Evaluation and optimization of an image-subtraction method for the analysis of the repositioning accuracy of female patients with breast cancer

To analyze the repositioning accuracy in female patients with breast carcinoma, two different setups of an image-subtraction system (Positioning System FIVE) were devised using different numbers and alignments of lasers. The applicability of the system was tested for repositioning of the breast in normal volunteers. Horizontal translations as well as breathing-related movements in the vertical direction were measured. The mean repositioning accuracy was found to be 2.9 mm for the first setup and 1.5 mm for a second, optimized setup. For this second setup, a gating function was implemented which evaluates the position of the breast twelve times per second. The simulation of a gated treatment showed that the breathing-related displacement of the breast can be reduced to 45-70% of the displacement without gating. This implies a significant improvement of the positioning accuracy.