Dose-volume analysis of regular moving source interstitial implants and comparison of stepping source dosimetry system with Paris System

Stepping Source Dosimetry System (SSDS) is recently developed after the invention of HDR moving source brachytherapy machines. This system makes it possible to optimize isodoses around the source according to the target volume. Although geometrical relationships of brachytherapy volumes - reference volume(D100), irradiated volume(D50) and overdossage volume(D200) - is described earlier and well known for Paris System (PS) implants, same data for SSDS system where geometric optimization is used, still remains unclear. In this study to analyse dose-volume relations and uniformity of the regular implants where geometric optimization is used and to compare with PS, 168 imaginary double-plane regular interstitial implants were prepared and entered to the Nucletron-PLATO planning computer. Of these implants PS was used in 84 and SSDS with geometric optimization was used in the remaining 84. For the implants where the PS was used 5 to 9 needles were arranged at the corners of the triangles where distances between them were 10mm to 19mm, and lengths of the needles were 4 to 10 cm. Every implant was unique by means of its geometry. Dose- volume analysis have been performed; D100, D200 and D50 volumes and uniformity index (UI) was calculated for each implant. The same geometries were also used for SSDS implants, the only difference was that lengths of the needles were modified to obtain the same treated length(TL) with PS, so that the length of a reference volume was same for both a PS implant and its counterpart at SSDS. Geometric optimization was performed and brachytherapy volumes and UI were also calculated for SSDS; then relations were analysed, comparisons with PS have been done. Mean UI was 1.763 ± 0.145 for PS and 1.776 ± 0.129 for SSDS (p=0.074), mean TL was 0.7448 ± 0.056 for PS and 0.9187 = 0.052 for SSDS (p<0.001). (D200(D100)) ratio was increased by increasing the distance between needles, decreased by increasing the number and length of the needles for both PS and SSDS; the mean (D200(D100)) ratio was 0.0887 ± 0.020 for PS and 0.0986 ± 0.019 for SSDS (p<0.001). (D50(D100)) ratio was increased by increasing the distance between needles and length of needles, decreased by increasing the number of the needles for both PS and SSDS; the mean (D50(100)) was 2.7553 ± 0.074 for PS and 2.6305 ± 0.037 for SSDS respectively (p<0.001). These results show that when the same treated lengths are obtained with PS and SSDS, overdosage volume ((D200(D100))) is significantly higher (11.1%) and irradiated volume ((D50(D100))) is significantly lower (4.7%) with geometric optimization. Geometric optimization has no effect on the uniformity of the implant. When optimization was done needles or catheters can be safely implanted to the skin just near the tumor because treated length is more than 90%. This may be especially important for the tumors which were localised near critical structures