The dosimetric effect of different source patterns in case of tumor shrinkage after 125 I seed implantation / 中华放射医学与防护杂志

ABSTRACT

Objective To study the dosimetric effect of two source patterns, including equal spacing and peripheral dense intermediate sparse by assuming a tumor shrinking speed of 20%per month after 125 I seed implantation. Methods A virtual cylindrical tumor with 4 cm in height and 5 cm in diameter was contoured on a three-dimensional treatment planning system ( TPS ) . Two groups of preoperative plans were made with 1. 85 × 107 Bq 125 I seeds using two source patterns respectively. It was assumed that the tumor height was unchanged, while the diameter of tumor would decrease at a speed of 20%per month, and the locations of seeds would concentrate towards the tumor core. The 90%target volume dose ( D90 ) , the ratio of 90%isodose volume over the target volume ( V90 ) , and the ratio of 150%isodose volume over the target volume (V150) were calculated at 0, 1, 2, 3 months after 125I implantation respectively. Results In equal spacing group, 85 seeds were implanted. The values of D90 were 126. 20, 130. 41, 133. 82 and 139. 48 Gy after 0, 1, 2 and 3 months respectively. The values of V90 were 97. 0%, 98. 1%, 99. 3%and 100%, while those of V150 were 70. 2%, 69. 9%, 71. 1% and 71. 5%. The dense in-periphery and sparse-in the middle group was loaded with 75 seeds. The D90 values were 126. 46, 125. 41, 123. 50 and 128. 83 Gy, the V9095. 2%, 95. 7%, 94. 9%and 97. 6%, and the V15052. 8%, 60. 4%, 62. 7%and 59. 3%after 0, 1, 2 and 3 months, respectiviely. Conclusions When the tumor diameter reduces at a rate of 20%per month after 125 I seed implantation, the expected tumor dose absorption will gradually increase using the equal spacing sources pattern. However, the expected dose does not vary withsource distribution of dense-in the-surrounding and sparse-in-middle, which also reduces high dose volume more than the equal spacing pattern.