Customized planning for radioactive 125I seed implantation

Abstract

Objective To customize the optimal plans for radioactive 125I seeds volumetric implants in selected regular target volumes.Methods 125I seeds were symmetrically and uniformly implanted into 3 spherical targets with the diameters of 1,2 and 3 cm and 7 ellipsoidal targets with the 3 dimensions of 1 cm×1 cm×2 cm,1 cm×1 cm×3 cm,1 cm×2cm×2cm,1 cm×2 cm×3 cm,1 cm ×3 cm×3 cm,2 cm×2 cm×3 cm and2 cm×3 cm×3 cm.The activity and inter-space of seeds were adjusted to obtain the conformal and uniform dose distribution,with the prescribed D90 (the dose delivered to 90％ of the targets) greater than 145 Gy.The inter-space of seeds was changed from 1 cm to 0.75 cm,to improve the conformity and uniformity of dose distribution.Plan quality was assessed using homogeneity index (HI),external index (EI) and conformal index (CI).The activity and number of seeds implanted were also recorded and compared.Results For the spherical target with the diameter of 1 cm,when seeds were implanted with the inter-space of 1 cm and 0.75 cm,the HI were 40.0％ and 55.9％,the EI were 98.3％ and 95.1％,the CI were 0.44 and 0.44,respectively.For the spherical target with the diameter of 3 cm and the target with the 3 dimensions of 1 cm × 2 cm × 2 cm,the implant with the inter-space of 1 cm provided better indices of HI,EI and CI than those with the inter-space of 0.75 cm.For the other targets,the implants with the inter-space of 0.75 cm provided better indices of EI and CI than those with the inter-space of 1 cm,although they displayed a little worse homogeneity in terms of HI.The activity per seed was 17.0-27.8 MBq and 30.0-58.8 MBq in the implants with the inter-spaces of 0.75 cm and 1 cm,respectively.2-10 more seeds were needed in the implants with the inter-space of 0.75 cm.Conclusions For the studied targets except the spherical targets with the diameter of 1 cm and 3 cm and the ellipsoidal target with the dimension of 1 cm × 2 cm × 2 cm,125I seeds implanted with the inter-space of 0.75 cm could provide more conformal dose distribution.It could be the better customized plans for uniformly spaced seed implantation. Key words: 125I; Radioactive seeds; Seeds implant; Treatment planning; Customized planning