A method to improve the dose distribution of interstitial breast implants using geometrically optimized stepping source techniques and dose normalization

Background and purpose: The standard linear source breast implant of our in stitution was compared with alternative linear source implant geometries an d a stepping source implant, to evaluate the possibility of minimizing the treated volume. Normalization to a higher isodose than the conventional 85% of the mean central dose (MCD) was investigated for the stepping source im plant to reduce the thickness of the treated volume and to increase dose un iformity. The purpose of this study was to develop an implant geometry yiel ding a high conformity and a more uniform dose distribution over the target volume. Materials and methods: The dose distributions of four implant geometries we re compared for a planning target volume (PTV) of 48 cm(3). Implants #1 (st andard) and #2 had linear sources arranged in a triangular pattern of equal lengths and lengths adapted to the shape of the PTV. Implants #3 and #4 we re squared pattern arranged implants with linear sources and a stepping sou rce with geometric optimized dwell times. The active lengths were adapted t o the shape of the PTV. Using implant #4 for PTVs of different volumes, the reference dose (RD) was normalized to 85 and 91% of the MCD. Results: Comparing implants #2, #3, and #4 with #1, the treated volume (V(1 00)) encompassed by the reference isodose was reduced by 22, 35, and 37%, r espectively. The Volumes receiving a dose of at least 125% (V(125)) of the reference dose was reduced by 16, 30, and 30%, respectively. The conformati on number increased being 0.30, 0.39, 0.47, and 0.48 for implants #1, #2, # 3, and #4, respectively. The average reduction of V(125) when the dose was normalized to 91% compared with 85% of the MCD was 18%. Conclusions: a conformal treatment to a PTV could be best achieved with a g eometrically optimized stepping source plan with needles arranged in a squa red pattern. Reduction of high dose volumes within the implant was obtained by normalizing the RD to 91% instead of 85% of the MCD. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.