[The effect of pre-operation (125)I seed activity measuring on dose accuracy]

Objective: To measure the activity of (125)I seed and compare the dose difference of ten patients treated with seed implantation in pre-plan with actual seed activity and calibrate activity. Method: The activity of 100 seeds from company A and B was measured with a well-type ionization chamber 1 day before operation and named group A and B. The activity of two groups was compared and the error between actual and calibrate activity (22.2 MBq, group C) was calculated. Ten patients implanted with (125)I seeds from November 1 st to 30 th, 2017, solstice 30 were selected in Hebei General Hospital. Firstly, pre-plans were designed with 22.2 MBq, prescribed dose were 100-140 Gy. The dose parameters of 90% volume absorbed dose (D(90)), 150% volume fraction (V(150)) and 100% volume fraction (V(100)) were calculated. Then changed 22.2 MBq to actual activity of group A and B, calculated the dose parameters above again. Then dose parameters of D(90), V(150), V(100) in group C were compared with those in group A and B respectively. Result: The actual activity 1 day before the operation was(22.6±0.7)and(23.9±0.9)MBq in group A and B respectively. Compared with 22.2 MBq, the difference was statistically significant(t=5.7, P<0.05 and t=19.2, P<0.05), and the activity error of group B was greater than 5%. The D(90) of group A, B and C were (124.3±9.7) , (131.2±10.2) and (121.9±9.5) Gy respectively.The V(150) were 58.4%±9.4%, 63.7%±8.9% and 56.5%±9.2% respectively. The V(100) were 88.9%±5.0%, 92.0%±4.1%, 88.1%±5.2% respectively.The difference of D(90) between calibrate activity(group C) and actual activity(group A and B) were statistically significant (t=40.2, P<0.05 and t=40.3, P<0.05). The difference of V(150) between group C and group A and B were statistically significant (t=7.5, P<0.05 and t=24.7, P<0.05). The difference of V(100) between group C and group A and B were statistically significant (t=6.6, P<0.05 and t=7.3, P<0.05). Conclusion: There is difference between the actual activity and calibration activity. The difference affects the dose parameters in pre-plan.The seed activity should be measured before operation strictly and the pre-plan should be designed with the actual activity.