EFFECT OF GEOMETRICAL OPTIMIZATION ON THE TREATMENT VOLUMES AND THE DOSE HOMOGENEITY OF BIPLANE INTERSTITIAL BRACHYTHERAPY IMPLANTS

Background and purpose: The isodose distributions of HDR stepping sour ce brachytherapy implants can be modified by changing dwell times and this procedure is called optimization. The purpose of this study is to evaluate the effect of geometrical optimization on the brachytherapy volumes and the dose homogeneity inside the implant and to compare the m with non-optimized counterparts. Material and methods: A set of bipl ane breast implants consisting of 84 different configurations have bee n digitized by the planning computer and volumetric analysis was perfo rmed for both non-optimized and geometrically optimized implants. Trea ted length (T-L), treated volume (V-100). irradiated volume (V-50), ov erdose volume (V-200) and quality index (QI) have been calculated for every non-optimized implant and compared to its corresponding geometri cally optimized implant having a similar configuration and covering th e same target length.Results: The mean T-L was 74.48% of the active le ngth (A(L)) for non-optimized implants and was 91.87% for optimized im plants (P < 0.001). The mean QI was 1.83 for non-optimized implants an d 2.17 for optimized implants (P < 0.001). The mean V-50/V-100 value w as 2.71 for non-optimized implants and 2.65 for optimized implants (P < 0.001) and the mean V-200/V-100 value was 0.09 for non-optimized imp lants and 0.10 for optimized implants (P < 0.001). Conclusions: By per forming geometrical optimization it is possible to implant shorter nee dles for a given tumour to adequately cover the target volume with the reference isodose and thus surgical damage is reduced. The amount of healthy tissues outside the target receiving considerable radiation is significantly reduced due to the decrease in irradiated volume. Dose homogeneity inside the implant is significantly improved. Although the re is a slight increase of overdose volume inside the implant, this in crease is considered to be negligible in clinical applications. (C) 19 97 Elsevier Science Ireland Ltd.