ESTRO 38 Abstract book

S1202 ESTRO 38

A CBCT was acquired before each treatment fraction to correct for inter-fraction positioning errors. We reported those position displacements for the three axes (AP, LR, SI). We finally correlated the magnitude of the displacements with the tumor localization (upper-, medium, or low lung lobe). Results Figure 1 shows that the largest displacement was observed along the vertical direction. The medium lung lobe was the localization with the lowest dispersion of the displacements. The largest range of displacements (in vertical direction between -1.8 cm and 0.3 cm and in lateral direction between -0.6 cm and 1.2 cm) was observed in the lower lobe.

whether omitting the skin marks and positioning based on tattoos only, would have an influence on the initial setup error within the context of image guided RT (IGRT). Within our department lung cancer patients will be treated according an online IGRT protocol, therefore the rotations are of most interest. In case an offline IGRT protocol is used in other departments, besides the rotations, the translations are also relevant when omitting skin marks. Material and Methods To analyze the results we selected 2 groups of lung cancer patients, treated with a fractionated radical dose prescription and a daily online IGRT protocol, aligned using 5 tattoos (3 anterior, 2 lateral) and positioned on a thorax support. Group A, the control group with skin marks, included 37 patients. Group B, a prospective cohort, included 30 patients irradiated without skin marks. For both groups, grand mean, systematic (Σ) and random (σ) errors were calculated using the initial set up errors from 1388 Cone Beam Computed Tomography (CBCT) scans. This study was communicated to all RTT’s within the department, but no additional set up instructions were given. The statistical significance for the systematic and random errors were tested using Levene’s test and Mann-Whitney U-test, respectively. Results The mean for the translations in both groups are < 0.1 cm. The Levene’s test and Mann-Whitney U test showed no statistically significant difference between the group standard deviations. Conclusion Omitting skin marks for patients treated in the thoracic area results in an equivalent initial setup error for both translations and rotations. Patients might benefit, as there will be no risk of any allergic reactions from the ink and they do not have to retrace the skin marks. EP-2176 Analysis of inter-fraction tumor position variability in lung SBRT A. Coral 1 , N. Espinosa 1 , A. Latorre 1 , S. Bermejo 2 , X. Nolla 1 , G. De Segura 1 , C. Diaz 1 , M. Mancera 1 1 Hospital de la Santa Creu i Sant Pau, Servei de Radiofísica i Radioprotecció, Barcelona, Spain; 2 Hospital de la Santa Creu i Sant Pau, Servei d'Oncologia Radioteràpica, Barcelona, Spain Purpose or Objective The aim of our study was to compare the tumor position differences ( displacements ) between the simulation TC and the CBCT performed prior each treatment fraction. We also correlated the magnitude of the displacements and the tumor location (lobe) in the lung. Material and Methods We retrospectively analyzed 112 oligometastasic lung cancer patients between December 2013 and August 2018. The patients underwent a simulation 4D-TC with limited free breathing obtained by abdominal compression, using Respiratory Belt (CIVCO). The positioning devices ware a thorax support (Posirest-2, CIVCO) and vacuum mattress (Urethane MTVLPLV01, CIVCO).The positioning in the treatment unit was based on the simulation skin marks (tattoos) followed by the isocenter displacements as indicated by the treatment plan.

Conclusion Tumors located in the lower lobe have important mobility in all directions. We suggest performing a stronger diaphragmatic compression or other respiratory control strategies with the objective to reduce this movement. The largest displacements in all lung locations (upper-, medium-, lower lobe) were observed in the vertical axis. To reduce the displacement of the upper lobe, attributable to the positioning of the arms, implementation of the new systems of immobilization, as a thermoplastic mesh, or surface recognition systems are recommended. IGRT techniques are essential to ensure a good reproducibility of the treatment. EP-2177 4D CBCT based determination of tumor amplitude variation in lung cancer SBRT J. Papp 1,2 , M. Simon 1,2 , E. Csiki 1 , E. Csobán 1 , A. Molnár 1 , P. Árkosy 3 , Á. Kovács 1,2 1 University of Debrecen Med. and Health Science Centre- Clinic of Oncology, Department of Radiotherapy, Debrecen, Hungary ; 2 University of Pécs Faculty of Health Sciences, Doctoral School of Health Sciences, Pécs, Hungary; 3 University of Debrecen Med. and Health Science Centre- Clinic of Oncology, Department of Oncology, Debrecen, Hungary Purpose or Objective Stereotactic Body Radiation Therapy (SBRT) is often performed with complex immobilization systems paired