D90, V90, V100, V150 of group B were 52.84±37.76 cc, 40.78±11.90 Gy, 75.48±7.03%, 69.44±8.18%, 42.16±10.66%. There were significant difference between the two groups of V, D90, V90, V100, V150, P<0.05. The percentage difference of V(Vd%) were 6.7%, 13.8%, 22.3%, 23.4%, 38.3%, 41.3%, 42.6%, 51.0%, 51.2%, 54.5%, 61.5%, 65.5%, 77.9%, 82.5%. The percentage difference of D90(D90d%) were 4.4%, 22.5%, 33.6%, 30.6%, 25.1%, 63.5%, 4.5%, 64.4%, 70.0%, 61.8%, 141.7%, 138.0%, 164.3%, 163.2%. The more of the tumor target volume changed in 1 month after implant, the greater of the peripheral dose changed.Conclusions: The reduction of tumor target volume has significant effect to the peripheral dose; the post-implant dose verified immediately can not reflect the actual absorbed dose of tumor. The more of the tumor target volume decrease, the greater of the peripheral dose change in the same time. We need consider tumors’ reduction rate of post-implant, when we make pre-implant treatment plan. We should determine prescribed dose(PD) according to the radiation sensitivity of tumor.