ESTRO 2023 - Abstract Book

S2046

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ESTRO 2023

Twenty patients that received treatment on the MR-Linac in the abdominal region were retrospectively selected for this study. 10 patients were positioned using laser (at St John of God Murdoch Hospital, Perth) and 10 patients without lasers (at St Vincent’s, Sydney). All patients received SABR treatment delivered in 5 alternate daily fractions. Daily MR-Linac scans were rigidly registered to the reference CT plan or previous adapted MR plan, with the 3D isocentre position shifts recorded for analysis. Differences in the daily positioning errors were examined using paired Student’s t tests. The mean (M), and standard deviation (SD) of the setup errors were calculated for each patient. The SD of population M expressed the population systematic error ( Σ ), and the root mean square of the SD describes the random error ( σ ). The percentage of errors exceeding 0.5 cm was also recorded. Results Differences in the daily positioning errors between the two patient groups were not statistically significant in the superior- inferior direction (p = 0.0001) but not in the left-right (p = 0.8) or anterior-posterior (p = 0.08) directions. Table 1 outlines the mean, systematic ( Σ ) and random ( σ ) errors for the two patient groups. In the superior-inferior direction, the systematic and random errors were higher in the group without laser by 0.74 cm and 0.41 cm, respectively. The measured errors were comparable between patients positioned with and without lasers in the left-right and anterior-posterior directions. The percentage of errors exceeding 0.5 cm is displayed in Figure 1.

Conclusion Setup accuracy between the two groups showed significant large variation in the superior-inferior direction was for patients positioned without laser but comparable in the left-right and anterior-posterior directions. While these offsets can be corrected using the standard MR-Linac adaptive workflow, minimising these errors with laser during patient positioning phase may improve image registration, contour propagation and plan reoptimisation time, ultimately reducing the overall treatment time.

PO-2275 Single-centre implementation of radiotherapy-dedicated bra for large and pendulous breast

S. Trivellato 1 , M.C. Daniotti 1 , P. Caricato 1 , V. Pisoni 2 , S. Terrevazzi 2 , A. Mantovani 3 , M. Castellano 3 , E. Ierman 2 , T. Brandolese 2 , S. Meregalli 2 , R.M. Niespolo 2 , E. Bonetto 2 , E. De Ponti 1 , S. Arcangeli 4 1 ASST Monza, Medical Physics Department, Monza, Italy; 2 ASST Monza, Department of Radiation Oncology, Monza, Italy; 3 University of Milan Bicocca, School of Medicine and Surgery, Milano, Italy; 4 University of Milan Bicocca, (4) School of Medicine and Surgery, Milano, Italy Purpose or Objective Dose conformality and homogeneity is commonly obtained in breast field-in-field (FIF) 3D-CRT but large and pendulous breasts can be challenging due to large inframammary fold and lateral displacement. This affects the risk of toxicity and organs at risk (OARs) doses. This study aimed to evaluate the geometric and dosimetric effects of a radiation bra during radiotherapy for large and pendulous breasts.

Materials and Methods