ESTRO 2024 - Abstract Book

S554

Clinical - Breast

ESTRO 2024

A total of 211 patients were included in the inter-fractional study, with 101 positioned using laser and 110 SGRT. All of them underwent a CBCT after positioning (total of 3,579 images/fractions). The systematic errors (vertical, longitudinal, lateral) using laser were (0.21, 0.22, 0.21) cm and SGRT (0.11, 0.15, 0.11) cm, respectively. The random errors using laser were (0.37, 0.33, 0.35) cm and SGRT (0.20, 0.21, 0.21) cm. The targeting accuracy for both methods are represented as 2D density histograms in Figure 1 . The calculated PTV margin was 0.8 cm for laser and 0.5 cm for SGRT. There was a statistically significant difference between SGRT and laser for the SD and the M error vector (p<0.001). In the multivariate analysis for the SD of the error vector (a measure of random error), the variables that showed statistical significance were the positioning technique and breast volume (p<0.001 and p=0.007). As for the M error vector (a measure of systematic error), in addition to the mentioned variables, age was also significant (p=0.021). To assess the breast intra-fractional motion, the results of SGRT for 99 patients and a total of 1557 fractions were evaluated. The monitoring time (M±SD) was 184 ± 100 seconds. The intra-fraction measurements (M±SD) in the vertical, longitudinal, and lateral directions, followed by the vector, were, in millimetres (mm): systematic deviations -0.25±0.71, 0.01±0.64, 0.03±0.47 and 1.14±0.65 and variability 0.55±0.44, 0.54±0.53, 0.34±0.48 and 0.60±0.72. These results are presented per treatment minute in Figure 2 , where it can be observed an increase at the systematic error (M vector) with time while the random error (SD) remains stable. The deviation intervals (in mm) with their respective percentages of occurrence observed were: 0 to 1 (55.1%), 1 to 2 (32.2%), 2 to 3 (9.1%), 3 to 4 (2.5%), 4 to 5 (0.6%), and greater than 5 (0.5%). In the multivariate analysis, only breast volume showed statistical significance for systematic deviation (p = 0.003).