ESTRO 2021 Abstract Book

S178

ESTRO 2021

Conclusion Very high precision in CBCT guided and OSMS-monitored spinal SABR was observed in this cohort. The lowest intrafraction motion was seen in patients treated with arms above their head and a body mask. In future clinical practice, patients will preferably be positioned with arms above their head. Special attention is necessary for cervical spine locations to reduce (rotational) intrafraction motion. The use of OSMS is an added value for patient monitoring during radiation treatment. *AlignRT, VisionRT LtD, London, UK PH-0269 Intrafractional monitoring of patients in four immobilization mask systems for cranial radiotherapy D. Reitz 1 , J. Muecke 1 , V. Mendes da Silva 1 , G. Landry 1 , M. Reiner 1 , M. Niyazi 1 , C. Belka 1 , S. Corradini 1 , P. Freislederer 1 1 University Hospital, LMU Munich, Department of Radiation Oncology, Munich, Germany Purpose or Objective Patients receiving cranial radiotherapy are immobilized with a custom-made thermoplastic mask in order to restrict patient motion, especially when using very tight margins in stereotactic radiotherapy. Depending on the target volume margins and individual dose applied, different mask systems are used for irradiation. Intrafractional movements of the head can be monitored using the ExacTrac Dynamic (Brainlab AG, Germany), which is a combination of X-ray imaging and hybrid surface and thermal imaging scanner. The goal of the present work is comparing the magnitudes of intrafractional deviation for all the different mask systems in order to determine the best possible solution for the specific treatment modality. Materials and Methods A total of 40 patients with four different head mask systems ( Brainlab Cranial 4Pi Open Face Mask [BLOF], IT V iCAST Head Double Micro Open Mask – Open mask [OM], Brainlab Cranial 4Pi Stereotactic Mask [BLSRS], IT V iCAST Head Maxi Double - Double mask [DM] ) were observed during the irradiation series. In total 490 treatment fractions and 3708 X-ray images were monitored. Mainly malignant brain tumors and brain metastases were treated, but also benign diseases such as meningiomas or vestibular schwannomas. The results of intrafractional X-ray corrections for all 3 translations and 3 rotations were compared between the different mask systems. Stereoscopic X-rays images were acquired for each treatment beam, at possible gantry positions 0°, 90°, 180°, and 270°. We calculated mean, standard deviation and the 95% confidence interval of all 3708 patient datasets.