Abstract Book

S1061

ESTRO 37

expressions utilized for margin calculations were the following:

Figure 1: Equations to calculate different contributions of the margin recipe. Equations [1] and [2] represent the effective contributions to the systematic and random components due to the reduced number of fraction. Equation [3] includes the penumbra contribution in lung tissue and equation 4 is the total margin expression used. Results Results yielded by equations [1-4] and systematic contributions are presented in table 1.

Conclusion We confirmed that repainting can be used effectively to mitigate the interplay effect. Furthermore, the intrinsic repainting of POne shows to be more effective and reliable in mitigating motion effects than the controlled repainting performed with PPlus. Future work includes the confirmation of these results for a different number of controlled repaintings. Also the use of a range shifter for the PPlus machine will be further evaluated to exclude any bias in the comparison between PPlus and POne. Ref: [1] C. Ribeiro, “Comprehensive prospective evaluation tool for treatments of thoracic tumours with scanned protons”, EP-1625, ESTRO 2017 EP-1952 CTV-PTV margin calculation for lung SBRT treatments. A. Prado 1 , G. Pozo 2 , A. Milanés 2 1 Hospital Universitario 12 de Octubre, Radiofísica y Protección Radiológica, Madrid, Spain 2 Hospital Universitario 12 de Octubre, Servicio de Oncología Radioterápica. Sección de Radiofísica., Madrid, Spain Purpose or Objective To obtain CTV-PTV margins for lung SBRT considering inter-fraction and intra-fraction motion, error delineation, respiration, penumbra contributions, number of fractions, a 90% confidence level (α=2.5) and 95% prescription isodose (β=1.64). Material and Methods SBRT treatments were performed on a Varian Clinac iX (Varian Medical Systems, Palo Alto. CA) with an On Board Imager (OBI) unit. A cohort of 10 NSCLC patients was selected, with a dose prescription of 60 Gy in 8 fractions (7.5 Gy/fx). After a proper patient immobilization using a thermoplastic body mask, a cone beam (CBCT) was performed. Following the registration procedure a second CBCT was acquired to account for set up variations. At the end of the treatment delivery a third CBCT was made. Intra-fraction motion was estimated as the difference between pre and post treatment CBCT registrations (Δx lat , Δx vert , Δx long ). Inter-fraction motion was calculated as the difference between pre-treatment CBCT registrations of consecutive fractions. Delineation error was set to 1.5 mm. Respiration uncertainties were estimated as 1/3 of the standard deviation in each direction obtained as of the 10 GTVs delineated using every 4DCT phase. Penumbra standard deviation was obtained from published data (6.4 mm for lung). The

Table 1: Results obtained for random, systematic and total contributions. For hypofractionated treatments the error in estimating the average value is palpable when calculating the systematic and random standard deviations contributions, as shown in equations [1] and [2]. This effect diminishes for increasing number of fractions. The inclusion of the penumbra factor clearly reduces the random contribution to the margin, as σ P >> σ (eq.3). A more exhaustive analysis might take into account tissue heterogeneities nearby the tumour regarding penumbra contributions. Margin asymmetry in the lateral coordinate is directly correlated with lower respiratory amplitude in this direction as expected. Conclusion Margins were obtained for a cohort of 10 patients with NSCLC undergoing lung SBRT treatments. Inter-fraction and intra-fraction motion, delineation error, respiration, penumbra contribution, number of fractions and set up errors were considered. Margin calculations were carried out using a 90% confidence level (α=2.5) and 95% prescription isodose (β=1.64). Values obtained are in agreement with published results that appear in the literature. EP-1953 Dosimetric effect of using FFF-VMAT beams for left-sided breast radiotherapy in breath hold C. Tamburella 1 , G. Faessler 1 , G. Guibert 1 , L. Hirschi 1 , L. Pion 1 , P. Weber 1 , P. Tsoutsou 1 1 Hôpital Neuchâtelois, radiotherapie, la chaux de fonds, Switzerland Purpose or Objective The association of volumetric arc therapy (VMAT) and deep inspiration breath hold (DIBH) is increasingly used to reduce the dose to the heart during left-sided breast radiotherapy (RT). Using flattening-filter-free (FFF) beams allows a higher dose rate which can speed up the delivery time in DIBH. In this study, we evaluate the dosimetric influence by using FFF-VMAT beams compared to Flattening Filter (FF) VMAT.