ESTRO 2020 Abstract Book

S885 ESTRO 2020

for treatments. The intrafraction motion evaluation has been performed by investigating the residual isocenter shifts during beam on time. Mean, minimum, maximum and SD displacements in isocenter position over all the treatment fractions of each patient were evaluated for the two sites separately. Results Intra-fraction mean isocenter shift over all the patients was 2.7±0.5 mm for tangential and 2.5±0.4 mm for locoregional treatments. The isocenter average shift for each patient ranged between 1.8mm-3.4mm for tangential and 1.9mm-3.1mm for locoregional treatments. The box plot of interfraction mean shift during beam on time of treatment delivery is shown in fig.1. Results indicate that locoregional treatments are characterized by slightly lower isocenter shifts than tangential although this difference is not statistically significant (t test p-value 0.2). This is probably due to the tighter tolerance set for the gating window amplitude and the isocenters shifts for locoregional treatments. However, locoregional treatments have usually a longer duration than tangential ones (4 beams for locoregional instead of 2 for the tangential treatments) and therefore the patient movements during the treatment can be larger. locoregional

may lead to under/overdosage of the target/organs-at-risk (OARs), compromising the patient outcome. The research aims to evaluate the impact of positioning errors in the dosimetry of volumetric modulated arc therapy (VMAT) for left-sided PMRT. Material and Methods Eleven VMAT treatment plans clinically delivered were included in this study. For each plan, eighteen perturbations were introduced shifting the isocenter from its reference position of 3, 5, 10 mm, in the inferior, superior, anterior, posterior, left and right directions. The dose-volume histogram (DVHs) of the target, left lung and heart were obtained for each perturbation introduced and case studied. A total of 209 DVHs were analysed. The absolute difference of the mean dose (Dm) and of the DVH endpoints Vx and Dy (percentage volume receiving x Gy, and dose covering y% of the volume, respectively), were used to compare the dosimetry of reference versus perturbed plans. Particularly Dm, V25, and V40 for the heart, Dm, V20, and V40 for the left lung, and D95, D98, Dm, and D1 for the clinical target volume (CTV). In Figure 1, the DVHs obtained for isocenter shifts of 10, 5, 3 mm, for a representative patient were shown.

Conclusion DIBH procedure guided by optical systems for left breast irradiation is a reproducible and stable tecnique with a limited intra-fraction DIBH variability both for tangential and locoregional treatments.

Poster: Physics track: Adaptive radiotherapy and inter- fraction motion management

PO-1622 Impact of positioning errors in the dosimetry of VMAT left-sided post mastectomy irradiation X. Liao 1 , F. Wu 1 , J. Wu 1 , Q. PENG 1 , X. Yao 1 , S. Kang 1 , Y. Zhao 1 , L.C. Orlandini 1 1 Sichuan Cancer Hospital and Institute, Radiation Oncology, Chengdu, China Purpose or Objective Post-mastectomy radiation therapy (PMRT) is technically difficult given the complexity of the target volume and its proximity to critical structures. Intensity modulated radiation therapy can achieve highly conformal dose distributions, nevertheless, inaccuracy in the positioning