ESTRO 2021 Abstract Book

S517

ESTRO 2021

PTV_Low respectively. Table 1.shows all 95 plans complied with the planning constraints at the dose metrics for each structure at the 25 Gy plan and each dose level (27.5Gy, 30Gy, 32.5Gy and 35Gy). Figure 1a. shows all patients achieved the planning constraint for PTV_High D95% > 95% at each dose level. Figure 1b. shows for PTV_Low D50% = 99-101% the majority of plans achieved the optimal constraint up to the 32.5.Gy level and at 35Gy dose level all plans were within mandatory constraints.

Conclusion Hypofractionated dose escalation to the primary tumour up to 35Gy is feasible in SCRT for rectal cancer. All plans met our a priori definition of feasibility at 32.5Gy (EQD2 53.2Gy). Above this at the 35Gy level, dose to PTV_Low exceeded the optimal constraints, but achieved mandatory. Future analysis will explore this effect. 1.Bahadoer et. al. SCRT followed by chemotherapy before total mesorectal excision versus preoperative chemoradiotherapy, TME, and optional adjuvant chemotherapy in locally advanced rectal cancer (RAPIDO): Lanc Oncol.2021;22(1):29-42.

Poster highlights: Poster Highlights 24: Imaging acquisition and processing

PH-0649 Left-ventricular MRI navigation: a novel approach for motion-free thoracic and cardiac radiotherapy A. van Lier 1 , O. Akdag 1 , S. Mandija 1 , P. Borman 1 , E. Alberts 2 , M. Fast 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands; 2 Philips, Philips, Best, The Netherlands Purpose or Objective Cardiac motion adversely affects the accuracy of thoracic and cardiac radiotherapy. Depending on the target location, cardiac motion in excess of 1 cm may pose a larger challenge than respiratory motion. Ideally, radiotherapy treatment delivery is tailored to minimize the effect of both types of motion. A 1D navigator is commonly used to minimize respiratory motion in MR images by measuring the liver/lung interface position to gate each MRI acquisition. Here, we hypothesize that the left-ventricular (LV)/lung interface can be used to capture both cardiac and respiratory motion and that this interface can be used as a 1D navigator structure to obtain motion free end- exhale/diastolic images of the thorax. This would allow cardiac gated imaging without the need for additional cardiac imaging hardware. This is especially appealing for MR-linac treatments, in which integration of the cardiac hardware is not standard and raises concerns about radiation interaction (e.g. hardware malfunctioning). We will quantify the potential benefit of LV navigation over the conventionally used liver/lung navigation with respect to obtaining a motion-free thoracic MRI. Materials and Methods First, real-time (10 Hz) free-breathing 2D coronal TFE images of the heart were acquired on a 1.5T Ingenia (Philips) or a 1.5T Unity MR-linac (Elekta) for 3 healthy volunteers. Next, LV-navigator gated coronal TFE images (2 mm gating window) were acquired of the same plane of the cine scans (Figure 1, top). For benchmarking, a liver/lung navigated gated scans were performed. Residual motion between the dynamics was quantified using rigid registration based on normalized cross correlation on the liver dome and the LV