Conformality and homogeneity of dose distributions in interstitial implants at idealized target volumes: a comparison between the Paris and dose-point optimized systems.

Background And Purpose: The use of high dose rate stepping source in interstitial brachytherapy provides more possibility to conform the dose distribution to the target volume compared to the classical systems. The purpose of this study was to evaluate implants made according to the Paris, the stepping source and the conformal dosimetry system with respect to dose homogeneity and conformality, and to compare these systems using volumetric parameters.

Materials And Methods: Single-plane and double-plane implants with catheters arranged in square and triangle pattern were used in the analysis. Twenty-seven idealized planning target volumes (PTV) were generated. They formed slabs with rectangular or trapezoidal cross-section. The lengths were 3, 5 and 7 cm, the widths and heights were determined according to the Paris system for catheter separation of 1, 1.5 and 2 cm. The dose specification was selected such that the coverage index was 0.95 for each implant. Optimal active lengths were determined according to the best conformality at the optimized implants. From the dose-volume histogram (DVH) the following indices were calculated for every implant: conformal (COIN), external volume (EI), relative dose homogeneity (HI) and overdose volume (OI). Furthermore, the mean central dose (MCD) and minimum target dose (MTD) was also determined. The dosimetry systems were compared through the mean values of these parameters and the volumetric indices were analyzed according to the geometry of the PTV.

Results: For the optimized systems the optimal active length was 0.5-1.0 cm shorter than the target volume length, depending on the catheter separation and geometry of the PTV. For the Paris, the stepping source and the conformal dosimetry system, the mean COIN was 0.66, 0.82 and 0.82; the mean HI was 0.71, 0.68 and 0.68; the mean EI was 0.44, 0.17 and 0.17; the mean OI was 0.11, 0.13 and 0.12, respectively. The statistical analysis showed that the Paris system differed from the optimized systems significantly. For the Paris, the stepping source and the conformal dosimetry system, the mean reference isodose was 85, 90 and 95%, the MCD was 100, 100 and 109%, the MTD was 67, 71 and 73%, respectively. Regarding geometry of the PTV, the most conformal and homogeneous dose distributions occurred when the catheter separation was small, the target volume was long and its shape was a thick rectangular slab.

Conclusions: Positioning the catheters according to the rules of the Paris system, but applying optimization on dose points placed either between the catheters in the whole target volume or on the surface of the target volume, and selecting the reference isodose by DVH, can provide highly conformal dose distribution to the target volume, with only a slightly worsened dose homogeneity compared to the Paris system.