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.

Objective To systematically evaluate the efficacy and safety of 125I seed implantation on the treatment of refractory thyroid carcinoma.Methods A total of 14 patients with refractory thyroid carcinoma received CT and ultrasound image-guided 125I seed implantation in our hospital from Apr.2003 to Oct.2016.The radioactive activity ranged from 0.3 to 0.8 mCi,and the prescribed doses were 80 to 120 Gy.11 patients were diagnosed as differentiated thyroid carcinoma and 3 patients were medullary thyroid carcinoma.The local control rate,pain relief and complications were observed.Results The patients were followed up from 12 to 96 months.Two months after treatment,complete remission happened in three patients,partial remission in nine,and disease stability in two,no patients with progression.The half year,one year,two year local control rate was 100％,92％ and 42％,respectively.Seven patients were evaluated using number rating scales and obtained a score of 4.14±1.68 before therapy.Two months after treatment,the score was reduced to 2.00±1.15 (P＜0.000).No usual complications occurred during or after treatment except for I acute skin radio reaction in 4 cases.Conclusion 125I seed interstitial implantation for refractory thyroid carcinoma under guidance of ultrasound or CT is efficient and safe.

Objective To investigate the dosimetric effect of same activity and same number of 125I seeds arranged in axial train but with different spacing.Methods A total of 27 film dosimeters were randomly and equally divided into group A,B and C.Each film was irradiated by three 125I seeds (activity of 1.48×107 Bq).The seeds were arranged in line,and their axial spacing was 1 mm,5 mm and 10 mm respectively.Image analysis software was used to draw iso-gray contour curves of 20,25,30 and 40 gray value on the films,and to calculate the areas contained by each curve.Results Multi-sample mean comparison variance analysis showed that the differences in area contained by 20 and 25 iso-gray contour curves were statistically significant between each other among the three groups (P＜0.001).The difference in area contained by 30 iso-gray contour curve between group A and group B was not statistically significant (P＞ 0.05),while the difference in area contained by 30 iso-gray contour curve between group A and group C as well as between group B and group C was statistically significant (P＜0.001).No statistically significant difference in area contained by 40 iso-gray contour curve existed between each other among the three groups (P=0.99).Conclusion Different spacing arrangement of same activity and same number of 125I seeds can directly influence the peripheral dose distribution.In specific dose range,seed-spacing of 1 mm arrangement may obtain better dose distribution than seed-spacing of 5 mm or 10 mm arrangement can do.