The dosimetry evaluation of 3D printing non-coplanar template-assisted CT-guided ¹²⁵I seed stereotactic ablation brachytherapy for pelvic recurrent rectal cancer after external beam radiotherapy

The aim of this study was to investigate the safety and accuracy of computed tomography (CT)-guided I-125 seed implantation assisted by a three-dimensional printing non-coplanar template (3D-PNCT) for treating pelvic locally recurrent rectal cancer (LRRC) patients. A total of 13 patients with 18 masses received I-125 seed implantation. The dosimetric parameters of pre-implantation and post-implantation were calculated to evaluate the quality of I-125 seed implantation. Doses delivered to the organs at risk (OAR) were also calculated. Differences between pre-implantation and post-implantation were compared by the paired t-test. The mean number of I-125 seeds pre-implantation and post-implantation was 67.1 and 68.8, respectively. The mean values of D-90 (dose that was delivered to 90% of the target volume), D-1(00) (dose that was delivered to 100% of the target volume), V-1(00) (the target volume receiving 100% of the prescription dose) and V-150 (the target volume receiving 150% of the prescription dose) pre-implantation and post-implantation were 136.6 and 135.2 Gy, 63.5 and 71.0 Gy, 90.3% and 90.3% and 62.1% and 62.2%, respectively. Dosimetric outcomes were evaluated quantitatively using the dose volume indices, i.e. coverage index (CI), external volume index (EI) and relative dose homogeneity index (HI). The mean values of those indices pre-implantation and post-implantation were 0.62 and 0.61, 0.31 and 0.33, and 0.31 and 0.31, respectively. The mean doses delivered to OAR pre-implantation and post-implantation for the bladder (D-2cc) and bowel (D-2cc) were 33.4 and 34.4 Gy, and 58.6 and 61.8 Gy, respectively. The parameters mentioned above fitted well, and no significant difference was found among them. It is concluded that CT-guided I-125 seed implantation assisted by 3D -PNCT could be a safe and accurate salvage modality for treating LRRC patients; the ideal pre-prescription dose could be achieved. Also, addition of 3D-PNCT could minimize radiation damage to the surrounding normal tissues.