ESTRO 2021 Abstract Book

S1476

ESTRO 2021

“VMAT technique” or “younger than 55” cases seem to show lower uncertainties than “3D technique” or “older than 55”.

Conclusion The results for the DIBH margins are similar to those obtained in other studies.

There are some little differences between DIBH and non-DIBH cases. The larger margins for DIBH cases seem to be coming from the trend of the patients staying above the upper half of the gating window while holding the inspiration and losing amplitude towards the end of the irradiation. In addition, an inter-inspiration fatigue often appears towards the end of the fraction, causing further deviation from the reference. The number of treatment fields could explain the differences between 3D and VMAT. VMAT treatment consists in two arcs of treatment, while 3D treatments can content between 6-14 treatment segments. The difference in number of fields seem to produce a fatigue during the fraction, which leads to a larger uncertainty. Regarding age differences, older patients show more difficulty to hold breath inspiration along the whole fraction. This could be the reason behind the uncertainty difference. It is remarkable how longitudinal displacements are the biggest in the DIBH cases. This could be due to the quick patients’ relaxation after irradiation, inducing small movements in the patients’ position. Reducing the gating window could reduce these uncertainties and margins, while still minimizing the cardiopulmonary dosimetry that this technique already offers. PO-1752 Intrafractional motion detection for spine SBRT via X-ray imaging using ExacTrac Dynamic J. Mücke 1 , D. Reitz 1 , G. Landry 1 , M. Reiner 1 , C. Belka 1 , M. Niyazi 1 , P. Freislederer 1 1 University Hospital, LMU Munich, Department of Radiation Oncology, Munich, Germany Purpose or Objective Spine stereotactic body radiotherapy (SBRT) is becoming a standard of care for selected patients with spine metastases. Especially for those in the oligometastatic state of disease, SBRT can prolong progression-free survival through the delivery of high biological effective doses (BED). Consequently, it can potentially delay entry to the next line of systemic therapy. Due to its close vicinity to critical structures, especially the spinal cord, standards for safety for spine SBRT should be high. Recent advancements in technology have made intrafractional motion detection and correction of positioning during each session of radiotherapy possible. Here we present the first results of high accuracy patient monitoring via X-ray imaging with ExacTrac Dynamic (Brainlab, Munich, Germany) during spine SBRT for patients without any additional immobilization. Materials and Methods Intrafractional x-ray data was collected from patients receiving spine SBRT. Metastases were located in the thoracic and lumbar spine. No immobilization devices or vacuum mattresses were used. Target volume definition was performed according to the international spine radiosurgery consensus guidelines. Fractionation schedules included 2x 9/12 Gy (CTV/SIB) and 5x 5/6 Gy applied in VMAT technique with the maximum dose limited to 125%. Intrafractional motion was detected in six degrees of freedom using ExacTrac Dynamic, which combines an optical surface and thermal imaging with real-time x-ray tracking (Figure 1). Tolerance for intrafractional correction was 0.7 mm for translational errors (lateral, longitudinal, and vertical) and 0.5° for rotational errors (pitch, roll, and yaw).