ESTRO 2020 Abstract book

S963 ESTRO 2020

Between May 2019 and October 2019 a total of 14 patients were treated with cranial SRT. All patients were immobilized in a QFix (Avondale, PA, USA) stereotactic open face mask. The system consists of a baseplate, a moldable head cushion and an open face thermoplastic mask. The masks are strengthened with kevlar and has a thickness of 2.4 mm. Open face masks reduce anxiety and increase patient comfort. All patients were treated with modulated arc therapy with coplanar and non-coplanar arcs on a linear accelerator equipped with cone-beam CT (CBCT) and a six degree-of-freedom (6DoF) treatment couch. A CBCT scan was acquired after positioning the patient on the treatment couch using the reference marks. Translational (x,y,z) and rotational(yaw, pitch, roll) offsets were identified, recorded and carried out. Offsets from pre-treatment registration can be analyzed in terms of systematic error (SE) and random error (RE) components. Overall population mean set-up error, population SE and population RE components were derived from the acquired data. Results The total number of CBCT scans was 75. The displacements ranged from Superior-Inferior (SI) (-0.7 to 0.3), Anterior- Posterior (AP) (-0.5 to 0.6), Left-Right (LR) (-0.6 to 1.0), yaw (-2.9 to 2.9), pitch ( -2.9 to 3.7), roll (-3.1 to 2.9). Overall population mean error values were identified to reveal any underlying common errors in this patient cohort. Translational components were: -0.07 cm, +0.08 cm and -0.01 cm in SI, AP and LR directions respectively. Rotational overall population mean error values were: - 0.09°, 0.06°and 0.26° in yaw, pitch and roll directions respectively. SE component values were: 0.12 cm, 0.13 cm, 0.21 cm in SI, AP, LR and 1.07°, 1.19° and 1.32° in yaw, pitch, roll directions. RE component values were: 0.15 cm, 0.14 cm, 0.21 cm and 0.99°, 0.9° and 1.17°. Applying the van Herk formula for treatment margins the components are 0.4 cm, 0.41 cm and 0.66 cm in SI, AP and LR directions respectively. Conclusion The van Herk formula for translational errors slightly overestimates the necessary treatment margins in case of daily online IGRT, but the rotational components are not included. In case of non-spherical target volumes, rotational components can have dosimetric impact on the disease site. Population mean error values indicate, there are no common errors included in the system, but both, translational and rotational, SE and RE components suggest that daily IGRT should be carried out for this group of patients. Future work would include utilizing a bite block to reduce rotational components. PO-1662 Influence of ITV-PTV margins on interfraction dosimetric variations in lung SBRT. A. Garcia Perez 1 , A. Gonzalez-Pose 1 , A. Lopez-Medina 1 , A.G. Teijeiro-Garcia 1 , J. Vazquez-Rodriguez 1 , B. Andrade- Alvarez 1 , D. Medal-Francesch 1 , F.J. Salvador-Gomez 1 , M. Salgado-Fernandez 1 1 Hospital Meixoeiro, Radiofisica y PR, VIGO, Spain Purpose or Objective Ablative doses are delivered in lung Stereotatic Body Radiation Therapy (SBRT), hence margins must be reduced as much as possible to reduce doses to organs at risk (OARs), but tumor coverage must remain unchanged during the whole treatment. We perform a dosimetric analysis of the GTV coverage in each fraction varying the ITV’s margin to generate different PTVs. Material and Methods Ten patients treated with lung SBRT have been analyzed. Prescribed dose was 50 Gy (5 fractions x 10 Gy). Seven of them have been treated with dynamic arctherapy, and the remaining three with VMAT. Lung SBRT treatment plans were generated based on an average CT from a 4D-CT. To create the ITV, a radiation oncologist contours the GTV on all 10 breathing phases, as well as their subsequent union.

region margins due to expected differences in range of motion. The applicability of a specific PTV margin was quantified by calculating the overlap of the generated PTVs with the CTV delineated on the post-treatment MRI. The margin with 95% overlap in 90% of fractions was considered acceptable, as used by Kleijnen et al., Acta Oncologica, 2019.

Results In total 30 fractions of 6 patients were analyzed. The min/max range of overlap in all 30 fractions of the mesorectum PTV margin was 99.9-100% for PTV A, 98.8- 100% for PTV B and 97.7-100% for PTV C. For the lymph node PTV margin, the range of overlap in all 30 fractions was 100% for PTV A, 99.0-100% for PTV B and 96.4-100% for PTV C. All applied margins had more than 95% overlap in 90% of fractions. More specific, the smallest PTV margin applied (PTV C) had both for the mesorectum as for the elective lymph node regions >99% overlap with the post- treatment CTV in 93% of fractions (figure 1).

Conclusion Based on intrafraction motion as observed in this patient cohort the 2&3mm PTV margin expansion (PTV C) seems large enough to compensate for the observed intrafraction variations. This indicates that a significant PTV margin reduction with online adaptive MR-guided radiotherapy on a MR-linac is feasible. PO-1661 Treatment setup errors in cranial SRT patients with non-invasive immobilization technique M. Simon 1,2 , E. Csiki 1 , J. Papp 1,2 , D. Solymosi 1 , Á. Kovács 1,2 1 University of Debrecen, Clinic of Oncoradiology, Debrecen, Hungary ; 2 University of Pécs, Faculty of Health Sciences- Doctoral School of Health Sciences, Pécs, Hungary Purpose or Objective Linac based fractionated cranial stereotactic radiotherapy treatments (SRT) require an immobilization that facilitates planning and accurate treatment delivery. Non-invasive immobilization techniques are preferred mainly because of patient comfort. The aim is to quantify the setup error components for a special stereotactic mask based non- invasive SRT treatment immobilization technique. Material and Methods