ESTRO 2023 - Abstract Book

S291

Sunday 14 May 2023

ESTRO 2023

terms of dosimetric measures. Next steps will include the integration of this work in the context of an effective implementation of an adaptive workflow from OARs annotation to full scale dose simulation and replanning.

MO-0390 Evaluating intra-breath-hold variation in IMAT breast boost patients delivered in DIBH C. Maguire 1 , S. Moinuddin 2 , A. Webster 1 , R. Simoes 2 , S. Wickers 2 1 University College London Hospital , Radiotherapy, London, United Kingdom; 2 University College London Hospital, Radiotherapy, London, United Kingdom Purpose or Objective Numerous breath-holds are performed within each fraction for deep inspiration breath-hold (DIBH) tumour bed (TB) breast boost delivery; 2 for the corrective imaging protocol (kV/kV pair clip matching), and between 2-3 for IMAT delivery. The intra-breath-hold (IBH) variation was unknown, and therefore it could not be assumed that translating the 5mm isotropic TB clinical target volume (CTV) to planning target volume (PTV) margin from the free-breathing protocol would be sufficient. The purpose of this work is to determine if a TB PTV margin of 5mm is appropriate when treating breast boost patients with IMAT in DIBH. Materials and Methods Patients with left-sided breast cancer requiring TB boost (TBB) with 12Gy in 4 fractions were prospectively identified. A 5mm TB PTV margin was applied. Residual/IBH error was quantified from post-treatment kV orthogonal paired images (according to excision cavity clip-matching). Vertical, longitudinal and lateral residual errors were determined by calculating the population mean, systematic and random errors. The Marcel Van Herk formula (2.5 Σ + 0.7 σ ) was used to confirm the appropriateness of a 5mm PTV expansion. Results 30 patients were identified between August 2020 and January 2022. This yielded 98 post-treatment kV paired images (mean 3.3 per patient, range 2-4). Two patients had only one post-treatment dataset (protocol deviation) and were excluded from the data analysis. Therefore, 96 image pairs from 28 patients are included in this analysis. The systematic error ( Σ ) was 1.1mm, 1.4mm, and 0.8mm and random error ( σ ) was 1.2mm, 1.3mm, and 0.9mm in the vertical, longitudinal and lateral directions, respectively. Therefore, the IBH variation was <5mm, except for one outlier where the residual error was 6mm in the lateral direction for 1 post-treatment image pair (Figure 1). The Van Herk formula calculates a vertical, longitudinal and lateral TB-PTV margin of 3.6mm, 4.2mm, and 2.8 mm respectively; confirming that the IBH variation is within the 5mm margin.

Conclusion