Optimization of catheter's implementation in the mold, in the case of vaginal HDR brachytherapy treatment

The purpose of this study was to evaluate and compare results obtained in high dose rate (HDR) brachytherapy treatment of vaginal cancer. Different catheters distributions inside the custom mold were explored. The difference between those distributions is the position of the posterior catheter located near the rectum in the actual custom mold applicator used in different hospitals, each one having a catheter displacement of 0.5 which is equal to the length of a step position. The best catheters distribution offering an optimal dose distribution: better coverage of the clinical target volume (CTV), while reducing the dose received by organs at risk (OARs), were discussed.

A group of 60 patients treated with HDR brachytherapy, alone or in combination with external radiotherapy, was investigated. A custom mold is normally used for HDR brachytherapy vaginal cancer treatment. Three different geometrical positions of the catheters (G1, G2 and G3) and, consequently, 3 different dosimetries were simulated out for each patient on the CT images, using the Oncentra planning system. The coverage of the CTV was studied.

The average volume treated was 30.46 cc (min = 9.8 cc, max = 70.86 cc). The total prescribed dose, including external and internal radiotherapy, was 80 Gy. We evaluated conformity index (CI), dose homogeneity index (DHI) and conformality index (COIN) indices for the three implantation geometries to reach the same coverage criteria of the CTV. The D2cc parameter allowed the evaluation of the dose received by the OARs. For the rectum, a dose reduction of 9.67% (range 0.29-32.86) was obtained with the second geometry of implantation compared to 10.14% (range 1.43-28.33) with the third geometry. For the bladder, the second geometry of implantation showed a better preservation for this organ [15.93% (range 0.86-58.71) vs. 8.35% (range 0.33-30.43) with the third geometry]. The sigmoid was more protected using the second plan of implantation as well [6.33% (range 0.14-40.71) for the second implantation compared to 5.95% (range 0.33-36) for the third implantation].

G2 and G3 catheters' distribution, having catheter position farther from the mold wall and so from the vaginal wall compared to the catheter position applied showed a better protection for the OARs while giving the same prescribed dose for the CTV.